Geoffrey J. Dimego

Changes to the Operational ''Early'' Eta Analysis / Forecast System at the National Centers for Environmental Prediction

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.

The Transformation of Tropical Storm Agnes into an Extratropical Cyclone. Part I: The Observed Fields and Vertical Motion Computations

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 Krishnamurtis (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 sto...

An Examination of the Frequency and Mean Conditions Surrounding Frontal Incursions into the Gulf of Mexico and Caribbean Sea

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...

The Regional Analysis and Forecast System of the National Meteorological Center

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 NMCs 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.

The Transformation of Tropical Storm Agnes into an Extratropical Cyclone. Part II: Moisture, Vorticity and Kinetic Energy Budgets

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. Advectio...

Four-Dimensional Variational Data Assimilation for the Blizzard of 2000

Abstract Four-dimensional variational (4DVAR) data assimilation experiments for the East Coast winter storm of 25 January 2000 (i.e., “blizzard of 2000”) were performed. This storm has received wide attention in the United States, because it was one of the major failures of the operational forecast system. All operational models of the U.S. National Weather Service (NWS) failed to produce heavy precipitation over the Carolina–New Jersey corridor, especially during the early stage of the storm development. The considered analysis cycle of this study is that of 0000 to 1200 UTC 24 January. This period was chosen because the forecast from 1200 UTC 24 January had the most damaging guidance for the forecasters at the National Weather Service offices and elsewhere. In the first set of experiments, the assimilation and forecast results between the 4DVAR and the operational three-dimensional variational (3DVAR) data assimilation method are compared. The most striking difference is in the accumulated precipitation...

A Case Study of the Sensitivity of the Eta Data Assimilation System

Abstract A case study is utilized to determine the sensitivity of the Eta Data Assimilation System (EDAS) to all operational observational data types used within it. The work described in this paper should be of interest to Eta Model users trying to identify the impact of each data type and could benefit other modelers trying to use EDAS analyses and forecasts as initial conditions for other models. The case study chosen is one characterized by strong Atlantic and Pacific maritime cyclogenesis, and is shortly after the EDAS began using three-dimensional variational analysis. The control run of the EDAS utilizes all 34 of the operational data types. One of these data types is then denied for each of the subsequent experimental runs. Differences between the experimental and control runs are analyzed to demonstrate the sensitivity of the EDAS system to each data type for the analysis and subsequent 48-h forecasts. Results show the necessity of various nonconventional observation types, such as aircraft data,...

The National Meteorological Center Regional Analysis System

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 obser...

The Weather Research and Forecasting Model: Overview, System Efforts, and Future Directions

AbstractSince its initial release in 2000, the Weather Research and Forecasting (WRF) Model has become one of the world’s most widely used numerical weather prediction models. Designed to serve both research and operational needs, it has grown to offer a spectrum of options and capabilities for a wide range of applications. In addition, it underlies a number of tailored systems that address Earth system modeling beyond weather. While the WRF Model has a centralized support effort, it has become a truly community model, driven by the developments and contributions of an active worldwide user base. The WRF Model sees significant use for operational forecasting, and its research implementations are pushing the boundaries of finescale atmospheric simulation. Future model directions include developments in physics, exploiting emerging compute technologies, and ever-innovative applications. From its contributions to research, forecasting, educational, and commercial efforts worldwide, the WRF Model has made a s...

Fine-Resolution 4DVAR Data Assimilation for the Great Plains Tornado Outbreak of 3 May 1999

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.