First, a list of 30 potential lake-effect precipitation events between 01 September 2008 and 31 December 2013 is compiled. These events were characterized by a difference between the LST und the mean 2-m air temperature in Bregenz of 2 to 10 K. Maximum observed precipitation accumulated over 36 h varied between about 4 and 126 mm. More than 70 % of the events occurred between July and December. One event isThis is one of the first case studies of a snowstorm at Lake Constance, located between Austria, Germany and Switzerland, which assesses the influence of the lake and the orography on the generation of heavy precipitation. The analysis is based on surface and radar observations as well as numerical simulations with the Weather Research and Forecasting (WRF) Model. Sensitivity experiments are conducted with modified surrounding orography, land-use and lake surface temperature (LST) as well as various different microphysics parameterizations to determine the role of individual physical processes.
First, a list of 30 potential lake-effect precipitation events between 01 September 2008 and 31 December 2013 is compiled. These events were characterized by a difference between the LST und the mean 2-m air temperature in Bregenz of 2 to 10 K. Maximum observed precipitation accumulated over 36 h varied between about 4 and 126 mm. More than 70 % of the events occurred between July and December. One event is analyzed in detail in this work.
The selected event occurred on 08 February 2013 during postfrontal conditions with northwesterly low-level flow along the long axis of the lake. It is characterized by a rather stationary and banded radar reflectivity pattern. The associated snowband reached a length of up to 30 km and affected the downstream shore and the adjacent mountainous region. The maximum of approximately 36 mm snow water equivalent accumulated within about 5 hours occurred in Bregenz at the eastern shore of the lake close to sloping terrain. The analysis of surface observations revealed a low-level convergence between the northeastern and southeastern lakeshore during the period of banded precipitation.
The numerical simulation of the event captures essential features including the formation of a convergence line and the associated snowband close to the northeastern shoreline and the adjacent complex terrain. A conditionally unstable stratification is formed at low levels by lake-induced latent and sensible heat fluxes, with the LST being about 5 K higher than the 2-m air temperature. Conditional instability is released by lifting of low-level air by orographically and thermally-induced convergence triggering shallow convection. Orographic enhancement of precipitation occurs downstream of the lake as the northwesterly flow is lifted over the adjacent mountains.
Sensitivity experiments show that the lake is a crucial factor for the formation of precipitation for this specific case, controlling the amount and distribution of snowfall. However, neither the lake nor the orography alone would have been able to trigger convection and to form a snowband. The major role of the lake is to destabilize the air mass. The role of the surrounding larger-scale terrain is to cause flow deflection and, hence, convergence necessary to trigger convection. The downstream terrain invigorates convection and therefore increases precipitation intensity. It was also found that the microphysic parameterization used in the numerical simulation influences the amount and distribution of precipitation. All of the five sensitivity experiments with different microphysics parameterizations form a snowband, but with different intensities and durations. The maximum value of accumulated precipitation downstream of the lake over 5 h during banded precipitation varies between 12.9 and 46.7 mm. This study highlights the complex interaction between lake and orographic effects and is a first proof that lake-effect snow is able to occur at Lake Constance despite the relatively small size of the lake.