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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.
The products judged most useful in preparing short-range terminal forecasts included (1) a station model time series display, (2) conventional geographic data displays presented simultaneously as four quadrant panels guidance on one screen, (3) mesoscale surface objective analyses, and (4) a forecast guidance prodecure based on 2-D upper-air trajectories and sensible weather algorithms based on imagery from the GOES satellite. The importance of half-hourly visible and IR imagery from the GOES satellite in short-range terminal forecasting was confirmed in this experiment. The participating forecasters relied more heavily on it to prepare their forecasts than any other source. The manipulation of digital imagery in a computer-based interactive graphic system through time-series looping, color enhancements, and overlaying conventional plots and analyses on it, provides a wealth of qualitative and quantitative guidance for forecasting. The numerical forecasts yielded superior rms errors compared to persistence at all forecast intervals except 1 h. At 4 h, the improvement over persistence ranged from about 21 percent for wind forecasts to about 34 percent for total cloud amount, while the 6-h quantitative precipitation forecasts yielded a 39 percent improvement.
Skill-scores, relative to climatology, for some parameters such as ceiling/visibility and precipitation are much lower than others, such as minimum temperature and pressure gradients. Also, the skill-scores have been improving appreciably faster for forecasts of 36 h (and more) than for forecasts of 24 h (and less). At the shortest ranges, less than 12 h, skill-scores relative to persistence are rather low, with values of 0.0 to 0.5 as typical. Power spectra for wind, temperature, dew point, rainfall rate, cloud reflectivity, and extinction coefficient (inversely related to visibility) were computed for periods of 10 min to 20 days, using fall season data from northeast United States. Analyses of these spectra indicate some of the problems in forecasting. Wind, temperature, and dew point spectra all had considerably more power at periods longer than 24 h than did rainfall rate, cloud reflectivity, and extinction coefficient, which relates to differences in forecast skill-scores. The greatest contribution to change for 2- to 8-h forecasts comes from disturbances with periods of about 8 to 32 h. Disturbances with periods shorter than about 24 h are purposedly filtered from current operational numerical models, in order to improve performance over longer ranges. The disturbances filtered out may be relatively unimportant to wind and temperature forecasts but quiet important for cloud and precipitation forecasts. Disturbances with periods less than about 2 h cannot be adequately resolved temporally or spatially using current weather data, yet these disturbances have sufficient amplitude to contribute noise in the analyses of longer period disturbances.

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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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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.