COALITION-2: Context and Scale Oriented Thunderstorm Satellite Predictors Development version 2
During the warm season of the year, intense thunderstorms regularly affect the Alpine region. They are often accompanied with hail, heavy rain and gale-force wind gusts. Furthermore, lightning and flash floods could cause severe damages at properties and infrastructure and may lead to live threatening situations. Numerical weather prediction models provide good forecasts on regional to global scales, but have difficulties in predicting the exact time and location of small-scale phenomena like thunderstorms. Therefore, nowcasting methods are applied using recent observations to intelligently extrapolate the current state into the near future. In contrast to the first version of COALITION, the second version is based on satellite observations only. Hence, it is possible to detect early signs of thunderstorms before the onset of rain. As an independent method, COALITION-2 serves as backup for radar observations, but can also be applied to regions beyond the radar range.
Objectives and input datasets
The overall goal of COALITION-2 is to identify, track and nowcast the position and intensification of convectively active regions in an accurate, continuous and robust manner by exploiting satellite observations. COALITION-2 uses the infrared channels of the MSG/SEVIRI instrument, wind speed and direction as predicted by the weather model COSMO-CH and some products of the Nowcasting SAF software, such as the cloud top height & pressure:
- MSG SEVIRI 3.9 µm channel (rapid scan each 5 min)
- MSG SEVIRI 6.2 µm channel (rapid scan each 5 min)
- MSG SEVIRI 7.3 µm channel (rapid scan each 5 min)
- MSG SEVIRI 8.7 µm channel (rapid scan each 5 min)
- MSG SEVIRI 9.7 µm channel (rapid scan each 5 min)
- MSG SEVIRI 10.8 µm channel (rapid scan each 5 min)
- MSG SEVIRI 12.0 µm channel (rapid scan each 5 min)
- MSG SEVIRI 13.4 µm channel (rapid scan each 5 min)
- COSMO wind (u,v) at 800, 500, 300 hPa (COSMO output every hour)
- Nowcasting SAF Cloud Top Height (rapid scan each 5 min)
- Nowcasting SAF Cloud Top Pressure (rapid scan each 5 min)
- Nowcasting SAF Cloud Type [Optional] (rapid scan each 5 min)
Methods
The SEVIRI instrument onboard of Meteosat Second Generation is sensitive to clouds (and not to precipitation). Therefore, COALITION-2 monitors changes of the cloud properties that are associated with thunderstorms. The algorithm observes how the cloud becomes thicker (increase of the cloud optical depth), how droplet at the upper part of the cloud freeze (cloud top glaciation), and how the cloud top is rising (updraft strength). During daytime one additional tests is performed for small ice crystals (indicating strong convection). For the updraft strength tests, temporal trends of the brightness temperatures are used. These temporal differences are calculated with a correction of the thunderstorms movement using the COSMO wind field as motion vectors. The following table show the threshold tests to monitor the development of the thunderstorm.