Underestimates of Orographic Precipitation in Idealized Simulations. Part I: Evidence from Unidirectional Warm-Sector Environments

Heavy precipitation in midlatitude mountain ranges is largely driven by the episodic passage of weather systems. Previous studies have shown a high correlation between the integrated vapor transport (IVT) in the airstream striking a mountain and the precipitation rate. Using data collected during the Olympic Mountain Experiment (OLYMPEX) project from a pair of sounding stations and a dense precipitation network, we further document the tight relation between IVT and precipitation rate and obtain results consistent with earlier work. We also survey previous studies that simulated orographic precipitation forced by unidirectional shear fl ows. Some of these simulations were performed using models that produce reasonably accurate rainfall totals in nested simulations of actual events driven by large-scale fl ows. Nevertheless, the increase in precipitation with IVT in all the simulations with unidirectional upstream fl ows is far lower than what would be expected based on the observationally derived correlation between IVT and precipitation rate. As a fi rst step toward explaining this discrepancy, we conduct idealized simulations of a midlatitude cyclone striking a north-south ridge. The relationship between IVT and rainfall rate in this "Cyc+Mtn" simulation matches that which would be expected from observations. In contrast, when the conditions upstream of the ridge in the Cyc+Mtn case were used as upstream forcing in a horizontally uniform unidirectional fl ow with the same IVT over the same mountain ridge, far less precipitation was produced. These idealized simulations will, therefore, be used to study the discrepancy in rainfall between simulations driven by unidirectional shear fl ows and observations in a companion paper. SIGNIFICANCE STATEMENT: Idealized simulations of orographic precipitation using horizontally uniform environmental forcing fail to capture the observed relationship between integrated water vapor fl ux impinging on the mountain and the precipitation rate. This suggests we need to improve the design of such idealized simulations.