On the dependence of boundary layer ventilation on frontal type

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114,, doi: /2008jd010694, 2009 On the dependence of boundary layer ventilation on frontal type A. Agustí-Panareda, 1,2 S. L. Gray, 1 and S. E. Belcher 1 Received 1 July 2008; revised 13 November 2008; accepted 30 December 2008; published 10 March [1] Case study simulations with idealized tracers have been used to determine the relationship between the dynamics and conceptual representations of different midlatitude frontal systems and the amount, distribution, and time scale of boundary layer ventilation by these systems. The key features of ventilation by a kata and ana cold frontal system are found to be quantitatively and also often qualitatively similar to the main ventilation pathways, which are the conveyor belts, cloud head, and other convection. The conveyor belts and cloud head occur within cloud, implying that they can be identified using satellite imagery. Differences in the transport by the two systems can be related to their conceptual representations and include a sensitive dependence on the diurnal cycle for the kata- but not the ana-cold frontal case. Citation: Agustí-Panareda, A., S. L. Gray, and S. E. Belcher (2009), On the dependence of boundary layer ventilation on frontal type, J. Geophys. Res., 114,, doi: /2008jd Introduction [2] The ventilation of the boundary layer is important because of its effects on air quality control, long-range transport, the chemistry in the free troposphere and also climate change. Frontal systems are key mediators for the transport of air from the boundary layer to the free troposphere in midlatitudes [Vaughan et al., 1998]. Trajectory analyses have shown that warm conveyor belts (WCBs) ahead of cold fronts in particular can transport boundary layer air to the mid and upper troposphere on a time scale of days and over large distances [e.g., Esler, 2003; Vaughan et al., 2003; Eckhardt et al., 2004]. This mechanism has been extensively studied using aircraft campaigns [e.g., Bethan et al., 1998; Cooper et al., 2001] and tower observations have shown rapid changes (compared to biological processes) in CO 2 mixing ratios during frontal passage [e.g., Hurwitz et al., 2004]. It is also believed to dominate ventilation into the free troposphere during winter in the midlatitudes [Hess, 2005]. A range of scales are active in frontal cyclones: synoptic scale (conveyor belts), mesoscale (cross-frontal circulations and convection) and boundary layer scale (turbulence and surface fluxes condition convection). There is an interaction between and gradual change of circulations associated with the different scales (both in space and time) in frontal zones. Besides frontal cyclones, other transport mechanisms are deep convection, coastal and topographical effects including sea breezes, and entrainment across the boundary layer top. These can also be important in the midlatitudes, as shown by the following two examples, but are not considered in this study. First, Dacre et al. [2007] found that a sea breeze circulation, turbulent mixing and shallow convection together ventilated 52% of constant 1 Department of Meteorology, University of Reading, Reading, UK. 2 Now at ECMWF, Reading, UK. Copyright 2009 by the American Geophysical Union /09/2008JD emission passive tracer into the free troposphere (and 26% to above 2 km) during a high-pressure event. Second, Henne et al. [2004] found that in the Alps on average three times the valley air mass was exported per day by topographic venting from the boundary layer to an injection layer (which became part of the free troposphere) under fair weather conditions. [3] There are two main types of cold fronts: kata-cold fronts and ana-cold fronts [Samson, 1951; Browning, 1986]. In this paper, numerical experiments are performed on two case studies with these two types of cold fronts to investigate the boundary layer ventilation associated with each frontal type. A longer-term ambition for classifying boundary layer ventilation due to different types of frontal cyclones is to pave the way to extrapolate from our results to a potential future climatology of ventilation by frontal systems. [4] Previous numerical experiments performed on a katacold front in the summer showed that convection associated with a Spanish plume event ahead of the kata-cold front was the dominant mechanism for boundary layer ventilation [Agustí-Panareda et al., 2005]. Spanish plume events occur when a tongue of warm relatively dry low-level air is advected northward from the Spanish plateau on the forward side of an eastward propagating upper level trough. This air caps the moister surface air allowing convective instability to build; once released this instability can lead to mesoscale convective systems and thunderstorms [Morris, 1986]. However, that kata-cold front result might have been more influenced by the summer conditions (as Spanish plume events are common in the summer months) than by the kata-cold front itself. The aim of that work was to compare the modeled boundary layer ventilation with observations from the EXPORT (European Export of Precursors and Ozone by Long-Range Transport) field campaign. Thus, the model forecast was initialized with idealized tracers in the boundary layer and a tracer source was introduced only over land. The results from the numerical experiments were found to be consistent with the observations. By contrast, 1of17

2 some verification of its ability to transport tracers realistically has been performed. The ana-cold front case has also been previously successfully simulated (without boundary layer tracers) using this model [Browning et al., 2001; Gray, 2003]. A review of the two types of frontal structures investigated is given in section 2. The case studies involving these two types of fronts are summarized in section 3. In section 4 the numerical experiments are described. The results of these experiments are presented in sections 5 and 6 and the two cases are compared in section 7. Finally, some conclusions are drawn in section 8. Figure 1. Schematic of principal airflows in the mature stage of flat trough, confluent cyclogenesis. (Browning [2004], # Royal Meteorological Society. Reproduced with permission. Permission is granted by John Wiley and Sons Ltd. on behalf of RMETS.) The flows are drawn relative to the cyclone system which is traveling toward the top righthand corner of the page. The two main cloud features are shown stippled: (1) the polar front cloud band associated with the W1 flow which travels parallel to the primary cold front (CF1) and (2) the cloud head (to the left of the bent back front WF2) associated with the W2 flow (represented by three broad diffluent arrows originating to the right of WF2/CF2) and the cold conveyor belt (CCB) (broad dashed arrow representing flow beneath W1 and W2). the numerical experiments performed here have been simplified by having zero tracer concentrations initially and tracer sources everywhere in the model domain to facilitate the comparison between different case studies. [5] Experiments have been performed on a kata-cold front in the spring [Browning, 1995; Browning et al., 1995] and an ana-cold front in the winter [Browning et al., 2001]. These case studies have been chosen because they epitomize midlatitude systems with the two types of cold front. They have also already been carefully analyzed in published literature. [6] The aim of the numerical experiments is to address the following questions: [7] 1. How do the dynamics of frontal systems affect the boundary layer ventilation and transport of pollutants? Which mechanisms are most important? We want to link these mechanisms with the dynamical features of fronts. [8] 2. Do the different dynamical features of fronts affect the horizontal and vertical distribution of tracers in the free troposphere? [9] 3. How much polluted boundary layer air is ventilated by different systems? [10] The model used is the Met Office Unified Model (UM). This is the same model as used in the Agustí- Panareda et al. [2005] study described above and hence 2. Conveyor Belt Conceptual Model and Ana and Kata Cold Fronts: A Review [11] A conveyor belt can be defined as a principal airstream of a frontal system. They can be visualized by plotting streamlines on moist isentropic surfaces (quantified by the wet bulb potential temperature, q w ) and their dimensions are typically 1 3 km deep, km wide and often thousands of km long [Bader et al., 1995]. There are three such conveyor belts in midlatitude cyclones: the warm conveyor belt, cold conveyor belt and dry intrusion. A conceptual model of the conveyor belt flows for the mature stage of flat trough, confluent flow cyclogenesis is shown in Figure 1. The main warm conveyor belt, referred to here as WCB, [Harrold, 1973] (labeled W1 in Figure 1) is a largescale, high q w, ascending air flow that advances poleward ahead of the cold front (labeled CF1). This is often the main rain producing flow. The cold conveyor belt [Carlson, 1980] is air with much lower q w that flows rearward relative to the system motion around the poleward side of the low center ahead of the bent-back warm front (labeled WF2 and extending into a secondary cold front, CF2) and beneath the warm conveyor belt. In conceptual models of cyclogenesis the WCB flow is often divided into two separate flows termed W1 and W2. W1 is the main conveyor belt flow along the axis of the polar front cloud band. The W2 flow is a flow that has q w between that of W1 and the cold conveyor belt and its configuration depends on the system under consideration. Frictional retardation and turning can account for the rearward motion of the W2 flow [Browning and Roberts, 1994]. The dry intrusion [Reed and Danielsen, 1959] is the final principal airstream and is a descending, low q w, dry airflow of lower stratospheric and upper tropospheric origin. [12] Cold fronts are regions where a relatively cold and dry air mass (low q w ) displaces a relatively warm and moist air mass (high q w ). Two types of cold fronts can be distinguished depending on whether there is a general ascent (Figure 2b) or descent (Figure 2a) at midlevels (around 6 km) in the region of the surface cold front. In practice a spectrum of fronts exists between pure ana and pure kata. These two types of cold fronts were named as ana (meaning up in Greek) and kata ( down ) cold fronts by Bergeron [1937]. [13] A detailed description of the conceptual models of anafronts and katafronts is given by Browning [1986] from which much of the following summary is drawn. In anacold fronts the WCB experiences a rearward sloping ascent above the cold frontal zone relative to the movement of the cold front, with cold and dry air associated with the dry 2of17

3 Figure 2. Schematic cross-sectional representations of (a) kata-cold front and (b) ana-cold fronts. Arrows show transverse flow relative to the front. Grey shading represents boundary layer or extruded boundary layer air. Adapted from Browning [1999]. intrusion descending beneath the WCB. The surface anacold front is generally associated with a sharp gradient in temperature and humidity, as well as an abrupt change in wind and relatively high precipitation. When the warm and moist boundary layer air in the lower part of the WCB encounters the surface cold front, the air is lifted abruptly at a few meters per second up to a height of 2 to 3 km. This abrupt upright ascent occurs within the narrow band along the surface cold front and it is consistent with the convergence of the warm and moist low-level jet which depends on surface friction (Ekman layer convergence) and release of latent heat [Hsie et al., 1984]. After this abrupt ascent, the air in the WCB then continues its ascent in a slantwise fashion at a few tens of centimeters per second above the cold wedge of air. The abrupt upright ascent is associated with a narrow band of line convection and very heavy rain collocated with the surface cold front; the slantwise ascent is associated with a broad belt of light to moderate rain. [14] In kata-cold fronts the WCB undergoes a forward sloping ascent relative to the movement of the surface cold front and the dry intrusion overruns the WCB [Browning, 1986; Browning and Monk, 1982]. This dry intrusion results in the development of an upper cold front in the midtroposphere and leads to the creation of potential instability in the area of the upper cold front. This instability can be released by large-scale ascent along the WCB leading to convective ascent from the top of the WCB along the upper cold front. The existence of the cold fronts at the surface and midtroposphere has been named the split front model by Browning and Monk [1982]. Both surface and upper cold fronts are characterized by a sharp drop in humidity but usually a small drop in temperature [Parker, 1999]. Browning [1986] distinguished different types of precipitation associated with the kata-cold fronts. On the surface cold front there is shallow precipitation. Between the surface cold front and the upper cold front there is a shallow moist zone characterized by scattered outbreaks of light rain and drizzle. Ahead of the upper cold front there is an organized band of convection giving a wide band of moderate-to-heavy rain. Finally, ahead of this convective band there is warm frontal precipitation associated with the ascent of the WCB over the warm frontal zone. [15] Ana-cold fronts are considered to be the classic frontogenetic fronts. Kata-cold fronts are more difficult to identify [Browning and Monk, 1982]. Thorpe [1990, p. 417] notes that it is tempting to suggest that an ana-front is frontogenetic and a kata-front is frontolytic because of the respective thermally direct and indirect circulations associated with the respective ascending and descending warm air. However in practice, there is a gradual change from an anacold front to a kata-cold front and some fronts have an intermediate structure with features from both categories [Samson, 1951]. These transitions can either occur with time as the front develops or along the length of the front [Samson, 1951; Browning, 1986]. After analyzing 50 cold fronts that crossed the British Isles [Samson, 1951] found that ana-cold fronts are generally associated with the initial stages of frontal development and kata-cold fronts with the final stages when the parent cyclone becomes more occluded [see also Thorpe, 1990]. Samson [1951] also concluded that katafronts are the more common fronts in the British Isles, especially in summer. 3. Synoptic and Mesoscale Overview of the Case Studies 3.1. Winter Ana Cold Front [16] The ana-cold front was part of a frontal wave development that crossed the British Isles on 10 February The frontal wave developed on the trailing cold front of a mature low associated with a large-amplitude upper level trough to the west of the cold front. Figure 3 shows the satellite image from which the different features linked to frontal wave can be identified. The upper level trough was associated with small-scale scattered convection (labeled T in Figure 3a). A cloud leaf (labeled C in Figure 3a) that had developed on the cold sector ahead of the upper level trough merged with the polar front cloud band (labeled Figure 3. Infrared satellite images of the system producing the ana-cold front at (a) 1921 UTC on 9 February 2000 and (b) 1408 UTC on 10 February 2000 (courtesy of Dundee satellite station). Labels explained in text. 3of17

4 Figure 4. Infrared satellite image of the system producing the kata-cold front at 0417 UTC on 28 April 1992 (courtesy of Dundee satellite station). Labels explained in text. F, a so-called instant occlusion ). This cloud leaf remained in the cold sector and developed into a distinct hook of cloud with an associated flow separate from that of the polar front cloud band and associated cold front. [17] This case study and the associated ana-cold frontal circulation (in which multiple slantwise circulations were observed by very high resolution microwave Doppler radar) has been extensively described by Browning et al. [2001]. The time evolution and broader-scale synoptic environment of this case are described by Gray [2003] Spring Kata Cold Front [18] The kata-cold front was part of a frontal cyclone that approached the UK on 27 April This development is consistent with the conceptual model described as flat trough, confluent cyclogenesis by Bader et al. [1995]. This type of cyclogenesis is characterized by the development of a cloud head (labeled C in Figure 4). The cloud head is defined as a broad well-defined cloud with a convex poleward edge that is separated from the polar front cloud band (labeled F ) by a dry slot (labeled D ). The katacold front develops in the region where the dry intrusion overruns the surface cold front, creating a split cold front with a strong moisture gradient aloft (or upper front) ahead of the surface front. The position of this upper front is marked (using open triangles) on Figure 4. The release of potential instability led to vigorous moist convection along the upper level front. [19] The mesoscale structure of this cyclone was extensively studied during the FRONTS-92 experiment [Browning et al., 1995]. In particular, the kata-cold front was analyzed in association with the development of narrow bands of convective cloud. Rearward slantwise ascent was found to be embedded in the broader forward slantwise ascent associated with the kata-cold front Conveyor Belt Flows [20] The conveyor belt patterns associated with the mature phase of these two types of cyclones are rather similar. Both are composed of a W1 flow and a W2 flow. The first flow, W1, is the conveyor belt associated with the cold front and it can be identified with the large-scale deep ascent originating from the warm sector. The second flow, W2, is a smaller-scale and shallower flow that can differ in the origin of air mass depending on the case study. Conceptual representations of these specific system types [e.g., see Bader et al., 1995, Figures (b) and (b)] suggest subtle differences between the systems in their mature stages. For an instant occlusion case (the winter ana-kata front case), W2 originates in the cold sector behind and ahead of the cold front. For a flat trough confluent flow cyclogenesis case (the spring kata-cold front case), W2 originates from the base of the warm conveyor belt and peals off rearward and forward (relative to the system motion) to form the upper part of the cloud head or hook; a cold conveyor belt forms the lower part of this cloud head. The evolution of the tracer supports the presence of these conveyor belts (see sections 5.1 and 5.2). 4. Methodology [21] Idealized tracer experiments have been performed using version 4.5 of the Met Office UM on limited area domains over Europe (shown in Figure 5). This version of Figure 5. Model domains for (a) the ana-cold front and (b) the kata-cold front case studies. 4of17

5 the Unified Model uses hydrostatic primitive equations and was the operational version of the model prior to the introduction of the new nonhydrostatic dynamical core in August The horizontal resolution used is 0.11 in latitude and longitude which corresponds to approximately 12 km in the horizontal. It has 38 levels in the vertical on a stretched grid ranging from the surface to 5 hpa. The resolution in the vertical corresponds to approximately 10 m for the first model level, 100 m in the boundary layer and 500 m in the mid troposphere. The model evolution is not sensitive to the hydrostatic assumption made in this version of the model at this resolution. Upright convection is parameterized using the mass flux scheme of Gregory and Rowntree [1990] based on a bulk cloud model with a CAPE (Convective Available Potential Energy) buoyancy-based closure. For explicit large-scale cloud and precipitation the microphysically based scheme of Wilson and Ballard [1999] is used. The surface heat fluxes and boundary layer turbulent mixing are parameterized using bulk aerodynamic formulae and a first-order local scheme, respectively [Smith, 1993]. A more detailed description is given by Cullen [1993]. [22] The model was initialized at 0000 UTC on 10 February 2000 for the ana-cold front case and at 0000 UTC on 27 April 1992 for the kata-cold front case. These times were chosen to be during the developing phases of the cyclones when the conveyor belt flows are anticipated to be strongest. The ana-cold front case was initialized from an operational analysis from the global version of the UM. The kata-cold front case was initialized from an ECMWF analysis as a UM analysis was not available. For both cases boundary conditions for the limited area domains were provided by global UM simulations. [23] Idealized tracers were initialized with zero mixing ratio. All the tracers had the same source (an emission rate of kg m 2 s 1 into the first model layer) but two different sinks were considered. The tracer sources were present over the whole domain to enable a clean comparison between the two cases and the sources were introduced 1 h into the model run to allow the boundary layer to develop in the model prior to their introduction. Tracer sinks are represented using an exponential decay term with an e- folding lifetime of t following the methodology of Agustí- Panareda et al. [2005] (see this paper for further details and derivations of the following equations). Thus, these idealized tracers can approximate real atmospheric tracers, which have characteristic rates of chemical loss, without introducing explicit chemistry into the model. Tracer decay time scales of 3 h and 24 h were chosen as they are characteristic time scales for transport by convection and advection, respectively, in midlatitude synoptic-scale systems. The shorter time scale is longer than that typically assumed for convection (perhaps h). However, as noted by Agustí-Panareda et al. [2005], far from the source the mixing ratio in the model becomes dominated by numerical dispersion for a tracer with the shorter 1 h time scale. [24] For two tracers with identical source strengths and source distributions at the ground but different lifetimes (t 1 and t 2 where t 2 > t 1 ) an age of an air mass can be defined by t s ¼ ln q 2 ; ð1þ t 2 t 1 q 1 where q 1 and q 2 are the instantaneous mixing ratios of tracers with lifetimes t 1 and t 2, respectively. It is assumed that tracers are conserved following an air mass except for the loss at rate t, so this is strictly only an age estimate in the limit of no mixing between air parcels away from the source region. However, it also applies if it can be assumed that both tracers mix with a clean background with zero tracer (as might be the case in a polluted plume leaving a localized source). [25] Assuming negligible loss across lateral boundaries, integration of the tracer evolution equation with time yields an equation for the evolution of the tracer mass, m: h i mðtþ ¼St 1 e t t ; ð2þ (for zero initial tracer mass) where S is the product of the tracer emission rate and the emission area. Thus, as time t increases we expect the total tracer mass will evolve to a steady state value given by St. [26] Tracers are transported by combinations of three different schemes in the model which represent different mechanisms of transport between the boundary layer and the free troposphere. The tracer transported by advection (all resolved processes including explicit convection) only is labeled as tracer A, by parameterized mixing and advection as tracer B, by parameterized convection and advection as tracer C and by mixing, convection and advection (all processes) as tracer D. Tracer mixing ratios were not considered below a threshold mixing ratio (10 14 ) where numerical diffusion was found to dominate the numerical modeling results. [27] A problem occurred with the age diagnostic when the tracer remained close to the source for a long time (i.e., t t). In this case the ratio of the longer-lived to shorterlived tracer mixing ratios would be greater than unity when the air mass containing the tracers was transported away from source because of the more rapid decay of the shorterlived tracer (contradicting the assumption that the mixing ratios of the tracers were the same on leaving the source region). This led to a slight bias toward larger ages very near to the surface in a few locations. To avoid this problem the model was run with no tracer sink in the first five model levels when diagnosing age of air. In contrast, it was necessary to allow tracer sinks in all model level for the budget calculations. [28] Finally we note that this calculation for age of air has been partially validated by Agustí-Panareda et al. [2005]. The spatial distribution of the air of air was found to be consistent with Lagrangian trajectory results and observations for the EXPORT case study. 5. Spatial Distribution of Tracers in the Free Troposphere 5.1. Winter Ana Cold Front [29] Figures 6 and 7 show the spatial distribution of the age of air (calculated using different tracers) on q w surfaces of 286 K and 280 K and the geopotential height of these surfaces. The 286 K q w surface was chosen because it characterizes air parcels ascending along the main warm conveyor belt, W1, which flows northeastward across the 5of17

6 Figure 6. Age of air (hours, color shaded) and geopotential height (m, labeled contours, contour interval 1000 m) on q w = 286 K surface which characterizes the W1 branch of the WCB for the winter ana-cold front case at 1200 UTC on 10 February Age is calculated for tracers A, B, C, and D which are transported by advection only, advection and mixing, advection and convection, and all processes, respectively. The low-pressure center and the approximate path of the W1 flow are marked. Very faint lines mark boundaries at which mixing ratios fall below the threshold value; age is not calculated outside these regions. 6of17

7 Figure 7. As for Figure 6 but on q w = 280 K surface which characterizes the W2 branch of the WCB. The low-pressure center and the approximate path of the W2 flow are marked. domain from about 40 N, 20 W ahead of the surface cold front. The 280 K q w surface characterizes the air parcels ascending along the second warm conveyor belt, W2, fanning out in the cloud hook between northern Scotland and northern Norway (the approximate paths of W1 and W2 are marked on Figures 6a and 7a, respectively). The height of both surfaces goes from the top of the boundary layer (1 km) to the mid-upper troposphere (7 km for the W1 flow and 5 km for the W2 flow). Thus, both surfaces can show the transport of tracer from the boundary layer to the free troposphere. [30] The roles of different transport processes can be assessed by analyzing the spatial distribution of the age of air calculated using tracers transported by different combination of schemes in the model. Resolved advection alone can only transport tracer from the surface to the free 7of17

8 Figure 8. Vertical cross section of age of air (hours, color shaded) across the cold front (7 W, 53 N to 3 E, 47 N), q w (thin contours, contour interval 1 K), and boundary layer top (thick line) for the winter ana-cold front case at 1200 UTC on 10 February Age is calculated from tracers A, B, C, and D which are transported by advection only, advection and mixing, advection and convection, and all processes, respectively. The position of the surface cold front (SCF) and path of the rearward sloping ascent are marked. troposphere where there is resolved vertical ascent at the surface. This occurs in the line convection associated with the rearward edge of W1 (Figure 6a) and in both forward and rearward flowing branches of W2 (Figure 7a). It also occurs in the resolved convection over Scotland and in a cyclone over the Mediterranean (Figure 7a). With mixing and advection the tracer is transported from the top of the boundary layer along the leading edge as well as the rearward edge of W1 (Figure 6b). The spatial extent of ventilated air in the free troposphere associated with W1, W2 and the resolved convection over Scotland and in the Mediterranean cyclone is much larger when there is mixing (Figure 7b). Transport by advection and parameterized convection is concentrated on the rearward edge of W1, along both branches of W2, in the region of scattered convection associated with the upper level trough between Scotland and Iceland (also associated with high values of convective available potential energy (CAPE), not shown) and in the region of the Mediterranean low (Figure 6c). Rapid transport (young air) by shallow convection is also seen in the cold sector behind the cold front (Figure 7c and to a lesser extent in Figure 6c). Some of this transport (over Northern England, southern Scotland and Ireland) is in the region of the undercutting low-q w air (the dry intrusion) shown schematically in Figure 2a; satellite observations (Figure 3b) show only partial and generally shallow (dark gray in infrared imagery) cloud cover in this region and calculations of model relative humidity on the 280 and 286 K q w surfaces (not shown) show that this air is unsaturated on the grid scale. Transport by all processes (advection, mixing and convection) within the conveyor belts W1 and W2 (Figures 6d and 7d) is very similar to that 8of17

9 Figure 9. Vertical cross sections of relative humidity (shaded) and q w (contours, contour interval 1 K) for the winter ana-cold front and spring kata-cold front cases, respectively. The cross sections plotted are at the same locations and times as those in Figures 8 and 12, respectively. of the transport by advection and mixing (Figures 6b and 7b). The areas of additional transport are those in which convective transport are important (Figures 6c and 7c). [31] Avertical cross section through the cold front (Figure 8) shows that the transport of tracer from the boundary layer to the free troposphere follows the rearward ascent associated with a typical ana-cold front. Transport by advection only (Figure 8a) and mixing and advection (Figure 8b) compare well with conceptual model of an ana-cold front (Figure 2b) with air aging in the rearward ascent. This paradigm is masked by the addition of convective transport (Figures 8c and 8d). Figure 9a shows that the rearward ascent region is a region of high relative humidity (cloud). Transport by convection behind the surface front (in the dry intrusion air) leads to transport in (and to above) potentially unstable (negative vertical gradient in q w ) but unsaturated air. This is consistent with convective parameterization scheme triggering here but failing to saturate the model on the grid scale. [32] In summary, the numerical experiments of tracer transport for this particular midlatitude cyclone show that the tracer is indeed exported from the boundary layer into the free troposphere within the regions of the two conveyor belts (W1 and W2), the convection associated with the upper level trough, and the shallow convection behind the cold front. Apart from the transport found in the dry intrusion region, regions of transport map strongly onto regions of cloud visible in the satellite images (Figure 3) Spring Kata Cold Front [33] Figures 10 and 11 show the spatial distribution of the age of air (calculated using different tracers) on q w surfaces of 286 K and 281 K and the geopotential height of these surfaces. On the 286 K q w surface this depicts the main warm conveyor belt (W1, most clearly seen in Figures 10b and 10d and approximately marked on the former of these). This is to the south of the low-pressure center and transports tracer toward to western corner of Iberia from the top of the boundary layer to approximately 6 km height. The cold conveyor belt and W2 flow are evident on the 281 K surface (most clearly seen in and approximately marked on Figures 11b and 11c, respectively). The cold conveyor belt transports tracer from the boundary layer in the cold sector up to a maximum height of 5 km south of the British Isles, just to the north of the low-pressure center (originating at around 8 W 47 N). The W2 flow transports tracer from the boundary layer near the center of the cyclone up to about 4 km height. Thus, in this kata-cold front case, all three airflows (the two warm conveyor belts and the cold conveyor belt) contribute to the transport of air from the boundary layer to the free troposphere. The dry intrusion originates well above the boundary layer and therefore it cannot contribute to the ventilation of boundary layer air. [34] The roles of different transport processes are also assessed for this case. Resolved vertical ascent at the surface only occurs in a very small region of the cold front close to the low-pressure center where there is a very steep gradient of q w (probably linked with line convection) and is shallow and rapid (this is evident in Figure 11a but not Figure 10a). With mixing and advection the tracer is transported from the top of the boundary layer by all the main airflows: W1, W2 and the cold conveyor belt (see description above, and Figures 10b and 11b). Transport by parameterized convection occurs predominantly near the southern tip of the cloud head (around 12 W and 47 N) and in the cold air to the north and east of the system (see Figures 10c and 11c). The position of the cloud head inferred from the convective transport compares well with its position in infrared satellite data at the same time and scattered convection can be seen to the north of the cloud head [see Browning et al., 1995, Figure 2 (right)]. Comparison between the regions of tracer transport and calculated model relative humidity on the 281 and 286 K q w surfaces (not shown) also shows a strong correspondence between regions of young air to the north of the cloud head on the lower surface in Figure 11c and cloud (the upper surface is generally cloud free in this region). The age of the air transported by convection is typically young (9 h or younger). Convection in the cloud head transports air rapidly up to heights of 7 km. At these heights the upper level jet associated with the large-scale flow advects the air downstream as shown by the aging plume over the British Isles. Downstream flow can also be seen to follow the convective transport to the North and East of the system. 9of17

10 Figure 10. Age of air (hours, color shaded) and geopotential height (m, labeled contours, contour interval 1000 m) on q w = 286 K surface which characterizes the W1 branch of the WCB for the spring kata-cold front case at 1800 UTC on 27 April Age is calculated for tracers A, B, C, and D which are transported by advection only, advection and mixing, advection and convection, and all processes, respectively. The low-pressure center and the approximate path of the W1 flow are marked. Very faint lines mark boundaries at which mixing ratios fall below the threshold value; age is not calculated outside these regions. Shallow convection can be seen in the cold air behind the cold front (Figure 11c). In contrast to the summer kata-cold front case studied by Agustí-Panareda et al. [2005], there is no evidence of boundary layer ventilation due to convective transport in the region where the dry intrusion overrides the warm conveyor belt. This suggests that midlevel potential instability in this region (diagnosed by Browning et al. [1995]) cannot lead to the ventilation of boundary layer air in the absence of strong summer surface heat fluxes. As with the winter ana-cold front, the spatial distribution of the age of air transported by all processes (Figures 10d and 11d) reflects the separate contributions from advection with boundary layer mixing (to the conveyor belts W1, W2 and the cold conveyor belt) and advection with convection (to the cloud head and other convective regions). [35] A vertical cross section through the cold front shows that the transport of tracer from the boundary layer to the free troposphere follows the forward ascent associated with a typical kata-cold front (Figure 2a) when only advection and mixing transport air (Figure 12b). Frontal ascent is negligible in the absence of mixing for this cross section (Figures 12a and 12c, consistent with Figures 10a and 10c). As with the winter ana-cold front, convection masks the paradigm of ascent for this type of front but this is only seen 10 of 17

11 Figure 11. As for Figure 10 but on q w = 281 K surface which characterizes the CCB. The low-pressure center and the approximate path of the CCB (dashed) and W2 flow (solid) are marked. here when convection is combined with mixing and advection (compare Figures 12b and 12d). Figure 9b shows that the forward ascent region is cloudy. Behind this region transport is occurring predominantly in moist air (compare Figure 9b with Figure 12d). [36] In summary, the numerical experiments of transport for this particular midlatitude cyclone show that the boundary layer ventilation can be attributed to advection from the top of the boundary layer (requiring advection and boundary layer mixing) by three main airstreams (W1, W2 and the cold conveyor belt) as well as deep convection in the cloud head (and to the north of the cloud head) and shallow convection in the cold sector behind the cold front. As with the winter ana-cold front case, regions of transport map strongly onto regions of cloud visible in the satellite image (Figure 4). 6. Budget of Boundary Layer Ventilation [37] The budget of boundary layer ventilation by the warm conveyor belts has been calculated for the two case studies by integrating the tracer mass in the boundary layer and in different vertical layers of the free troposphere within the region of the warm conveyor belts. The aim is to have a quantitative comparison between the two cases. The areas of the warm conveyor belts are shown as grey masks in Figure 13. These have been defined by using backward and forward Lagrangian trajectories [Wernli and Davies, 1997] together 11 of 17

12 Figure 12. Vertical cross section of age of air (hours, color shaded) across the cold front (12 W, 40 Nto 13 W, 46 N), q w (thin contours, contour interval 1 K), and boundary layer top (thick line) for the spring kata-cold front case at 1800 UTC on 27 April Age is calculated from tracers A, B, C, and D which are transported by advection only, advection and mixing, advection and convection, and all processes, respectively. The path of the forward sloping ascent is marked. with information about the tracer distribution in the mid and upper troposphere as follows. Ensembles of forward and backward trajectories were released 1 day after the beginning of the simulation. The region from which the trajectories were released covered an area in which the tracer concentrations at the time of the trajectory release were relatively high in the midtroposphere (500 hpa) and spanned a depth from 250 to 1000 hpa. The backward trajectories were run for 21 h and the forward trajectories for 18 h. Finally, a mask associated with the region of the conveyor belts was produced by broadening the area covered by the backward and forward trajectories by 3 to the west, to the east, to the north and to the south. This was done to ensure that all the tracer in the free troposphere originating from the warm sector was present in the budget. The areas of the masks are m 2 for the winter ana-cold front case and m 2 for the spring kata-cold front case. It is noteworthy how similar these values are given the very different cyclones. [38] The variation of the mean (averaged over the mask region) boundary layer heights with time in these regions is shown in Figure 14. The mean boundary layer height for the ana-cold front shows no diurnal cycle (Figure 14a) whereas for the kata-cold front it appears to follow the diurnal cycle with a maximum at 1800 UTC (recall that the forecasts for both cases are initiated at midnight). This is consistent with expectations as the boundary layer height is affected by the passage of the cold front as well as the diurnal cycle. We would expect the ana-cold front to have a stronger influence on the boundary layer height than the kata-cold front because the kata-cold front is primarily a midlevel front (this 12 of 17

13 AGUSTI -PANAREDA ET AL.: BOUNDARY LAYER VENTILATION Figure 13. Masks for the (a) ana-cold front and (b) kata-cold front warm conveyor belts used to calculate the integrated tracer budget. The masks have been obtained from backward and forward Lagrangian trajectories (see text for more details). can be seen by contrasting the boundary layer tops shown in Figures 8 and 12). We would also expect the impact of the diurnal cycle on the boundary layer height to be more pronounced in spring than in winter Winter Ana Cold Front [39] The tracer budgets are plotted in Figure 15 using a logarithmic scale for the tracer mass to emphasize the comparatively small amounts of tracer in the upper layers. The expected total tracer mass is marked by the arrows with values of ln(st) equal to 26.2 and 24.1 for the longer-lived and shorter-lived tracers, respectively (yielding a ratio of 8 in St, the ratio of lifetimes). The total tracer mass asymptotes rapidly to the expected value for the shorter-lived tracers but falls short of the expected value for the longer-lived tracers as this tracer is exported outside the warm conveyor belt within the lifetime of the tracer. As expected, the greatest amount of boundary layer ventilation occurs for the tracers transported by all processes (tracers D) and more ventilation occurs for the longer-lived tracers. Approximately 20 and 148 times more tracer are transported to the mid and upper troposphere, respectively, for the longer-lived tracer compared to the shorter-lived tracer. [40] Free tropospheric tracer increases rapidly over the first 6 12 h of the simulation and then either increases more slowly (for the longer-lived tracer) or decreases slightly (in most layers for most shorter-lived tracers). For the shorterlived tracers the tracer decay can thus exceed the tracer flux into a layer for some layers and tracers. Boundary layer mixing is vital for boundary layer ventilation; the total tracer is only visibly distinguishable from the boundary layer tracer on this logarithmic scale for tracers B and D (compare Figures 15b and 15d with Figures 15a and 15c). Transport by parameterized convection leads to an increase in the tracer mass in the mid and upper troposphere which is particularly noticeable in the first 12 h of the simulations. Figure 14. Mean boundary layer height for the (a) ana-cold front and (b) kata-cold front warm conveyor belt regions (see Figure 13). 13 of 17

14 Figure 15. Time series for the ana-cold front of tracer integrated within the warm conveyor belt mask (see Figure 13a) in the boundary layer (thin solid line), lower free troposphere (between boundary layer top and 762 hpa, see triangles), mid-free troposphere (between 725 hpa and 475 hpa, see asterisks), and upper free troposphere (between 430 hpa and 202 hpa, see squares). The total integrated tracer is shown as a dashed line. The arrows indicate the predicted steady state values of the total tracer mass. Convection leads to more transport into the upper troposphere during this time period than boundary layer mixing (compare Figures 15b and 15c and Figures 15f and 15g). A local peak in the upper tropospheric tracer mass occurs 9 h into the simulations (particularly for the shorter-lived tracer, see Figures 15f and 15h). The observed impact of convective transport before dawn implies that the convection leading to the transport is not strongly diurnally driven. There is no clear signal of the diurnal cycle in any of the time series as the boundary layer height is mainly dominated by the passage of the front (Figure 14a) Spring Kata Cold Front [41] The tracer budgets have an expected total mass given by ln(st) values of 25.4 and 23.3 for the longer- and shorter-lived tracers, respectively (Figure 16). Neither the longer- or shorter-lived set of tracers reach the expected values because of the export of tracer from the warm conveyor belt region with the disparity being worse for the longer-lived tracers as expected. As with the winter anacold front case, most ventilation occurs for the longer-lived tracer transported by all processes and the tracer decay exceeds the tracer flux for the shorter-lived tracer (particularly into the mid tropospheric layer) toward the end of the simulations. [42] Boundary layer mixing and convection are essential for transport to the upper troposphere with negligible tracer into this layer for the advection only tracer (Figures 16a and 16e). This contrasts with the winter ana-cold front case for which much greater transport occurred into this layer for this tracer (Figures 15a and 15e). Convection has a strong diurnal signature in the spring kata-cold front case with transport by parameterized convection only leading to significant transport into the upper troposphere after dawn (Figures 16c and 16g). Boundary layer mixing leads to more transport into the lower troposphere and midtroposphere than convection before dawn (compare Figures 16b and 16c and Figures 16f and 16g). There is a further increase in 14 of 17

15 Figure 16. As for Figure 15 but for the kata-cold front. upper tropospheric tracer due to convective transport between 1800 and 2100 UTC. The diurnal variation in boundary layer height (Figure 14b) is reflected in diurnal variability of the mass in the lower troposphere (defined as being between the boundary layer top and 762 hpa). This is most noticeable for the advection only tracers (Figures 15a and 15e) and results in a decrease in tracer mass as the boundary layer deepens during the day and an increase in mass when the boundary layer collapses at dusk. 7. Comparison of the Kata and Ana Cold Fronts [43] The surprise is that boundary layer ventilation by these two rather different frontal systems is qualitatively (and often quantitatively) very similar. For example, the percentage of long-lived ventilated tracer transported by advection, convection and mixing is 40% for the ana-cold front and 42% for the kata-cold front after 24 h (see Table 1). Note that these values are similar to the 50% ventilated tracer found by Donnell et al. [2001] for an initial condition tracer in two frontal case studies (also over 24 h). This suggests that the behavior of an initial condition tracer would be similar to that found here for the long-lived tracer (which had the same lifetime as the duration of the simulations). More recent evidence for the role of cyclones in boundary layer ventilation has come from idealized baroclinic life cycle experiments. Sinclair et al. [2008] found 42% and 50% of an initial condition tracer was ventilated by entire idealized LC1 and LC2 baroclinic life cycles in the absence of moist processes; Polvani and Esler [2007] found much smaller ventilation rates in similar experiments but without a boundary layer parameterization scheme, suggesting the importance of boundary layer processes for effective ventilation (as found here). For the short-lived tracer the difference between the two fronts is a bit larger, 14% for the ana-cold front and 20% for the kata-cold front (see Table 2). Table 1. Percentage of Total Tracer in the Free Troposphere 1 Day After Starting the Simulation: 24-h Tracer Lifetime a Type of Cold Front Tracer A Tracer B Tracer C Tracer D Kata Ana a Tracer A is transported by advection; tracer B is transported by advection and mixing; tracer C is transported by advection and convection; and tracer D is transported by advection, convection, and mixing. 15 of 17

16 Table 2. Percentage of Total Tracer in the Free Troposphere 1 Day After Starting the Simulation: 3-h Tracer Lifetime a Type of Cold Front Tracer A Tracer B Tracer C Tracer D Kata Ana a Tracer A is transported by advection; tracer B is transported by advection and mixing; tracer C is transported by advection and convection; and tracer D is transported by advection, convection, and mixing. In both systems the main ventilation regions are those of the conceptual conveyor belts and cloud head (the pathways of which can be inferred using knowledge of the type of frontal system) and other convective regions. Transport predominantly occurs within the cloud in these systems; a possible exception is the transport found in the dry intrusion region in the winter ana-cold front case (only shallow partial cloud cover is observed in this region and the model is unsaturated on the grid scale). Boundary layer mixing is essential to obtain ventilation by the main warm conveyor belt (W1). In the absence of mixing, transport only occurs in line convection (which is present in the ana-cold front case but not the kata-cold front case). Transport by boundary layer mixing and parameterized convection are both essential for ventilation by the second warm conveyor belt (W2). The cold conveyor belt can lead to relatively shallow transport (up to 2 or 3 km) if boundary layer mixing is present (the cold conveyor belt is only observed in the katacold front case). Convective transport occurs in the cloud heads and in shallow convection in the cold air behind the cold front in both systems. [44] The main differences between the cases are the pathways of the transport relative to the motion of the cold front and the impact of the diurnal cycle. Transport is rearward for the anafront and forward for the katafront. This is consistent with schematical representations of these fronts (Figure 2). The diurnal cycle has significant impact on both convective transport and boundary layer height in the katafront case but not in the anafront case. This is a consequence of two factors: the diurnal influence on convection is likely to be stronger in the warmer months and the katafront, being a midlevel front, does not affect the boundary layer height which means that the diurnal signature of boundary layer depth can be significant. 8. Conclusions [45] The aim of this work was to determine the relationship between the dynamics and conceptual representations of different midlatitude frontal systems and the amount, distribution and time scales of boundary layer ventilation by these systems. This was achieved by performing Met Office Unified Model forecasts with sources of tracers in the boundary layer. These tracers are passive but do have a specified decay time scale; this enabled us to calculate the age of the ventilated air. Ventilation mechanisms were attributed by passing different tracers through different combinations of the numerical schemes: advection (resolved transport including explicit convection), parameterized boundary layer mixing and parameterized convection. [46] Two typical, but structurally different, frontal systems were chosen for analysis: an ana-cold frontal case in winter and a kata-cold frontal case in spring. The key finding of this work is that the conceptual models of the air flows in these systems (the conveyor belts, frontal and cloud head convection, and cross-frontal transports) identify the main transport pathways found in the numerical simulations. Additional convective transport can also occur in the cold air behind the cold front but this tends to be shallow. Transport predominantly occurs through cloud which means that satellite images can be used to identify the main ventilation pathways; a caveat to this is that the convective parameterization scheme is leading to some, generally rapid, transport in regions that are unsaturated on the model grid scale (and in which only shallow partial cloud cover is observed). If this result can be extended to other frontal systems then this provides a paradigm through which climatological ventilation rates due to frontal systems can be estimated if a relationship between ventilation rates and the strengths of conveyor belts can be extracted. [47] Acknowledgments. The authors thank the Met Office for the use of their Unified Model. We also gratefully acknowledge the use of the diagnostic and plotting tools supported by Chang-Gui Wang via the National Centre for Atmospheric Science (NCAS). This work was funded by a NERC grant under the Polluted Troposphere thematic programme. 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