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

摘要

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℃ 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 radar and radiosondes, respectively. Shading from this forward-sloping cloud is believed to have produced a surface-based prefrontal inversion upon which a solitary gravity wave traveled. These and other observations reveal that the forward-sloping cloud generated by a shortwave trough aloft was producing precipitation that sublimated, melted, and evaporated in the dry subcloud air (dewpoint depression of 5°-10℃), causing the cooling aloft and the nonclassical frontal structure. Although the storm-total precipitation associated with this system was generally light (less than 20 mm at all observing sites), the amount of precipitation was strongly a function of elevation. During one 6-h period, precipitation at stations above cloud base (roughly 2000 m MSL) varied widely, mostly due to orographic effects, although precipitation amounts at most stations were about 7-11 mm. In contrast, precipitation amounts decreased with distance below cloud base, consistent with sublimation and evaporation in the dry subcloud air.