The Weather Research and Forecasting (WRF) model has been used to study the role of land-atmosphere coupling in influencing interannual summer climate variability over the contiguous U.S. Two long-term climate simulations are performed: A control experiment (CTL) allows soil moisture to interact freely with the atmosphere, and an additional experiment uncouples the land surface from the atmosphere by replacing soil moisture at each time step with the climatology of CTL. The CTL simulation reproduces well the observed temperature and precipitation variability, despite some discrepancies in daily mean and maximum temperature variability in the Midwest/Ohio Valley region and the adjacent areas, and precipitation variability in the Great Plains and some other areas. Strong coupling of soil moisture with daily mean temperature appears mainly over the transitional zone between cold and warm climates from the Southwest to the northern Great Plains to the Southeast, contributing up to about 30%-60% of the total interannual variance of temperature. There is a significantly different influence on daily maximum and minimum temperatures. Whereas soil moisture plays a leading role in explaining the variability of maximum temperature over the transitional zone, minimum temperature variability is highly constrained by external factors including atmospheric circulation and sea surface temperature almost everywhere over land. Soil moisture, mainly through its effects on convection, makes a dominant contribution to precipitation variability over about half of the northern U.S. The model’s behavior agrees generally well with land-atmosphere relationships diagnosed using available observations and soil moisture data from the Global Land Data Assimilation System.
Revised: March 13, 2009 |
Published: November 27, 2008