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Czech Republic: 21 June 2007
In this chapter we will have a closer look on the convection over the Czech Republic. We will again have a more detailed look on the different satellite images, including some RGBs, and some other nowcasting products from the
Convection Working Group. This chapter will be finalised by some radar images to pinpoint the areas where hail was reported.
21 June: Meteosat 9 Satellite Imagery
Meteosat 9 IR10.8: Time sequence
The first chapter dealing with the synoptic situation describes the convective developments seen over Central Europe on 21 June using plain Meteosat 9 infrared 10.8 μm. The channel is appropriate as it pictures the ice in high clouds quite clearly. In a sequence of 15 minutes the satellite images are presented and described.
Meteosat 9 Enhanced IR10.8: Time sequence
In this chapter again Meteosat 9 infrared 10.8 μm are shown, but the images have been artificially colour enhanced. This will improve the discrimination of where the evolution from water into ice is taking place during the several convective stages. In a sequence of 15 minutes the satellite images are presented and described.
Meteosat 9 WV6.2: Time sequence
The chapter presents the several convective developments over Central Europe using Meteosat 9 WV6.2 channel. The channel is suitable as it gives an idea of the upper tropospheric humidity (UTH) and gives a view of the upper air dynamics. For the 21st June a sequence of 15 minute images are presented.
Meteosat 9 HRVIS: Time sequence
This chapter will show the HRVIS images of 21st June. Especially with the smaller scale cells the high resolution channel allows us a very good monitoring of the convective development. In a sequence of 15 minutes the satellite images are presented and described.
Meteosat 9 composite WV6.2μm-WV7.3μm; IR3.9μm-IR10.8μm; NIR1.6μm-VIS0.6μm: selected time sequence
In this chapter the convective developments over Central and over South-East Europe is studied using the so-called
"severe convection RGB". On red the brightness temperature difference (BTD) of the two water vapour channels 6.2 and 7.3. On green the BTD of the infrared channels 3.9 and 10.8 and on blue the BTD of the two visible channels 1.6 and 0.6, respectively, are pictured.
Meteosat 9 composite WV6.2μm-WV7.3μm; IR9.7μm-IR10.8μm; WV6.2iμm: time sequence
In this chapter the convection is studied using the
"Airmass RGB". Unlike the Severe Convection RGB discussed previously this RGB does not use any visible channels making it thus also suitable to detect night time convection. The recipe of this RGB is that on red the brightness temperature difference (BTD) of the two water vapour channels 6.2 and 7.3 is pictured. On blue the BTD of the infrared channels 9.7 (Ozone channel) and 10.8 and finally on green WV channel 6.2. Using this channel combination different air masses are recognised. Polar and Tropical Airmass are pictured in blue and green, resp. The WV dark stripes discussed previously are pictured in red.
The "IR Cold Cloud Tops" is a product that can be very useful during summer convection season for the meteorologists. This product is generated from calibrated radiative temperatures of Meteosat 9 IR10.8 using the colour scheme from blue to turqoise to yellow and red applied over the fixed interval 200 to 240 K. Another speciality of the processing approach is the weighted average of MSG pixels during reprojection from satellite view into conical projection. Weighting function depends on satellite zenith angle. This procedure makes better picture quality even in areas of very high satellite zenith angle. The idea of the product is to smooth source pixels before reprojection, not smoothing of target image after reprojection. The effect of this approach is not only for areas with very high zenith angle but also in cases when zoom of reprojected target image is positive in comparison to source image. The method can be considered something like up scaling method. Addresses for set of 10 nearest pixels for each target pixel must be pre-computed due to huge number of arithmetic operations. It takes about 100 minutes on usual computer, but reprojection itself takes some few seconds, therefore it can be used operationaly, but of course only for geostationary satellites.
The MSG spectral channels allow the determination of cloud properties like cloud top temperature T and effective cloud particle size reff. The T-reff diagrams enable to discriminate different levels of storm severity. This product is applicable to an entire cloud field, where individual pixels show different stages of cloud development. The horizontal information is thus turned into a "virtual" vertical profile. You can find a detailed description on this method by studying the following article
GII
Global instability index (GII) is an air mass parameter indicating the stability of the clear atmosphere. The GII product should serve as a nowcasting tool to identify the potential of convection and possibly of severe storms in still preconvective conditions. The applied retrieval method makes use of six MSG SEVIRI thermal bands, and together with the a priori information of forecast profiles, the scheme infers an updated atmospheric profile for each MSG pixel, from which instability indices can be computed. Several instability indices are used in this case and presented. The images are presented in 1 hour sequence.
K-Index
The K-index is a widespread method amongst meteorologists to make a stability analysis of the atmosphere. In the above link the K-index as computed by the GII algorithm is presented in 1 hourly interval. To find out more on how the K-index is computed you can look at the following
animation. Since the K-index makes use of the Temperature and dew point Temperature at 850 hPa. the retrieval of the K-index over the Alps can be somehow problematic.
Lifted Index
A second index that is computed from GII is the Lifted Index. In 1 hour interval the Lifted Index is presented for the 21st June 2007 over Central Europa. If you want to learn more on Lifted Index and how it is derived click "
here".
Precipitable Water
One final product to be presented is the precipitable water. For a Meteorologist this product can be of extreme value when doing a nowcast. It represents the total atmospheric water vapour contained in a vertical column of unit cross-sectional area extending between any two specified levels, commonly expressed in terms of the height to which that water substance would stand if completely condensed and collected in a vessel of the same unit cross section.
21 June: Radar
Czech Republic Radar
Large hail is regularly observed in association with intense thunderstorms (as in this case study) and is often the cause of severe damage on e.g. crops, roofs and cars. Hail is a local phenomenon, in both time and space, so that it cannot be easily detected using satellite imagery or with surface observations (since the density is to coarse). Due to its wide spatial coverage and relatively fine spatial and temporal resolution, weather radar appears as a valuable tool for the real-time detection of hail (Holleman, 2002). For the 21st June the Radar is shown over the Czech Republic (
source CHMI).