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R. H. White, J. M. Wallace, and D. S. Battisti

Abstract

The impact of global orography on Northern Hemisphere wintertime climate is revisited using the Whole Atmosphere Community Climate Model, version 6 (WACCM6). A suite of experiments explores the roles of both resolved orography and the parameterized effects of unresolved orographic drag (hereafter parameterized orography), including gravity waves and boundary layer turbulence. Including orography reduces the extratropical tropospheric and stratospheric zonal mean zonal wind U¯ by up to 80%; this is substantially greater than previous estimates. Ultimately, parameterized orography accounts for 60%–80% of this reduction; however, away from the surface most of the forcing of U¯ by parameterized orography is accomplished by resolved planetary waves. We propose that a catalytic wave–mean-flow positive feedback in the stratosphere makes the stratospheric flow particularly sensitive to parameterized orography. Orography and land–sea contrast contribute approximately equally to the strength of the midlatitude stationary waves in the free troposphere, although orography is the dominant cause of the strength of the Siberian high and Aleutian low at the surface and of the position of the Icelandic low. We argue that precisely quantifying the role of orography on the observed stationary waves is an almost intractable problem, and in particular should not be approached with linear stationary wave models in which U¯ is prescribed. We show that orography has less impact on stationary waves, and therefore on U¯, on a backward-rotating Earth. Last, we show that atmospheric meridional heat transport shows remarkable constancy across our simulations, despite vastly different climates and stationary wave strengths.

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R. H. White, D. S. Battisti, and G. H. Roe

Abstract

The impacts of Asian orography on the wintertime atmospheric circulation over the Pacific are explored using altered-orography, semi-idealized, general circulation model experiments. The latitude of orography is found to be far more important than height. The Mongolian Plateau and nearby mountain ranges, centered at ~48°N, have an impact on the upper-level wintertime jet stream that is approximately 4 times greater than that of the larger and taller Tibetan Plateau and Himalayas to the south. Key contributing factors to the importance of the Mongolian mountains are latitudinal variations in the meridional potential vorticity gradient and the strength of the impinging wind—both of which determine the amplitude of the atmospheric response—and the structure of the atmosphere, which influences the spatial pattern of the downstream response. Interestingly, while the Mongolian mountains produce a larger response than the Tibetan Plateau in Northern Hemisphere winter, in April–June the response from the Tibetan Plateau predominates. This result holds in two different general circulation models. In experiments with idealized orography, varying the plateau latitude by 20°, from 43° to 63°N, changes the response amplitude by a factor of 2, with a maximum response for orography between 48° and 53°N, comparable to the Mongolian mountains. In these idealized experiments, the latitude of the maximum wintertime jet increase changes by only ~6°. It is proposed that this nearly invariant spatial response pattern is due to variations in the stationary wavenumber with latitude leading to differences in the zonal versus meridional wave propagation.

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D. E. Hagen, J. Schmitt, M. Trueblood, J. Carstens, D. R. White, and D. J. Alofs

Abstract

A systematic series of condensation coefficient measurements of water have been made using the University of Missouri—Rolla cooled-wall expansion chamber which simulates the thermodynamics of cloud. This coefficient is seen to decrease from a value near unity, at the outset of simulation, to a value in the neighborhood of 0.01 toward the end of a simulation. Final values of this coefficient are sufficiently low as to contribute significantly to the broadening of the drop-size distribution in cloud.

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D. J. Alofs, M. B. Trueblood, D. R. White, and V. L. Behr

Abstract

Nucleation experiments with monodisperse NaCl aerosols showed good agreement with the Köhler theory relating the critical super-saturation Sc to the dry size. Aerosols produced by condensing NaCl showed the same Sc as those produced by evaporating aqueous NaCl solution droplets. This indicates that if there is an energy barrier in going from a dry NaCl particle to a solution droplet, this energy barrier is small. The fact that the evaporation aerosol particles are cubical crystals and the condensation aerosols are amorphous spheres is shown to make no difference in the nucleation threshold.

The investigation also gives insights into the performance of the equipment used, especially the commercial electrostatic aerosol classifier and the vertical flow thermal diffusion chamber developed in this laboratory. When operating this chamber in the isothermal mode, a 36% upper limit was found on the uncertainty in Sc due to index of refraction sensitivity in sizing the water drops. Within this range of uncertainty, the isothermal mode data agreed with the Köhler theory.

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R. M. White, D. S. Cooley, R. C. Derby, and F. A. Seaver

Abstract

The design of efficient linear statistical operators for the 24-hour prediction of the sea-level pressure distribution over the United States is considered. Factor analysis techniques for reduction and selection of independent variables in regression analysis are used as a means of obtaining efficient statistical forecasting equations. The effects of the variations in data density in time and space, and the extent of geographical coverage upon the explained variance of the sea-level pressure are examined.

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A. E. White, R. M. Letelier, K. M. Björkman, E. Grabowski, S. Poulos, B. V. Watkins, and D. M. Karl
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EXECUTIVE COMMITTEE, R. G. Fleagle, R. E. Hallgren, R. M. White, J. Simpson, G. R. Hilst, D. S. Johnson, K. C. Spengler, and D. F. Landrigan
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F. M. Ralph, K. A. Prather, D. Cayan, J. R. Spackman, P. DeMott, M. Dettinger, C. Fairall, R. Leung, D. Rosenfeld, S. Rutledge, D. Waliser, A. B. White, J. Cordeira, A. Martin, J. Helly, and J. Intrieri

Abstract

The variability of precipitation and water supply along the U.S. West Coast creates major challenges to the region’s economy and environment, as evidenced by the recent California drought. This variability is strongly influenced by atmospheric rivers (ARs), which deliver much of the precipitation along the U.S. West Coast and can cause flooding, and by aerosols (from local sources and transported from remote continents and oceans) that modulate clouds and precipitation. A better understanding of these processes is needed to reduce uncertainties in weather predictions and climate projections of droughts and floods, both now and under changing climate conditions.

To address these gaps, a group of meteorologists, hydrologists, climate scientists, atmospheric chemists, and oceanographers have created an interdisciplinary research effort, with support from multiple agencies. From 2009 to 2011 a series of field campaigns [California Water Service (CalWater) 1] collected atmospheric chemistry, cloud microphysics, and meteorological measurements in California and associated modeling and diagnostic studies were carried out. Based on the remaining gaps, a vision was developed to extend these studies offshore over the eastern North Pacific and to enhance land-based measurements from 2014 to 2018 (CalWater-2). The dataset and selected results from CalWater-1 are summarized here. The goals of CalWater-2, and measurements to date, are then described.

CalWater is producing new findings and exploring new technologies to evaluate and improve global climate models and their regional performance and to develop tools supporting water and hydropower management. These advances also have potential to enhance hazard mitigation by improving near-term weather prediction and subseasonal and seasonal outlooks.

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F. M. Ralph, M. Dettinger, D. Lavers, I. V. Gorodetskaya, A. Martin, M. Viale, A. B. White, N. Oakley, J. Rutz, J. R. Spackman, H. Wernli, and J. Cordeira
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F. M. Ralph, S. F. Iacobellis, P. J. Neiman, J. M. Cordeira, J. R. Spackman, D. E. Waliser, G. A. Wick, A. B. White, and C. Fairall

Abstract

Aircraft dropsonde observations provide the most comprehensive measurements to date of horizontal water vapor transport in atmospheric rivers (ARs). The CalWater experiment recently more than tripled the number of ARs probed with the required measurements. This study uses vertical profiles of water vapor, wind, and pressure obtained from 304 dropsondes across 21 ARs. On average, total water vapor transport (TIVT) in an AR was 4.7 × 108 ± 2 × 108 kg s−1. This magnitude is 2.6 times larger than the average discharge of liquid water from the Amazon River. The mean AR width was 890 ± 270 km. Subtropical ARs contained larger integrated water vapor (IWV) but weaker winds than midlatitude ARs, although average TIVTs were nearly the same. Mean TIVTs calculated by defining the lateral “edges” of ARs using an IVT threshold versus an IWV threshold produced results that differed by less than 10% across all cases, but did vary between the midlatitudes and subtropical regions.

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