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Henry E. Fuelberg
and
James E. Hoke
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Richard A. Anthes
and
James E. Hoke

Abstract

The northward drift of a three-dimensional model hurricane is investigated by long-time numerical integrations of non-divergent and divergent barotropic vortices on a β-plane. Both vortices drifted in a general northward direction at speeds between 1.4 and 2.8 m s−1 during the first 48 hours, a movement similar to that of the model hurricane. An anticyclone developed east of the cyclone in both barotropic models. The combined circulation around the cyclone and anticyclone produced a spiral zone of confluence which extended southeast of the cyclone and resembled a slowly moving spiral band circulation found in the model hurricane experiment. Finally, the effect of closed lateral boundary conditions was to eventually turn the barotropic vortices toward the west.

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James E. Hoke
and
Richard A. Anthes

Abstract

A dynamic-initialization technique is tested with three models of fluid flow. In this technique data are assimilated through the inclusion of terms in the forecast equations which force the model atmosphere toward the observations. Results indicate that accurate, dynamically balanced mass and momentum fields can be obtained from unbalanced, inexact first guesses. In midlatitudes for horizontal scales less than 2000 km, observations of the wind are more important than observations of mass in producing a successful initialization for the systems presented here. There is also evidence that forcing the model winds toward the divergent wind component may slow the initialization process.

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Norman W. Junker
and
James E. Hoke

Abstract

The performance of the nested grid model (NGM) in predicting heavy rain is assessed for those cases in the cool season when moderato-to-strong low-level southerly inflow from the Gulf of Mexico is present. This study indicates that the NGM underpredicts precipitation maximum for heavier rainfall events, with the underprediction more common at 32°N than at 40°N. The NGM is also shown to have a slight slow bias in moving heavy precipitation bands to the east. Two case studies illustrate the model's difficulties in predicting heavy precipitation but also show that the NGM offers useful information in predicting major rainfall events. Several possible reasons for the NGM underprediction of heavy rainfall over the southern United States are presented.

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Ralph A. Petersen
and
James E. Hoke

Abstract

The response of the Regional Analysis and Forecast System (RAFS) low-level forecast fields to the geographical distribution of snow cover is discussed. The errors produced by an improper specification of this field in the forecast model can have a wide variety of local forecasting implications, ranging from poor forecasts of lee-side Great Lakes snowfalls, to errors in forecasts of the earth's surface temperature in areas where the snow cover is changing rapidly. The use of the snow-cover data in the forecast model and its effect on forecast guidance are described.

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James E. Hoke
and
Richard A. Anthes

Abstract

A technique of dynamic initialization previously studied with two-dimensional models is applied to a forecast of Hurricane Alma (1962) over a 24 h period. In the dynamic-initialization experiment, first-guess horizontal winds and the surface pressure in a four-layer primitive-equation model are driven, or relaxed, toward the observed fields during a 12 h preforecast integration, while the temperature and moisture fields are allowed to adjust freely. The observed data are successfully assimilated into the model during the dynamic initialization. A reasonable 12 h forecast is produced in terms of storm track and maintenance of storm structure.

The reduced dynamic imbalance at the beginning of the forecast utilizing dynamic initialization is beneficial. First of all, because the model state is not obscured by nonmeterological oscillations, this portion of the forecast is more useful than that of a forecast without dynamic initialization. Second, the meteorologically important features of the atmospheric circulation are not modified as much by a large mutual adjustment of the mass and momentum fields at the outset of the forecast. Also, the additional integration time in the dynamic-initialization experiment permits the small-scale forcing processes in the model to generate a finer scale precipitation band than present in the initial conditions.

The forecast of the storm track was slightly better during the first 6 h but was slightly worse by the end of the forecast, in the experiment with dynamic initialization, however. This latter result, together with the fact that these experiments pertain to only one case, makes the conclusion of the superiority of the dynamic-initialization scheme tentative.

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Norman W. Junker
,
James E. Hoke
, and
Richard H. Grumm

Abstract

This paper details the performance characteristics of the two regional dynamical models used at the National Meteorological Center to forecast for North America. Strengths and weaknesses of these models—the limited-area fine-mesh (LFM) model and the nested grid model (NGM) of the Regional Analysis and Forecast System (RAFS)—are presented in terms of their ability to predict such fields and features as 500-mb heights, surface lows and highs, precipitation events, and the diurnal cycle. The systematic characteristics of the models are emphasized.

Overall, the NGM was found to be more accurate than the LFM. Nevertheless, the LFM is a valuable forecast model because of its accuracy and longevity in providing operational guidance.

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William E. Gartner
,
James E. Hoke
,
Norman W. Junker
, and
Louis E. Wolf

Abstract

No Abstract Available

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James E. Hoke
,
Norman A. Phillips
,
Geoffrey J. Dimego
,
James J. Tuccillo
, and
Joseph G. Sela

Abstract

The three components of the Regional Analysis and Forecast System (RAFS) of the National Meteorological Center (NMC) are described. This system was implemented in March 1985 to supplement guidance from NMC's limited-area fine-mesh model (LFM), especially for precipitation forecasting. The three components of the RAFS are the regional optimum interpolation analysis, the Baer–Tribbia nonlinear normal mode initialization, and the nested grid model—a grid point, primitive-equation model in sigma coordinates. Postprocessing of model forecasts and plans for system improvement are also discussed.

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Norman W. Junker
,
James E. Hoke
,
Bruce E. Sullivan
,
Keith F. Brill
, and
Francis J. Hughes

Abstract

This paper assesses the performance of the National Meteorological Center (NMC) Nested-Grid Model (NGM) during a period from March 1988 through March 1990, and the NMC medium-range forecast model (MRF) in two 136-day tests, one during summer made up of two 68-day periods (19 July–25 September 1989 and 20 June–28 August 1990) and one during winter and early spring (12 December 1989–26 April 1990). Seasonal and geographical variations of precipitation bias and threat score are discussed for each model. Differences in model performance in predicting various amounts of precipitation are described.

The performance of the NGM and MRF varied by season, geographic area, and precipitation amount. The bias of the models varied significantly during the year. The NGM and MRF overpredicted the frequency of measurable precipitation (≥0.01 in.) across much of the eastern half of the United States during the warm season. Both models, however, underpredicted the frequency of ≥0.50-in. amounts across the South during the cool season.

The smooth orography in both models has a strong impact on the models’ precipitation forecasts. Each model overpredicted the frequency of heavier precipitation over the southern Appalachians, over portions of the Gulf-facing upslope areas east of the Rocky Mountains, and to the lee of the Cascade and Sierra ranges of the West. The NGM underpredicted the frequency of heavier amounts on the Pacific-facing windward side of the Cascade Range of Oregon and Washington.

Model performance also seems to be related to the synoptic situation. Threat scores were higher when the midlevel westerlies were more active, with the highest threat scores found north of the most frequent track of cyclones during the cool season.

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