Radar observations of orographic precipitation

Abstract

Operational Doppler radar archives provide insight into the structure and airflow within precipitating systems in two different mountain climates: the Mediterranean side of the European Alps and the coastal terrain of Northern California. Time-averaged reflectivity measurements reveal that climatological precipitation in both locations is the result of broad-scale upslope enhancement, upstream enhancement, and small-scale enhancement of precipitation. Within approximately a Rossby radius of each barrier's crest, precipitation amounts were greater than farther upstream. Over the broad windward slopes of each barrier, precipitation amounts were further enhanced and maximum over the first steep terrain rise of both ranges.Time-averaged radial velocity measurements during precipitation events revealed prevailing low-level flow that was perpendicular to the axis of each barrier and veered with height, indicating the presence of warm advection. Variations in the thermodynamic and dynamic properties of the impinging large-scale flow modulated the flow patterns and resulting rainfall. Precipitation was greatest when the direction of the impinging low-level flow (900--800 mb) brought copious low-level moisture and was perpendicular to the windward slopes. In the Alps, strong high Froude-number flow produced the greatest upslope enhancement over the lower windward slopes. In California, where high Froude-number flow was the norm, more rainfall occurred on the windward slopes and upstream when the flow was more stable. Likely in response to diurnal stability and humidity fluctuations, rainfall was more pronounced in the morning hours in both locations. Stronger, more humid airflow at mid levels (700--500 mb) also produced dramatically heavier precipitation over California.To facilitate this work, various software tools were developed to correct, process and display radar data, laying the groundwork for real-time radar data analysis during MAP. Techniques were configured for converting, interpolating, and finally displaying the radar data over terrain. In addition, a skillful yet efficient algorithm was designed to correct aliased Doppler velocity patterns over complex terrain.