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Michael L. Kaplan, John W. Zack, Vince C. Wong, and Glen D. Coats


Mesoscale model simulations with and without diurnal planetary boundary layer heat flux are compared to a detailed surface analysis for a case of an isolated tornadic convective complex development. The case study, 3-4 June 1980, is of particular interest because of the development of several destructive tornadic storms within the Grand Island, Nebraska metropolitan area during a period of relatively weak synoptic scale forcing. This type of case presents an opportunity for the mesoscale numerical simulation of the subtle interactions between an upper tropospheric jet stream and surface diabatic heating. Model simulations runwith and without diurnal surface sensible heating show marked differences in processes both within and above the planetary boundary layer (PBL). The results of the simulations indicate that the evolution of the subsynoptic scale low pressure system and its accompanying low level jet streak, areas of moisture convergence, and regions of convective instability are influenced by the interaction of the deep surface-heated PBL with a weak synoptic scale jet streak. The model simulations show that the distribution and evolution of tropospheric velocity divergence cannot be realistically decoupled from the thickness changes caused by PBL heating in this case of relatively weak dynamic forcing. Modifications in the simulated velocity divergence and low level warm advection caused by PBL heating led to a more realistic pattern of pressure falls, low level jet formation, and a significant reduction of the lifted index values near the region of observed convection. Comparisons with observations, however, also indicate that the modeling system still requires: I) enhanced soil moisture information in the data base utilized for its PBL parameterization to achieve the proper amplitude and distribution of surface sensible heat flux and 2) the proper parameterization of convective scale processes such as latent heating to completely capture the evolution of the subsynoptic scale low pressure system into a mesoscale low pressure system. The most significant implication of these modeling results is that previous dynamical models of upper and lower tropospheric coupling during the pre-stormenvironment should include consideration of the effects of diurnal surface sensible heating upon a pre-existing jet streak.

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