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Xie Yibing

Originally, the author intended to attack the problem of dynamic effects of the Tibet Plateau on the general circulation of the atmosphere. As the work was progressing, it appeared that the concept of assimilation of space and time had to be introduced. The result was a more general conclusion about the extent of the existence of planetary waves, the coupling of high- and low-level disturbances, and the criteria of space and time baroclinic instability. The classical theory of the baroclinic instability was revised and generalized.

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Eun-Pa Lim, Harry H. Hendon, Amy H. Butler, David W. J. Thompson, Zachary D. Lawrence, Adam A. Scaife, Theodore G. Shepherd, Inna Polichtchouk, Hisashi Nakamura, Chiaki Kobayashi, Ruth Comer, Lawrence Coy, Andrew Dowdy, Rene D. Garreaud, Paul A. Newman, and Guomin Wang

follow the canonical evolution of the upper-stratospheric winds and planetary wave activity for springtime polar vortex weakening, but the poleward heat flux anomalies at 100 hPa were extraordinarily strong in August and September 2019 ( Fig. 4d ), which was consistent with the resultant record weakening of the polar vortex. More details of the increased poleward heat flux during 2019 are provided in Fig. 5a , which displays the standardized amplitudes of the wave-1 poleward heat flux anomalies

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James R. Holton

The linear theory for stationary Rossby waves excited by flow over long ridges is reviewed. The traditional “textbook” model, in which there is no disturbance of the potential temperature surfaces or streamlines upstream of the ridge, is shown to be inadequate. For both the β-plane and the f-plane cases, pressure forces cause lifting of the isentropes and a cyclonic turning of the streamlines upstream of the ridge. The β-plane and f-plane solutions are, however, totally different for long ridges owing to the resonant excitation of stationary Rossby waves and the vertical propagation of the long-wave components in the β-plane case. Curiously, the vertical displacement of the isentropic surfaces is nearly identical in the β-plane and f-plane solutions in the lower troposphere where the response is dominated by the short-wave vertically decaying components.

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Albert V. Carlin

A clear-cut case of dispersion of energy downstream in the mid-tropospheric long-wave pattern during the first two weeks of November 1951 is examined with the help of plots of meridional wind components. The dispersion could be traced over more than half the hemisphere traveling with a speed about twice that of the zonal wind at 700-mb. Calculations of group velocity based on Rossby's work show good agreement with observed values of the speed of dispersion.

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Mitchell W. Moncrieff, Duane E. Waliser, Martin J. Miller, Melvyn A. Shapiro, Ghassem R. Asrar, and James Caughey

transported upward into the atmosphere. From there, the heat is radiated back to space and the moisture may condense and form clouds. Some of the condensate grows large enough to fall back to Earth's surface as precipitation. In this regard, moist convection plays a crucial role in the energy and water cycles of the tropics as well as the variability of the tropical climate system. In concert with its effects on the tropics per se, moist convection can generate planetary (Rossby) waves, which affect

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Elmar R. Reiter

Mountain ranges and high plateaus influence atmospheric circulation patterns on all scales, ranging from ultralong planetary waves to small turbulent eddies. Some of these effects are brought about simply by orographic obstacles acting as barriers to the flow. Of equal importance, however, are the thermal effects of elevated land masses, which can generate considerable baroclinicity. Various time scales have to be considered in the thermal forcing of the atmosphere by large elevated land masses. Diurnal variations of the heating and cooling cycle have been shown to be prominent factors over Tibet. On time scales from days to weeks, the Northern Hemisphere plateaus seem to influence the monsoon circulations. There are strong indications that interseasonal “memory” exists in the heat balance of plateaus that might affect seasonally abnormal monsoon behavior. Such “memory” could be caused by feedback between thermal effects of land masses and “near-resonant” planetary waves.

In order to assess the thermal impact of mountains and plateaus, we need considerably more detailed knowledge of the energy transfer processes between the valley atmosphere, the yet poorly delineated planetary boundary layer over mountains, and the “free atmosphere.” To achieve such knowledge, experimental and theoretical studies involving micro-, meso-, and macroscales will have to intermesh more closely than in the past.

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John W. Nielsen-Gammon

A 20-yr loop of the global tropopause, defined in terms of potential vorticity (PV), is constructed using the NCEP–NCAR reanalysis dataset. This method of visualizing observed upper-tropospheric dynamics is useful for studying a wide range of phenomena. Examples are given of the structure of jet streams and planetary-scale tropopause folds, the propagation of a high-amplitude Rossby wave packet partway around a hemisphere, several subtropical wave breaking events, the similarities between exceptional cases of rapid cyclogenesis, favorable regions for cross-equatorial propagation of Rossby waves, the annual cycle of the tropical tropopause, the structure of the Tibetan anticyclone and equatorial easterly jet associated with the Asian monsoon, the meridional structure of the upper branch of the Hadley cell, the interaction of a hurricane and midlatitude trough to form the “Perfect Storm,” and the upper-tropospheric PV changes associated with El Niño and La Niña.

Plumes of anticyclonic potential vorticity are frequently seen to be pulled from the subtropical reservoir and roll up into large anticyclones. These previously undescribed plumes may be particularly relevant to jet streak dynamics and stratosphere-troposphere exchange.

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Ernest M. Agee

Evidence has been presented and discussed to show a cooling trend over the Northern Hemisphere since around 1940, amounting to over 0.5°C, due primarily to cooling at mid- and high latitudes. Some regions of the middle latitudes have actually warmed while others, such as the central and eastern United States, have experienced sharp cooling. A representative station for this latter region is Lafayette, Ind., which has recorded a drop of 2.2°C in its mean annual temperature from 1940 through 1978. The cooling trend for the Northern Hemisphere has been associated with an increase of both the latitudinal gradient of temperature and the lapse rate, as predicted by climate models with decreased solar input and feedback mechanisms.

Observations and interpretation of sunspot activity have been used to infer a direct thermal response of terrestrial temperature to solar variability on the time scale of the Gleissberg cycle (~90 years, an amplitude of the 11-year cycles). Measurements at the Greenwich Observatory and the Kitt Peak National Observatory, as well as other supportive information and arguments, are presented to hypothesize a physical link between the sunspot activity and the solar parameter. On the time scale of the Gleissberg cycle when the mean annual sunspot number exceeds 50 it is proposed that global cooling may be initiated due to the decreased insolation. This is also supported by umbral-to-penumbral ratios computed and interpreted by Hoyt (1979a).

Observations of sensible heat flux by stationary planetary waves and transient eddies, as well as general circulation modeling results of these processes, have also been examined from the viewpoint of the hypothesis of cooling due to reduced insolation. The westerlies appear to have shifted southward and to have strengthened during the cooling period, which allows for arguments of a preferred wave number for stationary waves due to mountain interaction. This type of interaction may give rise to preferred regions of heat flux as seen observationally, e.g., the warming in the far west regions of the United States and the sharp cooling in central and eastern regions. Cyclone frequencies have also been observed to shift southward, with up to 25% reduction in January and July cyclone frequency during the cooling trend in the western border of the North America continent and in the Gulf of Alaska. This region corresponds to the location of the large amplitude ridge in the planetary wave that has been observed, especially during the winter season when the westerlies are stronger.

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Kristina B. Katsaros, Stuart D. Smith, and Wiebe A. Oost

HEXOS is an international program for the study of evaporation and spray-droplet flux from sea to air. The program includes measurements in the field at moderate-to-high wind speeds, wind-tunnel studies, instrument development, boundary-layer modeling, and subsequent development of parameterization for use in synoptic and climatic models of the atmosphere and the ocean. Present accomplishments of the program are 1) a wind-tunnel study of the flow distortion around the Dutch research platform, Meetpost Noordwijk, 2) a pilot experiment at this platform in November 1984, and 3) an investigation of processes near the air-sea interface in a wind-wave simulation tunnel. The main field experiment, taking place in the autumn of 1986 at and around the Noordwijk platform, includes measurements of the fluxes of water vapor, spray droplets, sensible heat, and momentum, as well as the structure of the planetary boundary layer and the state of the sea. This multidisciplinary effort involves direct measurements from the platform, a mast, a ship, a tethered balloon, moorings, and an aircraft, plus measurements obtained remotely by laser scintillometer, lidar, and radar.

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C. Nappo, J. Sun, L. Mahrt, and D. Belušić

numerical models. W hen : 23–27 July 2012 W here : Boulder, Colorado One of the most challenging problems facing atmospheric modelers is the parameterization of turbulence and gravity waves in the stable planetary boundary layer (PBL). Currently numerical models perform much worse for the stably stratified nocturnal PBL than the daytime convective PBL. The structure of the stable PBL has often been studied, including its dependence on the Richardson number, turbulence intermittency, surface

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