The relationship between 2-meter air temperature and lapse rate in the western U.S

One important component of high spatial resolution land surface modeling is the accurate downscaling of the typically coarse resolution atmospheric forcing data to better match the land surface complexity. We have investigated the temporal and spatial characteristics of lapse rate (defined here as the vertical changes of 2-meter air temperature along the terrain surface) using a total of 1189 in-situ stations in the western United States. Lapse rate magnitudes are found to be larger over the southern than the northern states, and larger in the summer than in the winter over continental regions. Additional differences were found for the coastal regions, where the oceanic influence reduced lapse rates further during the summer than in the winter. Such significant spatial and temporal variations in lapse rate suggest that a proxy is required to adequately predict these lapse rate variations for the downscaling of atmospheric forcing data to drive land models. We demonstrate a linear relationship between the lapse rate near the land surface and 2-meter air temperature. We also perform a fitting and validation experiment by deriving the relationship between temperature and lapse rate from 1991-2010, using 2011 as an independent test period. In combination with a high-resolution digital elevation map (DEM) data, this regression equation and the resulting lapse rates will support and improve a wide range of applications which are dependent on downscaling, such as the production of North American Land Data Assimilation System (NLDAS) forcing data for US drought monitoring.