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  • Author or Editor: A. E. MacDonald x
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J. L. Lee and A. E. MacDonald

Abstract

Mesoscale bounded derivative initialization (BDI) is utilized to derive dynamical constraints, from which elliptic equations are formulated to derive smooth initial fields over complex terrain for mesoscale models. The initialization is implemented specifically for the quasi-nonhydrostatic (QNH) model. This study presents the first real data application of the mesoscale BDI and the QNH model to simulate a mesoscale storm that produced heavy precipitation along the Colorado Front Range. In this study, the focus is on (i) smooth numerical solution over complex terrain, (ii) baroclinic instability associated with condensational heating and high mountains, and (iii) the simulation of orographic precipitation. Numerical results show that initial fields derived from BDI were smooth and evolved smoothly in the QNH model for 48 h. It is noteworthy that the smooth solution existed up to the lateral boundaries. During the 48-h simulation, the midtropospheric storm moved freely in and out of the limited-area domain as if there were no lateral boundaries. The mesoscale storm for northeast Colorado was initiated by the persistent upslope easterlies and strong upward motions that triggered heavy precipitation. The simulated precipitation amounts and pattern were in good agreement with those observed. In general, both the large-scale dynamic system and the mesoscale precipitation event evolved smoothly and accurately, which indicates the value of BDI and QNH for mesoscale weather prediction.

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A. E. MacDonald, J. L. Lee, and S. Sun

Abstract

A new mesoscale weather prediction model, called QNH, is described. It is characterized by a parameter that multiplies the hydrostatic terms in the vertical equation of motion. Models of this type are referred to generically as “quasi-nonhydrostatic.” The quasi-nonhydrostatic parameter gives the model a character that is essentially nonhydrostatic, but with properties that are theoretically thought to result in smoother, more accurate, and stable predictions. The model is unique in a number of other aspects, such as its treatment of lateral boundary conditions, the use of explicit calculation in the vertical direction, and the use of the bounded derivative theory for initialization. This paper reports on the design and test of the QNH model, which represents the first time the applicability of this type of model has been demonstrated for full-physics, mesoscale weather prediction. The dynamic formulation, discretization, numerical formulation, and physics packages of the model are described. The results of a comprehensive validation of the model are presented. The validation includes barotropic, baroclinic (Eady wave), mountain wave, tropical storm, and sea breeze tests. A simulation of a winter storm (with updated lateral boundary conditions) is presented, which shows that the model has significant skill in forecasting terrain-forced heavy precipitation. It is concluded that the QNH model may be valuable for mesoscale weather prediction.

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