Numerical simulation and analysis of a series of mesoscale convective systems

A case in which a series of mesoscale convective systems (MCSs) traversed across Kansas and Oklahoma (3-4 June 1985) was examined using observations and the MM4 model. The first MCS in the series (MCS1) began north of a stationary front in a region of strong low-level warm advection and moisture convergence near the terminus of a low-level jet (LLJ). Simulations of this system evolved realistically and show that the elevated convection fed on air riding over the frontal inversion. This source air originated in the heated boundary layer over Texas and Oklahoma the previous afternoon. The system-relative circulation of the mature system did not exhibit a front-to-rear upper-level flow nor extensive trailing stratiform region as has been observed with other systems.; The second MCS (MCS2) was linked to an upper-level jet streak throughout its lifetime. In agreement with observations, the simulation produced the two precipitation bands within MCS2 that gave it the appearance of a miniature occluded cyclone. The north-south band, located in the southern half of the system, consisted of surface-based convection forced by low-level convergence of convective downdrafts with the environmental flow, while the northeast-southwest band in the northern part of the system was comprised of elevated convection occurring in a frontogenetic zone. Convection in this region was initially triggered by a LLJ induced by transverse ageostrophic circulations about an approaching upper-level jet streak. The strong LLJ was decoupled from the surface by the frontal inversion and was able to advect high-{dollar}thetasb E{dollar} air northwestward, destabilizing the airmass.; The simulation of MCS1 was found to be sensitive to the initial low-level moisture distribution, suggesting that accurate prediction of elevated convection requires initial conditions that include mesoscale details aloft. In agreement with a previous study, the simulation was found to be quite sensitive to the convective trigger function. For weakly-forced cases such as 3-4 June, the trigger function is a critical component of the convective parameterization. The behavior of trigger functions based on low-level vertical motion, on the depth between source air and the level of free convection, and on the convective inhibition are examined.