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The thunderstorm gust front is an important feature for both the maintenance and initiation of storms. Previous studies have shown that the thunderstorm outflow producing the gust front can be treated as an atmospheric density current to a good approximation. In this study, a new version of the density current speed equation, based on the surface pressure rise, is derived. This equation is shown to give much better results than other commonly used forms when applied to twenty previously reported gust front observations. A two-dimensional numerical model is used to investigate the dynamics of atmospheric density currents. Simulations with this model show the effects of the environmental wind relative to the storm and the wind shear on the propagation of the gust front and the depth of the thunderstorm outflow. The results of these simulations are discussed in terms of the conditions necessary for the gust front to remain in a position that is beneficial for the maintenance of the storm. Moist processes are included in the model and simulations are made to investigate atmospheric density current propagation through a moist atmosphere. The lifting that occurs during gust front passage is calculated and it is found that even when this lifting is sufficient to bring moist parcels above their level of free convection; deep convection is prohibited by other aspects of the circulation. The generation of an arc cloud by the gust front is found to result in a less intense and slower gust front compared to an outflow that did not produce an arc cloud. This result is interpreted in terms of the gust front speed equation based on the surface pressure rise.
Previous experiments had shown that upper-level wind flows could be used to advect surface weather parameters to produce short-range (0-15 hours) forecasts. However, to achieve scores better than persistence, allowance had to be made for stationary weather patterns and also for diurnal changes in weather conditions. Two new forecast experiments were prepared and carried out, using data from 12 cases during March 1983. First, data were edited and adjusted to reduce effects of local conditions (altitude, surface roughness), and then were advected. Finally, the adjustment was removed. The forecasts using a 500 mb space-averaged flow with modified initial conditions produced improved advection forecasts, with some parameters better than persistence and MOS (Model Output Statistics) for 2-7 hours. In the second experiment, an improved objective-analysis procedure was introduced, one based on the 'Barnes' approach, which uses one-half degree (about 45 km) resolution and previous analysis as a first guess. (Prior analyses were 1 degree, single pass, 'Cressman'-type analyses.) These improved analyses resulted in a somewhat better score for 1-3 hours (using a 'change-advection' technique), but were slightly worse at longer periods. Apparently, the small-scale patterns recovered by the improved analyses were largely either short-lived or stationary. These conditions would not lead to better advection forecasts. Further examination revealed that those parameters most difficult to resolve in the objective analyses (visibility, ceiling, and wind speed) also had the lowest forecast skill scores for persistence. Keywords: Aviation forecasting; Meteorology; Mesoscale analysis and forecasting.
February issue includes Appendix entitled Directory of United States Government periodicals and subscription publications; September issue includes List of depository libraries; June and December issues include semiannual index

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United States. National Environmental Satellite, Data, and Information Service. Library and Information Services Division

Published: 1983

Total Pages: 606


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The products judged most useful in preparing short-range terminal forecasts included: (1) conventional geographic data displays presented simultaneously as four quadrant panels on one screen, (2) regional scale surface analyses, plots and data listings of basic variables, (3) satellite-based trajectory technique, (4) tailored plot displays such as the station-model time-series display, and (5) mapped displays of forecast guidance derived from the NMC LFM model. The importance of half-hourly visible and IR imagery from GOES in short-range terminal forecasting was confirmed in this experiment. The forecasters relied more heavily on it to prepare their forecasts than any other data source. The manipulation of digital imagery in a computer-based interactive graphics system through time-series looping, color enhancements, and overlaying conventional plots and analyses on it, provides a wealth of qualitative and quantitative guidance for fore- casting. The numerical forecasts yielded superior rmses compared to persistence for all forecast intervals and for each forecast element. Probability forecasts were substantially better than persistence probability and sample climatology. GEM showed skill relative to persistence probability but yielded larger rmses did persistence in its numerical form. MOS guidance was found to be useful for forecast intervals of 4 hr or longer.