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Numerical Simulations Initialized with Radar-Derived Winds. Pail II: Forecasts of Three Gust-Front Cases

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  • 1 National Center for Atmospheric Research, Boulder, Colorado
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Abstract

Numerical simulations of three gust-front cases that occurred in northeastern Colorado during the summers of 1991 and 1992 am presented. The simulations are initialized with radar-derived winds and, for the two cases in 1992, measurements from a surface mesonet. Thermodynamic retrieval is used to calculate the buoyancy in the boundary layer. The sensitivity of the retrieved buoyancy to the various constraints of real data was examined in Part I of this study.

In the first case, a large-scale gust front moved southward over the Denver region at a speed of 8–9 m s−1. The retrieved buoyancy field for this case exhibits a broad baroclinic zone, with a width of approximately 20 km centered about the radar-detected fine line. This baroclinic zone collapses to a width of about 5 km as the numerical model is integrated forward. The simulated gust front propagates at 7 m s−1, which is slightly less than the observed speed.

For the second and third cases, data from a 50-station surface mesonet were also available. In the second case, two gust fronts converged in the region of Mile High Radar but failed to generate significant convection. In the third case, three gust fronts converged and generated strong convection. Numerical simulations for both of these cases using surface and radar-derived winds are presented.

A verification analysis is performed on the forecasts of the two cases from 1992. Both surface observations and analyzed convergence fields are used to verify the forecasts. For the two cases, the numerical forecast of surface winds at 60 min improved over persistence by an average of 30%. The forecast surface convergence and temperature fields improved over persistence by an average of 25% and 28%, respectively.

Abstract

Numerical simulations of three gust-front cases that occurred in northeastern Colorado during the summers of 1991 and 1992 am presented. The simulations are initialized with radar-derived winds and, for the two cases in 1992, measurements from a surface mesonet. Thermodynamic retrieval is used to calculate the buoyancy in the boundary layer. The sensitivity of the retrieved buoyancy to the various constraints of real data was examined in Part I of this study.

In the first case, a large-scale gust front moved southward over the Denver region at a speed of 8–9 m s−1. The retrieved buoyancy field for this case exhibits a broad baroclinic zone, with a width of approximately 20 km centered about the radar-detected fine line. This baroclinic zone collapses to a width of about 5 km as the numerical model is integrated forward. The simulated gust front propagates at 7 m s−1, which is slightly less than the observed speed.

For the second and third cases, data from a 50-station surface mesonet were also available. In the second case, two gust fronts converged in the region of Mile High Radar but failed to generate significant convection. In the third case, three gust fronts converged and generated strong convection. Numerical simulations for both of these cases using surface and radar-derived winds are presented.

A verification analysis is performed on the forecasts of the two cases from 1992. Both surface observations and analyzed convergence fields are used to verify the forecasts. For the two cases, the numerical forecast of surface winds at 60 min improved over persistence by an average of 30%. The forecast surface convergence and temperature fields improved over persistence by an average of 25% and 28%, respectively.

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