Presented is a study on the role of land surface processes in determining the summertime climate over the
semiarid region of southwest Asia. In this region, a warm surface air temperature bias of 3.58C is simulated in
the summer by using the standard configuration of Regional Climate Model version 3 (RegCM3). Biases are
also simulated in surface albedo (underestimation), shortwave incident radiation (overestimation), and vapor
pressure (underestimation). Based on satellite measurements documented in NASA’s surface radiation
budget (SRB) dataset, a correction in surface albedo by 4% is introduced in RegCM3 to match the observed
SRB data. Increasing albedo values results in a nearly 18C cooling over the region. In addition, by incorporating
RegCM3’s dust module and including subgrid variability for surface wind, shortwave incident
radiation bias originally of about 45 W m22 is reduced by 30 W m22. As a result, the reduction of shortwave
incident radiation cools the surface by 0.68C. Finally, including a representation for the irrigation and
marshlands of Mesopotamia produces surface relative humidity values closer to observations, thus eliminating
a nearly 5-mb vapor pressure dry bias over some of the region. Consequently, the representation of
irrigation and marshlands results in cooling of nearly 18C in areas downwind of the actual land-cover change.
Along with identified biases in observational datasets, these combined processes explain the 3.58C warm bias
in RegCM3 simulations. Therefore, it is found that accurate representations of surface albedo, dust emissions,
and irrigation are important in correctly modeling summertime climates of semiarid regions.