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Autumnal Return of Tropical Air to the Gulf of Mexico's Coastal Plain

R. L. ThompsonNOAA/Houston Area National Weather Service Office, League City, Texas

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J. M. LewisNOAA/ERL/National Severe Storms Laboratory, Norman, Oklahoma

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R. A. MaddoxNOAA/ERL/National Severe Storms Laboratory, Norman, Oklahoma

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Abstract

The return of tropical air from the Gulf of Mexico is examined in the autumnal cool season. Results from the thermodynamic equilibrium model of Betts and Ridgway are used to calculate the equilibrium equivalent potential temperature (θ e ) over the gulf and the northwestern Caribbean Sea. With a climatological study as a backdrop, a case of severe weather outbreak in mid-November 1988 is analyzed with emphasis on the analysis of low-level θ e that flowed into the storm region from the Gulf of Mexico.

The primary results of the study are the following:

  1. The climatological distribution of equilibrium θ e over the gulf and the Caribbean in November serves as a useful tool for the analysis of the 1988 case study.

  2. Between 5 and 15 November 1988, equilibrium in the marine layer was established over the gulf due to the absence of any deep cold-air penetrations during this period.

  3. The high-valued θ e that streamed into the severe storm region on 15 November 1988 tracked from the Yucatán straits and the northwestern Caribbean over a three-day period.

  4. This air was able to maintain its high-θ e property because of an anomalously warm gulf.

  5. Significant increases in available energy for deep convection could have been anticipated by means of the upper bounds on coastal θ e predicted by the Betts and Ridgway formulation, which was supported by observations along the Texas coast.

Abstract

The return of tropical air from the Gulf of Mexico is examined in the autumnal cool season. Results from the thermodynamic equilibrium model of Betts and Ridgway are used to calculate the equilibrium equivalent potential temperature (θ e ) over the gulf and the northwestern Caribbean Sea. With a climatological study as a backdrop, a case of severe weather outbreak in mid-November 1988 is analyzed with emphasis on the analysis of low-level θ e that flowed into the storm region from the Gulf of Mexico.

The primary results of the study are the following:

  1. The climatological distribution of equilibrium θ e over the gulf and the Caribbean in November serves as a useful tool for the analysis of the 1988 case study.

  2. Between 5 and 15 November 1988, equilibrium in the marine layer was established over the gulf due to the absence of any deep cold-air penetrations during this period.

  3. The high-valued θ e that streamed into the severe storm region on 15 November 1988 tracked from the Yucatán straits and the northwestern Caribbean over a three-day period.

  4. This air was able to maintain its high-θ e property because of an anomalously warm gulf.

  5. Significant increases in available energy for deep convection could have been anticipated by means of the upper bounds on coastal θ e predicted by the Betts and Ridgway formulation, which was supported by observations along the Texas coast.

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