8 Orographic Precipitation in an Idealized Mid-latitude Cyclone

Tuesday, 26 June 2018
New Mexico/Santa Fe Room/Portal (La Fonda on the Plaza)
Lydia Helen Tierney, University of Washington, Seattle, WA; and D. Durran

Idealized work on orographic precipitation has primarily focused on simple airflow patterns encountering terrain. Few theoretical studies have been conducted examining the interaction of orography and pre-existing large-scale atmospheric disturbances. Here we present a novel idealized study of a realistic, archetypal mid-latitude cyclone encountering an isolated mountain ridge. The cyclone is initialized with a PV anomaly upstream of the 2-km ridge and encounters the terrain on the third day of the simulation, when the cyclone is mature and has developed a strong cold front. In addition to this experiment, two more simulations were run for comparison purposes; the first of these was initialized without topography, and the second was initialized without the initial PV anomaly that seeded the cyclone. By comparing the simulation including both the mountain and the topography to the two simpler systems, we are able to observe any nonlinearities in the amount of precipitation generated when the horizontal deformation and vertical velocity fields associated with the cyclone encounter topography.

The simulation including both the cyclone and the terrain produce substantially heavier precipitation than would be expected from the combined sum of the precipitation due to the cold front and the orographic uplift. At the time of the passage of the cold front over the ridge, observed rain rates are three times greater in the simulation that included the cyclone and the ridge than in either of the simpler control simulations. The interaction of the ridge with the mesoscale features of the cyclone also alters the geometry of the region that receives the greatest precipitation. The maximum rain rate is observed further south in the case study with the mountain and the cyclone than in either the no-mountain case or the no-cyclone case. The cyclone-plus-mountain case also shows a substantially different precipitation pattern downstream of the mountain, with less rain in the region occupied by the air mass that had been dehydrated by the forced uplift over the mountain. The importance of the nonlinear interaction of the cyclone and the topography is clear from these preliminary results, but the full scope of this effect is still being explored.

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