CHILL, Pariicle ID, and MM5: The role of the barrier jet in Meso-g-scale Precipitation distribution in an extreme snowstorm

The winter storm that progressed across the southern and central Rocky Mountains 16-20 March 2003 was characterized by tremendous forcing both synoptically and orographically, generating 50 yr record snowfall on the upwind side (220 cm max) and lee side (197 cm max) of the range simultaneously. In addition, meso-gamma-scale snowfall variability at nearly equal elevation above sea-level varied by nearly 100 cm. The total volume of precipitation is believed to occur only once per 100 years. The anomalous depth of the new snow that fell had a major impact on snowpack with increases in snow water equivalent amounting to over 25% of the annual snowpack accumulation and therefore in reducing the threat of ongoing drought.

In this work, using CHILL, WSR-88D, CoCoRAHS and standard meteorological observations and high-resolution MM5 numerical modeling, we examine the important role of the barrier jet in generating the unusual and extreme snowfall distribution observed during this event. Strong barrier jet periods are associated with maximum snowfall rates far upwind east of the barrier (over the plains) and immediately in the lee of the barrier, lower snow density, greater variation in meso-g-scale precipitation differences, and coincident changes to hydrometeor type as identified using PID algorithms in CHILL. Weaker barrier jet periods are found to correspond to higher snowfall rates immediately upwind of the barrier, less variability in snowfall at constant elevation and a more traditional decrease in snowfall in barrier lee. Since the barrier jet is a routine feature of extratropical cyclone systems in the vicinity of barriers, and in this region in particular, we discuss the implications of this study for the local climate.