Thursday, 2 August 2001
Simulated gravity waves produced by MCS-like heating profiles: MCS geometry and its effects on the environment
Among the MCSs studied by Parker and Johnson (2000), convective lines with parallel stratiform precipitation (PS) were found to be, on average, much shorter-lived than convective lines with trailing stratiform precipitation (TS); their mean lifetimes, respectively, were 6.3 h and 12.2 h. One possible explanation for this discrepancy pertains to the gravity wave dynamics associated with MCS heating profiles. As discussed by numerous authors in the published literature, deep convective heating produces gravity wave modes which entail subsidence throughout the depth of the troposphere (often called the n=1 mode). In like manner, the "heating over cooling" profile (resulting from the growth of ice above the freezing level and the melting and evaporation of hydrometeors below it) in the stratiform regions of MCSs produces gravity wave modes which entail subsidence in the upper troposphere, but ascent in the lower troposphere (often called the n=2 mode). The propagating n=1 mode serves to diminish the CAPE and increase the CIN of the surrounding environment. However, the propagating n=2 mode serves to remove CIN from low-level parcels in the surrounding environment, thereby facilitating continued convection. Notably, in TS MCSs the stratiform precipitation region, and its attendant heating, are proximate to the convective line, while in PS MCSs the stratiform precipitation region is laterally displaced (in the line-parallel direction) from the convective line.
The author has conducted numerical model simulations [using the Advanced Regional Prediction System (ARPS)] in which idealized, MCS-like heating profiles were placed within dry environments whose temperature and wind profiles ranged from very simple (i.e. resting isothermal states) to fairly realistic (i.e. having typical MCS wind and thermal profiles). Identical simulations were performed in which the imposed heating profiles were given either a quasi-TS or a quasi-PS horizontal shape. The modeling results suggest that gravity wave dynamics may indeed account for part of the observed variation of MCS lifetime with MCS geometry. In his proposed presentation and extended pre-print abstract, the author will describe the evolution of the idealized environments in response to the imposed MCS-like heating, and the resulting ramifications for the continual development of mesoscale organized convection.