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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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Louis W. Uccellini, Ralph A. Petersen, Paul J. Kocin, Keith F. Brill, and James J. Tuccillo

Abstract

A series of numerical simulations is presented for the February 1979 Presidents' Day cyclone in order to understand more fully the roles played by upper-level jet streaks the oceanic planetary boundary layer (PBL), and latent heat release in the development of a low-level jet (LU) and secondary cyclogenesis along the East Coast of the United States. Mesoscale model simulations with and without sensible and latent heating show that the diabatic processes, along with the jet streak circulation patterns, contribute to the enhancement of the low-level winds and the initial development of the coastal cyclone. However. none of the mechanisms acting alone is sufficient to yield a satisfactory simulation of the LIJ and secondary cyclogenesis. Furthermore, the model-based diagnostic analyses indicate that a synergistic interaction must exist among these processes to account for the substantial increase in the magnitude of the low-level winds and the decrease in the sea level pressure that mark the secondary cyclogenesis for this case.

The following sequence is derived from the model diagnostic study: 1) Temporally increasing divergence along the axis of an upper-tropospheric jet streak located near the crest of an upper-level ridge is associated with the development of an indirect circulation that spans the entire depth of the troposphere and is displaced to the anticyclonic side of the jet. The lower branch of the indirect circulation appears to extend northwestward from the oceanic PBL up sloping isentropic surfaces toward 700 mb over the Appalachian Mountains. 2) Sensible heating and associated moisture flux within the oceanic PBL warm and moisten the lower branch of the indirect circulation, enhancing precipitation rates and latent heat release west of the coastline. 3) The combination of a shallow direct circulation associated with a developing coastal front, sloping lower-tropospheric isentropic surfaces just to the west of the coastline, and latent heat release contributes to a vertical displacement of parcels within the lower branch of the indirect circulation as they cross the coastline. 4) The vertical displacement of the parcels in a baroclinic environment (in which the pressure gradient force changes with height) results in the rapid increase in the magnitude of the ageostrophic wind and associated unbalanced flow. This imbalance contributes to parcel acceleration resulting in the formation of a LLJ in the lower branch of the indirect circulation over a 2 to 4 h period. 5) The increasing wind speed associated with the developing LLJ is, in turn, responsible for an increase in the horizontal mass flux divergence in the entrance region of the LLJ. The increase in the mass flux divergence in the lower troposphere just above the boundary layer makes a significant contribution to the decreasing sea-level pressure that constitutes the initial development phase of the secondary cyclone along the coast.

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Robert M. Aune, James J. Tuccillo, Louis W. Uccellini, and Ralph A. Petersen

Abstract

Numerical experiments are conducted to assess the impact of incorporating temperature data from the VISSR Atmospheric Sounder (VAS) into a regional-scale numerical model using an assimilation technique developed by Gal-Chen. The technique uses a three-dimensional variational approach to combine the VAS observations with model temperature fields during the numerical integration. A nudging technique is also tested, whereby the model temperature field is constrained toward the VAS “updated” values during the assimilation cycle. Results of the experiments indicate that the Gal-Chen assimilation technique successfully combines actual VAS temperature observations with the dynamically balanced model fields without destabilizing the model during the assimilation cycle. Furthermore, increasing the temporal frequency of VAS temperature insertions during the assimilation cycle enhances the impact on the model forecast through successively longer forecast periods. These results support the conclusions of earlier experiments with simulated geostationary satellite data that show an increasing positive impact on numerical simulations as the insertion rate of the satellite-based temperature information is increased. Incorporating the nudging technique further enhances the impact of the VAS temperature data, permitting the model wind field to adjust to the updated temperature fields and increasing the impact on the VAS data through a longer portion of the model simulation.

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Michael L. Kaplan, John W. Zack, Vince C. Wong, and James J. Tuccillo

Abstract

The development of a comprehensive mesoscale atmospheric simulation system (MASS) is described in detail. The modeling system is designed for both research and real-time forecast applications. The 14-level numerical model, which has a 48 km grid mesh, can be run over most of North America and the adjacent oceanic regions. The model employs sixth-order accurate numerics, generalized similarity theory boundary-layer physics, a sophisticated cumulus parameterization scheme, and state of the art analysis and initialization techniques. Examples of model output on the synoptic and subsynoptic scales are presented for the AVE-SESAME I field experiment on 10–11 April 1979. The model output is subjectively compared to the observational analysis and the LFM II output on the synoptic scale. Subsynoptic model output is compared to analyses generated from the AVE-SESAME I data set.

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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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