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Geoffrey J. DiMego

Abstract

The National Meteorological Center (NMC) Regional Optimum-Interpolation (ROI) analysis is described. The ROI is the analysis component of the Regional Analysis and Forecast System (RAFS) and is specially designed to provide initial conditions for the Nested Grid Model (NGM), the forecast component of the RAFS. The ROI is an attempt to overcome weaknesses in the Limited-area Fine Mesh (LFM) and Global Optimum-Interpolation (GOI) analysis systems, to provide the NGM with more detailed and balanced analyses and to do so in a three-dimensional and dynamically consistent manner.

Among the unique aspects are its hemispheric domain with regional-scale (150–200 km) horizontal resolution and improved vertical resolution (16 levels with the first 12 below 250 mb). The analysis is geostrophically coupled and is multivariate in geopotential and wind; it utilizes the same sigma coordinate as the NGM prediction model. All significant-level radiosonde data are now used, as well as many more of the surface observations. The first-guess fields are adjusted to an improved terrain representation and treated directly in the sigma coordinate. Single-level observations are combined in a simple “super observation” technique which averages nearby reports.

The first-guess error correlation functions are more peaked in response to the increased resolution of the analysis. The observation error standard deviations have also been reduced to account for decreased errors of representativeness. The degree of geostrophic coupling, the relative error levels of mass versus wind data and the scale of the horizontal correlation function are varied during the analysis at the uppermost levels and at the levels nearest the surface. The observed surface temperature is now used in the analysis of heights. Finally, in all stages of the analysis, there is an increased dependence on observation quality codes.

Verifications versus North American radiosonde data indicate a somewhat looser fit to the data than that of the LFM analyses, but the fit is tighter than the GOI. The fit of moisture data is better than either the LFM or the GOI analyses. Examples of analyses are shown with comparisons to the LFM analyses and first-guess fields. Although the regional analyses tend to underestimate wind speeds at 250 mb, they exhibit increased detail in the moisture and vorticity fields.

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Geoffrey J. DiMego and Lance F. Bosart

Abstract

This is the first of two papers dealing with the transformation of tropical storm Agnes (June 1972) into an extratropical cyclone. Synoptic analyses and vertical motion patterns are used to describe the behavior of Agnes over a five-day period subsequent to initial landfall during which time Agnes regained tropical storm strength prior to transforming into an extratropical cyclone and dissipating.

Input data for all calculations are obtained from optimum interpolation objective analyses with a resolution of 1° latitude-longitude in the horizontal and 100 mb in the vertical. A modified, 9-level version of Krishnamurti's (1968a) diagnostic balance model is discussed and applied to the data set to determine the fields of vertical motion.

Redevelopment of Agnes subsequent to initial landfall is the result of the spread of an area of appreciable cyclonic vorticity advection aloft over the periphery of the low-level circulation devoid of significant baroclinicity. Unlike a corresponding midlatitude storm development, however, the initial presence of a warm, moist, high vorticity environment at low levels greatly aids the development. Ascent induced by thermal advection becomes increasingly important as the circulation intensifies. It is responsible in large part for the observed cyclonic rotation of the total vertical motion ascent-descent dipole around the storm.

The unique aspect of the regeneration of Agnes is provided by diabatic (primarily latent heat release) heating in the area of heavy rains extending well north of Agnes to the east of the Appalachian mountains. In conjunction with ascent due to thermal (warm) advection in the lower troposphere and cyclonic vorticity advection in the upper troposphere, a quasi-stationary region is produced along the eastern slopes of the mountains favorable for height falls, low-level convergence and vorticity generation. Agnes, after regaining tropical storm strength over open water, responds to this favorable forcing by redeveloping westward and rapidly transforming into an extratropical cyclone.

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Geoffrey J. DiMego and Lance F. Bosart

Abstract

Moisture, vorticity and kinetic energy budgets are constructed to diagnose the transformation of tropical storm Agnes (June 1972) into an extratropical cyclone in this second of two papers on Agnes. The vertical motions and divergent wind components used in the computations are taken from the solution of the nonlinear balance model described in Part I [DiMego and Bosart (1982)]. The budget equations are formulated in a quasi-Lagrangian reference frame centered with respect to the moving surface cyclone for several storm volumes. The results are displayed spatially as well as in time-section format.

Synoptic-scale transport and moisture convergence dominate the moisture budget and all terms together define well the areas of observed precipitation. Both budget and model-computed precipitation, particularly the latter, underestimate the observed precipitation. The discrepancy is attributed to the sub-grid scale convective processes and model underestimation of the divergent wind components.

Advection of vorticity by the non-divergent wind in the upper troposphere, vertical advection and convergence in the middle troposphere and low-level convergence in the presence of intense precipitation are the principal source terms in the vorticity budget. Reintensification of Agnes to tropical storm strength and the subsequent transformation to an extratropical cyclone with a westward displacement of the surface center is a near-classic example of a Petterssen and Smebye (1971) type-B cyclone development with one crucial difference, namely, the presence of a tropical storm with its large in situ vorticity beneath a region of prominent cyclonic vorticity advection aloft. Cyclonic rotation of areas of vorticity generation by diabatic processes and destruction by thermal advection effects in the lower troposphere are consistent with the observed westward looping of the storm center accompanying transformation.

Near Agnes the prominent kinetic energy source is in situ generation through cross-contour flow, primarily by the non-divergent wind. The generation of kinetic energy during the reintensification stage of Agnes (18–20 W m2) is about 40% of that computed by Palmén (1958) in his study of the transformation of hurricane Hazel. Most of the generated energy is used to increase the storm kinetic energy content and offset dissipation. Horizontal flux convergence of kinetic energy is the dominant source term in regions surrounding Agnes where the generation is negative. Upscale exchange processes associated with convection near the storm center and anticyclonic conditions in the storm periphery, coupled with an underestimation of vertical motion and divergent wind components, are associated with positive residuals prior to transformation and occlusion.

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Deirdre M. Kann, Geoffrey J. DiMego, and Ralph A. Petersen

Abstract

No abstract available.

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Geoffrey J. DiMego, Lance F. Bosart, and G. William Endersen

Abstract

Maps of mean monthly frequency and duration of frontal incursions into the Gulf of Mexico and Caribbean Sea are presented for the 1965–72 period. The transition from the low-frequency regime of summer to the high-frequency regime of winter is quite sharp in the fall, occurring between September and October. A more gradual decrease in activity occurs in spring. During the cooler months, relative maxima in frequency exist over the western Gulf of Mexico and east of Florida, while an arch-shaped region of maximum duration extends northeastward from the Yucatan Peninsula into the central Gulf and then southeastward along the north coast of the Greater Antilles with a second maximum in the central Caribbean. The frequency and degree of penetration of cold fronts are directly related to topographic features and the position, strength and amplitude of the mid-latitude circulation.

Time-sections centered around the time of frontal passage are used to present mean data for three regions. Tropical stations experience veering winds, rising temperatures, falling pressures, and increasing moisture content in the 1000–700 mb layer as fronts approach the area in winter. The stability of the atmosphere decreases and the trade-wind inversion is lifted and weakens in intensity. After the passage of the front, cold advection, subsidence and ridging produce an abrupt reversal of all these trends. A composite-case study shows that the depth of cold air to the rear of the front decreases southward and is accompanied by the development of a low-level anticyclone over the Gulf coast. Ahead of the front, the combination of an inverted trough in the central Caribbean and the subtropical anticyclone in the Atlantic produces a return flow of warm, moist tropical air into mid-latitudes.

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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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Dusanka Zupanski, Milija Zupanski, Eric Rogers, David F. Parrish, and Geoffrey J. DiMego

Abstract

The National Centers for Environmental Prediction fine-resolution four-dimensional variational (4DVAR) data assimilation system is used to study the Great Plains tornado outbreak of 3 May 1999. It was found that the 4DVAR method was able to capture very well the important precursors for the tornadic activity, such as upper- and low-level jet streaks, wind shear, humidity field, surface CAPE, and so on. It was also demonstrated that, in this particular synoptic case, characterized by fast-changing mesoscale systems, the model error adjustment played a substantial role. The experimental results suggest that the common practice of neglecting the model error in data assimilation systems may not be justified in synoptic situations similar to this one.

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Eric Rogers, Geoffrey J. DiMego, Joseph P. Gerrity, Ralph A. Petersen, Brian D. Schmidt, and Deirdre M. Kann

Analyses and forecasts for the first 2 weeks of the Genesis of Atlantic Lows Experiment (GALE) are described. These fields were produced using the National Meteorological Center (NMC) Regional Analysis and Forecast System (RAFS). Two sets of analyses and forecasts were constructed: one using the NMC operational database only (Level IIIa), and one using the NMC data merged with high-density observations taken during GALE (Level IIIb).

During the first 14 days of GALE, supplemental data were collected throughout two Intensive Observing Periods (IOPs). Comparisons of the Level IIIa and IIIb analyses over the GALE observing region in the southeastern United States indicated a worsening of the geopotential height analysis at operational NWS rawinsonde sites using the supplemental IIIb data. This was caused by inconsistencies in the height measurements at the high-density GALE rawinsonde sites. Such patterns were not observed in the wind and temperature analyses.

During IOP No. 1, the Level IIIa and IIIb Nested Grid Model (NGM) forecasts were nearly identical. For IOP No. 2, one forecast cycle saw an improvement in the Level IIIb forecasts due to offshore GALE dropwindsonde data, while another was improved by the inclusion of late-arriving rawinsonde data in the IIIb analysis. The inland, high-density GALE soundings, however, had a negligible impact on NGM forecasts during the entire 12-day period.

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Eric Rogers, Thomas L. Black, Dennis G. Deaven, Geoffrey J. DiMego, Qingyun Zhao, Michael Baldwin, Norman W. Junker, and Ying Lin

Abstract

This note describes changes that have been made to the National Centers for Environmental Prediction (NCEP) operational “early” eta model. The changes are 1) an decrease in horizontal grid spacing from 80 to 48 km, 2) incorporation of a cloud prediction scheme, 3) replacement of the original static analysis system with a 12-h intermittent data assimilation system using the eta model, and 4) the use of satellite-sensed total column water data in the eta optimum interpolation analysis. When tested separately, each of the four changes improved model performance. A quantitative and subjective evaluation of the full upgrade package during March and April 1995 indicated that the 48-km eta model was more skillful than the operational 80-km model in predicting the intensity and movement of large-scale weather systems. In addition, the 48-km eta model was more skillful in predicting severe mesoscale precipitation events than either the 80-km eta model, the nested grid model, or the NCEP global spectral model during the March-April 1995 period. The implementation of this new version of the operational early eta system was performed in October 1995.

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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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