1Lab. Física de la Atmósfera. Instituto de Medio Ambiente. Universidad. de León - Spain
2ADV Terres de Ponent. Lleida - Spain
Sever thunderstorms hit the Ebro Valley during the summer months, and hail precipitation is frequently registered. In this region the formation of storms is usually linked to prefrontal situations and/or to low pressures that introduce moist air masses from the Mediterranean on surface levels.
In this area, the spatial scale of individual storm cells is usually of 10 to 40 km. Nevertheless, in conditions of deep convection there may be precipitation systems on spatial scales from 40 to 500 km or more. These types are known as Mesoscale Convective Systems (MCS). Occasionally there are long-lived convective systems termed as Mesoscale Convective Complexes (MCC). The precipitation is intense and may produce hail in the case of MCS as well as in the case of MCC.
The analysis of the convective cloud masses allows us to distinguish between the different types of convective storms. One of the techniques that may be employed is the enhanced satellite imagery. The high-resolution IR images from the METEOSAT provide the necessary information to recognise those cloud masses that are termed as MCS or MCC. In the case of middle latitudes the classification criteria used for MCS and MCC are usually based on enhanced IR images, following Augustine and Howard.
Our aim has been to analyse and classify the METEOSAT images in the Ebro Valley for the events when it is known for sure that there has been hail precipitation. The database consists of 66 of these events. The classification by Augustine and Howard was applied and the results were:
• 28 cases of MCS and 5 of MCC.
• In the remaining 36 cases the convective air masses produced hail precipitation, but did not fulfil the classification criteria of Agustine and Howard.
Furthermore, it has been possible to establish conclusions about the structure of the storms and of the systems classified as MCC or MCS.