NUMERICAL PREDICTION OF AN INTENSE CONVECTIVE SYSTEM ASSOCIATED WITH THE JULY 1987 MONTREAL FLOOD .2. A TRAILING STRATIFORM RAINBAND

In this study, the internal circulation structures of the 14 July 1987 intense mesoscale convective system (MCS) are investigated using an i mproved high-resolution version of the Canadian regional finite-elemen t model. It is found that although the MCS is characterized by a leadi ng convective line followed by a trailing stratiform rainband, the ass ociated circulation structures differ substantially from those in the classical midlatitude squall system. These include the rapid propagati on and separation of the leading convection from the trailing rainband , the development of a surface-based instead of an elevated rear-to-fr ont descending flow and a shallow front-to-rear ascending flow associa ted with the stratiform precipitation, the generation of low- and mid- level rather than mid- to upper-level stratiform cloudiness and the de velopment of a strong anticyclonic vorticity band at the back edge of the stratiform region. It is shown that the trailing stratiform rainba nd is dynamically forced by frontogenetical processes, and aided by th e release of conditional symmetric instability and local orographical lifting. The intense leading and trailing circulations result from lat ent heat released by the convective and explicit cloud schemes, respec tively. Sensitivity experiments reveal that the proper coupling of the se two cloud schemes is instrumental in obtaining a realistic predicti on of the above-mentioned various mesoscale components. Vorticity budg et calculations show that tilting of horizontal vorticity contributes the most to the amplification of the anticyclonic vorticity band, part icularly during the squall's incipient stage. The sensitivity of the s imulated squall system to other model physical parameters is also exam ined.