Orographic Precipitation in an Idealized Midlatitude Cyclone

Abstract

In mountainous areas in the midlatitudes, the majority of terrain related precipitation occurs in conjunction with large scale storm systems. As noted in Smith 2006, ``Mountains have their most profound influence on precipitation during brief events when significant atmospheric disturbances move into mountainous areas''. Despite the established importance of synoptic dynamics, idealized studies in the past have generally neglected the large scale flow. The difficulty of modeling a complex synoptic scale system in an idealized numerical simulation has been a barrier. In this study, we aim to rectify this by presenting a series of idealized experiments of a realistic, prototypical midlatitude cyclone encountering an idealized ridge. The simulation is configured to have a canonical midlatitude cyclone located in an east-west channel flow. The cyclone encounters an isolated 2-km ridge after 2.5 days of simulation time. Two different ridge orientations were used in different regions of the cyclone; a north-south barrier was placed parallel to the front of the cyclone, and in a separate experiment, an east-west barrier was placed in the warm sector of the cyclone. In addition to the cyclone-and-mountain simulations, two control test cases were conducted. One test case included the cyclone but had flat terrain; the other test case did not include the cyclone, but was initialized with parallel shear flow over the terrain. For the simulation with the north-south mountain and the cyclone, simulations conducted with microphysics show an enhancement of precipitation over the mountain equal to the sum of the flat terrain and the shear-flow-only test cases. For the east-west mountain case, the precipitation observed in the cyclone+mountain case was 1.5-2 times the total combined precipitation in the flat-terrain and shear flow cases. In the case of the east-west ridge, the precipitation enhancement resulted from the updrafts due to the flow over terrain interacting with vertical motion in the larger system. The seeder-feeder effect also played a role. In the case of the north-south ridge along the front, the broadening and deepening of convective updrafts contributed to an increase in precipitation. In both cases, horizontal velocity gradients present in the cyclone generated greater convergence and up slope flow than just forced shear flow control experiments.