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Earl G. Droessler
,
John M. Lewis
, and
Thomas F. Malone
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John M. Lewis
,
Matthew G. Fearon
, and
Harold E. Klieforth

Herbert Riehl, known as the “father of tropical meteorology,” certainly made outstanding contributions to this field of study. Yet, when his oeuvre is examined retrospectively, there is strong evidence that his view was global and encompassed processes that cut across the latitudinal bands of the tropics, subtropics, and midlatitudes. His pathway into meteorology was unique as a Jewish man who immigrated to the United States from Germany in 1933—that point in time when the fascist regime in Germany gained significant power. Meteorology was not his first choice as a career, but circumstances related to imminent world war led him to the study of meteorology. He was inspired by his teaching and research experiences at the Institute of Tropical Meteorology in Puerto Rico during World War II. Further, he found his scientific calling in the milieu of “Rossby's School” at the University of Chicago (U of C) following the war. Particular attention is paid herein to his early work from the mid-1940s through the late 1950s while professor at the U of C—a period when he ventured into the relatively unknown field of tropical meteorology. The strength of his early research contributions along with his mastery of language and adeptness in scientific debate drew many first-rate students into the field. However, his unorthodox brand of mentorship and his hard-edged nature created challenges that are further examined through first-person verbal portraits or vignettes. This article explores in some detail the interaction between Riehl and one of his students, Joanne Simpson. Finally, Riehl's scientific legacy is discussed.

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Suomi

Pragmatic Visionary

John M. Lewis*
,
David W. Martin
,
Robert M. Rabin
, and
Hans Moosmüller

The steps on Verner Suomi's path to becoming a research scientist are examined. We argue that his research style—his natural interests in science and engineering, and his methodology in pursuing answers to scientific questions—was developed in his youth on the Iron Range of northeastern Minnesota, as an instructor in the cadet program at the University of Chicago (U of C) during World War II and as a fledgling academician at University of Wisconsin—Madison. We examine several of his early experiments that serve to identify his style. The principal results of this study are 1) despite austere living conditions on the Iron Range during the Great Depression, Suomi benefited from excellent industrial arts courses at Eveleth High School; 2) with his gift for designing instruments, his more practical approach to scientific investigation flourished in the company of world-class scientific thinkers at U of C; 3) his dissertation on the heat budget over a cornfield in the mid-1950s served as a springboard for studying the Earth–atmosphere energy balances in the space-age environment of the late 1950s; and 4) his design of radiometers—the so-called pingpong radiometer and its sequel, the hemispheric bolometer—flew aboard Explorer VI and Explorer VII in the late 1950s, and analysis of the radiances from these instruments led to the first accurate estimate of the Earth's mean albedo.

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Michael L. Kaplan
,
Ramesh K. Vellore
,
Phillip J. Marzette
, and
John M. Lewis

Abstract

This study focuses on the meso-α- and meso-β-scale manifestations of the latent-heat-induced reduction of windward-side blocking to two flood-producing precipitation events on the leeside of the Sierra Nevada. Two simulations were performed—one employing full microphysics [control (CTRL)] and a second in which the latent heating terms are turned off in the microphysics [no latent heating (NLH)]. The differences between the CTRL and NLH are consistent with upstream latent heating—the moist, divergent, and ascending flow dominates the leeside of the mountain range in the CTRL producing copious spillover precipitation while dry high-momentum/downslope-descending flow dominates the NLH simulation on the leeside. A comprehensive sequence of events for spillover precipitation is formulated as follows: 1) Ascent within the exit region of a polar jet streak develops in response to velocity divergence aloft. 2) This ascent phases with ascent from the windward-side flow to create a mesoscale region of heavy upslope precipitation. 3) The latent heat release from the upslope precipitation reduces the upstream static stability and blocking. 4) A mesoscale ridge in the thickness field builds in the upper troposphere and induces subgeostrophic flow in the jet’s exit region above the mountain range. 5) Adjustments to this ridge result in a cross-mountain midlevel jet that facilitates a river of midlevel moisture advected over to the leeside. 6) Stretching of moist isentropic surfaces in proximity to the plume of moisture fluxes causes destabilization on the leeside and formation of a leeside mesolow. 7) Boundary layer air accelerates into the leeside mesolow to form a mountain-parallel low-level flow.

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Michael L. Kaplan
,
Christopher S. Adaniya
,
Phillip J. Marzette
,
K. C. King
,
S. Jeffrey Underwood
, and
John M. Lewis

Abstract

The synoptic structure of two case studies of heavy “spillover” or leeside precipitation—1–2 January 1997 and 30–31 December 2005—that resulted in Truckee River flooding are analyzed over the North Pacific beginning approximately 7 days prior to the events. Several sequential cyclone-scale systems are tracked across the North Pacific, culminating in the strengthening and elongation of a polar jet stream’s deep exit region over northern California and Nevada. These extratropical cyclones separate extremely cold air from Siberia from an active intertropical convergence zone with broad mesoscale convective systems and tropical cyclones. The development of moisture surges resulting in leeside flooding precipitation over the Sierra Nevada is coupled to adjustments within the last wave in the sequence of cyclone waves. Stage I of the process occurs as the final wave moves across the Pacific and its polar jet streak becomes very long, thus traversing much of the eastern Pacific. Stage II involves the development of a low-level return branch circulation [low-level jet (LLJ)] within the exit region of the final cyclone scale wave. Stage III is associated with the low-level jet’s convergence under the upper-level divergence within the left exit region, which results in upward vertical motions, dynamic destabilization, and the development of mesoscale convective systems (MCSs). Stage IV is forced by the latent heating and subsynoptic-scale ridging caused by each MCS, which results in a region of diabatic isallobaric accelerations downstream from the MCS-induced mesoridge. During stage IV the convectively induced accelerating flow, well to the southeast of the upper-level jet core, organizes a midlevel jet and plume of moisture or midlevel atmospheric river, which is above and frequently out of phase with (e.g., southeast of) the low-level atmospheric river described in Ralph et al. ahead of the surface cold front. Stage V occurs as the final sequential midlevel river arrives over the Sierra Nevada. It phases with the low-level river, allowing upslope and midlevel moisture advection, thus creating a highly concentrated moist plume extending from near 700 to nearly 500 hPa, which subsequently advects moisture over the terrain.

When simulations are performed without upstream convective heating, the horizontal moisture fluxes over the Sierra Nevada are reduced by ∼30%, indicating the importance of convection in organizing the midlevel atmospheric rivers. The convective heating acts to accelerate the midlevel jet flow and create the secondary atmospheric river between ∼500 and 700 hPa near the 305-K isentropic surface. This midlevel moisture surge slopes forward with height and transports warm moist air over the Sierra Nevada to typically rain shadowed regions on the lee side of the range. Both observationally generated and model-generated back trajectories confirm the importance of this convectively forced rapid lifting process over the North Pacific west of the California coast ∼12 h and ∼1200 km upstream prior to heavy leeside spillover precipitation over the Sierra Nevada.

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