Abstract: Eleven years of dual-polarization weather radar data, complemented by satellite and lidar observations, were used to investigate the origin of areas of localized intensification of precipitation spotted in the vicinity of Helsinki-Vantaa airport. It was observed that existing precipitation is enhanced locally on spatial scales from a few kilometers to several tens of kilometers. The precipitation intensity in these localized areas was 6-14 times higher than the background large-scale precipitation rate. Surface observations and dual-polarization radar data indicate that snowflakes within the ice portion of the falling precipitation in the intensification regions are larger and more isotropic than in the surrounding precipitation. There appears to be an increase in the ice particle number concentration within the intensification region. The observed events were linked to arriving or departing air traffic. We advocate that the mechanism responsible for intensification is aircraft-produced ice particles boosting the aggregation growth of snowflakes.
Plain Language Summary: By analyzing 11 years of dual-polarization weather radar observations in the Helsinki region, we have discovered that airplanes landing in or departing from the Helsinki-Vantaa airport could locally increase precipitation rate by as much as 14 times. The observed phenomenon is related to the hole-punch clouds, which are also forming with the help of airplanes. The reported observations allow us to have a better understanding of precipitation formation processes that take place in ice and mixed phase clouds. They show that falling ice crystals from upper clouds could seed lower clouds and therefore increase rain or snowfall intensity through the process called snowflake aggregation. During snowflake aggregation bigger faster falling particles are formed by ice particles colliding and sticking together.
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