Synoptic-scale flow patterns associated with high lapse rates over North America

Atmospheric instability, in the presence of low-level moisture and forcing for ascent, has been demonstrated to be an important ingredient when forecasting the development of deep, moist convection (DMC). This instability can be produced by the superposition of elevated high lapse rates (e.g. diabatically-induced mixed layers generated over an elevated semi-arid source region and/or dynamical-driven processes associated with upper-level troughs and jet streaks) over a region of low-level warm, humid air. Given that instability is an important ingredient to the development of intense DMC, the purpose of this poster is to examine continental and regional synoptic-scale flow patterns associated with occurrences of high lapse rates over North America.

In order to examine the synoptic-scale flow patterns associated with high lapse rates, an objective climatology of the occurrence of high lapse rates was generated using the North American Radiosonde network data from 1974–2007, archived at the National Climatic Data Center. The objective climatology recorded occurrences of high lapse rates when the 700–500-hPa lapse rate was ≥ 8.0 K km-1. The climatology was restricted to radiosonde observations taken during the time period 1000–1400 UTC to avoid contamination by the presence of DMC as much as possible.

Preliminary results highlight a warm season maximum in high lapse rates over the Intermountain West that expands poleward during March through August. These high lapse rates, which are mostly surface-based during the warm season, tend to advect eastward over the southern Great Plains and southeast United States (US) in early spring in conjunction with westerly/southwesterly flow in association with eastward moving disturbances. High lapse rates also tended to advect eastward over the northern Great Plains and Great Lakes region during June through August in conjunction with strong westerly flow aloft over the northern US on the poleward side of the climatological upper-level anticyclone over the southern-central US. Further results document occurrences of high lapse rates along the California and Northwest US coasts in association with deep easterlies established by the poleward migration of the aforementioned upper-level anticyclone.