Föhn ow and stable air mass in the Rhine valley: The beginning of a MAP event

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1 Q. J. R. Meteorol. Soc. (24), 13, pp doi: /qj Föhn ow and stable air mass in the Rhine valley: The beginning of a MAP event By GUILLAUME BEFFREY, GENEVIEVE JAUBERT and ALAIN DABAS CNRM/GMEI, Météo-France, Toulouse, France (Received 23 December 22; revised 3 June 23) SUMMARY The föhn event of the 8th Intensive Observation Period (2 21 October 1999) of the Mesoscale Alpine Programme is examined at small scale with the help of both instrumental data (especially from a ground-based Doppler lidar) and of high-resolution simulation performed with the non-hydrostatic model meso-nh. The threedimensional characteristics of the ow in the Rhine valley at the beginning of the event are studied. Special attention is given to the interaction between the föhn and a stable air mass, whose presence drives the penetration of the warm and turbulent föhn air to the ground. A quanti cation of the simulated features of the ow is given in terms of mass budgets over the target area, and a three-dimensional picture representing the arrival of the föhn air in the Rhine valley area is proposed. KEYWORDS: Cold pool IOP8 Mesoscale Alpine Programme 1. INTRODUCTION Föhn is a warm, dry, turbulent, downslope wind blowing in valleys downstream of mountain chains (Seibert 199). It is particularly strong and frequent in the valleys perpendicular to the main crest downstream of a pass. The upstream wind crossing the mountain ridge penetrates the valley through the pass. It ows down along the valley until it meets cool, stable air masses. At rst, the cold air prevents the föhn from reaching the bottom of the valley. After a phase of gradual erosion and constant feeding of the cold pool, the föhn ow eventually reaches the valley oor. The transition from the cold, stable, to warm, windy and unstable meteorological conditions is sudden. Its forecast is a real challenge to weather services. Synoptic conditions favouring föhn are known and detectable (Hoinka 198), but the small-scale mechanisms involved in its penetration down the valleys are still unclear. An important dataset relevant to föhn was collected in the high Rhine valley during the Mesoscale Alpine Programme (MAP) experiment (Bougeault et al. 21). It gives a detailed, mesoscale picture of how the föhn penetrated the valley during 11 different föhn cases observed during the campaign. In the following, we focus on the 8th Intensive Observation Period (IOP) which started on 2 October 1999 early morning and ended on 21 October afternoon. The comprehensive dataset collected during the IOP is used to build a picture of the onset of föhn. Particular attention is paid to three-dimensional wind data provided by the Transportable Wind Lidar (TWL), a ground-based Doppler lidar which observed the ne-scale structure of the wind eld inside the valley. The structure is compared to a simulation of a mesoscale, non-hydrostatic model which gives a general picture of meteorological conditions in and outside the valley. A ne-scale output (horizontal resolution 625 m) is used to quantify air-mass exchanges and elaborate a three-dimensional picture of the mechanisms leading to the arrival of föhn to the ground. In this paper, section 2 presents the principal tools used for the study, i.e. TWL and the Meso-NH numerical model. The synoptic meteorological frame and the temporal evolution of föhn is discussed in section 3. In section 4, a three-dimensional picture of föhn penetration is proposed. Corresponding author: CNRM/GMEI, 42 Av Coriolis, 3157 Toulouse Cedex, France. guillaume.beffrey@cnrm.meteo.fr c Royal Meteorological Society,

2 542 G. BEFFREY et al. 2. INSTRUMENTATION AND MODELLING (a) The Transportable Wind Lidar The instrumental set-up deployed in the high Rhine valley during MAP is in part described in Bougeault et al. (21). For the rst time in a eld experiment, the TWL, a Doppler lidar, was involved. Deployed in Vilters, Switzerland (9.46 B E, 47.3 B N), the TWL was dedicated to the observation of the wind eld at the intersection of the Rhine and Seez valleys. A Doppler lidar basically measures radial winds, i.e. the component of wind along the laser beam. The radial velocity is derived from the frequency shift between the emitted laser wave and the radiation scattered back to the instrument by illuminated aerosol particles. According to the Doppler equation, the Doppler shift ±º is proportional to the radial wind v r ±º D 2v r ; (1) where D 1:6 ¹m is the laser wavelength. To get different wind components and explore three-dimensional volumes around the instrument, the laser beam is scanned. A two-axis, automated, step scanner allows the line of sight to be directed anywhere in the upper hemisphere. The range resolution is limited to 25 m, i.e. the length of emitted pulses. The maximum range depends on the aerosol loading. During föhn episodes the air was rather clear, so the maximum range was 5 km. The rst range gate is 5 m away from the instrument. Two different kinds of scan patterns were used during MAP (Drobinski et al. 21). The raster scan combines azimuth sweepings at increased elevation angles (from 2 B to 28 B with 2 B increments). The azimuth was varied from 98 B to 142 B in 5 B steps, so the full raster scan sounded the upper and lower parts of the Rhine valley and the Seez valley. A full scan was completed in about one hour. The second scan pattern is the Plan Position Indicator (PPI). In a PPI, the elevation angle of the laser beam is xed while the azimuth is rotated. The volume scanned around the instrument has consequently the shape of a cone. Using the velocity azimuth display method or geometrical projections, vertical pro les of horizontal winds can be retrieved, and are directly comparable to standard wind pro les from radiosondes. Note, however, that the retrieval assumes a horizontally homogeneous wind eld within the volume scanned by the laser beam. For a 7 B elevation angle (the standard elevation angle during MAP), the radius of the cone is 3 km at a height of 3 km above the instrument. The homogeneity of the wind eld at this scale may be questioned, but it should not be a problem closer to the instrument, and we believe vertical wind pro les are reliable at heights up to 1 or 2 km. The time needed for a full PPI scan is 15 to 3 min. The retrieval of frequency Doppler shifts from detected signals is made in the digital domain with dedicated, frequency estimating algorithms. A detailed description of the estimators used for MAP data can be found in Dabas et al. (1998) and Dabas (2). The accuracy and probability of bad estimates were estimated before the campaign as a function of the signal-to-noise ratio (SNR) with the method proposed by Mayor et al. (1997). For SNR > db (SNR D db was typically reached at ranges of about 6 m), more than 9% of radial wind measurements are good. At SNR D 5 db, one radial wind out of two is a bad, unreliable measurement. The accuracy for SNR > db is better than.8 m s 1 for both 5 s and 5 s time resolutions. At SNR D 5 db (range of about 5 m), it is.5 m s 1 for a time resolution of 5 s and.2 m s 1 for 5 s.

3 FÖHN FLOW AND STABLE AIR MASS IN THE RHINE VALLEY 543 (b) Other instruments The Alpine domain was largely instrumented during MAP. In the Rhine valley and the so-called FORM target area, a dense set of instruments was available, especially during the different IOPs. In addition to the network of surface stations distributed along the valleys, several radiosounding stations were deployed and operated every three hours during the IOPs (see Fig. 3 for their location). Other instruments were available: a sodar (at Luzisteig, south of Balzers), a scintillometer (across the valley, around Weite, see Furger et al. (21)). Also, in situ data from several ights of the Merlin research aircraft were available, some of them at low levels inside the valleys. (c) Modelling The Meso-NH mesoscale atmospheric model (Lafore et al. 1998) is used in this paper to simulate the rst phase of IOP8 (the onset of föhn). The parametrizations for convection, microphysics and surface and radiation schemes are the same as in Jaubert and Stein (23) (for the simulation of the IOP2 föhn event). Three nested grids are used. The model runs on the grids at the same time in a two-way interactive mode (Stein et al. 2). The largest grid covers western Europe at a horizontal resolution of 1 km; the intermediate grid focuses on the Alpine region with a horizontal resolution of 2.5 km; the nest grid the horizontal resolution is 625 m is restricted to the Rhine valley (Fig. 3). The initial and lateral conditions for the outer grid are interpolated from the global, operational Arpège model of the French weather service. Lateral boundary conditions are updated every six hours. For large and intermediate resolutions, turbulence is parametrized using the Bougeault Lacarrère scheme (Bougeault and Lacarrère 1989). For the ne-scale domain, a three-dimensional turbulent scheme is used with a mixing length according to Deardorff (1972). The vertical grid is stretched to obtain a maximal resolution close to the ground. The horizontal resolution of the topography is 1 km. The simulation covers the time period from to 18 UTC. During the rst few hours of simulation, only two domains the larger ones are active. During this period the model stabilizes the simulation by creating all the small-scale atmospheric features generated by the high-resolution topography: a boundary layer above the highresolution topography, and an initial cold, stable air mass inside the valleys. The third, smaller-scale domain is activated only between 7 and 94 UTC. This is the time period during which a raster scan was completed by the TWL. At this time, the föhn ow had penetrated the Rhine valley but had not yet reached the surface. 3. IOP8: SYNOPTIC DESCRIPTION, CONTEXT OF THE STUDY (a) IOP8: classical föhn-generating synoptic situation Three föhn episodes occurred in the high Rhine valley from 2 to 24 October; IOP8 covers the rst one. The föhn is generated by a warm, humid ow coming from the south-west caused by a deep low approaching the Alps from the west (see Fig. 1). These are typical föhn conditions (Seibert 199; Hoinka 1985). The föhn stops when the cold front associated with the deep low passes over the target area. As far as IOP8 is concerned, the deep low (989 hpa at sea level) approached the west Mediterranean Sea on 2 October. It is centred on the north of the Iberian peninsula in the late afternoon (Fig. 1). The south-westerly synoptic ow above the Alps at the beginning of the day turned gradually to southerly in the afternoon as the low pressure moved Föhn in the Rhine valley during MAP.

4 544 G. BEFFREY et al. Figure 1. Analyses for 12 UTC 2 October 1999 showing pressure contours (a) at ground level and (b) at 5 hpa. The dashed contours in (b) represent temperature at 5 hpa. eastward. The meridional component of the wind reached 16 m s 1 at 4 m above mean sea level (ASL) at 2 UTC (according to the UHF pro ler in Lonate, south of the Alps). The maximum velocity was observed at about 6 UTC 21 October. Strong precipitations occurred south of the Alps and in the French Rhône valley during the night between 2 and 21 October following big thunderstorms along the Mediterranean coast.

5 FÖHN FLOW AND STABLE AIR MASS IN THE RHINE VALLEY 545 5Ê 1Ê 15Ê 5Ê 1Ê 27 m s -1 Figure 2. Arpège analysis for 12 UTC 2 October Horizontal wind at 8 m ASL and sea-level pressure (the contour interval is 2 hpa). The rectangle represents the domain of the high-resolution simulation (in Fig. 3). Ground above 8 m ASL is marked with hatching. (b) Temporal characterization of the study To characterize the speed of the incoming ow, we take the average meridional wind U observed in Lonate between 1.5 and 4 km ASL. These upper and lower bounds were chosen for the following reasons: below 1.5 km, the air is blocked and does not penetrate the Rhine valley, while above 4 km, the synoptic ow is undisturbed by orography. The meridional wind increases during the day, from 6.5 m s 1 at 6 UTC to 1 m s 1 at 12 UTC and 14 m s 1 at UTC (21 October). The Brundt Vaïsälä frequency N is derived from the radiosounding pro le in Milano by considering the mean potential temperature and the vertical potential-temperature gradient in the 15 4 m ASL layer. The non-dimensional height H D Nh (2) U is 5.6 at 6 UTC, 3.7 at 12 UTC and 2.7 at UTC (we take 25 m for the average mountain height h). In the early morning of 2 October, the incoming ow is too weak, resulting in a blocking situation (barrier wind) in the Pô valley (Pierrehumbert and Wyman 1985) associated with a north south pressure gradient. The incoming air mass cannot cross the Alpine chain, which generates a strong easterly ow. In the afternoon, as the low approaches the area, H gets close to 1 and a mixed regime sets in with one part of the incoming ow crossing over the Alps through the main passes, and the other part owing around the west part of the mountain chain. It is similar to the regime observed during IOP2 and described by Jaubert and Stein (23).

6 546 G. BEFFREY et al. Figure 3. (a) Wind at 7 m ASL in the high-resolution simulation domain. (b) An enlargement of the area of (a) around the intersection between the Rhine and the Seez valley. Ground stations (point) and radiosounding launching sites (stars) are plotted. The transportable wind lidar location (Vilters) is indicated. AA is the crosssection along the lower Rhine valley axis in Fig. 8. BB is the cross-section along the upper Rhine and Seez valleys axes in Fig. 9. The rectangle on the right represents the domain of Fig. 5. The pressure eld takes the banana shape of the topography (Fig. 2). The strong north south pressure gradient increases during the day: the difference between the pressures, reduced to 369 m in Lugano in the Pô valley and in Vaduz in the lower Rhine valley (see Fig. 3 for their location), rises from 7 hpa in the morning to 13 hpa at 18 UTC. These values are strongly correlated to the drag along the transect (Bessemoulin et al. 1993). Associated with this, a south föhn develops in the north valleys of the Alps. At synoptic scale, the low-level, south wind over the Mediterranean Sea turns east before it reaches the Pô valley, and ows around the western part of the Alps. At this stage, it takes a south-south-east direction. North of the Alps, it merges into an easterly ow generated by a high pressure system located at the east of the domain. This kind of situation is reported in the climatology of Hoinka and Rösler (1987) based on measurements made in Munich. The presence at low levels of a cold ow coming from the east is a noticeable feature. We expect that it plays a key role in the penetration of föhn in the valleys north of the Alps because the warm and dry air brought by föhn is opposed to a cold, and stable air mass. In the following, particular attention is paid to this confrontation, to the mixing of these two very different types of air mass, and to the ability of our model to simulate it.

7 FÖHN FLOW AND STABLE AIR MASS IN THE RHINE VALLEY (a) (b) (c) ALTITUDE (m ASL) f1 1 Zonal component (m s f1 ) 5 TWL HEILIGKREUZ BUCHS fgrabs f1 1 Meridional component (m s f1 ) 5 f1 1 2 TWL signal to noise ratio (db) Figure 4. Vertical pro les of (a) the zonal and (b) the meridional components of the wind from the radiosoundings of Heiligkreuz (dotted line) and Buchs-Grabs (dashed line) for 12 UTC 2 October 1999, and from the transportable wind lidar (TWL) data (solid line) UTC 2 October (c) Vertical pro le of the mean signal-to-noise ratio from TWL. (c) Spatial characterization of the study The Rhine valley originates at the main ridge of the Alps and extends northwards, down to Lake Constance (Fig. 3) through a complex topography: oriented southwest/north-east at rst, the valley turns around the Calanda ridge and then splits into two valleys, the Seez valley to the west (Zurich), and the Rhine valley to the north (to Lake Constance). The processes involved in the penetration of föhn along the valley depends on the intensity of the mountain wave generated by the in ow, but also on the local topography (presence of tributaries valleys of different widths, etc.), and on the thermodynamic conditions of the different initial air masses inside the valleys. The east ow north of the Alps is expected to play a signi cant role in the penetration of föhn down to the valley oor. 4. THE MORNING OF 2 OCTOBER 1999: FIRST STEP OF THE FÖHN EVENT LIFE CYCLE The channelling of the ow inside the Rhine valley is a key process for the development of föhn. Observed during all MAP föhn events, it is studied and discussed in Drobinski et al. (21, 22). Two other key processes are the orographic waves and the adiabatic warming (Lothon 22). The wind eld simulated by meso-nh and the observations all show that the föhn ow was actively channelled by the Rhine valley during IOP8. This can be seen on the 7 m ASL (2 m above valley oor) horizontal cross-section of the wind eld simulated by Meso-NH (Fig. 3). The various pro les observed by the 12 UTC radio-soundings, or derived from the TWL (from 156 to 1128 UTC PPI) con rm the simulation (Fig. 4). In Buchs-Grabs for instance, the wind direction is 17 B, which is the axis of the high Rhine valley at this place, while it is 11 B at Heiligkreuz, that is, the axis of the Seez valley. Concerning the TWL at the

8 548 G. BEFFREY et al. intersection of the Seez and Rhine valley in Vilters, the direction of föhn is 15 B ; the air blowing over the instrument thus goes into the Rhine valley rather than into the Seez valley. Note that the altitude of the föhn jet varies from one place to another. The statistical distribution of the wind as a function of height is described by Häberli et al. (21). Based on the radiosoundings released during MAP, it reveals the presence of the major layers: at the lower levels the ow is channelled by the valley, above comes a transition layer, then the upper layer where the synoptic wind direction is unpertubed by the orography. Note that the altitude of the layers varies from one valley to the other, and speci c features like cold pools sometimes add an additional degree of complexity. These features are discussed below. (a) Intersection of the Rhine and Seez valley: meso- and local study Drobinski et al. (21) show that föhn winds in the Rhine valley are almost certainly generating a ow splitting at the intersection of the Seez and Rhine valleys. The föhn jet blows from the south-west above the TWL and turns along the valley axis downstream, with a direction of around 1 B at Heiligkreuz (Seez valley) and around 17 B at Buchs- Grabs (lower Rhine valley). A raster scan of the TWL was performed between 837 and 943 UTC on 2 October 1999, i.e. during the rst stage of föhn. The radial velocity eld is displayed in Fig. 5 at the altitudes of 1 and 16 m ASL. Positive values of radial velocities indicate winds blowing away from the lidar while approaching winds have negative values. At 1 m ASL, the radial velocities in the high Rhine valley show a pool of strong winds approaching the lidar from above. The air mass it carries splits between the Rhine and the Seez valleys (as will con rmed in the quantitative study). At 16 m ASL, the situation is different. In the upper part of the Rhine valley, there is still a pool of strong winds blowing, but the radial velocity intensity seems stronger in the lower Rhine valley at that height than at 1 m ASL. This is not the case in the Seez valley. Note that the strength of the jet in the upper Rhine valley is weaker at 16 m than at 1 m ASL, which suggests that the core of the jet is closer to 1 m ASL in this area. The height of the jet in the different valleys deduced from the lidar data are consistent with the radiosonde data, as represented in Fig. 7. Vertical pro les of the averaged radial velocities along the axes of each valley axis are represented in Fig. 6 at their location along the AA and BB cross-sections (see Fig. 3). The altitude of the jet in the Rhine valley is about 13 m ASL whereas it is about 11 m ASL in the Seez valley. Another noticeable feature concerns the strength of the jet. It is stronger in the Rhine valley (2 m s 1 ) than in the Seez valley (16 m s 1 ). Lastly, the vertical extension is much ner in the Seez valley. The radial velocities predicted by the model at 9 UTC are shown in Fig. 5. Note that the wind eld was rather stationary at this time of the day, so the time needed by the lidar to complete the raster scan is not a problem and meso-nh data can safely be compared with lidar data. The radial-velocity eld simulated by the model is consistent with the observations (Fig. 5). A strong jet of incoming air (radial velocities up to 15 m s 1 ) can be found in the upper Rhine valley. At the junction of the Seez and Rhine valleys it splits into two branches, one along the Seez valley and the other towards Lake Constance. In the Seez valley, a strong, transverse gradient can be observed in the radial-velocity eld with heavy winds along the northern wall and almost no wind in the south. This particular noticeable feature, which is correctly predicted by the model, was observed during other föhn episodes (Drobinski et al. 23). At higher levels, radial velocities are somewhat underestimated by the model (by about 5 m s 1, see Fig. 5). A possible explanation could be an insuf cient channelling by the model due to the smoothed topography (the resolution for the prescribed topography is 1 km). Another possible explanation could

9 FÖHN FLOW AND STABLE AIR MASS IN THE RHINE VALLEY 549 (a) (b) (c) (d) Figure 5. Horizontal cross-section at (a) (b) 1 m ASL and (c) (d) 16 m ASL of the radial velocities (m s 1 ): (a) and (c) from the transportable wind lidar UTC 2 October 1999; (b) and (d) simulated by Meso-NH 9 UTC 2 October Arrows represent the simulated wind eld; the arrow on the bottom lefthand corner represents 2 m s 1. The topography is shown by the contours (6 22 m ASL) with a contour interval of 2 m. be the direction of the simulated wind which is slightly different from the real direction. Although there are small, but signi cant discrepancies, it can be noted that the salient features of the dynamic eld revealed by the TWL are well recovered by the model. The radiosonde pro les at Buchs-Grabs and Heiligkreuz reveal the vertical extension of the föhn jet. They con rm that the föhn layer is thicker downstream in the Rhine valley (1!3 m ASL against!15 m ASL in the Seez valley) (Fig. 7). To compare radiosonde and model pro les, sonde trajectories have to be taken into account because the wind eld is highly heterogenous. The sondes were thus virtually released in the model and advected according to the predicted wind eld (the ascending speed is 5 m s 1 ). This technique was applied to the four radiosounding stations of Malans, Heiligkreuz, Buchs-Grabs and Diepoldsau (Fig. 7). The altitude of the jet predicted by the model is in good agreement with the Heiligkreuz radiosonde. At Buchs-Grabs, it is slightly underestimated. A similar conclusion applies to potential temperature. As far as the wind maximum is concerned, it is underestimated everywhere by 5 to 6 m s 1. The only exception is Diepoldsau at the northern end of the Rhine valley where the intensity of the föhn is overestimated. The wind direction, the potential temperature and the humidity are well represented.

10 55 G. BEFFREY et al. (a) (b) (c) ALTITUDE (m ASL) (m s -1 ) (m s -1 ) (m s -1 ) Figure 6. Vertical pro les of the mean transportable wind lidar radial velocities averaged on three lines of sight in the direction of the valley axes between 2 and 3 km from the lidar (dashed line), and of the simulated radial velocities (solid line): in the direction of (a) the upper Rhine valley (location in Fig. 8); (b) the Seez valley; (c) the lower Rhine valley (locations in Fig. 9). Two mixed layers can be observed on the radiosonde released at Heiligkreuz. The rst layer is between 7 and 14 m ASL (i.e. from the bottom of the valley up to a height of 7 m); it is a channelled jet with a potential temperature of 289 K equal to the potential temperature measured in the jet at Malans. We thus anticipate that it contains air coming from the upper part of the Rhine valley. In the second mixed layer between 15 and 18 m ASL, the wind is weak, from the south, and the potential temperature µ D 294 K is higher. A similar temperature is observed at Buchs-Grabs above the cold pool between 15 and 22 m ASL. This suggests that the air in these two layers has the same origin and comes from higher altitudes. This could mean that some of the air blocked south of the Alps has managed to pass over the mountain range and is brought down to lower altitudes downstream by a mechanism that needs to be determined. Figure 8 is a model cross-section along the AA line shown in Fig. 3. It can be seen that the 294 K iso-µ line is at the altitude of the mountain chain (about 2 m), so we expect that cooler air is either blocked or gets through mountain passes, while warmer air passes over. The 294 K iso-µ line exhibits a wave-like structure downstream and loses 2 to 3 m of altitude. This descent of iso-µ lines associated with a wave structure could be the explanation for the warmer air in the northern, lower part of the Rhine valley because the wave and the descent are taking place north of the Seez valley. This could explain also why the potential temperature near Vilters or above in the Rhine Valley is much lower because the air there originates from the many mountain passes that cut through the main crests. The presence of cooler air in the Seez valley con rms this because, as we have seen with the radiosoundings, it comes from the Rhine valley above Vilters and is advected in the Seez valley by the powerful föhn jet. This can be seen in Fig. 9 which represents the BB cross-section along the Seez valley axis

11 FÖHN FLOW AND STABLE AIR MASS IN THE RHINE VALLEY ALTITUDE (m ASL) (a) ALTITUDE (m ASL) (b) f1 1 Zonal component (m s f1 ) f1 1 Meridional component (m s f1 ) Potential temperature (K) f1 1 Zonal component (m s f1 ) f1 1 Meridional component (m s f1 ) Potential temperature (K) ALTITUDE (m ASL) (c) ALTITUDE (m ASL) (d) f1 1 Zonal component (m s f1 ) f1 1 Meridional component (m s f1 ) Potential temperature (K) f1 1 Zonal component (m s f1 ) f1 1 Meridional component (m s f1 ) Potential temperature (K) Figure 7. Vertical pro les of the zonal and meridional components of the wind and of the potential temperature (K) from the radiosoundings of (a) Malans, (b) Heiligkreuz, (c) Buchs-Grabs and (d) Diepoldsau, 9 UTC 2 October Data are shown by dashed lines and simulated radiosounding by solid lines.

12 552 G. BEFFREY et al. Figure 8. Cross-section along the lower Rhine valley axis (AA in Fig. 3) 94 UTC 2 October 1999; the topography is shown by hatching. (a) Potential temperature (K), vertical wind lower than 1 m s 1 (light shading) and higher than 1 m s 1 (dark shading) and turbulent kinetic energy higher than.8 m 2 s 2 (thick line). (b) Tangential wind (shaded every 5 m s 1 ), orthogonal wind higher than 4 m s 1 (thick line). The arrow on the right represents the location of the vertical pro le of radial velocities in Fig. 6(a). Figure 9. Cross-section along the Seez and upper Rhine valleys axes (BB in Fig. 3) 94 UTC 2 October 1999; the topography is shown by hatching. (a) Tangential wind (shaded every 5 m s 1 ); arrows represent the location of the vertical pro les of radial velocities in Figs. 6(b) and (c). (b) Potential temperature (K), and intensity of the wind, lower than 5 m s 1, between 5 and 1 m s 1 (light shading) and higher than 1 m s 1 (dark shading). (see Fig. 3). As shown earlier, the föhn ow near Vilters is highly three-dimensional. The complex topography and the associated mesoscale mountain waves drive the ow and result in strong, local discrepancies in the different valleys. (b) The cold pool in the lower Rhine valley The presence of a cold pool in the lower Rhine valley is a key element for the penetration of the föhn ow inside the Rhine valley. Such a con guration was already studied at other locations: Zängl (1999) has simulated a front-like structure separating

13 FÖHN FLOW AND STABLE AIR MASS IN THE RHINE VALLEY 553 the föhn from a cold air mass in the Sill and Inn valleys (Austria). In the lower Rhine valley, the cold pool generated strong local gradients of temperature and humidity. This was observed during several IOPs, so we believe it is a typical meteorological feature. During IOP8 it was observed in the morning of 2 October until late afternoon. The air above the ground in the north part of the Rhine valley is cooler and moister than the high turbulent föhn air located at the intersection of the Rhine and Seez valleys, or above the cold air mass. This cold pool can be observed at 9 UTC at Diepoldsau or Buchs- Grabs (Fig. 7). It is characterized by a weak north wind. At the beginning of the föhn event, the high-resolution simulation overestimates the potential temperature in the cold pool but the main features such as the weak north wind are properly rendered. The cold pool was not present in the initial elds. It was generated by the model after a few hours of simulation. The most probable explanation for it is the combination of the north-east wind at the northern base of the Alps and the presence of cold, moist air above Lake Constance. The north-east wind is the consequence of the pressure eld which forces the mesoscale ow north of the Alps to align itself with the strong south wind owing to the ow around the western part of the Alps (Fig. 2). It enhances the penetration of a cold and stable air mass inside the lower Rhine valley. The use of a third domain (625 m mesh) is necessary to improve the spatial representation of the cold pool. With a simulation with a 2.5 km resolution, the horizontal extension and thickness of the simulated cold pool is insuf cient and the north wind does not exist in the valley. The high resolution allows the thermodynamical gradients at the interface with the föhn air to be enhanced. From the intersection with the Seez valley to the location of the confrontation with the cold pool in the direction of the Lake of Constance, the föhn jet affected the entire lower Rhine valley in its vertical extension, from the ground to about 18 m ASL. The jet is then suddenly rejected above the cold pool, in a hydraulic jump behaviour, as revealed by the potential-temperature eld and the tangential wind along the valley axis (Fig. 8). This behaviour is frequent in favourable areas (Pettré 1982). Far to the north, the jet is observed between 14 and 2 m ASL and is simulated between 1 and 2 m ASL. The radiosounding of Buchs-Grabs displays the characteristics of the cold pool inside the valley. The thickness there is evaluated at 5 m (from 5 to 1 m ASL) and is simulated at 3 m (Fig. 7). The intensity of the maximum north component of the wind in the cold pool is slightly underestimated (by around 4 m s 1 at Buchs-Grabs). The thickness of the observed cold pool at Buchs-Grabs decreases slowly after 9 UTC until the high-reaching föhn. The potential temperature at Buchs-Grabs in the cold pool increases from 282 K at UTC to 29 K at 12 UTC, according to the diurnal evolution. The high-reaching föhn at the end of the day is characterized by the disappearance of the cold pool, a strong south wind and an increasing potential temperature. At Buchs-Grabs, this happens between 15 and 18 UTC. The transition from cold pool to föhn is characterized by strong temperature and humidity gradients and a sharp wind shear. The location of the interface cannot be determinated precisely in spite of the dense instrumental set-up. At 9 UTC, the interface is somewhere between Vilters and Weite. Looking at the vertical cross-section in Fig. 8, one would conclude that the simulated interface is further north, but at 94 UTC, the simulated interface is not oriented west east. The penetration of föhn is not homogeneous across the valley, but is deeper along the western side (Fig. 3). The vertical cross section AA is on the west part of the valley while the surface stations are on the east (location represented by the AA segment in Fig. 3). One hour before, the interface was more west east oriented. Note that the slow movement of the interface induces strong and fast, local, thermodynamical changes at the surface. The Merlin

14 554 G. BEFFREY et al. aircraft has measured these patterns during its low-level ight on 2 October afternoon (not shown). For a leg along the lower valley axis from north to south, the ight report indicates that turbulence increases and becomes strong near Vilters, and for a leg across the lower Rhine valley in the cold pool, it is reported that the turbulence is weak in the central part of the leg and increases on the two extremities into the small valleys. The haze is increasing inside the Rhine valley. The wind is very weak, the direction changes from south to north, and the smoke observed near the ground level shows a northerly ow. The area of interface corresponds to the place where the width of the valley is the smallest (before it becomes larger around Buchs-Grabs). At ground level, the arrival of föhn is clearly detectable by a signi cant warming associated with a strong drying of the air and an increase of the wind which takes a well-de ned direction. Figure 1 shows the time evolution of temperature, humidity and meridional wind for different stations in the Rhine valley (see locations in Fig. 3). In the morning, the temperature increases and the humidity decreases at all stations except Chur. We attribute this to the diurnal cycle as no change in the wind direction or speed is observed. At Chur, the föhn is already present. The Chur data are shown to highlight the differences in temperature and humidity between ground stations in and out of the cold pool. The föhn arrives at Weite and Balzers at 17 UTC and at Vaduz and Ruggell at 2 UTC. Weite and Balzers are rather close and it is interesting to note the 5 m s 1 difference in the intensity of the föhn wind. This proves the high local differences that can be observed. On 2 October, Heerbrugg is not reached by the föhn and the evolution of temperature and humidity in the evening is very different from other stations considered above. Note, however, the 4 B of difference from the preceding night. While the evolution of the cold pool is slow in the morning or the early afternoon, its afternoon retreat is sudden. It corresponds to the new increase of the sea-levelpressure difference between Lugano and Vaduz (which reaches its maximum), and to the beginning of the high-reaching föhn. The moving of the interface to the north at the end of the day is a direct consequence of the larger scale changing with the increase of the incoming ow south of the Alps. The model simulates the sudden penetration of the föhn, but earlier in the afternoon. This is due to what happens at a larger scale. In fact, the simulated incoming ow increases about four hours before the observed one. However, in the morning and at midday the simulated interface is well represented, in terms of location and gradient intensity. (c) First step of the föhn ow: quanti cation The simulation has been validated at high resolution by using lidar and other sensors at the intersection of the Rhine and Seez valleys, and radiosonde and surface station data further north in the lower Rhine valleys. In order to con rm the three-dimensional ow described by the model, a Lagrangian trajectory study (Gheusi and Stein 22) has been performed to identify the origin of the föhn air in the Seez and the lower Rhine valley and of the cold air mass. Let us rst consider particles located at 7 UTC in a box in the upper Rhine valley (Figs. 11 (a) and (b)). After one hour of simulation the locations of particles originally under 2 m ASL show a descending ow and a strong channelling by the upper Rhine valley. After two hours, they are all con ned to the Seez valley under 15 m ASL. By contrast, the particles originally above 2 m can be found either in the Seez valley or in the Rhine valley at about 15 m ASL in the föhn jet. Most of these particles have passed over the Calanda, the last ridge before the intersection of the two valleys. Other particles have taken a south-west to north-east direction consistent with the synoptic ow. This study reveals a signi cant three-dimensional aspect of the ow, and the föhn air in the Seez valley and the lower Rhine valley is not fed only by

15 FÖHN FLOW AND STABLE AIR MASS IN THE RHINE VALLEY 555 Potential temperature (K) (a) Time (UTC) Humidity (%) (b) Time (UTC) 15 (c) Meridional wind (m s ± 1 ) 1 5 Chur Balzers Weite Vaduz Ruggel Heerbrugg Time (UTC) Figure 1. Temporal evolution of (a) the potential temperature, (b) the humidity and (c) the meridional component of the wind, on 2 October 1999 for different ground stations of the Rhine valley (locations in Fig. 3). the air coming from the upper Rhine valley. The same study has been performed north of the domain. This time, the box is placed north of the lower Rhine valley, east of Lake Constance between 5 and 1 m ASL, i.e. closer to the ground (Fig. 11 (c)). Some particles are advected inside the east part of the lower Rhine valley at very low level and feed the cold pool. After two hours they are located closer to the ground in a layer approximately 25 m thick. The others particles stay under 1 m ASL, i.e. under the föhn layer. Note that the displacement is very weak in two hours. The simulation allows such a ow to be quanti ed in the FORM domain. A similar study as in Jaubert and Stein 23 (for the IOP2B föhn case) has been carried out. The target area is divided into several boxes and the mass uxes are computed for all the sides of each box at 94 UTC. The uxes across the lateral faces are evaluated by integrating the product between the orthogonal wind and the reference density, and the ux across the top by integrating the product between the vertical wind and the reference density. The unit of mass uxes is thus 1 6 kg s 1 and the accuracy for each face is around kg s 1. Figure 12 represents mass uxes across the different faces of the 25 2 km boxes de ned in the target area between the ground and 2 m ASL. The box located in the upper Rhine valley is mostly fed by the west (the major pass) and by the top. The channelling is signi cant and the maximum mass ux is found in the upper Rhine valley ( kg s 1 ). At the intersection, this ow splits between the two valleys downstream. The feeding of this box is completed by a signi cant vertical ux. The mass ux leaving this box is directed 7% to the lower Rhine valley and

16 556 G. BEFFREY et al. km 9 km km 9 km Figure 11. Location at 8 UTC of the particles originated at 7 UTC from the square area: (a) between 1 and 2 m ASL and (b) between 2 and 3 m ASL. (c) Location at 9 UTC of the particles originated at 7 UTC from the square area (between 5 and 1 m ASL) and their meridional displacement. The contours represent the topography from 6 m ASL, contour interval 4 m.

17 FÖHN FLOW AND STABLE AIR MASS IN THE RHINE VALLEY 557 Figure 12. (a) Mass uxes around the Rhine valley area between the ground and 2 m ASL: the number in the center (side) of each square is the downward (lateral) ux (1 6 kg s 1 ). (b) Longitudinal wind on the cross-section along the lower Rhine valley (between the two crosses on the left), with hatching representing the orography. Mass uxes across the boxes concerned by the cross-section are represented. The number in the centre of each rectangle represents the entering or leaving ux (classical representation) across the sides of the boxes located west of the lower Rhine valley axis. 3% to the Seez valley. Far to the north, the mass ux in the lower Rhine valley is the same as in the upper Rhine valley, thanks to feeding by the top and by a secondary valley (east west oriented). To better analyse the mass exchanges, especially on the vertical, the quanti cation of the ow is represented on a vertical cross-section along the lower Rhine valley axis (Fig. 12(b)). The vertical and horizontal mass uxes between the different boxes concerned in the cross-section are represented. The zonal mass uxes are those evaluated across the faces of the boxes west of the Rhine valley axis (Fig. 12(a)). At high levels, above 3 m ASL, the uxes are consistent with the synoptic south-west ow (south north and west east uxes are signi cant). The strong south ow concerned rst a layer between 2 and 3 m ASL, above the main ridges. Strong downward uxes are located above the upper Rhine valley (between Masein and Chur) and strong south north uxes are thus located at low levels in the upper Rhine valley. A little bit north, meridional uxes are signi cant even close to the ground (inside the valley). At the junction between the Rhine and the Seez valley, signi cant zonal mass uxes exist, even at low levels. This corresponds to the ow blowing toward the Seez valley. The dynamical aspect of the cold air mass and of its confrontation with the föhn jet is well represented by the mass uxes: one of the boxes is affected at low level by a convergence of mass from the south (föhn jet) and from the north (cold pool), which leads to a positive vertical motion towards the upper box. This box is characterized by a mass convergence from the bottom (confrontation between föhn air and cold pool at lower levels), from the top (falling of air in a mountain wave motion) and from the

18 558 G. BEFFREY et al. Figure 13. Three-dimensional picture representing the arrival of the föhn air in the Rhine valley target area. south (föhn jet). The consequence is an enhancement of the föhn jet towards the north (the meridional mass uxes increase from to kg s 1 ). 5. CONCLUSION The ability of the meso-nh model to simulate the IOP8 föhn case has been shown. The interaction from synoptic to local scales leads to a classical föhn event with strong local discrepancies in the meteorological parameters in the different valleys of the FORM target area. The rst step of the event has been studied here with the help of a comprehensive dataset collected during the IOP and with a high-resolution (625 m) simulation. A balance between föhn air and a stable and cold air mass in the lower part of the Rhine valley is observed and simulated, before the warm and turbulent air is able to reach the ground in all the valley. The complex contribution to the föhn air located inside the different valleys has been described and can be summarized by the three-dimensional picture shown in Fig. 13. The different mechanisms leading to the arrival of the föhn air on the ground have been described and the ow quanti ed by the computation of mass uxes. Special attention has been given to the cold pool whose behaviour is of much interest: it drives the penetration of the föhn air to the ground and is therefore responsible for strong local weather changing. The ability of the model to simulate this cold pool will allow the study in detail of the mechanisms which explain its feeding, its presence and its erosion, at the different scales.

19 FÖHN FLOW AND STABLE AIR MASS IN THE RHINE VALLEY 559 ACKNOWLEDGEMENTS The authors would like to thank the scientists of the MAP FORM working group, coordinated by H. Richner and R. Steinacker, and particularly P. Drobinski for fruitful discussions. We acknowledge J. Stein and J. P. Aubagnac whose comments helped us improve the paper, the MAP Data Center and the scientists involved in operating the TWL during the MAP eld experiment. We would also like to thank Pierre Flamant, the project manager for the development of the transportable wind lidar. Bessemoulin, P., Bougeault, P., Genoves, A., Jansa Clar, A. and Puech, D. REFERENCES 1993 Mountain pressure drag during PYREX. Beitr. Phys. Atmos., 66, Bougeault, P. and Lacarrère, P Parametrization of orography-induced turbulence in a meso-beta scale model. Mon. Weather Rev., 117, Bougeault, P., Binder, P., Buzzi, A., Dirks, R., Houze, R., Kuettner, J., Smith, R. B., Steinacker, R. and Volkert, H. 21 The MAP Special Observing Period. Bull. Am. Meteorol. Soc., 82, Dabas, A. 2 Velocity biases of adaptative lter estimates of heterodyne doppler lidar measurements. J. Atmos. Oceanic. Technol., 17, Dabas, A., Drobinski, P. and Flamant, P Chirp induced bias in velocity measurements by a coherent doppler CO 2 lidar. J. Atmos. Oceanic. Technol., 15, Deardorff, J. W Numerical investigation of neutral and unstable planetary boundary layers. J. Atmos. Sci., 29, Drobinski, P., Dabas, A., Häberli, C. and Flamant, P. 21 On the small-scale dynamics of ow splitting in the Rhine valley during a shallow foehn event. Boundary-Layer Meteorol., 99, Drobinski, P., Häberli, C., Richard, E., Lothon, M., Dabas, A., Flamant, P., Furger, M. and Steinacker, R. Furger, M., Drobinski, P., Prevot, A., Weber, O and Graber, W Scale interaction processes during the MAP IOP 12 south föhn event in the Rhine Valley. Q. J. R. Meteorol. Soc., 129, Comparison of horizontal and vertical scintillometer crosswinds during strong foehn event with lidar and aircraft measurements. J. Atmos. Oceanic Technol., 18, Gheusi, F. and Stein, J. 22 Lagrangian description of air ows using eulerian passive tracers. Häberli, C., Drobinski, P., Dabas, A. and Beffrey, G. Q. J. R. Meteorol. Soc., 128, Statistical characterization of the foehn ow in and above the Rhine valley during MAP-SOP using rawinsonde data. MAP Meeting 21, Schlirsee, Germany Hoinka, K. P. 198 Synoptic-scale atmospheric features and foehn. Cont. Atmos. Physics, 53, A comparison of numerical simulations of hydrostatic ow over mountain with observations. Mon. Weather Rev., 113, Hoinka, K. P. and Rösler, F The surface layer on the leeside of the Alps during foehn. Meteorol. Atmos. Phys., 37, Jaubert, G. and Stein, J. 23 Multiscale and unsteady aspects of a deep föhn event during MAP. Q. J. R. Meteorol. Soc., 588, Lafore, J. P., Stein, J., Asencio, N., Bougeault, P., Ducrocq, V., Duron, J., Fischer, C., Héreil, P., Mascart, P., Redelsperger, J. L., Richard, E. and Vilà-Guerau de Arellano, J The Meso-NH atmospheric simulation system. Part I: Adiabatic formulation and control simulations. Annales Geophysicae, 16, 9 19 Lothon, M. 22 Etude phénoménologique du foehn dans la vallée du Rhin dans le cadre de l expérience MAP. PhD thesis, Université Paul Sabatier Mayor, S., Lenschow, H., Schwiesow, R., Mann, J., Frush, C. and Simon, M Validation of NCAR 1.6 CO 2 doppler lidar radial velocity measurements and comparison with a 915 MHz pro ler. J. Atmos. Oceanic Technol., 14,

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