Deep Orographic Storms over the Sierra Nevada. Part I: Thermodynamic and Kinematic Structure

Abstract

Two deep, orographic storms were documented over the Sierra Nevada with an instrumented aircraft and with a single Doppler radar. In both storms the geographic winds were normal to the barrier with speeds of 15 to 30 m s?1. Because of blocking the measured wind component normal to the barrier was substantially less than the geostrophic wind component, especially below barrier crest height. In the very stable case, a barrier jet was present below 1 km AGL and had peak winds of 28 m s?1. In the slightly stable case a barrier jet was present at 1 km AGL and had peak winds of 21 m s?1. Other than the speed and altitude of the barrier jet, the other measured characteristics of the airflow and thermal fields were rather similar between these two storms. The forced ascent of stable air plus the diabatic process of melting caused the isotherms to descend near the barrier. The barrier jet can be expressed by the thermal wind relation and is, therefore, a quasi-geostrophic phenomenon. The derived vertical velocities were estimated to be 0.2 to 0.4 m s?1.