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David M. Schultz, Lance F. Bosart, Brian A. Colle, Huw C. Davies, Christopher Dearden, Daniel Keyser, Olivia Martius, Paul J. Roebber, W. James Steenburgh, Hans Volkert, and Andrew C. Winters

America, First World War, Treaty of Versailles, end of Second World War, and atomic era). Within this atmospheric continuum, we focus on extratropical cyclones, low pressure systems that are frequently born of and evolve with the jet stream, producing in some midlatitude locations as much as 85%–90% of the annual precipitation ( Hawcroft et al. 2012 ) and as many as 80% of extreme precipitation events ( Pfahl and Wernli 2012 ). Although extratropical anticyclones are the counterpart to extratropical

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Ronald B. Smith

jets, gap jets, wakes, thermally driven slope winds, and cold pools. In section 5 , orographic precipitation is summarized. In section 6 , we describe the generation of mountain waves that propagate deep into the upper atmosphere. In section 7 , we consider the global effects of mountains on climate over Earth’s history. 2. Atmospheric reference heights: How high is a mountain? Mountains are usually ranked by their peak heights. Citizens take pride in their nation’s highest peaks. Climbers

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Ulrich Schumann and Andrew J. Heymsfield

. 2011 ). The aircraft wake evolution is traditionally divided into the jet, vortex, and dispersion regimes ( Hoshizaki et al. 1975 ). After a roll-up phase ( Misaka et al. 2015 ) overlapping with the jet regime, the vortex regime exhibits two phases: a first phase of a coherent counterrotating vortex pair and a second phase with rapid hydrodynamic vortex breakup and subsequent turbulent dissipation ( Gerz and Holzäpfel 1999 ; Paoli and Shariff 2016 ). b. Formation of exhaust contrails 1) Formation

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John Marwitz

Abstract

The winter orographic storms over the San Juan Mountains and the Sierra Nevada are compared. The topography of the San Juans is complex while the Sierra barrier is comparatively simple. The barrier jet is well developed upwind of the Sierra Nevada and its development is restricted upwind of the San Juans. The major difference between the storms on the two barriers is that the Sierra Nevada storms are typically maritime while the San Juan storms are continental. The implications for seeding are discussed.

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Lance F. Bosart and, Alicia C. Wasula, Walter H. Drag, and Keith W. Meier

Abstract

This paper begins with a review of basic surface frontogenesis concepts with an emphasis on fronts located over sloping terrain adjacent to mountain barriers and fronts located in large-scale baroclinic zones close to coastlines. The impact of cold-air damming and differential diabatic heating and cooling on frontogenesis is considered through two detailed case studies of intense surface fronts. The first case, from 17 to 18 April 2002, featured the westward passage of a cold (side-door) front across coastal eastern New England in which 15°–20°C temperature decreases were observed in less than one hour. The second case, from 28 February to 4 March 1972, featured a long-lived front that affected most of the United States from the Rockies to the Atlantic coast and was noteworthy for a 50°C temperature contrast between Kansas and southern Manitoba, Canada.

In the April 2002 case most of New England was initially covered by an unusually warm, dry air mass. Dynamical anticyclogenesis over eastern Canada set the stage for a favorable pressure gradient to allow chilly marine air to approach coastal New England from the east. Diabatic cooling over the chilly (5°–8°C) waters of the Gulf of Maine allowed surface pressures to remain relatively high offshore while diabatic heating over the land (31°–33°C temperatures) enabled surface pressures to fall relative to over the ocean. The resulting higher pressures offshore resulted in an onshore cold push. Frontal intensity was likely enhanced prior to leaf out and grass green-up as virtually all of the available insolation went into sensible heating.

The large-scale environment in the February–March 1972 case favored the accumulation of bitterly cold arctic air in Canada. Frontal formation occurred over northern Montana and North Dakota as the arctic air moved slowly southward in conjunction with surface pressure rises east of the Canadian Rockies. The arctic air accelerated southward subsequent to lee cyclogenesis–induced pressure falls ahead of an upstream trough that crossed the Rockies. The southward acceleration of the arctic air was also facilitated by dynamic anticyclogenesis in southern Canada beneath a poleward jet-entrance region. Frontal intensity varied diurnally in response to differential diabatic heating. Three types of cyclogenesis events were observed over the lifetime of the event: 1) low-amplitude frontal waves with no upper-level support, 2) low-amplitude frontal waves that formed in a jet-entrance region, and 3) cyclones that formed ahead of advancing upper-level troughs. All cyclones were either nondeveloping or weak developments despite extreme baroclinicity, likely the result of large atmospheric static stability in the arctic frontal zone and unfavorable alongfront stretching deformation. Significant frontal–mountain interactions were observed over the Rockies and the Appalachians.

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Thomas J. Galarneau Jr.,, Lance F. Bosart, and, and Anantha R. Aiyyer

Abstract

The pioneering large-scale studies of cyclone frequency, location, and intensity conducted by Fred Sanders prompt similar questions about lesser-studied anticyclone development. The results of a climatology of closed anticyclones (CAs) at 200, 500, and 850 hPa, with an emphasis on the subtropics and midlatitudes, is presented to assess the seasonally varying distribution and hemispheric differences of these features. To construct the CA climatology, a counting program was applied to twice-daily 2.5° NCEP–NCAR reanalysis 200-, 500-, and 850-hPa geopotential height fields for the period 1950–2003. Stationary CAs, defined as those CAs that were located at a particular location for consecutive time periods, were counted only once.

The climatology results show that 200-hPa CAs occur preferentially during summer over subtropical continental regions, while 500-hPa CAs occur preferentially over subtropical oceans in all seasons and over subtropical continents in summer. Conversely, 850-hPa CAs occur preferentially over oceanic regions beneath upper-level midocean troughs, and are most prominent in the Northern Hemisphere, and over midlatitude continents in winter.

Three case studies of objectively identified CAs that produced heal waves over the United States, Europe, and Australia in 1995, 2003, and 2004, respectively, are presented to supplement the climatological results. The case studies, examining the subset of CAs than can produce heat waves, illustrate how climatologically hot continental tropical air masses produced over arid and semiarid regions of the subtropics and lower midlatitudes can become abnormally hot in conjunction with dynamically driven upper-level ridge amplification. Subsequently, these abnormally hot air masses are advected downstream away from their source regions in conjunction with transient disturbances embedded in anomalously strong westerly jets.

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Daniel Rosenfeld and William L. Woodley

Abstract

Spaceborne inferences of cloud microstructure and precipitation-forming processes with height have been used to investigate the effect of ingested aerosols on clouds and to integrate the findings with past cloud physics research. The inferences were made with a method that analyzes data from National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer (NOAA AVHRR) and Tropical Rainfall Measuring Mission Visible and Infrared Scanner (TRMM VIRS) sensors to determine the effective radius of cloud particles with height. In addition, the TRMM Precipitation Radar (PR) made it possible to measure the rainfall simultaneously with the microphysical retrievals, which were validated by aircraft cloud physics measurements under a wide range of conditions. For example, the satellite inferences suggest that vigorous convective clouds over many portions of the globe remain supercooled to near −38°C, the point of homogeneous nucleation. These inferences were then validated in Texas and Argentina by in situ measurements using a cloud physics jet aircraft.

This unique satellite vantage point has documented enormous variability of cloud conditions in space and time and the strong susceptibility of cloud microstructure and precipitation to the ingested aerosols. This is in agreement with past cloud physics research. In particular, it has been documented that smoke and air pollution can suppress both water and ice precipitation-forming processes over large areas. Measurements in Thailand of convective clouds suggest that the suppression of coalescence can decrease areal rainfall by as much as a factor of 2. It would appear, therefore, that pollution has the potential to alter the global climate by suppressing rainfall and decreasing the net latent heating to the atmosphere and/or forcing its redistribution. In addition, it appears that intense lightning activity, as documented by the TRMM Lightning Imaging Sensor (LIS), is usually associated with microphysically highly “continental” clouds having large concentrations of ingested aerosols, great cloud-base concentrations of tiny droplets, and high cloud water contents. Conversely, strongly “maritime” clouds, having intense coalescence, early fallout of the hydrometeors, and glaciation at warm temperatures, show little lightning activity. By extension these results suggest that pollution can enhance lightning activity.

The satellite inferences suggest that the effect of pollution on clouds is greater and on a much larger scale than any that have been documented for deliberate cloud seeding. They also provide insights for cloud seeding programs. Having documented the great variability in space and time of cloud structure, it is likely that the results of many cloud seeding efforts have been mixed and inconclusive, because both suitable and unsuitable clouds have been seeded and grouped together for evaluation. This can be addressed in the future by partitioning the cases based on the microphysical structure of the cloud field at seeding and then looking for seeding effects within each partition.

This study is built on the scientific foundation laid by many past investigators and its results can be viewed as a synthesis of the new satellite methodology with their findings. Especially noteworthy in this regard is Dr. Joanne Simpson, who has spent much of her career studying and modeling cumulus clouds and specifying their crucial role in driving the hurricane and the global atmospheric circulation. She also was a pioneer in early cloud seeding research in which she emphasized cloud dynamics rather than just microphysics in her seeding hypotheses and in her development and use of numerical models. It is appropriate, therefore, that this paper is offered to acknowledge Dr. Joanne Simpson and her many colleagues who paved the way for this research effort.

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Isaac M. Held

highlights the fundamentally wavelike character of the instability that, unlike gravitational and inertial (symmetric) instability, cannot be easily understood from a parcel perspective in isolation. The PV perspective allows one to unify barotropic and baroclinic instability, to understand why internal jets in the stratosphere (for which surface temperature gradients play a negligible role) can be stable despite the presence of substantial mean available potential energy, and conversely to understand

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Andrew J. Majda and Samuel N. Stechmann

cooperatively? This situation is reminiscent of the classic conundrum: Which came first: the chicken or the egg? In atmospheric science, such questions are most familiar from contexts such as midlatitude eddies and jets. Here, instead, the focus is on the tropical case described above, and the evidence will suggest that, in fact, there are cooperative interactions between the envelopes and the fluctuations within them. Fig . 10-1. A large-scale envelope with fluctuations embedded within it. 2. The chicken

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development are controlled by the low level jet, the return flow of moisture from the Gulf of Mexico during the winter and early spring months, the development of mesoscale convective complexes, frontal passages? What are the implications of the regional east–west gradients in altitude, soil type, vegetation, temperature, and precipitation on radiative fluxes? How important are seasonally varying distributions of aerosols and particulates (e.g., from regional oil refineries, or from burning of wheat

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