Characteristics of convective snow bands along the Swedish east coast

Julia Jeworrek,Lichuan Wu,Christian Dieterich,Anna Rutgersson

doi:10.5194/esd-8-163-2017

Abstract

Abstract. Convective snow bands develop in response to a cold air outbreak from the continent or the frozen sea over the open water surface of lakes or seas. The comparatively warm water body triggers shallow convection due to increased heat and moisture fluxes. Strong winds can align with this convection into wind-parallel cloud bands, which appear stationary as the wind direction remains consistent for the time period of the snow band event, delivering enduring snow precipitation at the approaching coast. The statistical analysis of a dataset from an 11-year high-resolution atmospheric regional climate model (RCA4) indicated 4 to 7 days a year of moderate to highly favourable conditions for the development of convective snow bands in the Baltic Sea region. The heaviest and most frequent lake effect snow was affecting the regions of Gävle and Västervik (along the Swedish east coast) as well as Gdansk (along the Polish coast). However, the hourly precipitation rate is often higher in Gävle than in the Västervik region. Two case studies comparing five different RCA4 model setups have shown that the Rossby Centre atmospheric regional climate model RCA4 provides a superior representation of the sea surface with more accurate sea surface temperature (SST) values when coupled to the ice–ocean model NEMO as opposed to the forcing by the ERA-40 reanalysis data. The refinement of the resolution of the atmospheric model component leads, especially in the horizontal direction, to significant improvement in the representation of the mesoscale circulation process as well as the local precipitation rate and area by the model.

Highlights

The roughness and temperature differences between land and water surfaces often lead to local sub-climates such as mesoscale circulation systems or stable and unstable conditions
In connection with the investigation of the mesoscale processes determining convective snow band events, this study was carried out in two parts: the statistical analysis of snow band events based on an 11-year RCA4 dataset and the evaluation of the use of different regional climate model systems
The atmospheric conditions which favour the development and maintenance of convective snow bands show a recurrent pattern in the Baltic Sea region

Summary

Introduction

The roughness and temperature differences between land and water surfaces often lead to local sub-climates such as mesoscale circulation systems or stable and unstable conditions. Mesoscale convective precipitation events like convective snow bands may develop. Convective snow bands are known and studied as lake effect snow (e.g. at the Great Lakes), cloud streets, horizontal convective rolls or vortices with solid precipitation. Convective snow bands develop commonly over the open water surface of lakes or seas when cold air approaches from the continent. Enhanced heat and moisture fluxes from the comparatively warm water body trigger shallow convection as the colder air mass travels across the sea. When the prevailing atmospheric conditions imply a strong development of convective snow bands, intense precipitation occurs locally where the snow bands hit the coast. The highest precipitation has, been shown to occur over the sea close

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