85th AMS Annual Meeting

Wednesday, 12 January 2005
Modeling the cloudy boundary layer at SHEBA with the GFDL single column model
John A. Beesley, NOAA/GFDL, Princeton, NJ
Low level stratiform clouds have a significant impact on the radiative budget of the sensitive ice-atmosphere climate system of the Arctic. What makes these clouds stand out from marine stratiform clouds in other environments is the common occurrence of ice particles, which are known to influence cloud radiative properties and longevity. At SHEBA, for example, liquid clouds were present at temperatures well below -10 C during the cold season, and ice crystals were observed falling from stratus clouds during the summer. This poses a challenge for both conceptual and numerical models of stratiform cloudiness. The GFDL global coupled climate model has an advanced atmospheric physics package, including an up-to-date mixed phase cloud microphysics parameterization and separate prognostic variables for cloud amount, and the specific humidity of cloud ice and cloud liquid. Nonetheless, the model has a significant positive bias in cloud amount throughout the year and excessive cloud optical depth during the summer in the Arctic.

In the present study, a single column version of the GFDL global model (including the sea-ice and atmospheric components) is used to simulate cloud and boundary layer processes at SHEBA. The goal is to identify and understand those physical processes that are essential for modeling the climate of the Arctic and its response to changes in external forcing. The model is forced with time dependent profiles of temperature specific humidity advection from the ECMWF forecast model, and incoming solar radiation at the top of the atmosphere. The results are composited according to prevailing meteorological conditions such as the presence of precipitation, temperature, and large-scale vertical motion. The model does a respectable job simulating the variations in cloud phase, precipitation and most components of the surface energy budget. The main problem is the tendency to overestimate cloud optical depth, which leads to a significant negative bias in downward shortwave radiation at the surface. Further experiments are performed to help identify the cause of this and other discrepancies.

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