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- Author or Editor: S. A. Hsu x
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Abstract
Severe flash flood storms that occurred in Las Vegas, Nevada, on 8 July 1999, were unusual for the semiarid southwest United States because of their extreme intensity and the morning occurrence of heavy convective rainfall. This event was simulated using the high-resolution Regional Atmospheric Modeling System (RAMS), and convective rainfall, storm cell processes, and thermodynamics were evaluated using Geostationary Operational Environmental Satellite (GOES) imagery and a variety of other observations. The simulation agreed reasonably well with the observations in a large-scale sense, but errors at small scales were significant. The storm's peak rainfalls were overestimated and had a 3-h timing delay. The primary forcing mechanism for storms in the simulation was clearly daytime surface heating along mountain slopes, and the actual trigger mechanism causing the morning convection, an outflow from nighttime storms to the northeast of Las Vegas, was not captured accurately. All simulated convective cells initiated over and propagated along mountain slopes; however, cloud images and observed rainfall cell tracks showed that several important storm cells developed over low-elevation areas of the Las Vegas valley, where a layer of fairly substantial convective inhibition persisted above the boundary layer in the simulation. The small-scale errors in timing, location, rain amounts, and characteristics of cell propagation would seriously affect the accuracy of streamflow forecasts if the RAMS simulated rainfall were used in hydrologic models. It remains to be seen if explicit storm-scale simulations can be improved to the point where they can drive operationally useful streamflow predictions for the semiarid southwest United States.
Abstract
Severe flash flood storms that occurred in Las Vegas, Nevada, on 8 July 1999, were unusual for the semiarid southwest United States because of their extreme intensity and the morning occurrence of heavy convective rainfall. This event was simulated using the high-resolution Regional Atmospheric Modeling System (RAMS), and convective rainfall, storm cell processes, and thermodynamics were evaluated using Geostationary Operational Environmental Satellite (GOES) imagery and a variety of other observations. The simulation agreed reasonably well with the observations in a large-scale sense, but errors at small scales were significant. The storm's peak rainfalls were overestimated and had a 3-h timing delay. The primary forcing mechanism for storms in the simulation was clearly daytime surface heating along mountain slopes, and the actual trigger mechanism causing the morning convection, an outflow from nighttime storms to the northeast of Las Vegas, was not captured accurately. All simulated convective cells initiated over and propagated along mountain slopes; however, cloud images and observed rainfall cell tracks showed that several important storm cells developed over low-elevation areas of the Las Vegas valley, where a layer of fairly substantial convective inhibition persisted above the boundary layer in the simulation. The small-scale errors in timing, location, rain amounts, and characteristics of cell propagation would seriously affect the accuracy of streamflow forecasts if the RAMS simulated rainfall were used in hydrologic models. It remains to be seen if explicit storm-scale simulations can be improved to the point where they can drive operationally useful streamflow predictions for the semiarid southwest United States.
Abstract
Accurate summertime weather forecasts, particularly the quantitative precipitation forecast (QPF), over the semiarid southwest United States pose a difficult challenge for numerical models. Two case studies, one with typical weather on 6 July 1999 and another with unusual flooding on 8 July 1999, using the Regional Atmospheric Modeling System (RAMS) nested inside the regional Eta Model, were conducted to test numerical weather prediction capabilities over the lower Colorado River basin. The results indicate that the rapid changes in synoptic patterns during these two cases strongly affect the weather and rainfall situation in the basin. The model illustrates that the midlevel sinking over the low elevation of the southwest area of the basin “capped” the development of deep convection in case 1; meanwhile, in case 2, a shear line and convergence over the Las Vegas area valley stimulated intense convective storms in the region. In both cases, the low-level jet (LLJ) stream from the Gulf of California was the major source of atmospheric moisture for the basin. Local topography and thermodynamics also play a significant role in the formation of the weather features. The “thermal low” over the Sonoran Desert is responsible for the LLJ stream, which led to the valley of the Colorado River becoming the warmest and moistest area in the basin. By nesting fine-resolution grids over the Las Vegas area, the representation of local topography in the region was improved in the RAMS model, compared with that in the relatively coarse resolution Eta Model. This appears to be the major reason that the RAMS model could predict intense convective storms over Las Vegas, while the operational Eta forecast could not.
Abstract
Accurate summertime weather forecasts, particularly the quantitative precipitation forecast (QPF), over the semiarid southwest United States pose a difficult challenge for numerical models. Two case studies, one with typical weather on 6 July 1999 and another with unusual flooding on 8 July 1999, using the Regional Atmospheric Modeling System (RAMS) nested inside the regional Eta Model, were conducted to test numerical weather prediction capabilities over the lower Colorado River basin. The results indicate that the rapid changes in synoptic patterns during these two cases strongly affect the weather and rainfall situation in the basin. The model illustrates that the midlevel sinking over the low elevation of the southwest area of the basin “capped” the development of deep convection in case 1; meanwhile, in case 2, a shear line and convergence over the Las Vegas area valley stimulated intense convective storms in the region. In both cases, the low-level jet (LLJ) stream from the Gulf of California was the major source of atmospheric moisture for the basin. Local topography and thermodynamics also play a significant role in the formation of the weather features. The “thermal low” over the Sonoran Desert is responsible for the LLJ stream, which led to the valley of the Colorado River becoming the warmest and moistest area in the basin. By nesting fine-resolution grids over the Las Vegas area, the representation of local topography in the region was improved in the RAMS model, compared with that in the relatively coarse resolution Eta Model. This appears to be the major reason that the RAMS model could predict intense convective storms over Las Vegas, while the operational Eta forecast could not.
Abstract
Two-dimensional simulations of the 11 January 1972 Boulder, Colorado, windstorm, obtained from 11 diverse nonhydrostatic models, are intercompared with special emphasis on the turbulent breakdown of topographically forced gravity waves, as part of the preparation for the Mesoscale Alpine Programme field phase. The sounding used to initialize the models is more representative of the actual lower stratosphere than those applied in previous simulations. Upper-level breaking is predicted by all models in comparable horizontal locations and vertical layers, which suggests that gravity wave breaking may be quite predictable in some circumstances. Characteristics of the breaking include the following: pronounced turbulence in the 13–16-km and 18–20-km layers positioned beneath a critical level near 21-km, a well-defined upstream tilt with height, and enhancement of upper-level breaking superpositioned above the low-level hydraulic jump. Sensitivity experiments indicate that the structure of the wave breaking was impacted by the numerical dissipation, numerical representation of the horizontal advection, and lateral boundary conditions. Small vertical wavelength variations in the shear and stability above 10 km contributed to significant changes in the structures associated with wave breaking. Simulation of this case is ideal for testing and evaluation of mesoscale numerical models and numerical algorithms because of the complex wave-breaking response.
Abstract
Two-dimensional simulations of the 11 January 1972 Boulder, Colorado, windstorm, obtained from 11 diverse nonhydrostatic models, are intercompared with special emphasis on the turbulent breakdown of topographically forced gravity waves, as part of the preparation for the Mesoscale Alpine Programme field phase. The sounding used to initialize the models is more representative of the actual lower stratosphere than those applied in previous simulations. Upper-level breaking is predicted by all models in comparable horizontal locations and vertical layers, which suggests that gravity wave breaking may be quite predictable in some circumstances. Characteristics of the breaking include the following: pronounced turbulence in the 13–16-km and 18–20-km layers positioned beneath a critical level near 21-km, a well-defined upstream tilt with height, and enhancement of upper-level breaking superpositioned above the low-level hydraulic jump. Sensitivity experiments indicate that the structure of the wave breaking was impacted by the numerical dissipation, numerical representation of the horizontal advection, and lateral boundary conditions. Small vertical wavelength variations in the shear and stability above 10 km contributed to significant changes in the structures associated with wave breaking. Simulation of this case is ideal for testing and evaluation of mesoscale numerical models and numerical algorithms because of the complex wave-breaking response.
