The key role of diabatic processes in modifying the upper-tropospheric wave guide: a North Atlantic case-study

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1 Quarterly Journal of the Royal Meteorological Society Q. J. R. Meteorol. Soc. 137: , October 2011 B The key role of diabatic processes in modifying the upper-tropospheric wave guide: a North Atlantic case-study Christian M. Grams, a * Heini Wernli, b Maxi Böttcher, b Jana Čampa, c Ulrich Corsmeier, a Sarah C. Jones a Julia H. Keller, a Claus-Jürgen enz a and ars Wiegand d a Institute for Meteorology and Climate Research (IMK-TRO), Karlsruhe Institute of Technology (KIT), Germany b Institute for Atmospheric and Climate Science, ETH Zürich, Switzerland c Institut für Physik der Atmosphäre, Universität Mainz, Germany d Institute for Climate and Atmospheric Science, School of Earth and Environment, University of eeds, UK *Correspondence to: C. M. Grams, Institute for Meteorology and Climate Research, KIT Campus Süd, Karlsruhe, Germany. grams@kit.edu This study highlights the importance of diabatic processes for the complex interaction of weather systems in the North Atlantic European sector during the week of 7 14 September A chain of events occurred including the extratropical transition (ET)of hurricanehanna, a subsequently developing extratropical cyclone, the formation of an upper-level potential vorticity (PV) streamer that protruded towards Europe and triggered intense rainfall, and the genesis of a Mediterranean cyclone. A PV perspective is adopted along with trajectory calculations to elucidate the diabatic modification of the midlatitude flow. Important diabatic PV modifications occurred at upper levels, associated with the cross-isentropic transport of low-pv air within warm conveyor belts (WCBs). These were diagnosed during the ET of Hanna and the development of the extratropical cyclone near Newfoundland. The WCBs contributed to the amplification of ridges downstream of each cyclone and to the subsequent elongation of Hanna s upstream trough into a PV streamer. This streamer eventually triggered the Mediterranean cyclogenesis. The second major effect of the diabatic processes occurred on smaller scales, in the low and middle troposphere. The remnants of Hanna s tropical PV core advected moist air towards the baroclinic zone leading to condensational PV production in the lower troposphere. In contrast, in the case of the extratropical cyclone, diabatic PV production occurred within its WCB at mid levels. These diagnostic analyses corroborate the potential of diabatic processes associated with extratropical flow systems for the modification of both the low-level vortices and the upper-level Rossby wave guide. Copyright c 2011 Royal Meteorological Society Key Words: potential vorticity; Rossby waves; warm conveyor belt; extratropical transition; diabatic processes; hurricane Hanna; extratropical cyclone Received 22 November 2010; Revised 27 June 2011; Accepted 30 June 2011; Published online in Wiley Online ibrary 17 August 2011 Citation: Grams CM, Wernli H, Böttcher M, Čampa J, Corsmeier U, Jones SC, Keller JH, enz C-J, Wiegand The key role of diabatic processes in modifying the upper-tropospheric wave guide: a North Atlantic case-study. Q. J. R. Meteorol. Soc. 137: DOI: /qj.891 Copyright c 2011 Royal Meteorological Society

2 Diabatic Modification of the Upper-Tropospheric Wave Guide Introduction It is well known that diabatic processes, and especially latent heat release due to the condensation of water vapour, can have a profound impact upon the development of extratropical cyclones (e.g. Danard, 1964; Tracton, 1973; Uccellini, 1990; Kuo et al., 1991; Davis et al., 1993). In the framework of potential vorticity (PV), the key effect of condensational heating is to produce anomalously high PV in the lower troposphere and to reduce the air parcel s PV in the upper troposphere (Hoskins et al., 1985; Stoelinga, 1996). Within rapidly ascending airstreams associated with extratropical cyclones, so-called warm conveyor belts (WCBs; e.g. Browning, 1990; Eckhardt et al., 2004), the continuous latent heating leads to a diabatic production of PV along the flow as long as the air parcels are below the level of maximum heating, and also to a diabatic reduction of PV during the further ascent close to the tropopause level (Wernli and Davies, 1997; Pomroy and Thorpe, 2000). At low and mid levels, a positive PV anomaly is created that contributes to the further poleward advection of warm and moist air downstream of the WCB. At upper levels, the typically low PV values (0 0.5 PVU) in the WCB outflow contribute to the formation and/or intensification of a negative PV anomaly below the tropopause. In some cases, these anomalies can have a significant impact upon the downstream flow evolution at the tropopause level and contribute to downstream Rossby wave breaking and the formation of stratospheric PV streamers (Massacand et al., 2001; Knippertz and Martin, 2005). In other cases of eastern North Atlantic wave breaking events (Meier and Knippertz, 2009), the impact of upstream latent heating was found to be less relevant. Diabatic processes play a crucial role in the interaction of a tropical cyclone with the midlatitude flow during extratropical transition (ET; Jones et al., 2003). In the early stages of ET, the inner-core convection and associated upper-level outflow of the TC can contribute significantly to downstream ridge building by the advection of low-pv air and result in the excitation of a Rossby wave train (RWT; Bosart and ackmann, 1995; Riemer et al., 2008; Torn, 2010). As ET progresses, diabatic processes become concentrated in the region of warm frontogenesis (Harr and Elsberry, 2000; Klein et al., 2000; Evans and Hart, 2003). Here latent heat release due to condensational processes leads to diabatic PV production in the low and middle troposphere and diabatic PV reduction in the upper troposphere. Both are enhanced by a WCB-like advection of tropical warm and moist air towards the baroclinic zone that separates the subtropics from the midlatitudes. As in the case of WCBs, the enhanced ridge building may promote downstream Rossby wave breaking and lead to cyclogenesis over Europe (Hoskins and Berrisford, 1988; Pinto et al., 2001; Agust í- Panareda et al., 2004; Riemer and Jones, 2010). Furthermore, an ET system may reintensify as an extratropical cyclone and have a direct impact in midlatitudes (Browning et al., 1998; Thorncroft and Jones, 2000; Agustí-Panareda et al., 2005). The modification of the upper-level PV structure due to diabatic processes in the western and central Atlantic has important consequences for the development of highimpact weather over Europe. For example, in the genesis of cyclones in the western Mediterranean, the key role of upper-level positive PV anomalies is well established (Bleck and Mattocks, 1984; Tafferner, 1990). These and later case-studies (e.g. Kljun et al., 2001; Romero, 2001; Buzzi et al., 2003) highlight the importance of the intense cyclonic flow and the reduced static stability induced by the approaching stratospheric PV disturbance for cyclogenesis to occur. Typically, these PV disturbances appear in the form of elongated PV streamers, which can be regarded as indicators of Rossby wave breaking near the extratropical tropopause. So far, only little attention has been given to the processes that occur upstream of these wave-breaking events associated with Mediterranean cyclogenesis. In their model experiment of a Mediterranean cyclone and heavy precipitation event in September 1993, Massacand et al. (2001) pointed to the importance of upstream diabatic effects for the formation of the PV streamer and associated high-impact weather. To date, the diabatic effects upon the evolution of cyclones and the downstream Rossby wave development have been investigated primarily in detailed case-studies of individual cyclones and wave-breaking events. One exception is the study of Cordeira and Bosart (2011), who investigated the impact of diabatic processes on the interaction of several cyclones in the northwestern Atlantic region. In the present study, the role of diabatic processes during a one week sequence of cyclonic weather events over the North Atlantic European sector in September 2008 is analysed. The period has been selected because of a reduced predictability in the North Atlantic and European region (section 3) and the diversity of the associated weather systems. It features the ET of hurricane Hanna, the development of an extratropical cyclone upstream of ex-hanna and downstream of Newfoundland, and the formation of an elongated PV streamer in the eastern North Atlantic. The PV streamer evolves into an upper-level cut-off low over the Mediterranean. This cut-off low in turn induces the genesis of a Mediterranean cyclone with associated heavy precipitation. A PV perspective is adopted to diagnose the effect of latent heating on these dynamically linked weather systems. A primary focus is on the role of WCBs associated with the tropical cyclone undergoing ET and the subsequent extratropical cyclone for the formation of the upper-level PV streamer. A secondary focus is on the linkage between the WCBs and the diabatic PV production in the low and middle troposphere in both developing cyclonic flow systems. The article is organised as follows. In section 2 we give a brief overview of the data and methods used. The motivation for the choice of the study period is discussed in section 3. In section 4 we describe the synoptic evolution during the period and the corresponding significant rainfall events in Europe and the Mediterranean region. In section 5 we discuss the key role of diabatic processes for the upper-level wave evolution and the formation of the PV streamer. This analysis, focusing on the diabatic reduction of PV at upper levels, is complemented by an investigation of the role of diabatic PV production at lower levels within the cyclonic systems. Finally, we summarise the results in section 6 and give our conclusions. 2. Data and tools 2.1. Data The main dataset used in this study consists of the 6-hourly operational analyses of the European Centre for Medium Range Weather Forecasts (ECMWF) with T799

3 2176 C. M. Grams et al. spectral resolution and 91 vertical levels, interpolated to a regular horizontal grid with 0.25 horizontal resolution. Meteorological quantities which are well suited to highlight the diabatic processes and the dynamical evolution of the weather systems discussed, are calculated from the ECMWF fields on model levels. These quantities are potential vorticity (PV), (equivalent) potential temperature (θ (e) ), and the PV production rate (PVR) due to condensational heating. The calculation of the diabatic heating rate follows Berrisford (1988): θ = c p κθω p dq s dt 1 + c p dq s dt { 1 exp ( h0 h 5 )}, (1) if ω<0 (ascending motion) and the relative humidity h > h 0 = 80%. Here denotes the latent heat of condensation of water, c p the heat capacity of water vapour at constant pressure, and q s the saturation mixing ratio. The basic assumption is that condensation occurs where ascending air is (nearly) saturated. The heuristic threshold function [1 exp{(h 0 h)/5}] has been introduced by Berrisford (1988) to account for potential saturation on the subgrid scale. The same diagnostic approach has been used previously in the studies, e.g. by Wernli and Davies (1997) and Rossa et al. (2000), and as in these studies, the diabatic PV production rate is then calculated according to the first term on the left-hand side of Eq. (70a) in Hoskins et al. (1985), reformulated in pressure coordinates. The intensity and position of cyclones are estimated by tracking the minimum mean sea level pressure (pmsl) in the ECMWF analyses, using the algorithm of Wernli and Schwierz (2006). Uncertainties in medium-range numerical weather predictions associated with the weather systems investigated in this study are assessed in section 3 using the THORPEX Interactive Grand Global Ensemble (TIGGE; Bougeault et al., 2010). Ensemble forecast data from ten different ensemble prediction systems (EPSs) are brought together in TIGGE to form a multi-model ensemble. The impact of the tropical cyclone Hanna on the midlatitude flow is studied using the mesoscale numerical weather prediction model COSMO (COnsortium for Small-scale MOdelling, Version 4.13; Steppeler et al., 2003; Schättler et al., 2009). Initial and boundary conditions for these simulations are taken from the ECMWF analyses. The horizontal resolution of the COSMO simulations is 0.25 and 80 vertical levels are used. The vertical levels are defined to be as close as possible to the vertical levels of the deterministic ECMWF model. Various physical processes (e.g. subgrid-scale turbulence, moist convection) are parametrised following the operational setup. The cloud structures along with the synoptic evolution are discussed based on observational data from channel 7 (thermal IR, wavelength µm) of the Spinning Enhanced Visible and Infrared Imager (SEVIRI) instrument on the Meteosat Second Generation satellite positioned at 0 E. The impact of weather systems in terms of surface precipitation is considered using the gridded daily precipitation E-OBS dataset from the EU-FP6 project ENSEMBES (Haylock et al., 2008). ocal extremes in surface precipitation are determined by 12-hourly accumulated data from the synoptic observations of the Global Telecommunication System (GTS) stations. When comparing these two datasets, one has to keep in mind that the gridded data smooth extremes and thus indicate the spatial extent rather than absolute values (Haylock et al., 2008) Tools A number of tools are applied to investigate the different datasets in more detail. The development scenarios in the medium-range forecasts provided by TIGGE are analysed using a clustering technique. WCBs and the large-scale impact of diabatic processes are identified with the help of trajectory calculations based on the ECMWF analyses. The impact of Hanna on the midlatitude flow is quantified using PV inversion. The technique which we use to analyse the TIGGE ensembles is based on an empirical orthogonal function (EOF) analysis of the multi-model EPS forecast, followed by a clustering of the principal components (PCs). This technique, originally applied to ensemble forecasts of ET by Harr et al. (2008) and Anwender et al. (2008), has been adapted for the TIGGE data by Keller et al. (2011). The EOF analysis is applied to the geopotential height at 200 hpa, because this field captures the tropopause region and is available for all TIGGE ensembles. The EOFs, calculated for the variance covariance matrix of the data, describe the regions of the highest variability in the whole TIGGE dataset at a particular time. Using the PCs of the first and second EOFs, the clustering algorithm allows us to group ensemble members that contribute in a similar manner to the patterns of uncertainty. In this way it is possible to extract the predominant scenarios out of the huge amount of information furnished by the TIGGE ensembles. In order to investigate intense cross-isentropic WCBs and the effect of cloud condensational heating on the uppertropospheric PV structures, extensive trajectory calculations are performed. Using the trajectory tool AGRANTO (Wernli and Davies, 1997), air parcels are selected that ascend more than 600 hpa within a sequence of 6-hourly shifted, 48-hour time periods. This criterion serves to identify WCBs (Wernli, 1997) that play a key role in the sequence of events investigated in this case-study. As a novel diagnostic, we will show isentropic PV (IPV) charts with so-called WCB intersection points. These points mark the location of all WCB trajectories, within a vertical distance of ±2.5 K from the selected isentropic surface at the time of the IPV chart. For instance, for IPV charts on 320 K, WCB intersection points are shown if the trajectory resides in the isentropic layer from K to K. This visualisation will serve to illustrate the influence of WCB outflows on the intensification of upper-level ridges. While ascent of individual air parcels might also fulfil the criteria for the trajectory and intersection point calculations, these illustrations highlight WCBs as coherent bundles of trajectories and clusters of intersection points. IPV charts also help to identify the midlatitude wave guide as a band with a sharp isentropic PV gradient centred around the 2 PVU contour. During the period of interest, the 320 K isentropic surface typically slopes from about 300 hpa on the northern side of this PV gradient to about 550 hpa on the southern side. The structure of the evolving cyclonic systems is studied with vertical profiles of PV. These are calculated along a cyclone s track, following Čampa and Wernli (2011). At

4 Diabatic Modification of the Upper-Tropospheric Wave Guide 2177 (a) (b) downstream ridge / Hanna s ridge upstream ridge upstream Hanna s trough / PV streamer Hanna downstream ridge / Hanna s ridge ds downstream trough upstream trough of upstream cyclone upstream ridge Hanna Hanna s trough / PV streamer Olivia Figure 1. Synoptic overview at 1200 UTC on (a) 9 September and (b) 11 September 2008, showing 500 hpa geopotential height (shading, gpdm), and pressure at mean sea level (black contours with 10 hpa interval). The surface positions and tracks of the cyclones at these times are in (a) and (b) the upstream cyclone ( upstream )and(ex-)hanna ( Hanna ), in (a) the downstream cyclone ( ds ), and in (b) the Mediterranean cyclone Olivia ( Olivia ). arge dots along the cyclone tracks mark the daily positions at 0000 UTC, labelled by the corresponding day of August/September 2008, and small dots mark positions at 1200 UTC. The data and tracks are taken from the six-hourly ECMWF analyses. At the time shown in (b), the Mediterranean cyclone Olivia has not yet developed. each data time (i.e. every 6 h), PV on a given level is averaged within a 200 km radius of the surface cyclone centre (as defined by the local pmsl). A vertical profile is produced from values calculated at 25 hpa intervals between 975 and 100 hpa. The impact of the tropical cyclone Hanna on the downstream midlatitude flow is investigated using a PV inversion technique to remove the positive PV anomaly associated with the tropical storm just before a strong interaction with the midlatitude jet occurs. This method is based on the nonlinear balance equations (Davis and Emanuel, 1991; Davis, 1992) and adapted for usage on tropical weather systems. In a first step, a PV inversion is calculated on the meteorological fields from which the positive PV associated with the TC has been removed while the domain average PV at each level is kept constant. The differences between the resulting balanced fields and the analysis are the horizontal wind, temperature and geopotential anomaly associated with the TC. A spatial mask is applied to define the moisture anomaly associated with the storm. In a second step, the anomalies are subtracted from the original analysis in a region confined to the vicinity of the storm. In this region the horizontal flow in the planetary boundary layer of the modified analysis is altered to account for wind speed reduction and change of wind direction due to frictional processes. These modifications result in an analysis without the TC while conserving the analysis fields in its environment. This modified analysis is used to initialise a COSMO model simulation without Hanna, referred to as NOTC. The application of this TC removal technique on various other cases showed that the removal of the positive PV associated with the TC is sufficient to remove the entire TC including its upper-level anticyclonic outflow. After computing the PV inversion, the outflow anomaly was either not present or it decayed in the first hours of the COSMO simulation. A second model run is initialised with the original analysis, referred to as CNTR. The differences between the CNTR and NOTC runs can be attributed to the effects of the tropical cyclone undergoing ET and thus enable us to quantify Hanna s local and downstream impact on the midlatitude flow. Both COSMO simulations have been computed for 150 h. An earlier version of the TC removal technique is described in Grams and Jones (2010). 3. Motivation for the choice of the study period Two aspects motivated us to study this particular time period starting with the ET of hurricane Hanna (7 September 2008) and ending with the development of the Mediterranean cyclone Olivia (about one week later). These are (i) the apparent dynamical linkages between a variety of weather system categories, and (ii) the reduced predictability associated with these systems. Our period of interest starts with the ET of Hanna ( Hanna in Figure 1(a)). As Hanna moves into the midlatitudes, a trough approaches from the west ( Hanna s trough in Figure 1(a)) and interacts with Hanna, initiating an extratropical reintensification at around 1200 UTC on 9 September We refer to the ET system subsequently as ex-hanna. During ET a ridge develops immediately downstream of Hanna ( downstream ridge or Hanna s ridge in Figure 1(a, b)). Further downstream a trough amplifies ( downstream trough ) and ahead of it a short-lived cyclone develops ( downstream cyclone, ds in Figure 1(a)). Over the following two days, the northern part of ex-hanna s trough wraps up cyclonically and the southern portion elongates significantly and becomes a PV streamer ( PV streamer in Figure 1(b)). Associated with the cyclonic wrap-up, the downstream ridge moves to the north of ex-hanna. At the same time, a new extratropical cyclone develops upstream of ex-hanna near Newfoundland ( upstream cyclone, upstream in Figure 1(a)). It intensifies ahead of a midlatitude trough ( trough of the upstream cyclone in Figure 1(b)) and builds a ridge upstream of (ex-)hanna ( upstream ridge ). Finally, ahead of the PV streamer, the Mediterranean cyclone Olivia develops ( Olivia in Figure 1(b)).

5 2178 C. M. Grams et al. Figure 2. Hovmöller plot of 200 hpa geopotential height standard deviation (gpm) of the ensemble members from the ensemble mean in the TIGGE ensemble forecast initialised at 1200 UTC on 5 September The data are meridionally averaged from 40 Nto60 N. The black star marks the initial growth of uncertainty in the eastern part of the ridge downstream of Hanna. The white line tracks the uncertainty associated with the PV streamer developing upstream of ex-hanna. And upstream marks the time and location of the cyclogenesis of the upstream cyclone. In the ECMWF ensemble forecasts, both the PV streamer and the Mediterranean cyclogenesis are well predicted for lead times of up to 5 days (not shown). However, the ensemble forecasts with longer lead times indicate significant uncertainty in the medium to extended range (not shown). We quantify this uncertainty using a representative TIGGE forecast initialised at 1200 UTC on 5 September The standard deviation of the 200 hpa geopotential height shown in a Hovmöller diagram highlights three regions of uncertainty (Figure 2). A first plume of uncertainty originates at around 30 W, 1200 UTC on 7 September 2008 in the eastern part of the ridge and the western part of the trough downstream of Hanna (marked by the black star in Figure 2). The standard deviation reaches a maximum of more than 90 gpm at 10 W at 1200 UTC on 9 September 2008 and then decreases. A second region of enhanced uncertainty at around 75 W at 1200 UTC on 9 September 2008 develops associated with the trough of the upstream cyclone and spreads downstream. A third pronounced region of uncertainty indicated by the white line in Figure 2 is linked to the evolution of ex-hanna s trough into a PV streamer. To highlight the link between the uncertainty seen in the Hovmöller diagram and the weather systems, we applied the EOF/clustering method to this TIGGE forecast at 1200 UTC on 9 September 2008 (+96 h). The ensemble members are grouped into four different clusters representing differing scenarios in the ensemble (Figure 3). At 1200 UTC on 9 September 2008, the first plume of uncertainty is characterised by major differences in the width and amplitude of the downstream trough between the clusters (25 5 W, Figure 3(a)). A day later the differences in the region of the downstream trough are still apparent but less pronounced (15 W 5 E, Figure 3(b)). The origin of the second region of uncertainty can be seen at 1200 UTC on 9 September 2008 at around 65 W as minor differences between the clusters directly to the east of the upstream cyclone (Figure 3(a)). During the next 36 h the clusters indicate increasing differences in the amplitude of the upstream ridge (Figure 3(b, c)). By 1200 UTC on 13 September 2008 the uncertainty is spread over a broader region associated with the ridge trough couplet of the upstream cyclone (70 20 W, Figure 3(d)). (a) geopotential height at 200 hpa, 12 UTC 09 Sep 2008 (b) geopotential height at 200 hpa,12 UTC 10 Sep 2008 Hanna ds upstream Hanna upstream (c) geopotential height at 200 hpa, 00 UTC 11 Sep 2008 (d) geopotential height at 200 hpa, 12 UTC 13 Sep 2008 upstream Hanna upstream Olivia Figure 3. Cluster mean (labelled with cluster number) and ECMWF-EPS control analysis (dot-dashed contours) of selected contours of geopotential height at 200 hpa (1190 and 1220 gpdm). Data are taken from the TIGGE forecast initialised at 1200 UTC on 5 September 2008 and clustered at 1200 UTC on 9 September Forecast times shown are (a) 1200 UTC on 9 September (+96 h), (b) 1200 UTC on 10 September (+120 h), (c) 0000 UTC on 11 September (+132 h), and (d) 1200 UTC on 13 September (+192 h). The labels mark the surface position of the cyclones discussed in this study.

6 Diabatic Modification of the Upper-Tropospheric Wave Guide 2179 An amplification of ex-hanna s trough between 30 Wand 20 W is consistently indicated in the clusters at 1200 UTC on 10 September 2008 (Figure 3(b)). However, small differences are seen indicating the origin of the third pronounced region of uncertainty. Twelve hours later, these differences in the intensity and tilt of ex-hanna s trough have increased (25 5 W, Figure 3(c)). At 1200 UTC on 13 September 2008, the clusters show major differences between 20 W and 20 E. In particular, cluster 1 shows a trough over the western Mediterranean while cluster 4 exhibits pronounced ridging in this region (Figure 3(d)). This indicates low predictability for the Mediterranean cyclogenesis event. None of the cluster solutions reproduce the analysis at this time, which is characterised by the extension of a deep trough into North Africa and a cut-off over southeastern France. In summary, the analysis of a representative TIGGE forecast showed that the predictability in the North Atlantic European sector in the first half of September 2008 was low. Similar results are found in other TIGGE forecasts. The forecast uncertainty was associated with the evolution of the midlatitude flow down- and upstream of hurricane Hanna undergoing ET and downstream of an extratropical cyclone developing in the western North Atlantic. In the following, we aim to understand the physical processes that governed the evolution of the midlatitude flow during this period by analysing the complex dynamical linkages between the weather systems involved. 4. Synoptic overview 4.1. The evolution of the North Atlantic wave guide In this section, a detailed synoptic overview on the evolution over the North Atlantic and Europe between 1200 UTC on 7 September 2008 and 1200 UTC on 13 September 2008 is given with the help of 48-hourly isentropic PV charts and satellite imagery (Figure 4). The 2 PVU contour approximately represents the midlatitude wave guide. At 1200 UTC on 7 September 2008, Hanna was seen at about 68 W, 43 N moving into a midlatitude ridge located between 60 Wand30 W, 40 Nand55 N (Figure 4(a)). Trajectory calculations suggest that the outflow of hurricane Gustav played a role in building this ridge during the previous three days (not shown). The outflow of Hanna broadened the ridge at its western flank (80 60 W). West of 75 W, the curvature in the 2 PVU contour indicates an upper-level trough approaching from the west. The lowlevel centre of Hanna was located southwest of Nova Scotia with a central pressure of 997 hpa. The WCB intersection points at the upper levels indicate that at this time warm moist air ascends in the eastern and northeastern sector of the decaying tropical cyclone. To the northeast of Hanna s surface centre, in the region of the WCB intersection points, a cloud shield indicates the formation of a warm sector over Newfoundland and Nova Scotia (at about 63 W, 48 Nin Figure 4(b)). During 8 September 2008, the number of WCB intersections at the 320 K isentropic level decreases, indicating a weakening of the ascent region east of Hanna s centre (not shown). Along with this, the associated cloud shield became smaller and separated from the low-level centre (not shown). The central pressure of Hanna remained almost constant at 996 hpa at 1200 UTC on 8 September On 9 September 2008, ex-hanna underwent an extratropical reintensification ahead of the midlatitude trough that approached from the west. The central pressure decreased by 12 hpa in 24 h to 984 hpa. The ridge downstream of ex-hanna was amplified by the advection of low-pv air, which started to wrap cyclonically around the vortex centre (as indicated by the blue shading in Figure 4(c) and the location of the WCB intersection points). To the north of ex-hanna s circulation centre, a new warm sector and associated cloud shield evolved as a consequence of the ascent of warm moist air in a newly developed extratropical WCB (Figures 4(c, d)). The decay of the former warm sector and associated cloud shield went along with the decay of the old WCB of tropical origin. Together with the ridge building, the trough downstream of ex-hanna thinned significantly. The associated downstream cyclone of moderate intensity over Ireland developed beneath this downstream trough and reached its mature stage after deepening by 12 hpa to 993 hpa (Figure 4(c)). The development of its frontal system is clearly reflected in the satellite imagery (Figure 4(d)). As will be shown later, the downstream cyclone had impacts on the European weather (section 4.2) and the downstream ridge building by Hanna was crucial for its evolution (section 5). At the same time, upstream of ex-hanna, a new extratropical cyclogenesis was initiated. In the next 24 h, ex-hanna and the associated trough ridge couplet moved about 20 eastward (not shown). Ex-Hanna intensified and reached its minimum central pressure of 978 hpa at 1800 UTC on 10 September 2008, which is almost as low as during the tropical stage (not shown). In addition to the far northward extension of ex-hanna s ridge, at the western flank of ex-hanna low-pv air wrapped cyclonically around its centre. The downstream cyclone over the British Isles had merged with ex-hanna. North of Newfoundland, the new extratropical cyclone (called the upstream cyclone in the remainder of the text) intensified and attained a central pressure of 994 hpa. A WCB evolved that significantly amplified the ridge upstream of ex-hanna, thus supporting the southeastward extension of ex-hanna s trough. Until 1200 UTC on 11 September 2008, the upstream cyclone in the northwest Atlantic deepened by 12 hpa in 24 h to a central pressure of 982 hpa at about 47 W, 55 N (Figure 4(e)). The trajectory intersection points indicate a WCB outflow to its north and east, amplifying the ridge upstream of ex-hanna. A second WCB to the southeast of the upstream cyclone ascended along its cold front, as reflected in the cloud structure (Figure 4(f)). The downstream ridge of ex-hanna extended over major parts of Greenland and, due to its northward and westward extension, almost cut off the trough of ex-hanna from the stratospheric PV reservoir (Figure 4(e)). This extension of the ridge was still affected by the WCB which became very weak. The southern part of ex-hanna s trough elongated to about 42 N and reached the northwestern edge of the Iberian Peninsula. The centre of ex-hanna was located between Scotland and Iceland and the central pressure increased to 984 hpa. The IR satellite picture reveals a prominent cold frontal structure from the eastern British Isles over western France and across the Iberian Peninsula ahead of ex-hanna s trough (Figure 4(f)). Ahead of this front, intermittent high clouds indicated local convection over eastern France, the Alps, and the western Mediterranean Sea.

7 2180 C. M. Grams et al. Figure 4. (a, c, e, g) show potential vorticity (shading, PVU) and wind vectors (black) on the 320 K isentropic level, and pmsl (black contours every 5 hpa) at 48-hour intervals. (b, d, f, h) show Meteosat IR satellite imagery (courtesy Dundee Satellite Receiving Station) with contours of PV on the 320K level of 2, 5, and 10 PVU at the same times. The satellite data do not cover the western and northwestern edges of the section shown. PV and pmsl are taken from the ECMWF operational analyses. The crosses in (a, c, e, g) mark the intersection of WCB trajectories with the isentropic layer between K and K. For clarity, not all intersection points are shown (except in (e)). Specifically every 30th point is displayed in (a) and (c), and every second point in (g). The labels mark the surface position of the cyclones mentioned in the text.

8 Diabatic Modification of the Upper-Tropospheric Wave Guide 2181 (a) (b) Hanna Hanna (c) (d) Hanna upstream Olivia Figure 5. Observed (24-hour) precipitation (0600 to 0600 UTC) over land (shading, mm), starting at 0600 UTC on (a) 10 September, (b) 11 September, (c) 12 September and (d) 13 September 2008, taken from the gridded precipitation dataset E-OBS. Also shown are vectors of horizontal moisture flux at 700 hpa, calculated from ECMWF analyses valid at the beginning of the 24-hour period; only vectors > 0.05 kg kg 1 ms 1 are shown. The black contours are the 0.05, 0.1, 0.15 and 0.2 kg kg 1 ms 1 values of moisture flux. The surface positions of the discussed cyclones at the beginning of the 24-hour period are indicated by the labels. During 12 September 2008, ex-hanna became embedded in the upstream cyclone over the central North Atlantic (not shown). The trough of ex-hanna was narrowing into a PV streamer over the British Isles due to strong ambient deformation. Its southern part began to cut off over France while the northern part was advected northwards along with the building of the upstream ridge (not shown). At 1200 UTC on 13 September 2008, the remnants of ex-hanna were completely absorbed by the upstream cyclone which had become the dominant weather system in the central North Atlantic. The ridge built by the upstream cyclone merged with the former ridge of ex- Hanna, maintaining a blocking situation over Scandinavia (Figure 4(g)). The northern part of ex-hanna s former trough was advected to the northern coast of Greenland at about 25 W, 82 N (not shown). This is consistent with the hypothesis of Riemer and Jones (2010) that systems downstream of an ET commonly migrate polewards. The southern part of ex-hanna s former trough had become an upper-level cut-off low. As will be shown in section 5.1, the cut-off process is supported by a WCB that evolved at the frontal zone of the upstream cyclone (seen at about 25 W, 45 N in Figures 4(g, h)). In the night of 13 September 2008 when the upper-level PV cut-off reached the Mediterranean region, cyclone Olivia developed over the igurian Sea (Figure 4(g)). This cyclogenesis led to large-scale ascent of moist air masses in eastern France, to the north of and over the Alps, as reflected in satellite imagery (Figure 4(h)) and by the location of the few WCB trajectory intersection points. South of Olivia, over the Mediterranean Sea, mesoscale convective systems were initiated over Sardinia and the Adriatic Sea (Figure 4(h)). Olivia reached its minimum central pressure of a little less than 1005 hpa at around 1800 UTC on 13 September In the following two days, the cyclone slowly crossed Italy and the Balkans where the sustained advection of moist air led to intense thunderstorms (not shown) Precipitation impacts in Europe and the Mediterranean The chain of events over the North Atlantic described in the previous section led to several heavy rainfall events in the European Mediterranean region (Figure 5). These are discussed in the following with a combined analysis of a gridded precipitation dataset (for the spatial extent), synoptic data (for the local extremes), and horizontal moisture fluxes at 700 hpa (q v v h ) calculated from ECMWF analyses. On 9 September 2008, the downstream cyclone moved across Ireland (Figures 4(c, d)) and brought large-scale rainfall of mm in 24 h to the British Isles (with maxima of up to 37 mm in 24 h in Cork, Ireland; not shown). This occurred immediately after a previous heavy rain period with major flooding in England on 5 7 September Ahead of the downstream trough, convective precipitation was initiated in northern and central Spain and the Atlantic coastal regions of western France. Here rainfall maxima reached more than 20mm in 24h and up to 61mm in 24 h at Navacerrada Pass (close to Madrid in central Spain) in the night of 10 September 2008 (not shown). On 10 September 2008 the horizontal moisture flux at 700 hpa (Figure 5(a)) reflects the frontal system of the downstream

9 2182 C. M. Grams et al. cyclone moving over the North Sea to Scandinavia where it brought large-scale rainfall with a maximum of 28 mm in 24 h at Torungen Fyr (southern Norway). Over central Europe, the horizontal moisture flux vectors indicate largescale advection of moist air which was partly embedded in the southern part of the frontal system and partly originated from the Mediterranean. The strong southwesterly horizontal moisture flux vectors approaching Ireland on 10 September 2008 indicate transport of moisture towards the British Isles by ex-hanna (Figure 5(a)). Ex-Hanna caused moderate rainfall of around 10 mm in 24 h along the western Irish coast and a maximum of 18 mm in 24 h in the western UK (Figure 5(a)). On 11 September 2008, heavy thunderstorms were initiated ahead of ex-hanna s cold front over France, as ex-hanna s trough moved further to the south and transformed into a PV streamer. The measured rainfall amounts reached from 42 mm in 24 h in Metz (northeastern France) to 68 mm in 24 h in Orange (southeastern France) in the night of 12 September 2008 (Figure 5(b)). The moisture flux vectors indicate that most of the humidity was advected from the Atlantic with the cold front. However, over southeastern France moisture flux vectors also indicate strong advection from the Mediterranean and North Africa. On 12 September 2008, the PV streamer cut off and, on its eastern and northern side, large-scale precipitation of more than 20 mm in 24 h occurred in the frontal region extending from England to Austria (Figure 5(c)). The rainfall amounts exceeded 45 mm in 24 h at several locations, with a maximum of 88 mm in 24 h at Roth near Nuremberg (southern Germany). Ahead of the cold front, mesoscale thunderstorms occurred over northern Italy and the western Mediterranean, with rainfall maxima of up to 47 mm in 24 h at Arezzo (Italy). The moisture flux vectors do not show significant transport of humidity to central Europe. This suggests that the heavy precipitation occurred due to the large-scale lifting of moist air which intruded into the region during the previous days. The dynamical forcing causing this large-scale lifting was induced by the approaching upper-level disturbance. Ahead of this disturbance, a strong horizontal moisture flux at 700 hpa extended from North Africa over the western Mediterranean Sea to the Adriatic Sea. On 13 September 2008, the Mediterranean cyclone Olivia formed in the Gulf of Genoa (Figures 4(g, h)). Warm air advection and associated lifting on Olivia s northern flank led to widespread precipitation of more than mm in 24 h over the Alps, Switzerland and eastern France (Figure 5(d)). Orographic and convective enhancement led to extremely high amounts of precipitation in northern Italy of up to 228 mm in 24 h (Torino, Italy). The region of strong horizontal moisture flux moved southeastwards. Thus the humidity causing the heavy precipitation originated from local sources. This moisture was probably advected mainly from the western Mediterranean and North Africa to the region over the previous days. During the next two days, associated with thunderstorms over the central Mediterranean region, local rainfall maxima reached up to 78 mm in 24 h (Palermo, Italy) on 14 September 2008 and up to 125 mm in 24 h (Herceg Novi, Montenegro) on 15 September 2008 (not shown). 5. Impact of diabatic processes on the midlatitude flow Diabatic processes played an important role during the chain of events detailed in the previous section. Warm conveyor belts associated with ex-hanna undergoing ET and with the upstream cyclone were crucial for the ridge building and the marked southward elongation of ex-hanna s trough into a PV streamer, which eventually triggered the Mediterranean cyclogenesis of Olivia and other intense precipitation events. The chain of events at upper levels will be investigated in section 5.1. On smaller scales and at lower levels, diabatic processes were essential for generating and invigorating the cyclonic vortices over the North Atlantic, as discussed in section The modification of the upper-level midlatitude flow by warm conveyor belts During the extratropical transition of Hanna (0600 UTC on 7 September 2008 to 0000 UTC on 9 September 2008), there was an intense cross-isentropic transport of low-pv air from the subtropics into midlatitudes, as indicated by the region of low-pv air and the intersection points in Figure 4(c). The ridge downstream amplified and the PV gradient near the tropopause wave guide strengthened. This resulted in an acceleration of the jet core speed from about 60 m s 1 at the 345 K isentropic level (1200 UTC on 6 September 2008) to more than 80 m s 1 (0000 UTC on 8 September 2008). The impact of Hanna on the downstream ridge building and on the acceleration of the midlatitude jet is quantified using PV inversion. A control run (CNTR with Hanna) compares well with ECMWF analyses and presents a realistic scenario in the first 72 h. In this control run, low-pv air (PV<0.2 PVU) emerged from the outflow region of Hanna and was advected northeastward and eastward contributing to the pronounced anticyclonic wavebreaking (dark blue shading in Figure 6(a) extending from about 55 W, 52 N eastward along the ridge). As a consequence, the downstream ridge amplified and the jet core speed reaches more than 80 ms 1 where the PV gradient was strongest. The midlatitude jet extended from the outflow region of Hanna along the intense PV gradient eastward to 30 W, 35 N and the downstream trough propagated southwards. In contrast, the removal of Hanna from the initial conditions (simulation NOTC, Figure 6(b)) led to a collapse of the TC outflow and thus of the poleward advection of low-pv air at upper levels. As a result, the downstream ridge was much less strongly amplified, the PV gradient was weaker, and the jet core speed reached only 60 m s 1 in a small region. Also, the downstream trough was not as pronounced, resulting in changes to the location and track of the developing downstream cyclone (not shown), which brought moderate rainfall to the British Isles. Thus, this model sensitivity experiment based upon a modification of the initial conditions corroborates the important impact of the ET of Hanna on the acceleration of the jet, the downstream ridge building, the amplification of the downstream trough, and the downstream cyclogenesis west of the British Isles. Trajectory calculations of WCBs, started every 6 h between 0000 UTC on 5 September 2008 and 0000 UTC on 7 September 2008, confirm that during the ET a WCBlike cross-isentropic airflow extended from Hanna to the midlatitude jet. Air parcels from low levels were advected

10 Diabatic Modification of the Upper-Tropospheric Wave Guide 2183 (a) (b) Hanna Figure 6. Potential vorticity (shading, PVU), wind speed (black dashed contours for values > 50 m s 1 at 10 m s 1 intervals), wind vectors (grey) interpolated on the 345 K isentropic level, and pmsl (black contours, 5 hpa intervals). Data are taken from (a) a COSMO run initialised with the ECMWF analysis (CNTR) and (b) a COSMO run initialised with an ECMWF analysis in which Hanna was removed using PV inversion (NOTC). Both runs are initialised at 0000 UTC on 6 September 2008 and the forecasts for 0000 UTC on 8 September 2008 (+48 h) are shown. In (a) the location of Hanna is marked by Hanna. (a) (b) (c) (d) Figure 7. End locations (black dots) of trajectories (green lines), PV (shading) and wind vectors (black) at the 320 K-isentropic level, and pmsl (black contours at 5 hpa intervals). In all four panels, the start times for the trajectory calculation are 48 h prior to the valid times of the meteorological fields and parcel locations, which are (a) 0000 UTC on 9 September 2008, (b) 0600 UTC on 10 September 2008, (c) 0000 UTC on 11 September 2008 and (d) 1800 UTC on 13 September The labels mark the surface centres of cyclones mentioned in the text. northwards on the eastern side of Hanna, ascended in the vicinity of the decaying tropical storm, and reached the jet core after 48 h (Figure 7(a) and discussion below). Two WCBs occurred in the later stages of ET (Figure 7(a)). The first is the continuation of the WCB which already started during the tropical stage of Hanna and advected tropical air into the jet region. The air originated from the inner core and the eastern sector of Hanna. The air rose to very high levels ( >345 K) and flowed anticyclonically to the east along the midlatitude jet until it curved around the subtropical high (cf. Figures 7(a) showing PV at the 320 K level and 6(a) showing PV at the higher 345 K level). This WCB contributed significantly to the amplification of the upper-level ridge downstream of Hanna during the early stage of ET. It also extended the upper-level trough downstream to the south (PV on =345 K in Figure 6, to be compared with Figure 7(a)). A second WCB originated from the northern sector of Hanna. Here air was advected towards a baroclinic zone located at around W, N at 1200 UTC on

11 2184 C. M. Grams et al. 8 September 2008, as revealed by the strong gradient of equivalent potential temperature at 850 hpa (Figure 11(b)). The diabatic heating was not as intense (and consequently the final isentropic level not as high) as for the first WCB with a more tropical origin. The trajectories ended in the ridge at =320 K and contributed significantly to the expansion of the ridge northwards to higher latitudes by advection of low-pv air (Figure 7(a) showing end location of trajectories of the northern branch and PV on =320 K). Similarly, Riemer and Jones (2010) in their idealised set-up identified the advection of high- air on the dynamic tropopause (2 PVU surface) by the divergent wind component of the outflow anomaly as the main indirect diabatic PV process that amplified the northern part of the ridge. Along with the downstream ridge building, the downstream trough extended southwards and triggered the downstream cyclogenesis southwest of the British Isles. Both the northward extension of the downstream ridge by low-pv air advection due to this WCB, and the eastward extension by low-pv air advection due to the WCB originating from the tropical phase of Hanna, were confirmed in the PV surgery experiments (Figure 6). A more quantitative analysis of the ascent along the trajectories and the transport of low-pv air to upper levels is conducted by considering the time evolution along the WCBs of PV, the average PV production rate (PVR), the condensational heating (CH), and pressure (Figure 8 and Table I). Since the y-axis of Figure 8 corresponds to the averaged pressure of the air parcels in the trajectory at a given time, the diagrams can be considered as pseudovertical profiles of the quantities along the path of the slantwise ascending air parcels. For the WCB associated with Hanna, starting at 0000 UTC on 7 September 2008, the profile of PVR shows an average diabatic PV production below 700 hpa in the first 12 h and average diabatic PV reduction afterwards at higher levels (Figure 8(a)). This is consistent with the profile of condensational heating which shows a maximum 12 h after the start of the WCB ascent at an average pressure of 780 hpa (Figure 8(c)). Thus diabatic PV production occurred below the level of maximum latent heating and diabatic PV reduction aloft (Wernli and Davies, 1997). As a consequence, the average PV profile (Figure 8(b)) has a maximum in the lower half of the troposphere. The maximum of PV is not exactly at the level of maximum diabatic PV production, which is most probably an effect of the non-stationarity of the flow. Most importantly, the PV evolution along the trajectories shows that at high levels (hours 42 48, above 350 hpa) low- PV air (average PV< 0.4 PVU) was transported within the WCBs to the jet level. This advection of low-pv air was responsible for the amplification of the downstream ridge and thus the elongation of the upper-level wave guide. The secondary maximum of condensational heating 30 h after the trajectory start time was due to the diabatic heating that occurred when the air parcels of the second WCB ascended along the baroclinic zone. The different intensity of the two WCBs starting at 0000 UTC on 7 September 2008 is reflected by the differing cross-isentropic transport (not shown), which reached from 308 K to 348 K for the first WCB and from 310 K to 330 K for the second one (not shown). For the calculations discussed in the following, the average profiles of p, PV, PVR, and CH are given in Table I. After ET (1200 UTC on 9 September 2008), the WCB activity associated with ex-hanna collapsed. No new WCB started to ascend in the vicinity of ex-hanna after this time. However, the downstream ridge building continued, associated with WCB trajectories that started in the vicinity of Hanna before 1200 UTC on 9 September 2008 as seen by the trajectory end locations north of 60 N in Figures 7(b, c). At 0600 UTC on 10 September 2008, a WCB originating from the eastern sector (centred at around 52 W, 45 N) and northern sector of Hanna (centred at around 45 W, 52 N) continued to extend the downstream ridge to the north and to the east, again by cross-isentropic transport of low-pv air (Figure 7(b)). The downstream trough became a narrow filament located over northern Scotland. In the western part of ex-hanna, low-pv air wrapped cyclonically around the vortex and reached into the centre. Here the cyclonic flow around the centre helped to thin ex-hanna s trough and to promote its southward extension. These trajectories resemble those identified by Martin (1999). The central pmsl of ex-hanna deepened to below 981 hpa. At the same time, a new WCB associated with the new upstream cyclone became apparent west of 45 W (Figure 7(b)). The cross-isentropic transport contributed to another ridge building upstream of ex-hanna, which went on over the next three days as new WCBs were continuously generated east of the upstream cyclone (Figure 7(c) shows the two WCBs west of 40 W). This new upstream ridge thins out the trough of ex-hanna at its western flank and helps to push it further to the south (Figure 7(c)). The average profiles of PVR, PV, and CH along the WCBs associated with ex-hanna and the upstream cyclone starting at 0600 UTC on 8 September 2008 reflect the evolution along the trajectories discussed above (Table I). The average PV maximum occurred at mid levels at the level of maximum average diabatic heating. At upper levels, the average PV attained a minimum of 0.6 PVU after 42 h which reflected the advection of low-pv air to upper levels and the upperlevel ridge building. At the end of the calculation period the average PV increased. However, the median (not shown) reached its minimum of 0.3 PVU reflecting the northward advection of low-pv air by most of the trajectories. The relatively high average PV value was probably due to a few air parcels that experienced troposphere-to-stratosphere transport, an aspect of WCB trajectories that has already been noted by Eckhardt et al. (2004) and which is not considered further in this study. At 0000 UTC on 11 September 2008, most of the remaining WCB trajectories originating from Hanna indicate that low-pv air from the downstream ridge was advected very far to the north (75 N) (Figure 7c). Trajectories to the west of ex-hanna correspond to air parcels moving cyclonically around its centre. This flow and the filling up of ex-hanna with lower-pv air in the upper troposphere show that the cyclone had reached its mature stage. The core pressure was at its minimum of 978 hpa. This is consistent with previous studies on extratropical cyclones, which highlighted that a strong lowlevel circulation in mature, quasi-barotropic extratropical cyclones goes along with the wrap-up of upper-level PV anomalies (Davis et al., 1993; Agustí-Panareda et al., 2005; Riemer et al., 2008). The advection of air around the western edge of the vortex, together with the ongoing upstream ridge building associated with the new upstream cyclone, further thinned ex-hanna s trough, which elongated into a southward extending PV streamer. As for the previous time steps, the temporal evolution of average PV along the

12 Diabatic Modification of the Upper-Tropospheric Wave Guide 2185 (a) (b) (c) Figure 8. Temporal evolution of (a) PV production rate (PVU (6 h) 1 ), (b) PV (PVU) and (c) diabatic heating (K (6 h) 1 ) along the WCB from the 48-hour trajectory calculation started at 0000 UTC on 7 September The values to the right of the diagrams denote the averaged pressure of the trajectories at the given time. The black solid line marks the average and the grey shading the average ±one standard deviation. The box-and-whisker plots show the minimum, the 25%, 50% (median), and 75% percentiles, and the maximum value at each time. Note that the x-axis concentrates on the most interesting range of values and becomes irregular outside the thin vertical lines. The y-axis shows values at 6 h timesteps after the initialisation of the calculation. Table I. Temporal evolution of average values along the WCB trajectories shown in Figure 7. Trajectory date/time: 07/0000 UTC 09/0000 UTC 08/0600 UTC 10/0600 UTC WCBs linked to: Hanna ex-hanna and upstream cyclone Time after start (h) p PVR PV CH p PVR PV CH /0000 UTC 11/0000 UTC 11/1800 UTC 13/1800 UTC upstream cyclone and ex-hanna upstream cyclone p = pressure (hpa), PV = potential vorticity (PVU), PVR = PV production rate (PVU (6 h) 1 ), and CH = diabatic heating (K (6 h) 1 ). The maximum diabatic heating for each WCB is shown in bold. WCB indicates the advection of low-pv air to upper levels (Table I). At 1800 UTC on 13 September 2008, the PV streamer was finally cut off by a WCB. This WCB started 48 h earlier ahead of a distinct trailing cold front over the Atlantic that developed with the upstream cyclone (Figure 7(d), also Figures 4(f, h)). The diabatic PV destruction in the outflow of this WCB (average PV of < 0.1 PVU at 330 hpa, Table I) cut off the southern part of the PV streamer and probably prevented the PV cut-off from being captured by the trough of the upstream cyclone (Figure 7(d), also Figure 4(g)). This upper-level PV cut-off is essential for the subsequent formation of the Mediterranean cyclone Olivia. In summary, in this section we have shown that the interplay of WCBs linked to Hanna and the upstream cyclone crucially modified the midlatitude wave guide such that a PV streamer formed over the eastern North Atlantic and evolved into an upper-level cut-off low triggering a Mediterranean cyclogenesis. Dirren et al. (2003) have shown that pronounced localised medium-range forecast errors occur along the tropopause-level PV wave guide, in

13 2186 C. M. Grams et al. (a) (b) (c) S N (d) SO NW SO NW S N Figure 9. Cross-sections through (ex-)hanna: PV (shading, PVU), equivalent potential temperature e (thin black contours at 4 K intervals), and PV production rate (PVR, thick blue contours at 1.0 PVU (6 h) 1 intervals, dashed for negative values). The crosses mark every fourth (every tenth in (a)) trajectory that crosses the section in a ±1 h time interval. The sections are: (a) at 0000 UTC on 7 September 2008 from 74 W, 30 Nto74 W, 55 N (cf. Figure 11(a)); (b) at 0000 UTC on 9 September 2008 from 29 W, 40 Nto54 W, 55 N (cf. Figure 11(c); (c) at 1200 UTC on 9 September 2008 from 34 W, 40 Nto43 W, 55 N (cf. Figure 11(d); and (d) at 0000 UTC on 10 September 2008 from 29 W, 35 Nto29 W, 60 N (cf. Figure 11(e). For slantwise sections (b, c), the horizontal axis is labelled by the horizontal distance (km). For the sake of clarity, at each grid point a horizontal average with the eight neighbouring grid points in a box is taken for PV, PVR and e The impact of diabatic processes on the evolution of cyclonic vortices Hanna ds On a smaller scale, diabatic processes played an important role in the (re-)intensification of ex-hanna and of the upstream cyclone. In this section we analyse in more detail the evolution of the PV structure in the lower and mid troposphere, close to the centre of the cyclonic vortices. Figure 10. As Figure 7(b), but for a valid time of 0000 UTC on 9 September 2008 from a start time of 0600 UTC on 8 September particular in the regions of upper-level ridges. Similarily, it is conceivable that the model representation of the crossisentropic WCB-like flows during our period of interest, which amplified the upper-level ridges, play some role in the low predictability of the upper-level flow evolution in the TIGGE forecast (section 3) The evolution of ex-hanna A north south vertical cross-section through Hanna at 0000 UTC on 7 September 2008 before ET shows an upright PV tower with PV values larger than 3 PVU extending from the surface up to about 350 hpa and with a warm core ( e >350 K, Figure 9(a)). On its northward side the PVR indicates erosion of the PV tower at mid and upper levels due to strong condensational heating. At low and mid levels, the PV core was supported by positive PVR. The tropopause

14 Diabatic Modification of the Upper-Tropospheric Wave Guide 2187 above Hanna was elevated to less than 150 hpa while, north of 47 N, high PV values at 150 hpa and a tropopause location at about 300 hpa indicate a sharp tropopause step. The trajectory intersection locations reveal very intense WCBlike ascent in the entire TC core region. The dense contours of e at 850 hpa to the north of Hanna in Figure 11(a) show that Hanna approached a strong baroclinic zone. Strictly speaking, a strong e gradient can emerge exclusively from moisture differences. Here we use e instead of because the combination of temperature and humidity helps to identify the baroclinic and frontal zones more distinctly. At 1200 UTC on 8 September 2008, during ET, the lowlevel PV had a maximum slightly south of the pressure minimum (Figure 11(b)). This is a remnant of the former tropical cyclone s core. Ex-Hanna approached an intense baroclinic zone with a strong horizontal gradient of e at around 50 N. The flow associated with ex-hanna s PV anomaly advected moist air towards the baroclinic zone and a new low- and mid-level PV anomaly began to evolve. This new diabatically produced PV appears as a band at about W, 48 N (Figure 11(b)) to the northeast of the low-level PV maximum. The ascent at the baroclinic zone and diabatic PV production are also reflected in the WCB trajectory calculations. All WCB trajectories that started between 0000 UTC on 7 September 2008 and 0000 UTC on 9 September 2008 show a concentration and strong lifting of the air parcels at the baroclinic zone between 1800 UTC on 7 September 2008 and 0600 UTC on 9 September An example of this is shown at 0000 UTC on 9 September 2008 for the calculation started at 0600 UTC on 8 September 2008 (Figure 10). The air parcels cluster between 40 Wand 50 W and their average PV reaches strongly elevated values of about 1.2 PVU (Table I). The location of the surface circulation centre associated with ex-hanna relative to the baroclinic zone is crucial for the flow downstream. The NOTC sensitivity experiment (section 5.1) shows that an extratropical cyclone develops even if Hanna is not present. However, the location of this new cyclone (at about 60 W, 49 N) and its intensity (1002 hpa at 1200 UTC on 8 September 2008) differ from those in the run with Hanna (55 W, 49 N, 996 hpa, not shown). In the absence of the tropical cyclone circulation impinging on the baroclinic zone, no diabatic PV production occurs and the ridge is not as pronounced as when Hanna is present. This differs from the findings of McTaggart-Cowan et al. (2004) who found only limited sensitivity of the upper levels. These differences underline the fact that the relative position of tropical TC remnants and extratropical features determine the strength of the interaction. In our case, the sensitivity is manifested in the diabatic amplification of the ridge downstream and an accelerated midlatitude jet. This results in a different trough downstream of Hanna and thus in a different impact on the British Isles of the downstream cyclone that develops ahead of this trough near 15 W. At 0000 UTC on 9 September 2008, the new diabatically produced PV at the baroclinic zone extends from about 50 Wto35 Wat48 N with particularly large values at the western edge of this band (Figure 11(c)). This is also the location of the surface pressure minimum. Thus the new PV replaced the remnants of tropical PV in the centre of ex-hanna, which will be referred to in the following as the tropical PV. To the north of the low-level centre, ex-hanna reached the frontal zone and a marked warm front was generated extending from about 50 W, 50 Nfar to the east, to the centre of a very short-lived extratropical cyclone near 22 W (labelled in Figure 11(c); this is not ds, developing at the expense of at 12 W at the same time). The tropical PV was advected southeastwards to about 43 W, 46 N, and started to form a banded structure. With the advection of the tropical PV, relatively cool air moved southeastwards contributing to the formation of a weak cold front. The orientation of the cross-section in Figure 9(b) has been chosen to illustrate the PV production at the baroclinic zone, the southeastward advection of the tropical PV, the decay of ex-hanna s tropical PV tower and the weak cold front. Only remnants of the tropical PV persist at lower levels ( hpa, Figure 9(b) near the horizontal position 1100 km). This is the low-level PV maximum seen in Figure 11(b) to the south and in Figure 11(c) to the southeast of the minimum sea level pressure. Figure 9(b) also reveals that at low and mid levels ( hpa), ascent at the baroclinic zone ( km) leads to diabatic PV production due to condensational heating in WCBs. The WCB intersection points indicate that the warm moist air ascended along the moist isentropes from low levels up to the tropopause. Above the level of maximum condensational heating, where diabatic PV reduction occurred, the WCBs advected low-pv air, enhancing the PV gradient at the tropopause (Table I). The PV at tropopause level also indicates that a positive PV anomaly associated with an upper-level trough approached the new low- and mid-level band-like PV anomaly. At 1200 UTC on 9 September 2008, the tropical PV had moved further southeastwards, evolved into a banded structure and formed a weak cold front (Figure 11(d)). The continuously regenerated warm-frontal PV in the baroclinic zone started to wrap up cyclonically, leading to the formation of a pronounced bent-back warm front. Note that marked bent-back warm fronts have been identified as a typical feature of intense extratropical cyclones and are frequently associated with strong near-surface winds (e.g. Neimann et al., 1993; Browning, 2004). The frontal structure becomes obvious when looking at a section across both the bent-back warm front and the less intense cold front (Figure 9(c)). The horizontal gradient of e from about 900 to 700 hpa in the southeastern edge of the cross-section indicates the cold front associated with the tropical PV located at about 350 km (35 W, 43 N). The front associated with the new PV extended from the surface up to 600 hpa, as seen in the strong gradient of e at about 1150 km (38 W, 50 N). The upper-level PV anomaly associated with the midlatitude trough became vertically aligned with the low-level PV anomaly of the bent-back warm front. This phasing of the upper- and low-level positive PV anomalies goes along with a strong extratropical reintensification of ex-hanna. The PV production rate in this section shows PV erosion in the region of maximum new PV. However, new PV continued to be produced at the baroclinic zone at low levels. This PV was most likely transported isentropically along the baroclinic zone at mid levels and thus sustained the new PV tower. The WCB intersection points are restricted to the upper troposphere, consistent with the collapse of the regeneration of WCBs after Hanna s ET. The WCBs continued to advect low-pv air to upper levels and to enhance the PV gradient at the tropopause.

15 2188 C. M. Grams et al. (a) (b) (c) (d) (e) (f) (g) (h) Figure 11. Horizontal overview for positions of the cross-sections (marked by straight black lines) in Figures 9 and 13. The horizontal sections shown concentrate on ex-hanna and the upstream cyclone; the time and cyclone are indicated above each panel. (a) (e): PV at 850 hpa (shading, PVU), equivalent potential temperature θ e (thin black contours at 4 K intervals) at 850 hpa, and pmsl (black contours at 5 hpa intervals). (f, g, h): PV at the isentropic level of θ = 320 K as Figure 7 but without WCBs. At 0000 UTC on 10 September 2008, the extratropical cyclone ex-hanna had generated a new PV tower-like structure with values exceeding 1 PVU extending through the entire troposphere (Figure 9(d)). The upper-level trough became vertically aligned with the low-level positive PV anomaly. The nose of lower-pv air and the WCB intersection points in the upper and mid levels at about 51 N indicate the wrapping up and advection of lower-pv air around the centre of ex-hanna. Further to the north, the remaining WCB intersection points coincide with regions of low-pv air and acted to extend ex-hanna s ridge northward. Condensational heating had almost ceased, which explains the lack of diabatic PV production. The low-level PV chart shows that the bent-back warm front, which continued to wrap up cyclonically, became collocated with the front of the remnant PV which had almost vanished (Figure 11(e)). Vertical profiles of PV averaged around the vortex centre within a radius of 200 km help to summarise the transition of the vertical structure of Hanna during ET and extratropical reintensification (Figure 12(a)). During the tropical stage Hanna is characterised by an upright PV tower with an upper-level negative PV anomaly in the outflow and a