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A case study of a snowstorm with multiple snowbands in southern Finland 23 November 2008

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Title: A case study of a snowstorm with multiple snowbands in southern Finland 23 November 2008
Author(s): Nevalainen, Katja
Contributor: University of Helsinki, Faculty of Science, Department of Physics
Discipline: Meteorology
Language: English
Acceptance year: 2012
A large number of studies have concerned banded precipitation structures in extratropical cyclones but these studies have focused on single banded events. Most of these studies have taken place in the United States, whereas this study investigates multiple snowbands formed in a snowstorm on 23th November 2008 in Southern Finland, where the large-scale dynamical features maybe different. The storm caused heavy snowfall, especially along the southern coast of Finland. The study is divided in two parts. The first part describes the observed characteristics of the snowstorm by identifying different stages of the storm evolution as well as the large and smallscale structures of precipitation. The second part aims to identify the forcing mechanisms, which lead to the formation of multiple snowbands by using high resolution model output. To achieve this results, radar composite data obtained from the Finnish Meteorological Institute's (FMI) Doppler radar network and the AROME mesoscale model simulation output are used. The radar composites revealed four different phases of the storm evolution. The storm exhibited 22 individual bands and 6 groups of bands during the first three phases. The AROME simulation was able to produce the storm evolution and precipitation features rather similar to those observed. Strong and widespread frontogenetical forcing, weak moist symmetric and potential stability and to a small extent moist symmetric instability and potential instability were important mechanisms for producing heavy precipitation and mesoscale bands. During the first two phases frontogenesis was the forcing mechanism for ascent. Precipitation along the warm front at middle troposphere during phase 1 was mainly caused by ascent along isentropic surfaces below 500 hPa were sufficient moisture was available. During phase 2, weak potential stability and to a small extent potential instability were present in the warm sector enhancing the vertical ascent and precipitation. At lower levels along the warm front also symmetric instability was found and most likely released, resulting in slantwise convection. Moist layer in the warm sector reached 500 hPa although there were fluctuations which made the precipitation field scattered. Based on the radar composites phases 3 and 4 differed dynamically from phases 1 and 2 but were similar to each other. Frontogenetical forcing reduced significantly after the cold frontal passage in phase 3. Despite of the shallow moist layer and potential instability, convection did not occur in the simulation in same extent than in observations.

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