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John M. Lewis

Abstract

The dynamical adjustment scheme of P.D. Thompson (1969) has been adapted to the two-parameter baroclinic model which has potential vorticity as the constraint. In contrast to Thompson's approach, which used a differential-difference form of the constraint in space-time, the governing equations are discretized. Analyses simulated from analytic functions and analyses derived at the National Meteorological Center (NMC) are used to test the adjustment procedure. The reduction in error variance is related to the characteristics of the analysis error and the consequences of discretization, i.e., truncation error in the constraint and associated Euler–Lagrange equations.

The principal results are as follows:

1) Significant reduction in mean square error of vorticity can be accomplished with systematic or random error sources when r = |V| Δts < 1, where |V| is the geostrophic advection speed, Δt is one-half the time interval between maps, and Δs is the spatial resolution along the steering contours.

2) The limit of error reduction is reached as r→0, and the limiting values obtained from experiment compare favorably with the theoretical results of Thompson.

3) Height fields that are post-processed from adjusted vorticities also exhibit reduced error variance.

4) Results from the two-parameter model indicate that the strategy of adjustment will be useful in assimilating a sequence of mean temperature (thickness) fields derived from the VISSR Atmospheric Sounder (VAS) which is to be carded on all GOES satellites during this decade.

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John M. Lewis

Abstract

The interaction between a squall line and its environment is examined by using the model of Ogura and Cho (1973). This model incorporates a continuous spectrum of cumulus clouds that are distinguished by their entrainment rates. Conversion of liquid water droplets into raindrops has been included in the cloud microphysical process, but the ice phase has been neglected. By virtue of the cloud spectrum, convective transport terms in the larger scale heat and moisture equations appear as functions of vertical mass flux within the clouds. Once the larger-scale distributions are determined from observations, the vertical mass flux can be found from the budget equations. The cloud populations, i.e., fractional area covered by each cloud category, and the cumulative rainfall rate are functions of this vertical mass flux.

A squall line observed in the National Severe Storms Laboratory (NSSL) network on 8 June 1966 is used to test the theory. This squall line encompassed approximately 10% of the area used in the budget calculations. Observed heat and moisture distributions in the larger scale environment of the squall line are explained in terms of the cumulus processes. A comparison between the theoretically-derived cloud population and observed population was made possible by the WSR-57 radar at NSSL. Cloud population was estimated using precipitation reflectivity data from hourly tilt sequences of this 10 cm radar. The observed and theoretical distribution of clouds compared favorably on 1) the relative frequency of tall clouds, and 2) total areal coverage by clouds.

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John M. Lewis

Abstract

Inaccuracy in the numerical prediction of the moisture content of return-flow air over the Gulf of Mexico continues to plague operational forecasters. At the Environmental Modeling Center/National Centers for Environmental Prediction in the United States, the prediction errors have exhibited bias—typically too dry in the early 1990s and too moist from the mid-1990s to present. This research explores the possible sources of bias by using a Lagrangian formulation of the classic mixed-layer model. Justification for use of this low-order model rests on careful examination of the upper-air thermodynamic structure in a well-observed event during the Gulf of Mexico Experiment. The mixed-layer constraints are shown to be appropriate for the first phase of return flow, namely, the northerly-flow or outflow phase. The theme of the research is estimation of sensitivity—change in the model output (at termination of outflow) in response to inaccuracies or uncertainties in the elements of the control vector (the initial conditions, the boundary conditions, and the physical and empirical parameters). The first stage of research explores this sensitivity through a known analytic solution to a reduced form of the mixed-layer equations. Numerically calculated sensitivity (via Runge–Kutta integration of the equations) is compared to the exact values and found to be most credible. Further, because the first- and second-order terms in the solution about the base state can be found exactly for the analytic case, the degree of nonlinearity in the dynamical system can be determined. It is found that the system is “weakly nonlinear”; that is, solutions that result from perturbations to the control vector are well approximated by the first-order terms in the Taylor series expansion. This bodes well for the sensitivity analysis. The second stage of research examines sensitivity for the general case that includes moisture and imposed subsidence. Results indicate that uncertainties in the initial conditions are significant, yet they are secondary to uncertainties in the boundary conditions and physical/empirical parameters. The sea surface temperatures and associated parameters, the saturation mixing ratio at the sea surface and the turbulent transfer coefficient, exert the most influence on the moisture forecast. Uncertainty in the surface wind speed is also shown to be a major source of systematic error in the forecast. By assuming errors in the elements of the control vector that reflect observational error and uncertainties in the parameters, the bias error in the moisture forecast is estimated. These bias errors are significantly greater than random errors as explored through Monte Carlo experiments. Bias errors of 1–2 g kg−1 in the moisture forecast are possible through a variety of systematic errors in the control vector. The sensitivity analysis also makes it clear that judiciously chosen incorrect specifications of the elements can offset each other and lead to a good moisture forecast. The paper ends with a discussion of research approaches that hold promise for improved operational forecasts of moisture in return-flow events.

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John M. Lewis

Abstract

The generation of a probabilistic view of dynamical weather prediction is traced back to the early 1950s, to that point in time when deterministic short-range numerical weather prediction (NWP) achieved its earliest success. Eric Eady was the first meteorologist to voice concern over strict determinism—that is, a future determined by the initial state without account for uncertainties in that state. By the end of the decade, Philip Thompson and Edward Lorenz explored the predictability limits of deterministic forecasting and set the stage for an alternate view—a stochastic–dynamic view that was enunciated by Edward Epstein.

The steps in both operational short-range NWP and extended-range forecasting that justified a coupling between probability and dynamical law are followed. A discussion of the bridge from theory to practice follows, and the study ends with a genealogy of ensemble forecasting as an outgrowth of traditions in the history of science.

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OOISHI'S OBSERVATION

Viewed in the Context of Jet Stream Discovery

John M. Lewis

Although aircraft encounters with strong westerly winds during World War II provided the stimulus for postwar research on the jet stream, Wasaburo Ooishi observed these winds in the 1920s. Ooishi's work is reviewed in the context of earlier work in upper-air observation and postwar work on the jet stream. An effort is made to reconstruct Ooishi's path to the directorship of Japan's first upper-air observatory by reliance on historical studies and memoirs from the Central Meteorological Observatory.

Archival records from Japan's Aerological Observatory have been used to document Ooishi's upper-air observations. The first official report from the observatory (written in 1926 and in the auxiliary language of Esperanto) assumes a central role in the study. In this report, data are stratified by season and used to produce the mean seasonal wind profiles. The profile for winter gives the first known evidence of the persistent strong westerlies over Japan that would later become known as the jet stream.

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John M. Lewis

Philip Thompson (1922–94) pioneered innovative approaches to weather analysis and prediction that blended determinism and probability. He generally posed problems in terms of simplified dynamics that were amenable to analytic solution. His preciseness in problem formulation and presentation in a forceful didactical manner are linked to his early home-schooling and experiences with a coterie of young intellectuals. Four of Thompson's contributions are examined with the intention of highlighting their impact on the current state of operational analysis and prediction.

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John M. Lewis

In the mid-1950s, amid heated debate over the physical mechanisms that controlled the known features of the atmosphere's general circulation, Norman Phillips simulated hemispheric motion on the high-speed computer at the Institute for Advanced Study. A simple energetically consistent model was integrated for a simulated time of approximately 1 month. Analysis of the model results clarified the respective roles of the synoptic-scale eddies (cyclones-anticyclones) and mean meridional circulation in the maintenance of the upper-level westerlies and the surface wind regimes. Furthermore, the modeled cyclones clearly linked surface frontogenesis with the upper-level Charney–Eady wave. In addition to discussing the model results in light of the controversy and ferment that surrounded general circulation theory in the 1940s–1950s, an effort is made to follow Phillips's scientific path to the experiment.

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John M. Lewis

The emigration of 11 young Japanese meteorologists to the United States following World War II is investigated. Their move is examined with the benefit of a historical backdrop that includes a study of the socioeconomic conditions in Japan and the education that they received at the University of Tokyo. Oral histories and letters of reminiscence from these scientists are used with standard source material to reconstruct the conditions of postwar Japan. The principal results of the study are that 1) these scientists were among the intellectual elite, because of the rigorous screening process in the Japanese educational system; 2) their scientific education was fundamentally grounded in traditional physics and a wide range of geophysical sciences; 3) they all experienced austere living conditions and poor job prospects in the war-torn Japanese economy; and 4) they made a strong scientific connection with U.S. researchers in the areas of numerical experimentation and numerical weather prediction, which facilitated their move to the United States.

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John M. Lewis

LeRoy Meisinger was a U.S. Weather Bureau meteorologist and aeronaut who worked vigorously to bring meteorology to the aid of aviation in the post–World War I period. He was killed at the age of 29 in a scientific ballooning accident that has been detailed in a companion paper by Lewis and Moore. Meisinger's personality and scientific profile are reconstructed by examination of his oeuvre, which contains research contributions augmented by popular articles in the magazines of the period.

Meisinger's personal characteristics were those of a quiet, scholarly man with strong interests in science, music, and art. His experiences as a Signal Corps weather officer during World War I inclined him toward a career in meteorology. While stationed at the Fort Omaha Balloon School, he became intrigued with the possibilities of using the free balloon as a platform for tracking air currents.

As a research meteorologist with the U.S. Weather Bureau after the war, Meisinger melded adventurous scientific ballooning with the more painstaking and arduous task of scrutinizing data from the limited upper-air network of kite stations. His principal research contribution was a form of differential analysis that extrapolated surface data to the 1- and 2-km levels by using climatological statistics from the upper-air network. The impressive line of research he pioneered at the bureau came to an immediate and abrupt end with his accidental death in 1924.

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John M Lewis

Meteorologist Carl-Gustaf Rossby is examined as a mentor. In order to evaluate him, the mentor–protégé concept is discussed with the benefit of existing literature on the subject and key examples from the recent history of science. In addition to standard source material, oral histories and letters of reminiscence from approximately 25 former students and associates have been used.

The study indicates that Rossby expected an unusually high degree of independence on the part of his protégés, but that he was exceptional in his ability to engage the protégés on an intellectual basis—to scientifically excite them on issues of importance to him. Once they were entrained, however, Rossby was not inclined to follow their work closely.

He surrounded himself with a cadre of exceptional teachers who complemented his own heuristic style, and he further used his influence to establish a steady stream of first-rate visitors to the institutes. In this environment that bristled with ideas and discourse, the protégés thrived.

A list of Rossby's protégés and the titles of their doctoral dissertations are also included.

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