The role of unbalanced mesoscale circulations in dust storms

Abstract

In this study, two dust storms in northwestern Nevada (February 2002 and April 2004) are investigated through the use of Weather Research and Forecasting (WRF) model simulations. The focus of the study is twofold: (1) Examination of dynamic processes on the meso-β scale for both cases, and (2) analysis of extreme upper-air cooling prior to storm formation and the development of a nearly discontinuous gust front in the 2002 case that could not be validated in an earlier synoptic-scale study. Results of the simulations suggest that the driving mechanism for dust storm dynamics derives from the breakdown and subsequent balance between the advection of geostrophic wind and total wind in the exit region of the polar jet. In this process, the deviation from quasi-geostrophic (Q-G) balance creates a plume of ascent along and to the right of the jet's exit region. The cold pool generation in the mid-lower troposphere in consequence of this adjustment sets up the kinetic energy in the planetary boundary layer and creates a forward leaning (slope from north to south) cold front under the jet exit region. Surface heating is coupled with this frontal structure, and rapid surface pressure falls (rises) occur initially (later) in response to diabatic (adiabatic) processes. The adjustments occur at fast time scales, scales that are radically different from those in studies that followed the Q-G tenets of the Danielsen paradigm. The results of this study indicate that meso-β scale features associated with subgeostrophy in the exit region of the curved jet aloft and associated thermal wind imbalance (700-500 hPa) lead to significant velocity divergence aloft. Mass/momentum adjustments and the associated cooling strengthen the baroclinic zone aloft. The restoration to thermal wind balance accompanying this cooling resulted in a narrow zone of surface pressure rise and strong low-level isallobaric winds. The turbulent momentum for dust ablation comes from this sequence of processes. Copyright 2011 by the American Geophysical Union.