Search Results

You are looking at 1 - 6 of 6 items for

  • Author or Editor: Richard L. Wobus x
  • All content x
Clear All Modify Search
Richard L. Wobus and Eugenia Kalnay

Abstract

In real time since 1990, the National Meteorological Center (NMC) has been running a system to predict the forecast skill of the medium-range forecasts produced by the NMC global spectral model. The predictors used are the agreement of an ensemble consisting of operational forecasts from various centers, the persistence in the forecast, and the amplitude of the anomalies. These predictors are used in a stepwise regression scheme, with the last 60 days used as training period, and the regional anomaly correlation of the 0000 UTC NMC global forecast is predicted from days 1 to 6. By far the most important predictor of skill is the agreement between the NMC global forecast started at 0000 UTC, out to 6 days, and four other 12-h “older” forecasts (Japan Meteorological Agency, United Kingdom Meteorological Office, and the European Centre for Medium-Range Weather Forecasts, as well as the average of the NMC forecast at 0000 UTC with the previous day's forecast). The other predictors have been selected to add to the predictive capability of the agreement alone, and together they quantify the factors that forecasters use subjectively when evaluating the available forecasts. These predictions are available to NMC forecasters on workstations and to outside users through the Internet.

The predictive ability of this system compares favorably with recent theoretical and experimental studies. The correlation between predicted and verifying forecast skill seems to be best in regions where forecast skin varies significantly. The seasonal variation in predicting the skill is small expect in the Tropics. The overall performance shows that these predictors include enough information about forecast skill to justify further development of skill predictions based on large forecast ensembles and on more sophisticated statistical techniques.

Full access
H. Jean Thiébaux, Lauren L. Morone, and Richard L. Wobus

Abstract

Results of a thorough study of the correlation structure of observation-minus-forecast increments for mandatory pressure level radiosonde observations of zonal and meridional wind components and geopotential, differenced with NMC's 6-hour global forecasts, are reported. Our work focused on the selection of a representation for spatial lag-correlations to be used in updating the multivariate statistical objective analysis algorithm of the global data assimilation system, with attention given to regional and seasonal dependence of the correlation structure, and on the degree to which the increments are in the same geostrophic balance as the signal and forecast fields individually.

We compare the performance of several candidates for representing autocorrelations of geopotential increments, on the one hand, and the auto- and cross-correlations of the wind component increments, on the other, for five mandatory pressure levels, for four regions of the Northern Hemisphere and for the Southern Hemisphere. A third-order auto-regressive correlation function is identified as optimal for NMC's global objective analysis. The parameters are shown to vary with level and season. Furthermore, the geopotential and wind correlation fits have identified important differences in corresponding parameter values. A single algorithm which covers the primary candidates in one fitting operation, for future semi-automatic updating, has been developed in the course of this work. Results of its use are presented and discussed.

Full access
Frederick H. Carr, Richard L. Wobus, and Ralph A. Petersen

Abstract

The Regional Analysis and Forecast System at the National Meteorological Center consists of an optimum interpolation objective analysis scheme, an adiabatic nonlinear normal model initialization (NNMI) and a hemispheric Nested Grid Model (NGM) to provide 48 h forecasts. We investigate here the effect NNMI has on the analyses and forecasts produced by this system. An eight vertical mode NNMI procedure led to significant reductions of the divergent component of the analyzed wind field in regions of heavy precipitation. This is shown to contribute to a systematic spinup error in NGM 0–12 h precipitation forecasts, especially from the 0000 UTC runs. Forecasts starting with no initialization had unacceptable noise levels. NNMI using two vertical modes yielded the best combination of noise-free forecasts and unsuppressed initial precipitation rates. A physical interpretation of this result is provided using the vertical structure functions of the normal modes. Tests of the two-mode NNMI in an operational setting confirmed that the 0–12 h NGM precipitation amounts increased along with a reduction of an excessive precipitation bias in the 12–36 h period. The two-mode procedure was implemented operationally in August 1987. It is suggested that if one has a reliable data assimilation system, especially if the 6 h forecasts used as background fields for the analysis have realistic precipitation rates and attendant divergent circulations, then a two-vertical-mode adiabatic NNMI provides many of the benefits of a traditional diabatic normal mode initialization procedure.

Full access
David D. Houghton, Ralph A. Petersen, and Richard L. Wobus

Abstract

Forecasts from different resolution versions of the National Meteorological Center Nested Grid Model (NGM) are compared for two case studies to assess an optimal ratio of model vertical and horizontal resolutions. Four combinations are considered: 1) 16 layers and 80-km horizontal grid over the United States (the operational version of the model), 2) 32 layers and 80-km horizontal grid, 3) 16 layers and 40-km horizontal grid, and 4) 32 layers and 40-km horizontal grid. Resolution impacts are evaluated for a number of weather system components such as extratropical cyclone evolution, baroclinic and frontal zone structure, jet-stream blow, moisture fields, and precipitation.

Resolution impacts for this limited sample are relatively small for synoptic-scale features such as the position of the extratropical cyclone and main jet-stream flows. Larger impacts are noted for smaller-scale horizontal structure and gradients, frontal zone associated circulations and hydrological cycle components. Vertical resolution enhancement effects on the NGM, which already has added resolution near the lower boundary, are less dramatic in the lower troposphere than those for horizontal resolution, but are important for defining upper-level frontal structures and circulations where the NGM's vertical structure is coarser. Conclusions concerning consistency of horizontal and vertical resolution impacts on baroclinic zone structure and spurious noise generation found in earlier studies with simpler models are confirmed and brought into perspective for comprehensive numerical models and operational weather prediction model applications for the two cases discussed. The effects of the improvements in small-scale forecast accuracy, however, are difficult either to generalize due to the limited number of case studies or to assess because of the lack of high-resolution verification information and evaluation techniques.

Full access
Ralph A. Petersen, Geoffrey J. DiMego, James E. Hoke, Kenneth E. Mitchell, Joseph P. Gerrity, Richard L. Wobus, Hann-Ming H. Juang, and Michael J. Pecnick

Abstract

The final set of changes to NMC's Regional Analysis and Forecast System (RAFS) is described. The changes include modifications to both the forecast model and the analysis model, as well as development of a Regional Data Assimilation System (RDAS). The forecast model changes were developed to correct a number of known deficiencies in the Nested Grid Model (NGM), while the RDAS development will allow the RAFS to take advantage of the new asynoptic data sets soon to be available. Several of the changes were implemented on 7 November 1990. The remaining changes (including the RDAS) are planned for implementation before mid 1991. Results from tests of the revised forecast model and the combined RDAS/NGM system are presented and discussed.

Full access
Geoffrey J. DiMego, Kenneth E. Mitchell, Ralph A. Petersen, James E. Hoke, Joseph P. Gerrity, James J. Tuccillo, Richard L. Wobus, and Hann-Ming H. Juang

Abstract

The recent implementation of changes to the National Meteorological Center's (NMC's) Regional Analysis and Forecast System (RAFS) is described. The changes include an expansion of the innermost grids of the nested-grid model (NGM) and the implementation of the Regional Data Assimilation System (RDAS). The new version of the forecast model and a 3-hourly RDAS analysis system were implemented on 7 August 1991. Some results from tests of the revised forecast model and the combined RDAS/NGM system are presented.

Full access