REGIONAL AND SEASONAL ESTIMATES OF FRACTIONAL STORM COVERAGE BASED ONSTATION PRECIPITATION OBSERVATIONS

Simulated climates using numerical atmospheric general circulation mod els (GCMs) have been shown to be highly sensitive to the fraction of G CM grid area assumed to be wetted during rain events. The model hydrol ogic cycle and land-surface water and energy balance are influenced by the parameter <(kappa)over bar>, which is the dimensionless fractiona l wetted area for GCM grids. Hourly precipitation records for over 170 0 precipitation stations within the contiguous United States are used to obtain observation-based estimates of fractional wetting that exhib it regional and seasonal variations. The spatial parameter <(kappa)ove r bar> is estimated from the temporal raingauge data using conditional probability relations. Monthly <(kappa)over bar> values are estimated for rectangular grid areas over the contiguous United States as defin ed by the Goddard Institute for Space Studies 4 degrees X 5 degrees GC M. A bias in the estimates is evident due to the unavoidably sparse ra ingauge network density, which causes some storms to go undetected by the network. This bias is corrected by deriving the probability of a s torm escaping detection by the network. A Monte Carlo simulation study is also conducted that consists of synthetically generated storm arri vals over an artificial grid area. It is used to confirm the ii estima tion procedure and to test the nature of the bias and its correction. These monthly fractional wetting estimates, based on the analysis of s tation precipitation data, provide an observational basis for assignin g the influential parameter <(kappa)over bar> in GCM land-surface hydr ology parameterizations.