Orographic Signature on Multiscale Statistics of Extreme Rainfall

The main objective of this work is to quantify the statistical signature imprinted by orography on the spatio-temporal structure of rainfall at multiple scales with the aim of developing a consistent theoretical basis for downscaling orographic precipitation. We report the results of an extensive analysis of the high resolution stage II Doppler radar data of the Rapidan storm, June 1995, over the Appalachian Mountains. By tracking the tail value statistics of the precipitation field during the storm evolution, it is documented that the mean and variance of extreme rain cells have a strong dependence on the elevation of the underlying terrain and that the variance grows faster than the mean intensity as the elevation increases. An analysis of spatial rainfall fluctuations over a range of scales shows that these are well parameterized by a generalized log-Normal distribution whose scale-dependent parameters can be easily characterized. Furthermore, studying the evolution of the generalized log-normal distribution in time with respect to the underlying topography indicates that the rainfall fluctuations become less heavy tailed and more dispersed while the storm moves toward high elevation orographic features. It is showed that by conditioning the multiscale characterization of orographic rainfall on the underlying topography, a more accurate downscaling precipitation formalism can be developed. Specific examples motivated by the GPM satellite precipitation products are presented.