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Keith D. Sashegyi and Rangarao V. Madala

Abstract

The vertical-mode initialization procedure of Bourke and MeGregor is applied to a limited-area weather prediction model that is formulated in flux form and is shown to be successful in reducing the undesirable gravity-wave oscillations in integrations of the numerical model. Alternative boundary conditions are developed for the scheme so that the changes to the wind at the lateral boundaries of the model are consistent with the changes in the integrated mass divergence and vorticity over the domain. The convergence of the modified scheme is shown to be rapid for two different grids. For a grid with significant topography along the lateral boundaries, use of increased diffusion in the boundary zone is shown to negatively impact the convergence of the scheme. Model integrations are performed to show the effectiveness of the scheme with improved boundary conditions in removing the gravity-wave oscillations. The results are compared with the damping of the gravity waves in the boundary zone by the time-integration scheme and by different lateral boundary treatments. The influence of noisy boundary values is also tested.

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Keith D. Sashegyi and John E. Geisler

Abstract

A linear model of the steady response of a stratified fluid to isolated heat sources on a sphere is developed. The model is used to examine the response to diabatic heating associated with summer monsoon precipitation in India and to low-level diabatic heating along the northeast coast of Africa. In a laterally unbounded, spherical domain, the summer monsoon heat source forces a cross-equatorial meridional cell that is about half as strong as the main response feature, which is a cell oriented zonally and situated on the west side of the source. The imposition of a meridional wall concentrates the cross-equatorial flow in the meridional cell into a western boundary current. For representative summer monsoon heating the northward transport in this simple East African Jet is comparable to what is observed. The cross-equatorial flew pattern forced by low-level, diabatic heating along the African coast consists of a western boundary current near the equator that turns into a geostrophically balanced sea breeze in low latitudes away from the equator. The northward mass flux in this locally forced jet is about an order of magnitude smaller than that which is forced by the summer monsoon heat source.

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Simon W. Chang, Teddy R. Holt, and Keith D. Sashegyi

Abstract

A numerical study is conducted using the Naval Research Laboratory (NRL) limited-area model to study the evolution and structure of a rapidly intensifying marine cyclone observed during intensive observing period 4 (IOP 4; 4–5 January 1989) of the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) over the North Atlantic Ocean.

The single grid version of the NRL model used in the study has 16 layers in the vertical with a horizontal resolution of 1/3° longitude and 1/4° latitude. The primitive equation, hydrostatic model in sigma coordinates includes parameterized physics of cumulus convection, radiation, and the planetary boundary layer. The National Meteorological Center (NMC) Regional Analysis Forecast System (RAFS) analysis is used to provide the initial and boundary conditions.

Starting from the 0000 UTC 4 January RAFS initialization, the control model simulates the ensuing cyclogenesis, deepening the initial disturbance from 998 to 952 mb in 24 h. While the simulated cyclone is about 15 mb weaker than that observed, the simulation reproduced many of the well-documented observed features of the IOP 4 cyclone, such as the remarkable comma-shaped precipitation pattern, bent-back warm front, warm-core seclusion, and secondary cold front. Control model results show that (i) the strongest temperature and water vapor gradients are aligned with the warm front and secondary cold front, not the primary cold front, (ii) the major precipitation and strongest vertical motion are along the warm front and its bent-back extension, (iii) the cyclonic circulation is displaced well to the southwest of the triple point, and (iv) the cellular convection occurs behind the secondary cold front accompanied by extreme surface sensible and latent beat transfer with a total maximum flux exceeding 3000 W m−2 over the Gulf Stream approximately 100 km offshore of the Carolinas. A detailed analysis of model results is performed and is found to be in excellent agreement with available satellite and mesoscale observations.

Sensitivity experiments are also conducted to identify the importance of various dynamical and physical processes contributing to the rapid intensification. Results from sensitivity tests show that (i) the dynamic processes are more responsible for the rapid intensification and unique structure of the marine cyclone than the physical processes, (ii) both the sea surface heat transfer and the release of latent heat in clouds contribute positively to the cyclogenesis, (iii) physical processes combine to intensify the storm in a nonlinear fashion, and (iv) the formation of unique features associated with the IOP 4 storm such as the bent-back extension of the warm front, warm-core seclusion, and westward development of the low pressure center away from the triple point are not sensitive to physical processes.

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Frank H. Ruggiero, Keith D. Sashegyi, Rangarao V. Madala, and Sethu Raman

Abstract

A technique is described that adds diabatic forcing from stratiform precipitation to a vertical normal-mode initialization of a mesoscale model. The technique uses observed precipitation amounts and cloud-top height estimations with analyzed thermodynamic and kinematic fields to vertically distribute diabatic heating that arises from stratiform precipitation. Simulation experiments reveal the importance of incorporating this heating into the initialization. An adiabatic initialization recovered about 65%–75% of the maximum upward vertical motions, whereas a diabatic initialization, with respect to stratiform precipitation, recovered nearly all the original vertical motions. A real-data case study is presented using combined rain gauge-satellite precipitation analyses with cloud-top heights estimated from Geostationary Operational Environmental Satellite infrared brightness temperatures. The short-term precipitation forecasts from a diabatically initialized model, with respect to stratiform precipitation, demonstrate improvement over forecasts from an adiabatically initialized model.

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Frank H. Ruggiero, Keith D. Sashegyi, Rangarao V. Madala, and Sethu Raman

Abstract

A system for the frequent intermittent assimilation of surface observations into a mesoscale model is described. The assimilation begins by transforming the surface observations to model coordinates. Next, the lowest-level model fields of potential temperature, relative humidity, u and v component winds, and surface pressure are updated by an objective analysis using the successive correction approach. The deviations of the analysis from the first guess at the lowest model layer are then used to adjust the other model layers within the planetary boundary layer. The PBL adjustment is carried out by using the model's values of eddy diffusivity, which are nudged to reflect the updated conditions, to determine the influence of the lowest-layer deviations on the other model layers. Results from a case study indicate that the frequent intermittent assimilation of surface data can provide superior mososcale analyses and forecasts compared to assimilation of synoptic data only. The inclusion of the PBL adjustment procedure is an important part of generating the better forecasts. Extrapolation of the results here suggests that two-dimensional data can be successfully assimilated into a model provided there is a mechanism to smoothly blend the data into the third dimension.

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Dewey E. Harms, Rangarao V. Madala, Sethu Raman, and Keith D. Sashegyi

Abstract

Diabatic forcing has been incorporated into a nonlinear normal-mode initialization scheme to provide more realistic initial conditions and to alleviate the problem of the spinup time of the Naval Research Laboratory Limited-Area Numerical Weather Prediction Model. Latent heating profiles are computed from the observed rainfall and from the model-generated convective rainfall at locations where there were no observations. The latent heating is distributed in the vertical according to the cumulus convective parameterization scheme (Kuo scheme) of the model. The results of a case study from the Genesis of Atlantic Lows Experiment indicated that model spinup of forecast rainfall can be reduced when diabatic initialization with merging of heat and/or rain is used.

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Keith D. Sashegyi, Dewey E. Harms, Rangarao V. Madala, and Sethu Raman

Abstract

The successive correction scheme of Bratseth, which converges to optimum interpolation, is applied for the numerical analysis of data collected during the Genesis of Atlantic Lows Experiment. A first guess for the analysis is provided by a 12-h forecast produced by integrating a limited-area model from a prior coarse operational analysis. Initially, univariate analyses of the mass and wind fields are produced. To achieve the coupling of the mass and wind fields, additional iterations on the geopotential are performed by extrapolating the geopotential to grid points, using improving estimates of the geostrophic wind. This improved geostrophic wind is then used to update the geostrophic component of the initial univariate wind analysis. Use of a background forecast produces much improved mesoscale structures in the analysis. Enhanced gradients of the geopotential and larger wind shears are the result of the coupling of the mass and wind fields, particularly in regions of lower data density. Application of the vertical mode initialization scheme of Bourke and McGregor is used to diagnose the divergent component of the mesoscale circulations produced with the analysis scheme.

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Frank H. Ruggiero, Keith D. Sashegyi, Alan E. Lipton, Rangarao V. Madala, and Sethu Raman

Abstract

A satellite–model coupled procedure for assimilating geostationary satellite sounder data was adapted to a mesoscale analysis and forecast system jointly developed by the Naval Research Laboratory and the Air Force Research Laboratory. The coupled procedure involves the use of the model output fields as the first guess for the thermodynamic retrievals. Atmospheric thermodynamic profiles and ground temperatures were retrieved from observed radiances of the VISSR Atmospheric Sounder (VAS) on board the Geostationary Operational Environmental Satellite. The successive corrections objective analysis scheme in the mesoscale analysis and forecast system was modified to consider the horizontal spatial correlation of the satellite data. The procedure was tested using a wintertime case from the 1986 Genesis of Atlantic Lows Experiment project. The retrievals generated by the coupled method were modestly improved relative to independent stand-alone retrievals. Coupled analyses and forecasts of temperature and moisture fields compared favorably to forecasts from a control run without the VAS assimilation.

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