The January 2004 explosive cyclogenesis over the Aegean Sea: Observations and model analysis

Transcription

1 QUARTERY JOURNA OF THE ROYA METEOROOGICA SOCIETY Q. J. R. Meteorol. Soc. 133: (2007) Published online in Wiley InterScience ( The January 2004 explosive cyclogenesis over the Aegean Sea: Observations and model analysis K. agouvardos,* V. Kotroni and E. Defer National Observatory of Athens, Institute of Environmental Research and Sustainable Development, Athens, Greece ABSTCT: During the period January 2004, an explosive cyclogenesis event occurred over the Aegean Sea. The minimum observed pressure was 972 hpa, a value which is among the three lowest observed over the entire Mediterranean Sea during the last 40 years. This paper is devoted to the investigation of the conditions that contributed to the rapid development of this low-pressure system through analysis of both observations and model results. It was found that the rapid development of the cyclone was associated with a two-trough system that, under the influence of a very intense upper-level jet, merged into a single trough and then acquired a negative tilt. Sensitivity tests with the MM5 model showed that the upper-level dynamic forcing was the main factor that led to the explosive cyclogenesis, while surface sensible and latent heat fluxes contributed to the cyclone deepening mainly during the storm s mature phase. Copyright 2007 Royal Meteorological Society KEY WORDS deep cyclone; mesoscale simulations Received 7 June 2006; Revised 20 March 2007; Accepted 27 May Introduction Explosive cyclogenesis is an interesting meteorological phenomenon as it is accompanied by high-impact weather. Sanders and Gyakum (1980) presented the first thorough climatological study on this subject and they introduced the notion of explosive cyclone development or bomb cyclogenesis. More recently, Gyakum and Danielson (2000) performed an analysis of the meteorological conditions (upper-air features as well as surface fluxes and the moisture budget) that characterized a large number of weak and explosive cyclogenesis events over the Pacific and they found that large surface fluxes are crucial in the case of explosive cyclogenesis. Strahl and Smith (2001) performed a diagnostic study of an explosive cyclone over the eastern USA, identifying the mechanism of upper-level trough merging as responsible for the fast rate of development of the cyclone. Martin and Otkin (2004) investigated the growth and decay of an explosive cyclogenesis case over the Pacific, based on the use of the MM5 model (fifth-generation National Center for Atmospheric Research/Pennsylvania State University mesoscale model), relating the rapid development to the latent heat release within the cyclone and the generation of a potential vorticity anomaly through diabatic effects. Nielsen and Sass (2003) in their study of a North Sea severe storm have also identified potential vorticity (PV) * Correspondence to: K. agouvardos, National Observatory of Athens, Institute of Environmental Research and Sustainable Development, ofos Koufou, P. Penteli, 15236, Athens, Greece. lagouvar@meteo.noa.gr generation by latent heat release as a crucial factor for the deepening of the cyclone under study. This paper is devoted to the study of an explosive cyclogenesis event in the eastern Mediterranean. Indeed, a record low pressure was set in the eastern Aegean Sea with 972 hpa at 12 UTC 22 January 2004 as measured by an automatic station in the area. Sustained winds in northern Aegean islands exceeded 30 m s 1 with gusts up to 45 m s 1, and resulted in the destruction of the major part of the power supply infrastructure in the area. Gradual filling of the low started at around 1800 UTC 22 January. On the back side of the low, blizzard conditions swept across the northern Aegean and the eastern coasts of continental Greece, with important snowfall (snowfall of about 1 metre was accumulated during the day of 22 January over the mountains in eastern continental Greece and about cm in the mountains north of Athens). A first approach to the synoptic setting of this cyclogenesis event was given in agouvardos et al. (2006). Inspection of European Centre for Medium-Range Weather Forecasts (ECMWF) analyses during this event revealed that the lowest analysed pressure at 1200 UTC 22 January was 976 hpa, 4 hpa higher than the observed value. According to the MEDEX (MEDiterranean EXperiment) database (constructed by ECMWF analyses and E-40 reanalyses, given at inm.uib.es/), this case was among the deepest cyclones observed in the entire Mediterranean basin at least for the last 40 years and the deepest in the eastern part of the basin. It is therefore of great importance for meteorologists and operational weather forecasters to investigate Copyright 2007 Royal Meteorological Society

2 1520 K. AGOUVARDOS ET A. the synoptic environment under which the deepening and evolution of this low-pressure system occurred, as well as to understand its mesoscale organization. This task is accomplished in the present paper through the investigation of ECMWF analyses and of all available observations (e.g. lightning activity, infrared imagery, spaceborne microwave and radar measurements, and scatterometer winds over the sea). Moreover, the analysis of the highresolution simulations performed with the MM5 model provides a better understanding of the mesoscale patterns and allows us to identify some of the mechanisms leading to the strong deepening of the cyclone. Section 2 is devoted to the synoptic description of the storm, based on ECMWF surface and upper-air analyses, together with synoptic surface data, lightning observations and scatterometer winds. An insight into the microphysical characteristics of the storm is given in section 3, based on microwave data from the Tropical Rainfall Measurement Mission (TRMM) and AQUA satellites. Analysis based on MM5 model output is given in section 4, together with results from sensitivity simulations, while concluding remarks and prospects are provided in the final section. 2. Synoptic overview The very deep low which affected the Aegean Sea developed from a low centre (with a central pressure of 999 hpa) that on 1200 UTC 21 January 2004 was located over the Gulf of Sidra, and it was associated with a 500 hpa cut-off low over ibya (not shown). The evolution of the cyclone during the next 24 hours up to the time of its maximum deepening is first documented through inspection of ECMWF analyses (at degree resolution) of mean-sea-level pressure and upperair features presented in Figures 1 and 2, along with Figure 1. ECMWF analysis of mean-sea-level pressure (solid lines at 3 hpa intervals) and of 500 hpa absolute vorticity advection (shading contours at s 2 intervals), valid at 1800 UTC 21 January Open dots denote the location of lightning flashes recorded by the UK Met Office ATD system ±30 min around the analysis time. As, but at 0000 UTC 22 January Temperature advection contour lines of and Ks 1 are also given in bold. As, but at 0600 UTC 22 January (d) As, but at 1200 UTC 22 January 2004.

3 EXPOSIVE CYCOGENESIS OVER THE AEGEAN SEA 1521 Figure 2. ECMWF analysis of 500 hpa geopotential height (solid line at 40 m intervals) and of 300 hpa wind speed (shaded contours at 10 m s 1 intervals, only values greater than 50 m s 1 are shown) UTC 21 January UTC 22 January UTC 22 January (d) 1200 UTC 22 January the lightning activity recorded by the UK Met Office Arrival Time Difference system (ATD hereafter, Holt et al., 2001). At 1800 UTC 21 January, the low-pressure centre that has deepened to 993 hpa was located in the maritime area south-west of Greece (Figure 1). ocalised lightning activity was sensed near the low-pressure centre (dots in Figure 1), while a belt with lightning flashes is also evident over the northern Aegean Sea. A short-wave trough with its axis extending north-east/south-west (positively tilted) over the area west of the Ionian Sea and a cut-off low over the Gulf of Sidra are evident at the 500 hpa level (Figure 2). A very strong upper-level jet is located at the base of the cut-off, reaching a value of 80 m s 1 (shaded contours in Figure 2). At 0000 UTC 22 January the low pressure is over the southern tip of continental Greece with a central pressure of 986 hpa (Figure 1). Regarding the 500 hpa two-trough system, the northern part (over the area west of the Ionian Sea) remained stationary in relation to its axis position but it also deepened (by about 80 gpm) while the southern part moved eastwards and its axis turned anticlockwise under the influence of the upper-level jet that at 300 hpa has intensified to 90 m s 1 just over the southern edge of the trough (Figure 2). Significant vorticity advection at 500 hpa is evident in the form of a band extending from the Gulf of Sidra towards the low centre, while a second but less pronounced band of vorticity advection is evident over the northern Aegean Sea (shaded contours in Figure 1). Warm advection at the same level is evident over the maritime area south of Greece (bold line in Figure 1). ightning activity is observed near the low-pressure centre (Figure 1) as well as along the area of vorticity advection over the northern Aegean Sea. ocalised convection is also reported over the western part of Crete. During the following 12 hours, the low-pressure system moved north-eastwards, rapidly deepening. At 0600 UTC 22 January the low centre (with a 982 hpa central pressure) is located over the Aegean Sea (Figure 1) while a line of significant convective activity (as derived from the lightning data) is evident along the western

4 1522 K. AGOUVARDOS ET A. Turkish coastline. At that time, vorticity advection at the 500 hpa level was maximised over the maritime area south and east of Crete (shaded contours in Figure 1), while warm advection at the same level is maximised mainly in the area in between the vorticity advection maxima (solid line in Figure 1). During this time, important modifications have occurred in the upper-level features as deduced by comparing the 500 hpa patterns six and twelve hours before. More specifically, the twotrough system merged into a single one, the curvature sharpened considerably, the gradient of geopotential height amplified and the trough axis became negatively tilted, thus generating cyclonic vorticity (Figure 2). Therefore the vorticity advection was maximised within the merged trough, as indicated by the vorticity advection pattern also shown in Figure 1. Gaza and Bosart (1990) in their climatological study of trough-merger events in North America found that the meridional tilt of the principal 500 hpa height trough axis changes from positive to negative prior to the rapid cyclogenesis. Further, Hart et al. (2006) in their study on the extratropical transition life cycle of North Atlantic tropical cyclones have identified negative tilting as an intensification mechanism once the extratropical transition has completed. At 1200 UTC 22 January (Figure 1(d)), the low-pressure system has reached its minimum pressure (976 hpa over the eastern Aegean, as analysed by ECMWF, and 972 hpa as reported by a nearby automatic station). According to Sanders and Gyakum (1980) explosive cyclogenesis occurs when the deepening of a cyclone exceeds one bergeron. In the area over the Aegean Sea (at 36 N latitude) one bergeron equals 16.3 hpa (24 h) 1. Therefore the 24-hour central pressure fall from 999 hpa at 1200 UTC 21 January to 976 hpa at 1200 UTC 22 January 2004 corresponds to 1.4 bergeron and thus the low pressure system can be referred to as a strong bomb. At that time the vorticity advection maximum, as well as the organized lightning activity, had shifted to over the eastern Mediterranean and the area of Cyprus (shaded contours in Figure 1(d)), while scattered convection (as identified from the lightning data) was observed west and south of the low-pressure system (Figure 1(d)). As inferred by the strong pressure gradient, the northerly winds over the Aegean Sea reached their maximum intensity; this feature will be further discussed in the next paragraph. A series of surface observations from the synoptic network over Greece and western Turkey is given in Figure 3. At 1800 UTC 21 January, the low centre was located south-west of Greece and the synoptic reports over Crete and southern Aegean islands show a moderate southerly flow of 10 m s 1 (not shown). ater on, at 0000 UTC 22 January 2004, when the low-pressure centre was located over the southern tip of continental Greece, the southerly sector surface winds over the southern part of the Aegean Sea have intensified, reaching locally 15 m s 1 while over the northern part of the Aegean north-easterly winds of the same intensity were prevailing (Figure 3). At that time, snow was reported by some stations located in northern and central Greece. Surface wind observations from both the QUIKSCAT scatterometer and from the surface synoptic network were available at 0300 UTC 22 January 2004 (the satellite passed over the area at 0315 UTC). At that time, the low was depicted over the southern Aegean, with very strong south-easterly surface winds at its eastern flank (winds exceeding 20 m s 1 ). Over the northern and central Aegean the north-easterly winds have further intensified with sustained winds up to 27 m s 1 as estimated by QUIKSCAT in good agreement with the surface reports (two islands in the northern Aegean, imnos and Skiros, reported 20 and 21 m s 1, respectively; their location is given in Figure 3(d)). At 0600 and 1200 UTC 22 January (Figure 3 and (d), respectively), the lowpressure system has migrated towards the east, snowfall continued over the northern and central part of Greece, while a mix of snow and rain was reported from stations in the northern part of the Athens area. Northerly sector surface winds prevailed all over the Aegean, with sustained winds ranging from 20 to 30 m s 1. Wind gusts at imnos and Skiros islands reached 45 m s 1. On both islands, a significant destruction of power and telephone lines was reported. At 1200 UTC, Samos island reported a surface pressure of 976 hpa (as analysed by ECMWF, see Figure 1(d)), while on Ikaria island 30 km to the west, the reported pressure from an automated station was as low as 972 hpa. A panel of four Meteosat infrared images, corresponding to the same instants as the ECMWF analyses (Figures 1 and 2) is shown in Figure 4. The location of lightning activity as recorded by the ATD system within a ±30 minutes time interval around the nominal time of each satellite picture is overlaid. At 0000 UTC (Figure 4) the lightning activity was important and was concentrated mainly over the northern Aegean but also over western Crete, south of the low-pressure centre (denoted by the letter in all Figure 4 panels). Note also a wide region of almost cloud-free conditions south of Crete, an indication of dry air masses that could be related to an upper-tropospheric intrusion, as discussed later, in section 4. During the next twelve hours (Figures 4 and (d)), the lightning activity was mainly organized along a line extending from north-west towards south-east following the eastward migration of the system. The cloud-free area south of Crete shrinks forming a wedge that extends northwards to the southern Aegean Sea (Figure 4), while at the same time the lightning activity is organized along a line on the eastward flank of this wedge. A notable feature is that almost all lightning activity (at all instants shown) occurs over the sea, typical behaviour during the cold season of the year (Holt et al., 2001). 3. Microwave and radar observations of the storm Passive microwave satellite observations are extensively used to estimate cloud properties. Some insight into

5 EXPOSIVE CYCOGENESIS OVER THE AEGEAN SEA FR UTC 22 JAN UTC 22 JAN 2004 (d) TS UTC 22 JAN imnos FR Skiros Samos Ikaria UTC 22 JAN 2004 Figure 3. Surface synoptic reports of temperature, mean-sea-level pressure and wind (one pennant: 25 m s 1, one barb: 5 m s 1, one half-barb: 2.5 m s 1 ). stands for rain, for snow and FR for freezing rain UTC 22 January UTC 22 January 2004, superimposed is the wind field given by the QUIKSCAT scatterometer UTC 22 January (d) 1200 UTC 22 January This figure is available in colour online at the cloud structure of the system between 0000 and 0100 UTC 22 January (during that time interval the low pressure was approximately over the south-eastern tip of continental Greece) can be assessed through inspection of observations provided by rainfall TRMM and AQUA satellites. TRMM carries three main instruments (for details see Kummerow et al., 1998): the Visible and InfraRed Scanner (VIRS), a fivechannel, cross-track scanning radiometer operating at 0.63, 1.6, 3.75, 10.8 and 12 µm, which provides highresolution observations of cloud coverage, cloud type and cloud top temperatures; the TRMM Microwave Imager (TMI), a multi-channel passive microwave radiometer operating at five frequencies: 10.65, 19.35, 37.0 and 85.5 GHz at dual polarization and GHz at single polarization. TMI provides information on the integrated column precipitation content, cloud liquid water, cloud ice, rain intensity and rainfall types; the Precipitation Radar (PR), an electronically scanning radar, operating at 13.8 GHz that measures the three-dimensional rainfall distribution over both land and ocean, and defines the layer depth of the precipitation. The Advanced Microwave Scanning Radiometer (AMSR- E) onboard the AQUA satellite is a twelve-channel, sixfrequency, total power passive-microwave radiometer. It measures brightness temperatures at 6.925, 10.65, 18.7, 23.8, 36.5 and 89.0 GHz in both vertical and horizontal polarization. Two overpasses over the area of interest are examined: the first at 0000 UTC 22 January by the AQUA

6 1524 K. AGOUVARDOS ET A. (d) Figure 4. Meteosat infrared image. Crosses denote the location of lightning flashes recorded by the UK Met Office ATD system ±30 min around the image time UTC 21 January UTC 22 January UTC 22 January (d) 1200 UTC 22 January satellite and the second one hour later by TRMM satellite. Figure 5 presents AQUA AMSR-E brightness temperature in the 89 GHz vertical polarization channel. This frequency is sensitive to the presence of ice and shows a thin line of low brightness temperatures (dark shading) over the maritime area north of Crete, just ahead of the low-pressure system centre. This low brightness temperature signature, indicative of high concentration of ice, is also associated with significant lightning activity (see Figures 1 and 4). Cold 89 GHz brightness temperatures are also evident within the cloud mass over the central and northern parts of the Aegean Sea and again this area was associated with important lightning activity. Figure 6 shows the observations from TRMM cloud sensors at 0100 UTC 22 January The VIRS 12.0 µm image shows a significant cloud mass ahead of the low-pressure centre which is located at the southeastern tip of Peloponnisos at the time of TRMM overpass (Figure 6). Figure 6 shows a vertical crosssection of reflectivity as measured by PR that is performed along the solid line given in Figure 6 where the PR reflectivity field at 4 km above mean sea level (a.m.s.l.) is given. The altitude of cloud masses, as depicted in the cross-section, ranged from 2 to 6 km a.m.s.l. In the area of important vorticity advection, i.e. south-west of Peloponnisos, a cloud band was sensed with a cloud top reaching 5 6 km a.m.s.l. as measured by PR. Within that cloud band 2A25(V5) near-surface rain rate exceeded 30 mm h 1 in some places. The clouds that had the highest vertical extent, within the PR field of view, were located north of Crete. Their cloud tops exceeded 7.5 km a.m.s.l. and the associated near-surface rain rate exceeded 40 mm h 1 in some places. ATD reported lightning flashes within those cells (Figure 4) indicating that updraughts and microphysics content were sufficient for electrification processes and lightning activity to occur. 4. Model simulations 4.1. Model setup Figure 5. AQUA brightness temperature image in 89 GHz vertical polarization channel at 0000 UTC 22 January Darker colours denote lower brightness temperature values. The MM5 model (Version 3.5) is a non-hydrostatic, primitive equation model using terrain-following coordinates (Dudhia, 1993). Several physical parametrization

7 EXPOSIVE CYCOGENESIS OVER THE AEGEAN SEA 1525 Figure 6. TRMM-VIRS infrared imagery at 0100 UTC 22 January Dashed lines denote the swath of PR measurements shown in and the solid line the position of the vertical cross-section shown in. Vertical cross-section of the PR reflectivity field, along the solid line shown in. Horizontal cross-section of the PR reflectivity field at 4 km height. schemes are available in the model for the boundary layer, the radiative transfer, the microphysics and the cumulus convection. For the simulations performed in the frame of this study the following schemes are selected: the Kain Fritsch scheme (Kain and Fritsch, 1993) for the convective parametrization, the scheme proposed by Schultz (1995) for the explicit microphysics and the scheme by Hong and Pan (1996) as also used by the NCEP (National Centres for Environmental Prediction) Global Forecasting System (GFS) for the planetary boundary layer. The selection of the combination of the Kain Fritsch scheme for convection and the Schultz scheme for explicit microphysics is based on the comparative study by Kotroni and agouvardos (2001). These authors had performed a comparison of various combinations of schemes for cases with important precipitation amounts over the eastern Mediterranean. They showed that the combination of the Kain Fritsch parametrization scheme with the highly efficient and simplified microphysical scheme proposed by Schultz (1995) provides the most skilful forecasts of accumulated precipitation for wintertime rain events in the area. For this study, two one-way nested grids have been defined: Grid 1 (with a grid spacing of 24 km) covering the major part of Europe, the Mediterranean and the northern African coasts, and Grid 2 (8 km grid spacing), covering the larger part of the eastern Mediterranean. In the vertical dimension, 30 unevenly spaced full sigma levels were selected. The simulations were initialised at 1800 UTC 21 January 2004 and lasted for 30 hours. The ECMWF gridded analyses fields at 6-hour intervals and at 0.5-degree latitude/longitude horizontal grid increments have been used for initial and boundary conditions (CNTR run hereafter). A further two additional sensitivity experiments have been performed. For the first experiment, the surface fluxes (both sensible and latent heat) have been turned off (NFX run hereafter), in order to study the role of surface fluxes in the evolution of the cyclone. For the second experiment, a piecewise PV-inversion technique has been applied (NPV run hereafter), in order to study the role of the dynamical forcing at upper levels and the relative contribution of potential vorticity anomalies to the surface cyclogenesis process. Some details of this latter experiment are given in the following paragraph. The piecewise PV inversion has been applied in numerous studies to investigate (among other issues) the effect of modifying the PV perturbations in the model initial conditions on the forecasted fields (Fehlmann and Davies, 1999; Huo et al., 1999; Romero, 2001). This technique is based on the PV invertibility principle (Hoskins et al., 1985) and was initially proposed by Davis and Emanuel (1991). The strategy followed in this work is similar to the one applied by Romero (2001) who studied the sensitivity of a heavy-rain-producing cyclone to embedded potential vorticity anomalies. The piecewise PVinversion technique has been applied to the initial conditions used for the MM5 simulations. First, from the meteorological fields at 1800 UTC 21 January 2004 (model initialisation time) the balanced flow associated with the distribution of Ertel s PV at that time is calculated. Then the reference state, defined as a time average from a three-day period beginning at 0000 UTC 21 January 2004, is also calculated. The total PV perturbation is computed simply as the departure from the time average. The technique allows partitioning of the total PV anomalies in order to isolate distinct perturbations of different origins. In the present work, the PV anomalies considered are the volumes of positive PV perturbation above 500 hpa in the whole model domain (Grid 1). The aim is to calculate a balanced flow associated with the defined anomaly that can be used to modify the model initial conditions in a physically consistent way. The sensitivity experiment (NPV) to be discussed in the following was

8 1526 K. AGOUVARDOS ET A. designed by subtracting the PV-inverted balanced fields of geopotential, temperature and wind from the model initial conditions, while the relative humidity was kept unchanged. Extensive details of the application of the method are given in Romero (2001) Model results: CNTR run Figure 7 shows a series of mean-sea-level pressures and 500 hpa cloud ice contents and Figure 8 a series of 700 and 500 hpa vertical velocities, at 0000, 0600 and 1200 UTC 22 January 2004, as provided by MM5 Grid 2 (CNTR run). At 0000 UTC 22 January (Figure 7), the low-pressure system is over the south-eastern tip of continental Greece, with a central pressure of 987 hpa, 1 hpa shallower than the ECMWF analysis at the same time (see Figure 1). At 500 hpa two major areas with important cloud ice content are evident (Figure 7). The first one is organized along a band over the maritime area west and south-west of Crete and coincides with the area of maximum vorticity advection at the same level, as analysed in the ECMWF charts (see shaded contours in Figure 1). Part of this band is evident in the PR reflectivity field (Figure 6 and ). The second area of important cloud ice content is depicted in the northern Aegean Sea and part of continental Greece. The band over the northern Aegean Sea presents high concentrations of cloud ice (exceeding 0.7 g kg 1 ), a feature also evident in the AQUA microwave measurements at the higher frequencies (89 GHz, Figure 5). This area, characterized by the presence of ice as derived by both MM5 model and satellite observations, was also associated with important lightning activity (Figure 1). The presence of lightning activity is highly correlated with depression of brightness temperature at high frequencies, as reported by many authors in the literature (Toracinta and Zipser, 2001; Toracinta et al., 2002; Katsanos et al., 2007, among others). At this point it should be noted that the model fails to reproduce the thin band of low brightness temperatures, and as a matter of fact high ice content, depicted by AMSR-E AQUA and PR TRMM in the maritime area north of Crete (Figure 5). This could be attributed to the fine scale of this band that could not be resolved by the model at the grid spacing of 8 km used for the simulation. The vertical velocities at the 700 and 500 hpa level reproduced by MM5 Grid 2 at 0000 UTC 22 January (Figure 8) show an extended band of ascending motion in the areas with the highest ice concentrations over the north-eastern Aegean and to a lesser degree south-west of Crete; over the north-eastern Aegean, the 0.2 m s 1 updraught isoline at the 700 hpa level is found to be displaced southwards with respect to the 500 hpa 0.2 m s 1 isoline, reflecting the shift of ascending motions as the cold air from the north impinges on the warmer air masses. Very strong updraughts, reaching 1 m s 1, are simulated over the south-eastern part of continental Figure 7. MM5 Grid 2 map (CNTR run) of mean-sea-level pressure (solid lines at 2 hpa intervals) and of 500 hpa ice content (shaded contours at 0.05 g kg 1 intervals) UTC 22 January UTC UTC. Copyright 2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: (2007)

9 EXPOSIVE CYCOGENESIS OVER THE AEGEAN SEA Figure 8. MM5 Grid 2 map (CNTR run) of 500 hpa vertical velocity (shaded contours at 0.5 m s 1 intervals, only values greater than 0.2 m s 1 are shown). Solid line denotes the 0.2 m s 1 isoline of 700 hpa vertical velocity UTC 22 January UTC UTC. Greece as well as over the northern coast of Crete, in good agreement with the collocated high number of lightning flashes over the area. Six hours later, the low pressure system has deepened to 979 hpa. The areas with the highest ice concentrations at the 500 hpa level as well as the most vigorous ascending motions at the 700 and 500 hpa levels are now reproduced over the eastern part of the Aegean Sea and the western Turkish coasts (Figures 7 and 8), following the rapid progression and deepening of the low-pressure system towards the east. The simulated ascending motions (that reach 2 ms 1 along the western Turkish coasts) at 0600 UTC 22 January are almost collocated with the line of significant lightning over the Turkish coasts (Figures 1 and 4). Finally at 1200 UTC 22 January, the low-pressure system as simulated by MM5 has reached its maximum depth (974 hpa over the eastern Aegean, Figure 7), a value between the ECMWF analysed sea-level pressure (976 hpa) and the report of an automatic weather station nearby (972 hpa). The pressure gradient over the Aegean Sea was highly intensified, in good agreement with the gale-force winds that were observed at that time over the area (Figure 3(d)). Further, the areas with the most significant ascending motions (Figure 8), and the higher concentrations of cloud ice (Figure 7) have migrated towards the eastern Mediterranean and the Cyprus area, again in agreement with the observed lightning activity in the same areas. It would be instructive here to look at the role of upper-level features in the organization of the cloud and precipitation structure of the system. Figure 9 presents a series of 300 hpa potential vorticity, at 0000, 0600 and 1200 UTC 22 January 2004, as simulated by MM5 model Grid 2 (CNTR run). More specifically the Ertel potential vorticity (defined as q = ρ 1 η θ, where ρ is the density, η is the absolute vorticity vector and θ is the potential temperature) is given in PVU (potential vorticity unit: 10 6 km 2 kg 1 s 1 ). At 0000 UTC 22 January (Figure 9), high PV values at the 300 hpa level are evident over the Ionian Sea, while a tongue of high PV values is evident in the area west and south-west of Crete. These high values, indicative of intrusion of dry stratospheric air towards the middle tropospheric layers, coincide well with the cloud-free area south of Crete, as depicted in the infrared imagery at the same time (Figures 4 and 6). Six hours later (0600 UTC) and as the upper-level trough becomes negatively tilted, the area of high PV values south of Crete has moved north-eastwards while narrowing and it is extending along a band in a north south orientation crossing eastern Crete. Hart et al. (2006), in their study

10 1528 K. AGOUVARDOS ET A. of the extratropical transition life cycle of North Atlantic tropical cyclones, have found that once the extratropical transition is completed, the negative tilting of the upperlevel trough results in the contraction and intensification of the PV flux and the intensification of the surface cyclone. The high-pv band depicted in Figure 9 almost coincides with the narrow cloud-free region that forms a wedge with a north-west/south-east orientation over the Aegean Sea evident in the respective infrared image (Figure 4). As pointed out by Browning and Golding (1995) and agouvardos and Kotroni (2000), this dry area may penetrate into the wet and warm air masses ahead of the low pressure system and thus contribute to the development of local instability and to the generation of strong convective motions. This feature is supported for the studied case by the presence of significant lightning activity (indicative of strong convection), as evidenced in Figure 4, that is aligned along the eastern edge of the area of high PV values shown in Figure 9. A vertical cross-section along the bold line in Figure 9 is given in Figure 9 showing the relative humidity as well as the location of the 1.5 PVU isoline that denotes the dynamic tropopause. The presence of the dry air masses (with relative humidity <40%) related to the descent of the dynamic tropopause is evidenced just in the western flank of the cloud band that was accompanied by important lightning activity (Figure 4). Finally, at 1200 UTC (Figure 9(d)), the model reproduced a band of high PV values north-west of Cyprus that correlates well with the observed lightning activity shown in Figure 4(d), which is almost aligned over the eastern flank of high PV air Model results: NFX run The role of surface heat fluxes has been investigated in many cases of explosive (as well as ordinary) cyclogenesis and it is considered to be a crucial feature during the cyclogenetical process, as explained by Gyakum and Danielson (2000) in their climatological study of explosive cyclogenesis in the Pacific. In order to investigate this role in the particular case of January 2004, results from the NFX experiment are discussed in the (d) Figure 9. MM5 Grid 2 map (CNTR run) of 300 hpa potential vorticity (shaded contours at 2 PVU intervals, only values greater than 2 PVU are shown) at 0000 UTC 22 January The solid lines denote the 500 hpa geopotential height (at 40 m intervals). As, but at 0600 UTC 22 January The solid line denotes the position of the vertical cross-section shown in. Vertical cross section of the relative humidity (shaded contours at 10% intervals) following the line shown in. The bold line denotes the 1.5 PVU isoline. (d) As, but at 1200 UTC 22 January 2004.

11 EXPOSIVE CYCOGENESIS OVER THE AEGEAN SEA 1529 following. Figure 10 shows a series of mean-sea-level pressures and 500 hpa cloud ice contents, at 0000, 0600 and 1200 UTC 22 January The evolution of the low-pressure system in the NFX experiment is quite identical with that of the CNTR run, however the central pressure is 2, 3 and 7 hpa higher at 0000, 0600 and 1200 UTC 22 January, respectively. Differences are also evident in the 500 hpa ice water content, with lower concentrations reproduced by the NFX experiment, but in general the structure of the icebands is very similar in both experiments. The differences are more pronounced for the vertical velocity fields: at 0000 UTC the ascending motions over the northern Aegean are less intense in the NFX run (not shown), while at 0600 UTC there is a clear decrease of the ascending motions over the western Turkish coasts, and this decrease is also evident at 1200 UTC (not shown). As an overall conclusion, it seems that surface fluxes played a role mainly at the mature stage of the deepening of the cyclone, as the absence of fluxes resulted in a shallower cyclone at that stage while the dynamic upper-air features played the primary role, since even in the NFX experiment the model was able to produce a vigorous and rapidly deepening cyclone Model results: NPV run The role of the upper-level features and the PV anomalies on the surface cyclogenesis is discussed in the following, through inspection of the NPV-run results. Indeed, Figure 11 presents a series of mean-sea-level pressures and 500 hpa cloud ice contents at 0000, 0600 and 1200 UTC 22 January At 0000 UTC the surface low is located over Crete with a central pressure of 992 hpa (Figure 11), that is, it is not only located to the south of the position of the CNTR run but it is also shallower by 5 hpa. Further, the NPV run underestimates considerably the pressure gradient over the Aegean Sea, while the simulated ice content at the 500 hpa level is unrealistic compared to both the CNTR and the NFX runs but also to the observations provided by the AQUA satellite (Figure 5). From that point onwards, the NPV-run simulated fields become even more unrealistic compared to both the CNTR run and to observations: six hours later, at 0600 UTC (Figure 11), the NPV run has moved the surface low quite quickly to the east (over the land of west-south-west Turkey) compared to its actual position, while it deepens it to 990 hpa, which is 11 hpa shallower than in the CNTR run. Accordingly the pressure gradient over the Aegean Sea and the ice content are also unrealistic. Finally at 1200 UTC the low with a central pressure of 988 hpa is positioned over northern Turkey (Figure 11). From the above, it is evident that the NPV run is not only unable to reproduce the deepening of the cyclone but results in a much shallower system that quickly moves out of the area of interest, Figure 10. MM5 Grid 2 map (NFX run) of mean-sea-level pressure (solid lines at 2 hpa intervals) and of 500 hpa ice content (shaded contours at 0.05 g kg 1 intervals) UTC 22 January UTC UTC.

12 1530 K. AGOUVARDOS ET A. Figure 11. MM5 Grid 2 map (NPV run) of mean-sea-level pressure (solid lines at 2 hpa intervals) and of 500 hpa ice content (shaded contours at 0.05 g kg 1 intervals) UTC 22 January UTC UTC. thus supporting the idea that the upper-level dynamics, through the distribution of the PV, have played the decisive role in the explosive cyclogenesis of 22 January Concluding remarks The present paper aims at synthesising the observational and model simulated features of a rapidly deepening cyclone in the Mediterranean. More specifically, on 22 January 2004 a low-pressure system with a central value of 972 hpa was reported over the Aegean Sea, a value that is considered as a record low over the eastern Mediterranean basin for at least the last 40 years. This case of rapid cyclogenesis was associated with gale-force winds that caused considerable damage over many islands of the Aegean Sea. The study was based on the use of ECMWF analyses, lightning observations, data provided by low-orbiting satellites (microwave measurements and precipitation radar) but also on results of simulations performed with the MM5 model. It was found that the rapid development of the cyclone was sustained by an intense upper-level jet that gradually produced a negatively tilted upper-level trough that in its turn produced considerable vorticity advection over the area of explosive cyclogenesis. Microwave observations permitted the identification of the areas with high cloud ice concentrations (as inferred by the presence of brightness temperature depressions at high frequencies) that were also associated with significant lightning activity. Analysis of the simulations produced with the MM5 model helped to detail the evolution of various features during the cyclogenesis process. An important intrusion of dry air towards the middle tropospheric levels has been identified based on the analysis of the PV field. As the upper-level trough was negatively tilted, this area of high-pv air became gradually narrower, forming a wedge that undercut warm air at low levels during the mature phase of the cyclone. This intrusion created instability locally and significant upward motions that led to the organization of the deep convection into a line on the eastern flank of the intrusion. This line was also characterized by significant lightning activity. In order to provide a further insight into the mechanisms that contributed to the cyclone development, two sensitivity experiments were performed with the MM5 model. For the first experiment the surface fluxes were switched off. This experiment showed that a vigorous cyclone was created, even though surface fluxes were switched off, although in that case the cyclone central pressure was about 7 hpa shallower than in the control

13 EXPOSIVE CYCOGENESIS OVER THE AEGEAN SEA 1531 simulation. For the second experiment a piecewise PV inversion technique was applied, in order to evaluate the role of the upper-level PV anomalies on the surface cyclogenesis. In that experiment the initial cyclone that formed was shallower, while after the first 12 hours of simulation it did not deepen any further as it quickly progressed north-eastwards over land. Consequently the resulting cyclone did not match any of the characteristics of the vigorous cyclone that was actually observed. These findings indicate that the upper-level features were the dominant mechanism for this explosive cyclogenesis, while surface fluxes played a role mainly during the mature phase of the storm leading to its further deepening. 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