The Effect of Decoupled Low-Level Flow on Winter Orographic Clouds and Precipitation in the Yampa River Valley

Abstract

Mountains often act as barriers to low-level flow creating regions of stagnant, decoupled flow within thermally stratified air masses. This paper addresses the question: how does a region of low-level decoupled flow affect the overlying orographic cloud? Three different methodologies were used to examine this problem. The first method involved analysis of one and a half months of precipitation and wind data from a 24-station mesonetwork located in the Yampa River valley and surrounding mountains of northwest Colorado during the winter of 1981/1982 as part of the third Colorado Orographic Seeding Experiment (COSE III). The second method was a case study analysis of two orographic storms using data from an instrumented cloud physics aircraft to supplement the data from the mesonetwork. The third method involved two-dimensional numerical simulations using Colorado State University's Regional Atmospheric Modeling System (RAMS). The results show that the presence of extensive low-level decoupled flow causes part of the orographic lift of the mountain barrier to be experienced upstream of the barrier. This changes the location of condensate production which in turn shifts precipitation upstream and appears to enhance the precipitation efficiency for the entire barrier.