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John Manobianco and Paul A. Nutter

Abstract

This paper describes a subjective evaluation of the National Centers for Environmental Prediction 29-km (Meso) Eta Model during the 1996 warm (May–August) and cool (October–January) seasons. The companion paper by Nutter and Manobianco presents results from an objective evaluation of the Meso Eta Model at three selected locations during the 1996 and 1997 warm and cool seasons. The overall evaluation is designed to assess the utility of the model for operational weather forecasting by the U.S. Air Force 45th Weather Squadron, National Weather Service (NWS) Spaceflight Meteorology Group, and NWS Office in Melbourne, Florida. In the subjective verification, limited case studies are used to highlight model capabilities and limitations in forecasting convective activity, the location and movement of cold fronts, and the onset of sea breezes over regions including east-central Florida. In addition, contingency tables and categorical scores are used to verify the occurrence of these phenomena throughout the season.

Results from the subjective verification demonstrate that model forecasts of developing weather events such as thunderstorms, sea breezes, and cold fronts are not always as accurate as might otherwise be implied by the seasonally averaged error statistics. Although the objective statistics do not indicate whether the model provides more accurate forecast guidance on average during either the warm or cool seasons, results from the subjective verification suggest that model forecasts over central Florida may be more useful during the cool season. This is because the Meso Eta Model resolution is not yet sufficient to resolve the small-scale details of sea and river/lake breeze circulations, thunderstorm outflow boundaries, and other phenomena, which play a dominant role in determining the short-term evolution of weather over east-central Florida during the warm season. Lessons learned from the subjective portion of the Meso Eta evaluation should apply equally as well to the recently upgraded “early” Eta Model running with a similar 32-km horizontal resolution.

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Paul A. Nutter and John Manobianco

Abstract

This paper describes an objective verification of the National Centers for Environmental Prediction 29-km Eta Model from May 1996 through January 1998. The evaluation was designed to assess the model’s surface and upper-air point forecast accuracy at three selected locations during separate warm (May–August) and cool (October–January) season periods. In order to enhance sample sizes available for statistical calculations, the objective verification includes two consecutive warm and cool season periods.

The statistical evaluation identified model biases that result from inadequate parameterization of physical processes. However, since the model biases are relatively small compared to the random error component, most of the total model error results from day-to-day variability in the forecasts and/or observations. To some extent, these nonsystematic errors reflect the variability in point observations that sample spatial and temporal scales of atmospheric phenomena that cannot be resolved by the model.

On average, Meso Eta point forecasts provide useful guidance for predicting the evolution of the larger-scale environment. A more substantial challenge facing model users in real time is the discrimination of nonsystematic errors that tend to inflate the total forecast error. It is important that users maintain awareness of ongoing model updates because they modify the basic error characteristics, particularly near the surface. While some of the changes in error were expected, others were not consistent with the intent of the model updates and further emphasize the need for ongoing sensitivity studies and localized statistical verification efforts.

Objective verification of point forecasts is a stringent measure of model performance, but when used alone, is not enough to quantify the overall value that model guidance may add to the forecast process. Therefore, results from a subjective verification of the Meso Eta Model over the Florida peninsula are discussed in the companion paper by Manobianco and Nutter.

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Weather Education for Disaster Recovery

Returning Control in a Time of Personal Crisis

Paul A. Nutter, Derek Gaarder, Jonathan Gunderson, and Curt Drennen
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Paul A. Nutter, Steven L. Mullen, and David P. Baumhefner

Abstract

The impact of initial condition uncertainty (ICU) on the onset and maintenance of eastern North Pacific blocking is examined within the framework of a general circulation model (GCM) and the perfect model assumption. Comparisons are made with the contrasting zonal flow regime. Twenty-member ensembles of perturbed simulations are run out to 15 days for the zonal flow, and for blocking at lead times of 8, 4, 2, and 0 days.

Blocking occurs in 95% of the 0-day lead simulations and declines monotonically to 65% for the 8-day lead simulations. The uncertainty in the exact time of onset among those simulations that form blocks also increases with lead time. The synoptic-scale features in both the blocking and zonal ensembles saturate, relative to climatological variance, and decorrelate (anomaly correlation coefficient < 0.5) by 6 days. The planetary-scale features, however, maintain skill relative to climatology beyond 10 days. The zonal simulations are generally the first to saturate and decorrelate, followed by simulations of blocking maintenance (0-day lead) and onset (2-, 4-, and 8-day lead), respectively. Thus, initial flows that project negatively (zonal flows) on the GCM’s Pacific–North American teleconnectivity pattern are more sensitive to ICU, and thus are less predictable than positive (blocking flows) projections.

While the results for this study demonstrate that error growth due to ICU ultimately imposes limits on the predictability of blocking, they also suggest that skillful ensemble predictions of transitions to a blocked state are possible at long lead times if the model error is held to a minimum. The majority of the perturbed simulations make the transition into a blocked state with an associated sustenance of skill even after the loss of skill in the synoptic-scale waves. The results are consistent with the hypothesis that the planetary-scale waves may need to be preconditioned toward the formation of blocking events. They also may, in part, help explain the poor performance of operational models in forecasts of blocking onset.

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