The goal of this research is to advance understanding of the orographic modification of precipitation systems as they pass over major mountain ranges. The study will integrate data from two recent field programs: the Mesoscale Alpine Programme (MAP), conducted over the European Alps in 1999, and the second phase of the Improvement of Microphysical PaRameterization through Observational Verification Experiment (IMPROVE II), conducted over the Oregon Cascades in 2001.
Data in both projects was collected from a variety of airborne and ground-based advanced weather radars. This collection of radars was effective at indicating the predominant precipitation mechanisms over the windward slopes in both projects. In MAP, mobile Dopplers provided further information in deep valleys hidden from the view of fixed site radars. In IMPROVE II the University of Washington Convair aircraft collected ice particle samples aloft, while ground observers collected ice particle samples at the crest of the Cascades.
The tentative conceptual model of orographic precipitation enhancement inferred from the data collected in MAP suggests that coalescence of drops below the 0 degree C level and riming that generates graupel, just above the 0 degree C level, are key processes in producing particles that fall out quickly on the windward slopes. This process seems to work best in unblocked flow over the terrain, i.e., when the flow is strong and weakly stable, so it possesses the energy required to rise directly over the barrier. The process works even better when the flow up the terrain is slightly unstable, so that embedded convective cells can form and accelerate the coalescence and graupel-forming processes over the first large windward peak of terrain. Preliminary analysis of the new IMPROVE II data set suggests that similar processes are at work over the Cascades.
The empirical conceptual models will guide the critical evaluation of high resolution numerical model runs for the MAP and IMPROVE II storms. Numerical model output provides a database which will be evaluated for consistency with the MAP and IMPROVE II empirical models of orographic enhancement. The numerical models also will be run for select cases to obtain finer spatial resolution and more details on the microphysical processes taking place in the model. The radar data will provide dynamical constraints on the model results and the data will be examined for consistency with the model-simulated microphysical fields.
Improvements to the numerical models will help to advance the forecasting of precipitation and runoff in mountainous regions.
