Simulations of hydrologically significant winter storms over a mountainous watershed

The largest watershed-to-basin-scale flood events in the Inland Pacific Northwest have typically occurred with a fairly rapid transition from anomalously cold to anomalously warm weather in the winter or early spring months. A research study has been conducted to analyze and simulate the variability of precipitation and snow distribution associated with these floods on a watershed scale. A goal of the study is to assess regional impacts of global climate change on land surface hydrology.;In conjunction with these research efforts, two winter precipitation events, one on 16-17 March 1993 (Case I) and the other on 28-31 December 1995 (Case II), were selected for analysis and simulations. These well documented cases at Reynolds Creek Experimental Watershed (RCEW) were simulated using the non-hydrostatic version of Colorado State University's (CSU) Regional Atmospheric Modeling System (RAMS) (Walko, et al. 1993). The associated wind fields of the first event also were simulated using a computationally-simple numerical model (MS-Micro/3) (Walmsley et al. 1992). The simulated flow fields, microphysical structure, and precipitation distribution compared quite well with observations.;MS-Micro/3 was evaluated as a potential model for developing realistic wind climatologies over complex terrain, as well as for simulating wind flow during a hydrologically-important event. A ten-hour spatial and temporal simulation was performed, starting at 07 UTC and ending at 16 UTC March 16, 1993. Simulated wind fields on 250 x 250 m grid spacing were displayed using a Geographic Information System (GIS). Statistical analysis on the simulated results showed that simulated wind speed and wind direction were in good agreement with the observations in the watershed.;RAMS was initialized by three nested grids and by both horizontally homogeneous initializations (Boise soundings) and initial fields derived from National Meteorological Center (NMC) upper-air grids and rawinsondes (multiple initializations). Preliminary sensitivity tests of grid resolution and land-surface characteristics were conducted using a homogeneous Boise sounding for Case I. Finest resolutions of 500, 800, 1000 and 1200 m were used with a domain size of 500 km for scaling sensitivity tests. The scale of the domain structure also showed strong influence on the simulation results. The simulated results showed small sensitivities to land-surface characteristics over the RCEW due to strong, synoptic-scale atmospheric dynamics and short temporal periods.;Of particular interest was investigating the feasibility of using a comprehensive numerical model for performing QPF at small scales. The results have shown that initializing the model with a sounding and with an isentropic surface which are representative of the upstream environment is most critical to the success of the simulations. Simulated precipitation patterns and wind fields were in good agreement with the observations during these the two storm events. (Abstract shortened by UMI.)...