Atmospheric flux computations in complex terrain

One component of a regional water budget analysis involves computing atmospheric moisture convergence from available data about the profiles of water vapor and wind in the atmosphere. An issue that arises in such computations, which are important in many GCIP studies, is the averaging or interpolation of observations between available data sites (such as rawinsonde stations). For flat terrain various procedures are available to deal with this issue. For complex topography, such as that in the mountainous areas of the GCIP LSA-NW region, however, questions arise about how to conduct the analysis when the terrain between observing sites is sloping, or even blocking. Simple schemes that produce plausible results in the free atmosphere can lead to discontinuities or other problems in situations with complex terrain. This paper discusses the topography issue in atmospheric flux computations and considers various approaches to deal with the problem. Data from the recent Upper Missouri River Basin Pilot Project, focusing on the Black Hills region of South Dakota and Wyoming, provide examples relevant to the GCIP ISA scales.