Climatology of high lapse rates over North America (1974–2007)

Several papers in the classic literature have demonstrated the utility of using an ingredients-based approach to forecasting the development of intense deep, moist convection (DMC). The three documented ingredients necessary for the development of intense DMC are lift, moisture, and instability. Instability is manifested as high

lapse rates generated by diabatically-induced mixed layers originating over elevated terrain. High lapse rates can also be generated through dynamical processes in association with upper-level troughs and jet-streaks. Previous work has shown that instability can be tracked simply by calculating the 700–500-hPa lapse rate from either raw sounding data or model data. The elegance of using the 700–500-hPa lapse rates lies in its simplicity since data is only needed at two levels, and ease in plotting on horizontal weather maps to assess where instability axes reside and overlap high boundary layer moisture values.

Given the importance of instability as an ingredient for the development of intense DMC, the purpose of this presentation is to examine an objective climatology of high lapse rates over North America during 1974–2007 to determine the temporal and spatial variability of this important ingredient. The lapse rate climatology was generated using the North American radiosonde network archive at the National Climatic Data Center. The objective scheme counted the occurrence 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 data 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, and extends eastward over the southern Great Plains during March through May and east over the Northern Great Plains and Great Lakes region during June through August. The maximum also extends westward to the southern California coast during July and August. A secondary maximum of high lapse rates occurs during the winter months over the Canadian Rockies south to the four-corners region and east to the central Great Plains. The warm season maximum in high lapse rates is likely associated with diabatic heating over elevated terrain, while the cold season maximum in high lapse rates is likely associated with cold upper-level troughs.