SMALL SCALE WV-FEATURE ABOVE FRANCE: NEXT DAY SEVERE WEATHER OVER THE NETHERLANDS F.P. Debie KNMI PO Box 201, AE de Bilt, The Netherlands ABSTRACT Due to economic reasons world-wide can be seen that the human observers have to withdraw. Despite of the reduction of human synoptic observations today more data sources are available to the modern synopticians. Satellite information is one of the most important sources of the forecaster. And thereby satellite meteorology is a hot issue in the training departments of the national meteorological services. As well known, the Meteosat satellites provide three kinds of images: VIS, IR and the WV. In former years WV-images were mostly used for a quick overview of the synoptical patterns. The last years the knowledge of pattern recognition in the WV-imagery is improved and it has found out that small-scale features can be very important for the weather forecast. The discussed case study of the 24 August 2002 will show the interaction of a mesoscale Water Vapour phenomenon with a synoptic frontal system. In initial state the IR- and VIS-images did not show any signal for developing a severe weather situation. However in the WV-channel a WV eddy was already seen. Starting as a separate phenomenon in the WV-image this quite remarkable WV feature was merging with a frontal wave. The impact was an enhanced activity of the wave and the developing of severe thunderstorms in the eddy area over the Netherlands. Hirlam, ECMWF- and UK-models, available at the KNMI, did not give any signal for the heavy precipitation (and the flooding risk). The overall conclusion is that this WV-feature was one of the most important triggers for the forecaster to be suspicious about the model forecasts. 1. INTRODUCTION The summer of 2002 was a wet summer in the Netherlands, as well as in whole West-Europe. In July and August the country encountered already several periods with very heavy precipitation with flooding as result. Some cities got already twice the amount of 100 mm precipitation within 24 hours. Civil services had to work hard to get rid of the water. During the last weekend of the holiday season, 23 25 August 2002, a lot of cultural manifestations were outdoors. E.g. in Amsterdam the cultural market for the new season, called Uitmarkt, was planned. More than a half million people are following the outdoor performances of the mostly Dutch artists and performers. The KNMI had to provide the weather forecast to the civil authorities as a safety task for the government. But not only in Amsterdam there were a lot of people outside, also in the central part of the Netherlands. Lowlands, a pop festival, included with camping facilities, expected more than 50000 people. The name of the yearly returning pop festival exactly covered the situation: in a polder with risk of flooding after heavy precipitation. So the weather forecast for this weekend was more important and more discussed than other weekends. Unfortunately, the models calculations were not uniform and they
were quite uncertain. The forecasters at the KNMI are aware of problems with NWP-models with southerly flows. Especially during summer seasons the southerly flows can produce a lot of rain, due to moist air coming from the Mediterranean area. The most problems can be gathered under the subject position and amount of precipitation. The risk of thunderstorms is well known, however the exact place to fall is hard to forecast. Also the exact position of the north-south orientated fronts, with waves as sub-features, are difficult to forecast. On global, synoptical, scale it make hardly any difference. However, on local, so-called mesoscale, it can complete destroy the forecast. The synoptical situation for this weekend was a southerly upper level flow with a frontal zone from the North Sea to France and North of Spain (Fig.1). From five days until one day ahead the model calculations were jumping between the ranges of a complete dry sunny weekend, heavy precipitation in the east-part or light rain in the west of Holland. Fig 1. KNMI surface analysis of 23 Aug 2002 12 UTC 2. INITIAL SYNOPTICAL SITUATION SEEN FROM MODEL AND SATELLITE The surface chart in Figure 1 gives an overview of the pressure distribution over the Atlantic Ocean and Europe. A strong surface high over the Atlantic has a ridge in the direction of West-France. In the WV-image (Fig. 2). a waving cold front from the North Sea over France towards Spain is the separation of the very warm and moist subtropical air and the drier polar air west of the front In this figure the white elongating band over Southwest and West-Europe can be associated with the moist air east of the polar front, the bulge over Southwest France is an indication of the wave in the polar front. An other quite remarkable feature, a socalled WV-Eddy, is seen over the Bay of Biscay. At this time, 12 UTC, this feature is clearly separated from the cold front by a dark stripe and has nothing to do with the front. The upper air pattern, seen in Figure 3, shows a South-westerly flow. The pronounced crowding zone can be associated with the frontal zone. Combining Figure 1 and Figure 3 and looking at the polar front over Western Europe and the 300 hpa isohypse pattern, the conclusion can be that the polar front is parallel to this flow and therefore the risk of wave development is quite significant. The wave over Central France moves slowly to the north. The days before August 23 rd the problems of the numerical models (and forecasters) were the uncertainties of the exact position of the polar front, the wave development and the wave displacement.
WV Eddy front Fig 2. 23 Aug 2002 12 UTC, WV-image : Front over SW- / W-Europe, WV-Eddy over Bay of Biscay Fig 3. 23 Aug. 2002 12 UTC. Hirlam 300 hpa field superimposed over WV-image
In the VIS-image (Fig..4) the frontal cloudiness is shown over eastern part of France as thin Cirrus shield and thick cloudiness below this shield. Over the Bay of Biscay (WV-eddy) there is almost clear sky.. Fig.4. 23 Aug. 2002 12 UTC VIS image with almost clear sky over Bay of Biscay Enhanced convection as an indication of a WV-eddy is not seen due to too low seawater temperatures. Above Bretagne light showers, cause by thermal heating, are already developed (Fig 5). The next 6 hours the WV-eddy moves slowly to the north and enhanced convection starts above land (Bretagne, France). The wave part of the polar front over central France shows the most intensive precipitation. Fig. 5. 23 Aug. 2002 12 UTC. Radar shows showers in the west and frontal rain over central France..
3. PHYSICAL BACKGROUND OF A WV-EDDY In WV imagery the most important features are the patterns. Pattern recognition is very important for the forecasters. The boundaries between these phenomena are given by dark stripes (dry air). Looking at a WVeddy there is a curl of moist (white) and dry air (dark). In this case we are interested in these boundaries. Main physical processes involved with small scale WV-eddies - Diabatic heating - Entrainment of dry air Reduction of convection by entrainment Area starting convection Growth of convective cells due to high humidity high Diabatic heating low Fig.6. Physical processes at the boundary of WV-eddy In the moist area the insolation is less than in the dry area (Fig. 6). So, the adiabatic heating is the strongest in dark area of the eddy. In the dark area entrainment of dry air, lack of moist, reduces the convection. In the white area moist can let grow the convection. There will be a transition zone with the most optimal situation for starting convection. In more than 80% of the cases (see Lit. 5) convection starts just in the white/dark boundary (in the transition zone). Dynamical features are the other involved physical processes. Shear and curvature vorticity are the features associated with an upper level trough. The movement of the Eddy (upper trough) provides PVA (Positive Vorticity Advection) and leads to UPWARD MOTION (see Lit. 6.) Shear vorticity Curvature vorticity Positive vorticity advection Upward motions
4. FINAL SYNOPTICAL DEVELOPMENT The medium range forecasts of the models for the Netherlands were rather inconsistent the days before. The range of positions of the waving polar front for the weekend of 23/24 August 2002 was between the North Sea, west of the Netherlands, and Central Germany. Also the amount of precipitation was varying between 0 mm 30 mm in 24 hours. So the forecast was jumping between two warm and dry summer days and a wet, rather cold and cloudy weekend. Hirlam calculations of 23 August, 24 hours ahead, showed the waving cold front just over the east border of the Netherlands. The small upper level trough was also calculated. However, no weather was involved with this trough. Fig 7. Pseudo WV-images of Hirlam 12 UTC 23 Aug. 2002. The arrows show the WV-eddy The pseudo WV-images (Fig.7) show the interaction of the WV eddy and the frontal system (dark stripe). The model resolution was not high enough to see the WV-eddy in the initial state. In the next 24 hours the pseudo model field images show a pronounced dark area which can be related to the eddy and the frontal system. Combining the right corner image (Fig. 7) with the real WV-image (Fig. 8) the similarities of the features are quite good. However the precipitation fields only show rain in the eastern part of the Netherlands and this fact could be associated with the frontal zone. The surface analysis of 24 August 18 UTC shows a wave and a trough over the Netherlands (Fig. 9). Radar image (Fig.10) shows showery precipitation on the top of the wave and showers in the WV-eddy. Locally the amount of precipitation was exceeding the 100 mm in a few hours in the west part of the Netherlands and around 80 mm in the east part. Radar images clearly indicate that the two features should be separated. However, the outflow of the showers and the wave were merging in the satellite images.
Fig. 8. WV-image, 24 Aug. 2002 18 UTC. Moist of wave and WV eddy are (partly) merged. Fig. 9. KNMI surface analysis of 24 Aug. 18 UTC. Wave and trough are separately analysed. Fig.10. KNMI Radar image 24 Aug. 2002 15 UTC. Frontal rain in the NE, showers in the SW (WV-Eddy)
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