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John M. Lewis

In the early 1930s, Heinz Lettau and Werner Schwerdtfeger made direct measurements of air motion in the lowest 4 km of the troposphere by using the manned free balloon as an instrumented platform. The experiment was motivated by Wilhelm Schmidt's and Ludwig Prandtl's work on Austausch (exchange) theory in the second and third decades of the twentieth century. As a prelude to investigating the Lettau–Schwerdtfeger experiment, historical developments that had bearing on the field program are reviewed. Following this review, the experiment is analyzed by 1) documenting the scientific goals, 2) discussing the strategy for data collection, 3) examining one flight in detail (the flight of 25 February 1934), and 4) summarizing results from the experiment. The paper ends with a retrospective view of Austausch theory.

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John M. Lewis

In the late 1960s, well before the availability of computer power to produce ensemble weather forecasts, Edward Epstein (1931–2008) developed a stochastic–dynamic prediction (SDP) method for calculating the temporal evolution of mean value, variance, and covariance of the model variables: the statistical moments of a time-varying probability density function that define an ensemble forecast. This statistical–dynamical approach to ensemble forecasting is an alternative to the Monte Carlo formulation that is currently used in operations. The stages of Epstein's career that led to his development of this methodology are presented with the benefit of his oral history and supporting documentation that describes the retreat of strict deterministic weather forecasting. The important follow-on research by two of Epstein's protégés, Rex Fleming and Eric Pitcher, is also presented.

A low-order nonlinear dynamical system is used to discuss the rudiments of SDP and Monte Carlo and to compare these approximate methods with the exact solution found by solving Liouville's equation. Graphical results from these various methods of solution are found in the main body of the paper while mathematical development is contained in an online supplement. The paper ends with a discussion of SDP's strengths and weaknesses and its possible future as an operational and research tool in probabilistic–dynamic weather prediction.

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SMAGORINSKY'S GFDL

Building the Team

John M. Lewis

Joseph Smagorinsky (1924–2005) was a forceful and powerful figure in meteorology during the last half of the twentieth century. He served as director of the Geophysical Fluid Dynamics Laboratory (GFDL) for nearly 30 yr (1955–83); and during his tenure as director, this organization substantially contributed to advances in weather forecasting and climate diagnostics/prediction. The purpose of this research is to explore Smagorinsky s philosophy of science and style of management which were central to the success of GFDL. Information herein comes from his early scientific publications, personal letters and notes in the possession of his family, several oral histories, and letters of reminiscence from scientists who worked within and outside GFDL.

The principal results of the study are that 1) early inspiration and development of Smagorinsky's scientific philosophy came from his contact with Jule Charney and Harry Wexler, 2) his doctoral dissertation ideally prepared him for appointment as director of the U.S. Weather Bureau's long-range numerical prediction project in 1955—the General Circulation Research Section (later renamed GFDL), 3) he masterfully assembled a team of researchers to attack the challenging problem of general circulation modeling, and 4) he exhibited an authoritarian style of rule tempered by protection of the scientists from disrupting outside influence while celebrating the elitism and esprit de corps that characterized the laboratory.

A list of Smagorinsky's management principles is found in the appendix. Several of these tenets have been interspersed in the main body of the paper in support of actions he took at GFDL.

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JOHN M. LEWIS

Abstract

Sasaki's variational analysis method is used to describe the subsynoptic surface conditions accompanying severe local storms. Observations are extracted from the network of surface stations that routinely report every hour. The variational analysis filters the observations by constraining the meteorological fields to satisfy a set of governing prognostic equations. The filtering is monotonic and is designed to admit space and time scales of the order of 500 km and 10 hr, respectively.

The analysis is applied to a severe storm situation on June 10, 1968. The development of an intense squall line from the incipient to mature stage is depicted by an index coupling vertical motion and surface moisture. The results demonstrate that dynamically consistent time continuity can be achieved by using the variational method.

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John M. Lewis

Abstract

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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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Vanda Grubišić
and
John M. Lewis

The Sierra Wave Project was the first post–World War II (WWII) mountain meteorology field experiment in the United States designed to study mountain lee-wave phenomena. In its concept, design, organization, and execution, this Air Force–funded project served as an important predecessor of modern mesoscale field experiments proving clearly that mesoscale phenomena could be studied effectively by combining high-density ground-based and airborne observations. In this historical overview of the Sierra Wave Project, we set the scientific motivations for the experiment in their historical context, examine the coupling of the Air Force interests with the sport of soaring and the science of meteorology in this experiment, and evaluate the impact of the observational and theoretical programs of the Sierra Wave Project on the meteorological and aviation communities. We also provide a link to the related past investigations of mountain waves and an outlook for the future ones.

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