Julien Savre
[Department of Meteorology, Stockholm University, Sweden

Large Eddy Simulations of stratiform Arctic mixed-phase clouds

Time and place
Tue 11 Dec 2012, 11.15
Room C609, Arrhenius Laboratory, 6th floor

(this event has taken place)


With occurrence probabilities of up to 80% in the spring season, low-level Arctic mixed-phase clouds are known to play a central role in the local but also in the global climate. These clouds constitute one of the main components of the Earth's energy budget over the Arctic where the high/low sunlight exposure in summer/winter times and the
presence of highly reflective ice at the surface during a major part of the year lead to a very unique and delicate balance. It is thus of crucial importance for climate models to accurately simulate the main macro- and micro-physical properties of such clouds to provide more realistic climate predictions, for instance with prescribed scenarios concerning greenhouse gas emissions. Modeling the proper partitioning between liquid and ice phase within mixed-phase clouds is of particular relevance to capture the correct cloud-radiative feedbacks given the different radiative properties (absorption, reflection...) of ice crystals and cloud droplets. Current climate models unfortunately show a large
spread in predicted liquid vs. ice partitioning, with full glaciation temperature thresholds (temperature below which no liquid is observed) ranging from -10˚C to -40˚C (liquid has been observed to naturally persist down to approximately -40˚C).

Large Eddy Simulation (LES) has only recently been successfully used to support observations and help identifying the major driving processes controlling the persistence and properties of Arctic mixed-phase clouds, but also to test and develop new parameterizations for large scale climate models. Within that framework, MIMICA, an LES model dedicated to cloud studies, has been developed for a year now at Stockholm University. The model has already been successfully validated on a precipitating marine
stratocumulus cloud deck and is now operated to determine the sensitivity of Arctic mixed-phase clouds to various physical processes.

The present seminar will be divided into four main parts. First of all, a background on Arctic mixed-phase clouds and their significance for the present-day climate will be given. A brief description of MIMICA will then be presented, with a focus on the microphysics parameterization. The biggest part of the talk will introduce some recent results obtained on an idealized Arctic cloud case. These simulations try to assess the sensitivity of modeled cloud properties on the treatment of ice crystal diffusional growth and surface fluxes. Future works regarding the sensitivity of mixed-phase clouds on ice nucleation mechanisms are finally discussed.

(this event has taken place)