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W. E. COBB, B. B. PHILLIPS, and P. A. ALLEE

Abstract

Atmospheric electric measurements were made during the summer of 1958 from the summit of Mt. Washburn in Yellowstone Park, Wyoming. These measurements included the electric field, the positive and negative electrical conductivities, the charge on individual raindrops, the size and charge of individual cloud droplets, and the corona discharge current from the earth' s surface associated with high electric fields beneath thunderstorms. Electric fields exceeding 600 v.cm.−1 were recorded. Specific results are presented and interpretations made of their significance.

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A. Cobb, A. Michaelis, S. Iacobellis, F. M. Ralph, and L. Delle Monache

Abstract

Atmospheric rivers (ARs) are responsible for intense winter rainfall events impacting the U.S. West Coast, and have been studied extensively during CalWater and AR Recon field programs (2014–20). A unique set of 858 dropsondes deployed in lines transecting 33 ARs are analyzed, and integrated vapor transport (IVT) is used to define five regions: core, cold and warm sectors, and non-AR cold and warm sides. The core is defined as having at least 80% of the maximum IVT in the transect. Remaining dropsondes with IVT > 250 kg m−1 s−1 are assigned to cold or warm sectors, and those outside of this threshold form non-AR sides. The mean widths of the three AR sectors are approximately 280 km. However, the core contains roughly 50% of all the water vapor transport (i.e., the total IVT), while the others each contain roughly 25%. A low-level jet occurs most often in the core and warm sector with mean maximum wind speeds of 28.3 and 21.7 m s−1, comparable to previous studies, although with heights approximately 300 m lower than previously reported. The core exhibits characteristics most favorable for adiabatic lifting to saturation by the California coastal range. On average, stability in the core is moist neutral, with considerable variability around the mean. A relaxed squared moist Brunt–Väisälä frequency threshold shows ~8%–12% of core profiles exhibiting near-moist neutrality. The vertical distribution of IVT, which modulates orographic precipitation, varied across AR sectors, with 75% of IVT residing below 3115 m in the core.

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John L. Beven II, Lixion A. Avila, Eric S. Blake, Hugh D. Cobb, and Richard J. Pasch

Abstract

The Best Track Change Committee of the National Hurricane Center evaluates proposed changes to the Hurricane Database (HURDAT) in the Atlantic and eastern North Pacific basins. In the companion paper, Gruskin documents a possible tropical cyclone that affected portions of the eastern United States on 27–28 June 2006 and proposes that it be added to HURDAT. The committee reviewed the aircraft, radar, rawinsonde, satellite, and surface data available on this system and found it to be a challenging and complex system. A reconnaissance aircraft flying in the system in real time failed to find a closed circulation before landfall, and kinematic parameters suggest the system was more likely to have the structure of an open wave, with any surface circulation at best being poorly defined. Because of the lack of conclusive evidence regarding the existence of a closed surface circulation before landfall, the committee has decided not to add this system to HURDAT as a tropical cyclone.

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Matthew Newman, Michael A. Alexander, Toby R. Ault, Kim M. Cobb, Clara Deser, Emanuele Di Lorenzo, Nathan J. Mantua, Arthur J. Miller, Shoshiro Minobe, Hisashi Nakamura, Niklas Schneider, Daniel J. Vimont, Adam S. Phillips, James D. Scott, and Catherine A. Smith

Abstract

The Pacific decadal oscillation (PDO), the dominant year-round pattern of monthly North Pacific sea surface temperature (SST) variability, is an important target of ongoing research within the meteorological and climate dynamics communities and is central to the work of many geologists, ecologists, natural resource managers, and social scientists. Research over the last 15 years has led to an emerging consensus: the PDO is not a single phenomenon, but is instead the result of a combination of different physical processes, including both remote tropical forcing and local North Pacific atmosphere–ocean interactions, which operate on different time scales to drive similar PDO-like SST anomaly patterns. How these processes combine to generate the observed PDO evolution, including apparent regime shifts, is shown using simple autoregressive models of increasing spatial complexity. Simulations of recent climate in coupled GCMs are able to capture many aspects of the PDO, but do so based on a balance of processes often more independent of the tropics than is observed. Finally, it is suggested that the assessment of PDO-related regional climate impacts, reconstruction of PDO-related variability into the past with proxy records, and diagnosis of Pacific variability within coupled GCMs should all account for the effects of these different processes, which only partly represent the direct forcing of the atmosphere by North Pacific Ocean SSTs.

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