EUMeTrain: Case Study on severe convection over Central and Eastern Europe

Nowcasting SAF products

The cloud type (CT), developed within the SAF NWC context, mainly aims to support nowcasting applications. The main objective of this product is to provide a detailed cloud analysis. It may be used as input to an objective meso-scale analysis (which in turn may feed a simple nowcasting scheme), as an intermediate product input to other products, or as a final image product for display at a forecaster 's desk. The CT product is essential for the generation of the cloud top temperature and height product and for the identification of precipitation clouds. Finally, it is also essential for the computation of radiative fluxes over sea or land, which are SAF Ocean and Sea Ice products.

The aim ASII and ASII-NWP as SAFNWC products is the Automatic Satellite Image Interpretation in terms of conceptual models (CM). CMs are a synthesis of physical processes and the typical features they cause in satellite images, as well as in other synoptic material like for instance in numerical model output parameters. A CM diagnosis can be used for a deepening of synoptic diagnosis and for the early detection of ongoing meteorological processes, as well as for the determination of the stage of development within those processes.

Automatic Satellite Image Interpretation from MSG SEVIRI satellite data alone

Automatic Satellite Image Interpretation from MSG SEVIRI satellite data supplemented by typical key parameters from the numerical model output (Numerical model used: ECMWF)

Airmass Analysis
Evaluates basic quantities describing air masses (upper and middle level humidity, mean temperature, atmospheric stability, cloud pattern, etc) to combine them into one integrated classification of the air mass.
Convective Rainfall Rate
The objective of the CRR product is to estimate the precipitation rate associated to convective clouds. The final output is a numerical calibrated product (in mm/hr) divided into classes in an image format. This product provides to forecasters complementary information to other SAF NWC products related to rain and convection monitoring as Precipitating clouds and Cloud type.
Total Precipitable Water
Total Precipitable Water (TPW) is the amount of liquid water, in mm, if all the atmospheric water vapour in the column were condensed. High values of TPW in clear air often become antecedent conditions prior to the development of heavy precipitation and flash floods. When high TPW values areas present a lifting mechanism and warm advection in low levels, heavy precipitation often occurs. These data can provide to forecasters an important tool for very short range forecasting. Within the SAF NWC context, the main goal is to provide TPW data in clear air pixel by pixel in image format for Nowcasting purposes.
Precipitating Clouds
The objective of the PC product is to support detailed precipitation analysis for nowcasting purposes. The focus is on the delineation of non-precipitating and precipitating clouds for light and heavy precipitation, rather than quantifying the precipitation rate. Particular attention will be given to the identification of areas of light frontal precipitation.

The product provides probability results, i.e. probabilities of precipitation intensities in pre-defined intensity intervals. From the probabilities a categorical estimate of precipitation intensity may be derived. It is not intended to provide information on the type of precipitation.

Stability Analysis Products

Convective systems can develop in a thermodynamically unstable atmosphere. Such systems may quickly reach high altitudes and can cause severe storms. Meteorologists are thus especially interested to identify such storm potentials when the respective system is still in a preconvective state. A number of instability indices have been defined to describe such situations. Traditionally, these indices are taken from temperature and humidity soundings by radiosondes. As radiosondes are only of very limited temporal and spatial resolution there is a demand for satellite-derived indices. Two of such products are now in operational use, which are GII and SAI.
The Stability Analysis Imagery (SAI) was developed by the NWC SAF. The central aim of the SAI is to provide estimations of the atmospheric instability in cloud-free areas. Among all potential indices the Lifted Index (LI) has been implemented and codified and presented in this case for central Europe on the 30th May 2005. Like the GII (with its K-index) also the calculus of the lifted index of SAI is only done for clear sky conditions. Therefor for SAI the first step is to compute the Cloud Mask product (CMa). This CMa allows the identification of cloud free and cloud contaminated areas. The SAI product itself uses the corrected normalized IR SEVIRI radiance values of the following channels WV6.2, WV7.3, IR8.7, IR9.7, IR10.8, IR12.0 and IR13.4μm), and provides as output the normalized lifted index.
Global instability index (GII) is an airmass 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. The instability index used in this case is the K-index (optional are also Lifted index and Precipitable Water). The images are presented in 30 minute sequence.

Summary of the investigations in this chapter

The set of SAI images showed the first signs of unstability on the flanks of the frontal system around 30 May 0730 UTC. The studies of the satellite images in the "synoptic situation" chapter however showed that these where not the areas where convection started. This area could therefor be interpretated wrongly! In fact it would last another two hours before SAI indicated negative lifted index values at the right positions in Austria, the Czech Republic and Poland. One hour later the convective event started.

Already at an early stage the GII calculates a wide unstable area at the leading edge of the front. Although the areas are included that do show the convective event, Bavaria is mistakenly interpretated as being unstable as it does not show any convective development for that day. Ground for such a misinterpretation might be the capped inversion around 660 hPa. (see radiosounding Munich: 30 May 2005: 12UTC) that prevented triggering the convection of that day. Around 0830UTC the GII shows high instability just north of the Alps in Austria. It will then be another two hours as the serious convective events starts. This gain of one hour in comparison to SAI (see SAI: 30 May 2005: 0830UTC is in the aspect of nowcasting thunderstorms a huge leap forward.