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  • Author or Editor: John M. Lewis x
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Charlie A. Crisp
and
John M. Lewis

Abstract

Return-flow events have been examined with the aid of a classification scheme that identifies each event with cold air masses that invade the Gulf during the cool season (February-March). These air masses were classified as either continental polar (cP), maritime polar (mP), or a mix of two or more of these basic types (MIX in future reference). Each event was viewed as a cycle in which the first phase represented an offshore flow typifying the cold-air outbreak over the Gulf and the second phase was associated with the return of modified air to the continent. Surface data for a 12-yr period, 1978–89, were used to make a statistical analysis of the event and each of its phases. The principal results of the study are 1) a total of 127 events occurred in this cool season over the 12-yr period. The relative percentages of mP, cP, and MIX air masses are 28%, 20%, and 52%, respectively. A median of 10.5 return-flow events occurred in the cool season where the MIX category was the dominant regime. The median duration for a return-flow cycle is 3,3, 5.2, and 6.2 days for mP, cP, and MIX, respectively, for the cool season. 2) The median duration of the offshore-flow phase for the cool season shows a wide range depending on airmass type with 30, 55, and 49 h as median times for mP, cP, and MIX, respectively. 3) The median duration of the return-flow phase for the cool season was significantly longer than the offshore-flow phase when all cases were examined en masse; but when the cases were segregated according to airmass type, the duration of the return flow for the cool season exhibited a wide range with 47, 57, and 62 h as median times for mP, cP and MIX, respectively.

In order to view the return-flow events in the Gulf of Mexico from a wider perspective, a historical summary of research on this event and similar events around the world is included.

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John M. Lewis
and
Charlie A. Crisp

Abstract

A return-flow case study is examined with the benefit of an unprecedented set of observations obtained during the Gulf of Mexico Experiment (GUFMEX). This case represents the return of modified continental air to the coastal plain in mid-February, and the work is designed to complement the classificatory study of return flow that is found in the companion paper by Crisp and Lewis.

Surface air trajectories are combined with land- and ocean-based upper-air data to methodically follow the airmass modification process from the exit point off the Gulf coastal plain to its subsequent entry point on land. Upper-air data from the U.S. Coast Guard (USCG) ship Salvia are especially valuable in this tracking process, but data from an oil platform at the edge of the continental shelf, as well as special onshore observations, significantly contribute to a macroscopic tracking of air involved in the return flow.

Results indicate that the warming and moistening process is complicated and requires careful assessment of both air-sea interaction processes and larger-scale vertical motion. The principal results are 1) the thermodynamic character of the returning air mass exhibits significant differences along the entire Gulf coast, and 2) the mixed-layer modeling theory appears to account for the warming and moistening processes for air in the central Gulf that tracks over the Loop Current. The processes determining the character and stratification of the air mass become very complicated, however, as the air approaches neutrally stable conditions and begins its northward track back toward land.

The paper concludes with a synopsis of the airmass modification process built upon a composite chart that combines analyses from the various observational platforms.

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John M. Lewis
and
Thomas H. Grayson

Abstract

The sea level pressure and surface wind fields operationally produced at Fleet Numerical Weather Central are adjusted by using numerical variational analysis. The analysis region is a global band extending from 40S to 6ON and the successive corrections method is used to generate the initial or input fields. These fields are then adjusted within the framework of the variational method by requiring that they satisfy certain governing dynamical equations.

A detailed study of this method is made in the Atlantic Ocean region on 4 January 1971. There is convincing evidence that small-scale wind information is incorporated into the pressure field and that the adjusted wind field has been modified to account for ageostrophic motion.

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John M. Lewis
and
Lee Panetta

Abstract

P.D. Thompson devised a scheme to correct imperfect analyses of a conservative quantity at two observation times. His scheme has been extended to include a sequence of observation times. When the times are equally spaced, the governing adjustment equations simplify to an equation in one variable, a weighted average of the conservative property at the various times. The weights are found from Pascal' rule. The primary advantage of adding more observation times is to reduce the mean square error in the analyses. The limiting value of mean square error reduction is ½,⅓¾,…,(k/k for 2,3,4,…k times, respectively. The applicability of this method to adjustment of a sequence of mean temperature (thickness) fields from the VISSR Atmospheric Sounder (VAS) is discussed.

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Qingfu Liu
,
John M. Lewis
, and
Jeanne M. Schneider

Abstract

The evolution of the mean characteristics of the marine boundary layer during cold-air outbreaks can be described with an integrated or slab model. In order to assess the practical applicability of this-type of model to flows over the Gulf of Mexico, we use the observations collected during the Gulf of Mexico Experiment (GUFMEX) by an instrumented National Oceanic and Atmospheric Administration (NOAA) P-3 aircraft and a Cross-chain Loran Atmospheric Sounding System (CLASS) onboard the U.S. Coast Guard vessel Salvia. The numerical results show that the model successfully reproduced the changes in mean characteristics of momentum, moisture, and temperature under unstable conditions. The largest differences between the predictions and measurements are 0.8°C for the potential temperature, 0.15 g kg−1 for the specific humidity, 47 m for the mixed-layer height, and 1.5 m s−1 for the horizontal velocity components. A sensitivity analysis shows that the modeled mixed-layer height is slightly sensitive to changes in the specified sea surface temperature, while the other mean characteristics are relatively insensitive to the input parameters. Based upon the results of this single case study, the slab model appears to be a promising approach to account for the moistening and heating processes at the air-sea interface during cold-air outbreaks over the Gulf of Mexico.

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