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Curtis H. Marshall
,
Kenneth C. Crawford
,
Kenneth E. Mitchell
, and
David J. Stensrud

Abstract

On 31 January 1996, the National Centers for Environmental Prediction/Environmental Modeling Center (NCEP/EMC) implemented a state-of-the-art land surface parameterization in the operational Eta Model. The purpose of this study is to evaluate and test its performance and demonstrate its impacts on the diurnal cycle of the modeled planetary boundary layer (PBL). Operational Eta Model output from summer 1997 are evaluated against the unique observations of near-surface and subsurface fields provided by the Oklahoma Mesonet. The evaluation is partially extended to July 1998 to examine the effects of significant changes that were made to the operational model configuration during the intervening time.

Results indicate a severe positive bias in top-layer soil moisture, which was significantly reduced in 1998 by a change in the initialization technique. Net radiation was overestimated, largely because of a positive bias in the downward shortwave component. Also, the ground heat flux was severely underestimated. Given energy balance constraints, the combination of these two factors resulted in too much available energy for the turbulent fluxes of sensible and latent heat. Comparison of model and observed vertical thermodynamic profiles demonstrates that these errors had a marked impact on the model PBL throughout its entire depth. Evidence also is presented that suggests a systematic underestimation of the downward entrainment of relatively warmer, drier air at the top of the PBL during daylight hours.

Analyses of the monthly mean bias of 2-m temperature and specific humidity revealed a cool, moist bias over western Oklahoma, and a warm, dry bias over the eastern portion of the state. A very sharp transition existed across central Oklahoma between these two regimes. The sharp spatial gradient in both the air temperature and humidity bias fields is strikingly correlated with a sharp west–east gradient in the model vegetation greenness database. This result suggests too much (too little) latent heat flux over less (more) vegetated areas of the model domain.

A series of sensitivity tests are presented that were designed to explore the reasons for the documented error in the simulated surface fluxes. These tests have been used as supporting evidence for changes in the operational model. Specifically, an alternative specification for the soil thermal conductivity yields a more realistic ground heat flux. Also, the alternative thermal conductivity, when combined with a slight adjustment to the thermal roughness length, yields much better internal consistency among the simulated skin temperature and surface fluxes, and better agreement with observations.

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David J. Stensrud
,
Geoffrey S. Manikin
,
Eric Rogers
, and
Kenneth E. Mitchell

Abstract

The cold pool, a pool of evaporatively cooled downdraft air that spreads out horizontally along the ground beneath a precipitating cloud, is often a factor in severe weather and heavy precipitation events. Unfortunately, cold pools are not well sampled by the present observational network and are rarely depicted in numerical model initial conditions. A procedure to identify and insert cold pools into the 29-km Eta Model is developed and tested on seven cases during 1995. Results suggest that when the large-scale forcing is strong, the inclusion of cold pools produces only slight changes in the forecasts. However, for the one case in which the large-scale forcing is relatively weak, the inclusion of cold pools produces significant changes in many of the model fields. These initial results, while not conclusive, suggest that the incorporation of cold pools, and other mesoscale features, may be important to the improvement of numerical guidance for severe weather and heavy precipitation forecasting.

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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.

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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.

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