Trees in mist over snow. Photo: Peter van de Lavoir
Trees in mist over snow. Photo: Peter van de Lavoir

Name

Martin Hagman, Licentiate candidate
Department of Meteorology, Stockholm University, Sweden

Title

Processes important for forecasting of clouds over snow

Abstract

The Swedish Armed Forces setup of the Weather Research and Forecasting Model (WRF) has problems to forecast low clouds in stably stratified conditions when the ground is covered by snow. The aim of this thesis is to understand what causes this deficit. Simulations during January and February 2018 are here compared with observations from Sodankylä in northern Finland. It is revealed that neither type of planetary boundary layer parameterization chosen nor vertical or horizontal interpolation are responsible for the deficiency. Instead, our experiments show that, to first order, poor initialization of Stratocumulus (Sc) clouds from the host model, Atmospheric Model High Resolution (HRES), of the Integrated Forecast System (IFS) is the missing link. In situations when Sc clouds are missing in the IFS analysis, although they exist in reality, we use information from vertical soundings from Sodankylä. In the initialization process we used the fact that liquid potential temperature is constant in a well-mixed cloud. Initializing cloud water and cloud ice from IFS HRES and from soundings with different methods improves the model performance and the formation of very low artificial clouds at the first model level is prohibited.

The diagnostic, used to assess the cloud fraction amount at every mass level in the model, even further worsens the cloud forecast. Cloud fraction is operationally used to assess the cloud base, which is a very important parameter when issuing flight weather forecasts. In this thesis we show that this diagnostic exaggerates low level cloud fraction during Arctic winter conditions, and propose an alternative approach.

Time and Place

Friday February 28, 2020, at 10.00

C609 Rossbysalen, Department of Meteorology, Svante Arrhenius väg 16C, 6th floor