Characterization of the atmospheric state: Lower boundary condition

It is convenient to consider 2 broad categories of climate-related modeling studies for which it is necessary to specify some kind of lower boundary conditions. The first of these categories is the use of general circulation or weather forecasting models, perhaps modified to carry out climate simulations. In these models, one normally has to specify something about the albedo of the surface to get the radiation balance right, the surface roughness to get the momentum exchange right, and the surface moisture availability to get the surface heat and water vapor fluxes right. Correctly specifying the surface moisture availability can be a major problem and may involve a sophisticated land surface parameterization scheme to take into account plant and soil characteristics. It is reasonable to expect that misrepresenting the water vapor flux by 10--20% on average over continental scales could lead to significant errors in simulated precipitation, temperatures, and circulation patterns. The Atmospheric Radiation Measurement (ARM) Program is focused, however, on clouds and radiation; and it has chosen Cloud and Radiation Testbeds (CART) as the principal tool with which to carry out its work. In this context, what the authors are concerned about for the lower boundary conditions is somewhat different. What they want to known is show the incoming radiation is partitioned into various components by surface processes, and--more importantly--what is the resultant sensitivity of the cloud and radiation fields to that partitioning. These features then determine the accuracy to which they need to describe the lower boundary conditions.