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Chung-Chieng A. Lai, Wen Qian, and Scott M. Glenn

The Institute for Naval Oceanography, in cooperation with Naval Research Laboratories and universities, executed the Data Assimilation and Model Evaluation Experiment (DAMÉE) for the Gulf Stream region during fiscal years 1991–1993. Enormous effort has gone into the preparation of several high-quality and consistent datasets for model initialization and verification. This paper describes the preparation process, the temporal and spatial scopes, the contents, the structure, etc., of these datasets.

The goal of DAMÉE and the need of data for the four phases of experiment are briefly stated. The preparation of DAMÉE datasets consisted of a series of processes: 1) collection of observational data; 2) analysis and interpretation; 3) interpolation using the Optimum Thermal Interpolation System package; 4) quality control and reanalysis; and 5) data archiving and software documentation.

The data products from these processes included a time series of 3D fields of temperature and salinity, 2D fields of surface dynamic height and mixed-layer depth, analysis of the Gulf Stream and rings system, and bathythermograph profiles. To date, these are the most detailed and high-quality data for mesoscale ocean modeling, data assimilation, and forecasting research. Feedback from ocean modeling groups who tested this data was incorporated into its refinement.

Suggestions for DAMÉE data usages include 1) ocean modeling and data assimilation studies, 2) diagnosis and theorectical studies, and 3) comparisons with locally detailed observations.

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Lance F. Bosart, Chung-Chieng Lai, and Robert A. Weisman

Abstract

This paper describes a case of unexpected weak cyclogenesis over the northwestern Gulf of Mexico from 16 to 19 September 1984 based upon manually prepared and European Centre for Medium-Range Weather Forecasts (ECMWF) gridded analyses. Noteworthy aspects of the case include: 1) upward of 50 cm of rain along the extreme southern coast of Texas and 2) the brief occurrence of minimal strength tropical-storm conditions in a weak baroclinic marine environment. A crucial antecedent condition to rainstorm formation was the creation of a low-level baroclinic zone over the northwestern Gulf of Mexico due to the southward advance of drier and slightly cooler air behind a cold front that penetrated into northeastern Mexico. Four factors were responsible for rainfall concentration along the coast: 1) a northward-moving 700-mb trough and embedded vorticity maximum in the easterlies over the western Gulf of Mexico, 2) an eastward-propagating upper-tropospheric disturbance in the midlatitude westerlies over the southern United States to the north of a subtropical ridge line over Texas and Louisiana, 3) the formation of a weak midtropospheric baroclinic zone over the extreme north-western Gulf of Mexico, along which cyclonic-vorticity advection by the thermal wind contributed to a favorable environment for deep convection and cyclogenesis, and 4) the existence and maintenance of a weak north-south-oriented baroclinic zone along the Mexican coast in the lower troposphere.

The coastal baroclinic zone was associated with a quasi-stationary axis of ascent that maximized at 700 mb and lay 200–300 km to the cast of a persistent band of frontogenesis along the Sierra Madre Oriental Mountains of Mexico. Frontogenesis (∼2−4×10−10°C m−1 s−1) was dominated by the twisting term as relatively cool air over coastal Mexico was forced to ascend in the 700-mb easterly flow, favoring the northward movement of an area of cyclonic vorticity along the coast.

The results from this study are compared and contrasted with a similar September (1979) heavy-rain event that also occurred in southern Texas. In both cases, cyclogenesis occurred in a weakly baroclinic environment with embedded convection concentrated along the boundary of the surface baroclinic zone. The paper concludes with a discussion of tropical-storm formation in a baroclinic environment. It is speculated that the apparent, but short-lived, minimal strength tropical-storm development in this case could not be sustained because of the absence of a significant upstream cyclonic-vorlicity maximum aloft, despite otherwise favorable indicators for tropical cyclogenesis.

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