Mesoscale simulations of cold season Alaskan atmosphere-surface interactions using the PSU/NCAR MM5 model coupled to the NOAH-LSM land surface model

The variation of snow cover and its effect on temperature anomalies, precipitation patterns and hydrological processes have been widely studied (e.g., Barnett et al.,1989). During the cold season, frozen soil can dramatically affect rainfall-runoff partitioning, evapotranspiration as well as surface temperature(e.g., Spaans and Baker,1996). All these processes are particularly important for Arctic and subArctic continental regions which contain permafrost and have frozen active layers and large snow covered areas during the long winter season. To obtain realistic depictions of land-atmosphere-ice exchange processes in model simulations, it is necessary for the model to account for such winter season processes (e.g.,freezing and thawing of soils, snow accumulation and ablation, etc.)

In this study, the land surface model NOAH-LSM, which has been recently developed by NCEP, Oregon State University, the Air Force Weather Agency and NOAA's Office of Hydrology (Koren et al.,1999), and which has a more comprehensive consideration of the above winter season processes, is coupled to the PSU/NCAR Mesoscale Model MM5. Two cold season case study periods over the Alaskan region , one with patchy snow cover and one with continuous snow cover , are simulated with the new coupled model MM5-NOAH-LSM model as well as the standard MM5 model using the standard Land Surface Model (a forerunner of NOAH-LSM with less sophisticated treatment of snow and frozen soil.). Comparisons between these two sets of simulation results show that MM5 coupled to NOAH-LSM can produce more reasonable and realistic results for land surface hydrological processes and thermodynamic processes. In turn, the NOAH-LSM simulations then provide more realistic depiction of land-atmosphere exchanges during the cold season.