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Carl W. Kreitzberg
and
Donald J. Perkey

Abstract

In Part I the convective processes important during the release of potential instability were described qualitatively and evaluated quantitatively in a parameterized cumulus model within a primitive equation model. Part II includes a more detailed examination of convective/mesoscale interactions through a basic simulation experiment and tests under different physical conditions and with different computational grids. The cumulus model was documented in Part I and the primitive equation model is documented herein. The example, for which detailed dynamical fields are shown, began with 6 h of convective activity that developed a saturated neutrally buoyant mesoscale updraft which produced the bulk of the precipitation by 12 h into the integration.

The potential instability process is readily understandable and verifiable in general terms by numerical simulation. Increasing moisture bandwidth or large-scale ascent results in a wider precipitation band. Permitting evaporation of convective precipitation above cloud base had surprising little effect on these rain-bands that are about 100 km wide. Decreasing cumulus updraft radius, thereby increasing entrainment effects, delays initial development of the mesoscale circulation and produces a much narrower and more intense circulation later on. Reducing the horizontal grid size from 20 to 10 km results in much narrower rainbands but does little to the area total precipitation. The rate of propagation inward of lateral boundary condition influences shows that rather large areas must be dealt with in mesoscale field projects and numerical weather prediction for phenomena with time scales of several hours.

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Carl W. Kreitzberg
and
Donald J. Perkey

Abstract

The release of potential instability by large-scale lifting and the subsequent interaction of cumulus convection and the hydrostatic mesoscale flow in a most complex scale-interaction process. This process is an essential part of tropical weather but it is also important in extratropical cyclones through the formation of mesoscale rainbands that contribute much of the precipitation. The purpose of this paper is to qualitatively and quantitatively clarify the potential instability release process within a framework that will permit calculation of convective/mesoscale interactions.

The approach is to use an extension of the Lagrangian form of the one-dimensional cumulus model to provide values of convective-scale changes to a hydrostatic primitive equation model. This cumulus sub-routine locates the base of the convection, computes the cumulus plume that will build, accounts for the environmental subsidence, and mixes the subsided environment with the cumulus plume after rainout. These plumes build sequentially when the subroutine is called every 20 min at each column in the hydrostatic model.

The convection model is explained in some detail along with its behavior within the hydrostatic model. The use of this scheme for convective adjustment is contrasted with other schemes; it is emphasized that this scheme is more generally applicable and includes the temporal evolution of mesoscale convective disturbances through consumption of pre-existing potential instability as well as the resupply of warm moist air (fuel).

Examples of convective/mesoscale interaction will he presented in Part II along with examples of the sensitivity of the results to variations in initial conditions and numerical coefficients.

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Yuh-Lang Lin
and
Donald J. Perkey

Abstract

A process of lee cyclogenesis associated with backsheared baroclinic flow is studied using a fully nonlinear, primitive equation numerical model. A region of low pressure and a narrow baroclinic zone develop to the southwest of the mountain in the model for surface northerly wind with constant backshear. These major features are consistent with theoretical results of other authors. The low-level flow splits around the mountain for low Froude number such as investigated in this study. The splitting process is less pronounced for flow over a lower mountain. The positive pressure deviation has its center displaced northeast with respect to the mountaintop because of the relatively strong ageostrophic advection of cold air. Our numerical simulation indicates that a theory of lee cyclogenesis proposed by Smith is valid, at least in the early stage of cyclogenesis.

The lee cyclogenesis is affected by both the low-level sensible heating and the turning of the wind associated with boundary-layer processes. The development of the low is weakened in the early stage, which is the result of the weakened warm advection associated with the mountain-induced anticyclone. Weak upward motion, instead of strong downward motion, is found near the lee low-pressure region. The upward motion is produced by the mountain acting as an elevated heat source. This upward motion may accelerate and strengthen the cyclogenesis if the low-level flow were saturated and latent heating were to become important.

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