Impact of Synoptic Scale Storm Structure on Orographic Precipitation

In mountainous areas in the midlatitudes, the majority of terrain-related precipitation occurs during the passage of midlatitude cyclones. Despite this, no prior idealized work has focused explicitly on the interaction of a complex synoptic scale flow and terrain. Here we present a series of idealized numerical simulations of a prototypical midlatitude cyclone encountering an isolated ridge. In an idealized WRF simulation, a canonical midlatitude cyclone develops from an isolated disturbance in an east-west channel flow and encounters a 2-km north-south ridge after 2.5 days of simulation time. In separate numerical simulations, isolated ridges are placed at two different latitudes with respect to the storm center. The terrain is positioned so that the low pressure center of the cyclone passes 500 km north of the northern ridge, and 2500 km north of the southern ridge. The idealized terrain is designed to be roughly comparable in height and extent to the cascade mountain range. In both experiments the combined circulations of the cyclone and the ridge increase orographic precipitation. For both terrain positions, the three dimensional mesoscale and synoptic scale features in the evolving flow strongly influence the flow pattern around the ridge, and therefore the precipitation distribution and amount. Additional test cases were conducted without terrain (cyclone-only) and with only cross mountain vertically sheared flow over the terrain. The greatest precipitation enhancement is observed during the passage of the cold frontal rainband over the southern ridge; at this time, precipitation rates over the ridge in the cyclone+ridge case are more than than sum of the rates in the shear flow and the cyclone with flat terrain cases. We also evaluate the contribution of microphysical orographic precipitation enhancement for both the northern and the southern mountain ranges.