A model for simulation of the climate and hydrology of the Great Lakes basin

The Coupled Hydrosphere-Atmosphere Research Model (CHARM) was developed by coupling the Regional Atmospheric Modeling System (RAMS) to models of the land hydrology of the Great Lakes basin and of the evaporation and thermodynamics of the Great Lakes. It is intended for running coupled atmosphere-surface climate scenarios for the Great Lakes basin, to gain a perspective that has been missed by running hydrologic models in off-line mode, driven by the output of global general circulation models. This paper presents validation of this model using historical atmospheric data to drive the regional embedded CHARM model. The current version of CHARM simulates the near-surface air temperature in the region quite well, with some positive bias during the winter and negative bias during the summer. Biases in the temperature averaged over 1 month and over the portion of the domain that is not directly forced by observations are less than or approximately 2 K. The annual precipitation has a positive bias of 6.6% and does well at placing the lake-effect precipitation areas, but may have too strong a west-east gradient. Simulation of annually averaged runoff meets well with expectations, but additional empirical fitting may be required to replicate the seasonal cycle. Aspects of the model that remain troublesome are the tendency for unrealistically low pressure at mean sea level and for persistent heavy low stratus clouds.