Student Presentation:
Sven-Erik Enno, Formation of damaging winds in thunderclouds, 2005 Estonia GLOBE Games, Kuuresaare, Estonia, 9-11 August, 2005.

Summary

This research studyes thunderstorm winds in Estonia. It gives detailed overview of damaging thunderstorms occurred between July 16-th and 19-th 2001. I study the reasons of damaging winds in front of thunderclouds. I also write about necessitate of public thunderstorm observations in Estonia.

Between July 16-th and 19-th 2001 many severe thunderstorms occurred in Estonia. They were associated with cold front that separated eastern tropical and western polar air masses. Estonian Meteorological and Hydrological Institute (EMHI) data was used to study these storms. I found that there were many stormwaves separated by placid periods with no thunderclouds. I was able to determine 6 bigger stormwaves in total between July 16-th and 19-th. Variety in thunderstorms frequency and severity between different parts of Estonia was also obvious. In the western part of country, most of the thunderstorms were weak or moderate and heavy gusts were rare. These clouds moved slowly from western coast to east. In the eastern Estonia there were a lot of heavy tuhnderstorms with damaging winds. These clouds were rapid in movement and moved from southern contys to north or northeast. Peak gusts measured in the souther and easter part of Estonia were as high as 24-30 m/s. Most of the storms began from southern parts of inland (Viljandi and Valga county) and then moved to north(east) across Jõgeva and Viru countys and finally withdrew to the Gulf on Finland. Only few thunderstorms occurred in western islands (they were most of the time behind the cold front in a polar air mass) and in southeastern corner of the contry (was almost all the time in the tropical air mass).

To study the formation of straight line winds in thunderclouds data from Tartu and Pärnu automatical observation points was used. The main reason of damaging winds seems to be the fact that thunderclouds consist of two very different parts. In the former part of the cloud strong updraft dominates, air pressure decreases, air temperature is high and (almost) no precipitation occur. In the rear part of the cloud strong downdrafts dominate, air pressure rises, temperatures are low and heawy precipitation occur. Stormy gusts are the result of cold air masses that sink with downdrafts in precipitation zone. Then they flow rapidly near the ground to fill the lower pressure area under the former part of thundercloud. If updrafts in the former part of cloud and downdrafts in the rear part are strong, damaging straight line winds will probably occur. Damaging winds are also possible between forming cloud updrafts and dissipating thunderstorm downdrafts. They can occur if distance between forming and dissipating thundercloud is 10-15 km or less.

Heavy straight line winds probability also depends on air mass temperature and velocityes of thunderclouds. Warmer air and thunderclouds rapid movement more commonly cause damaging winds. It is also possible to predict storm according to appearance of approaching thunderstorm. If there is a low and dark wallcloud ahead of precipitation zone, damaging winds will probably occur under the wall.

In 1960-s there was a highly organised public thunderstorm observers network in Estonia. After the end of these observations essential climate warming has occurred. So the preferred ways and main propertyes of thunderclouds can have changed. Distances between EMHI observation points are too high to study these changes. So I am very interested in organising new public thunderstorm observations in Estonia soon. .