Estimating extreme precipitation at high elevations in Colorado through mesoscale ensemble modeling

There is paleohydrographic evidence that current empirical models of Probable Maximum Precipitation (PMP) may significantly overestimate actual PMP at higher elevations in Colorado. As a result, engineering standards adopted for structures such as dams may be overdesigned and unnecessarily costly. In an exploratory effort to develop alternate and more accurate models of PMP in this region, a new methodology based on mesoscale ensemble modeling is being developed for estimating extreme precipitation in Colorado's complex terrain. In this approach, the Regional Atmospheric Modeling System (RAMS) is applied to numerous extreme precipitation events that have occurred in Colorado in the last 40+ years. For each case, interactive grid nesting is used to simulate the synoptic, mesoscale and cloud-resolving scales, respectively, over the western U.S., the Colorado region, and the meso-beta-scale area of the extreme precipitation event. NCEP reanalysis data are used for initialization and time-dependent boundary conditions. The ensemble simulations for each case consist of various perturbations imposed on the initial conditions. The perturbations are designed to account for favorable mesoscale features that were observed but not well resolved in the NCEP initialization, or which are even more extreme than were observed in the event but which are climatologically plausible in similar synoptic scenarios. This perturbation strategy is developed from composite analysis of several synoptic classes of the extreme events, inter-event variability, and climatological frequency distributions of parameters such as precipitable water and surface dewpoints. From the ensemble simulations of a given case, the maximum precipitation that might be expected to occur with slight variations of the observed event can be estimated. An alternate ensemble simulation approach is utilized wherein the initial meteorological fields are shifted relative to the topography, allowing the specific region of forcing in a given case to be shifted to other regions in Colorado. Combining both approaches, the systematic dependence of elevation and topography on extreme precipitation and its partition between flash-flood producing rainfall and slow-melting frozen precipitation can be assessed.