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Michael W. Kalb

Abstract

Three 15 h LAMPS (Limited Area Mesoscale Prediction System) model simulations were performed on a 70 km grid to examine the role of convective parameterization on the evolution of the Gulf Coast precipitation system of 6–7 March 1982. The first simulation neglected moist processes. The second used an explicit representation for moist physics as suggested by Orlanski and Ross (1984) and Rosenthal (1978; 1979). A third simulation employed the LAMPS sequential plume cumulus cloud model as convective parameterization for the mesoscale model. Results from these experiments indicate that the manner in which moist thermodynamics is included in a meso-alpha scale model has significant bearing on model depiction and evolution of mesoscale structures.

The dry simulation developed and maintained a low pressure area consistent with the observed Gulf Coast precipitation system; however, kinematic and thermodynamic fields were substantially modified by the inclusion of moist physics. The simulation with parameterized convection outperformed that with grid-resolved moist physics in several important respects. While precipitation in the grid-resolved simulation formed in the same location as the parameterized convection and as observed, it was slower to develop and was initiated about 2 h late over the Gulf Coast During the latter half of that simulation, the Gulf Coast system underwent explosive development, giving midtropospheric diabatic heating rates and vertical motions that were unrealistically large near the center of the main precipitation system. A superadiabatic layer was generated near the top of the model precipitation system for this experiment in response to the extreme midlevel heating. Overdevelopment appeared to be a result of a runaway CISK-like feedback between low-level moisture convergence, upward vertical motion and latent heat release. These difficulties reflected the inability of grid resolved explicit model dynamics to eliminate gravitational instability except through the generation of a grid-resolved convective circulation. The convective parameterization effectively acted as a convective adjustment to preclude development of the superadiabatic layer that led to overdevelopment in the explicit simulation.

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Michael W. Kalb

Abstract

Results are presented from a nine-hour limited area fine mesh (35 km) mesoscale model simulation initialize with SESAME-AVE I radiosonde data for 10 April 1979 at 2100 GMT. Emphasis is on the diagnosis of mesoscale structure in the man and precipitation fields.

The model simulated most of the life cycle of a propagating mesoscale short wave/precipitation band (≈250 km wide) which developed from a dry low-level (700 mb) mesoscale short wave trough in the initial state. The short wave, which was supported initially by balanced winds, was retained by the model, propagated around a synoptic scale trough from Oklahoma to Nebraska, while initiating and organizing precipitation into a mesoscale band along the trough axis. The short wave and precipitation propagated northward together at about 18 m s−1 The onset of precipitation was accompanied by a decrease in width of the short wave. The system displayed distinctive vertical structure including downwind tilt with height. The short wave precipitation band verified well with a similar band evident in NMC hourly radar summaries in terms of location, rate of movement and spatial dimensions.

Along the Texas/Oklahoma border, independent of the short wave, convective precipitation formed several hours into the simulation and was organized into a narrow band suggestive of the observed 10 April squall line. The orientation and location of model convective precipitation were generally consistent with observed convective weather areas over Texas and Oklahoma.

The good results of model simulation are attributed to a combination of comprehensive model physics, fine grid resolution and subsynoptic scale initial data.

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