Nonclassical cold-frontal structure caused by dry subcloud air in Northern Utah during the Intermountain Precipitation Experiment (IPEX)

David M. Schultz, Robert J. Trapp

Research output: Contribution to journalArticlepeer-review


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 in 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°C), 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.

Original languageEnglish (US)
Pages (from-to)2222-2246
Number of pages25
JournalMonthly Weather Review
Issue number10
StatePublished - Oct 2003
Externally publishedYes

ASJC Scopus subject areas

  • Atmospheric Science


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