IMPACT OF VEGETATION PROPERTIES ON US SUMMER WEATHER PREDICTION

Systematic biases in U.S. summer integrations with the Center for Ocea n-Land-Atmosphere Studies (COLA) atmospheric general circulation model (GCM) have been identified and analyzed. Positive surface air tempera ture biases of 2 degrees-4 degrees K occurred over the central United States. The temperature biases were coincident with the agricultural r egion of the central United States, where negative precipitation biase s also occurred. The biases developed in June and became very signific ant during July and August. The impact of the crop area vegetation and soil properties on the biases was investigated in a series of numeric al experiments. The biases were largely caused by the erroneous prescr iption of crop vegetation phenology in the surface model of the GCM. T he prescribed crop soil properties also contributed to the biases. On the basis of these results the crop model has been improved and the sy stematic errors in the U.S. summer simulations have been reduced. The numerical experiments also revealed that land surface effects on the a tmospheric variables at and near the surface during the North American summer are very pronounced and persistent but are largely limited to the area of the anomalous land surface forcing. In this regard, the mi dlatitude land surface effects described here are similar to those pre viously found for tropical regions.