Nonclassical Cold-Frontal Structure Caused by Dry Subcloud Air in Northern Utah during the Intermountain Precipitation Experiment (IPEX)

Abstract

Abstract The purpose of the Intermountain Precipitation Experiment (IPEX) is to improve understanding of precipitating systems in the Intermountain West. Instrumentation deployed during the field phase of IPEX sampled a strong cold front and associated convection that moved through northern Utah on 14–15 February 2000. The surface cold front was characterized by a sharp temperature drop (8°C in 8 min), strong pressure rise (3 hPa in 30 min), and gusts to 40 m s−1. The temperature drop at high-elevation surface stations (2500–3000 m MSL) preceded the temperature drop at low-elevation surface stations (1290–2000 m MSL) by as much as an hour, implying a forward- or downshear-tilting frontal structure. Consistent with the cooling aloft, a hydrostatic pressure rise and wind shift preceded the temperature drop at the surface. Radar captured the rapid evolution of the wind shift line into a gravity current. A forward-sloping cloud with mammatus and a 20-hPa-deep superadiabatic layer underneath were observed by r...