Role of Terrain and Convection on Microfront Formation Leading to Severe Low-Level Turbulence

摘要

Two low-level convectively-induced turbulence (CIT) events east of the Appalachian Mountains are investigated utilizing observations, satellite, radar, and numerical simulations. Both events had an inordinate amount of low- level turbulence reported, but one event had more than twice as many severe or greater reports. The events were compared-to include the 72 hours leading up to the turbulence reports-and similarities and differences at the various scales from the synoptic to meso-alpha, meso-beta, meso-gamma, and microscale are noted. The case of weaker turbulence featured a meridional wave pattern with ridging over the East Coast and a single upper-level jet closely coupled with the large-scale frontal system. The stronger turbulence case possessed a zonal wave pattern with a vortex over eastern Canada and both a polar jet and subtropical jet. These differences are reflected in the low-level temperature and potential vorticity patterns and affected the hydraulic structures as well- with the stronger turbulence environment more prone to a blocking-type regime. Hydrostatic mountain waves were observed for both events. Stronger cross- mountain flow combined with a strong low-level leeside inversion resulted in a more vigorous mountain wave with a stronger downstream isentropic upfold (mid- level cold pool) in the stronger turbulence event. This mid-level cold pool was deformed by the large-scale jet resulting in a mid-level cold front (downstream from the surface cold front), surface pressure rises to the lee of the Allegheny Mountains, and ultimately a surface cold surge (edgewave) that merges with warm air from the south. The phasing of the mid-level cold pool and the convergence with the northerly cold surge and southerly warm air results in kata- frontogenesis and cellular convection that transits the severe turbulence location in space and time. Convection in the weaker turbulence case was lineal in structure and tied to the large-scale cold anafront.