The relative airflow and accompanying precipitation structure of squall lines trailed by mesoscale regions of stratiform rain are examined with emphasis on the occurrence of “rear inflow,” i.e. the intrusion of environmental air into these storms across the trailing precipitation boundary. Three cases from Kansas and Oklahoma provide examples of “Strong Rear Inflow,” which crosses the back edge of the stratiform precipitation area at relative speeds exceeding 10 m s−1. The vertical profiles of relative flow at the trailing precipitation boundary of these three systems were remarkably similar, with the rear inflow confined to a jet-like layer centered at about 550 mb. Doppler radar data for two of these cases showed that the rear inflow jet occupied a continuous channel extending from middle levels at the back edge of the stratiform region to lower levels of the leading convective region, where it merged with outflow from convective downdrafts to bolster the leading gust front. Strong front-to-rear flow occurred both above and below this layer. The front-to-rear flow lying above the rear inflow jet was consistently strengthened in the vicinity of convective cells and was separated from the rear inflow by an interface of strong vertical wind shear that sloped upward toward the rear of the storm. Soundings indicate this interface marked the division between cloudy air (the trailing “anvil”) associated mesoscale updraft (above) and subsaturated air in the mesoscale downdraft (below).

An inclusive review of the literature reveals that these three midlatitude squall lines had stronger rear inflow than any previously described squall lines with trailing stratiform precipitation. Five “Weak Rear Inflow” cases had peak inflows at the back edge of the stratiform rain area between 5 and 10 m s−1. The vertical profiles of relative flow at the trailing edge exhibited a variety of structures, with the rear inflow maximum sometimes at higher altitude and sometimes at lower altitude than in the Strong Rear Inflow cases. The literature further reveals ten studies in which the relative flow at the back edge of the precipitation showed little if any rear inflow (<5 m s−1), suggestive of a stagnation of the midlevel system-relative flow as air at the back edge of the stratiform region moved at or near the speed of the system. These “Stagnation Zone” squall systems, like both the Strong and Weak Rear Inflow cases, exhibited maxima of front-to-rear flow at both upper and lower levels; however, the front-to-rear flow was not as strong as in the Strong Rear Inflow cases. The stagnation layer was located between 650 and 750 mb, considerably below the height of the inflow jet in the Strong Rear Inflow systems. While no appreciable rear inflow occurred at the back edge of Stagnation Zone cases, rear-to-front flow has been observed to develop at midlevels in the interior of their stratiform regions, suggesting that physical processes internal to the mesoscale system are capable of generating rear-to-front flow behind the convective line without the aid of ambient flow entering the storm.

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