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Nili Harnik and Richard S. Lindzen

focus on two kinds of variations of particular interest: a daily timescale variation in the phase tilt of the waves and a seasonal timescale change in the amplitude structure. Since wave activity is observed to be episodic, with episodes lasting a few weeks (e.g., Hirota and Sato 1969 ; Hartmann 1976 ), daily phase structure changes are observed within a given episode, while seasonal amplitude changes are in fact a variation in wave structure from one episode to the other. On seasonal timescales

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Eric Bembenek, Timothy M. Merlis, and David N. Straub

Abstract

A large fraction of tropical cyclones (TCs) are generated near the intertropical convergence zone (ITCZ), and barotropic instability of the related wind shear has been shown to be an important generation mechanism. The latitudinal position of the ITCZ shifts seasonally and may shift poleward in response to global warming. Aquaplanet GCM simulations have shown TC-generation frequency to vary with position of the ITCZ. These results, and that moisture plays an essential role in the dynamics, motivate the present study on the growth rates of barotropic instability in ITCZ-like zonal wind profiles. Base-state zonal wind profiles are generated by applying a prescribed forcing (representing zonally-averaged latent heat release in the ITCZ) to a shallow-water model. Shifting the latitudinal position of the forcing alters these profiles, with a poleward shift leading to enhanced barotropic instability. Next, an examination of how latent release impacts the barotropic breakdown of these profiles is considered. To do this, moisture is explicitly represented using a tracer variable. Upon supersaturation, precipitation occurs and the related latent heat release is parameterized as a mass transfer out of the dynamically active layer. Whether moisture serves to enhance or reduce barotropic growth rates is found to depend on how saturation humidity is represented. In particular, taking it to be constant or a function of the layer thickness (related to temperature) leads to a reduction, whereas taking it to be a specified function of latitude leads to an enhancement. Simple arguments are given to support the idea that moisture effects should lead to a reduction in the moist shallow water model and that a poleward shift of the ITCZ should lead to an enhancement of barotropic instability.

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R. S. Lindzen and S-S. Hong

b.y Lindzen andHong. Therefore, I am writing this note to clarify twoof their points. In their reply Lindzen and Hong made the statement,"Incidentally, it should be noted that Geller's attemptto use local seasonal temperature profiles to calculatelocal changes in the lunar semidiurnal tide is inconsistent since the main lunar semidiurnal Hough modeis global in extent." In fact, the seasonal temperatureprofiles that are shown in Geller (1970) are hemispheric averages calculated according to

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Marvin A. Geller

mannerthat, I am assured, was not intended b.y Lindzen andI-long. Therefore, I am writing this note to clarify twoof their points. In their reply Lindzen and Hong made the statement,"Incidentally, it should be noted that Geller's attemptto use local seasonal temperature profiles to calculatelocal changes in the lunar semidiurnal tide is inconsistent since the main lunar semidiurnal Hough modeis global in extent." In fact, the seasonal temperatureprofiles that are shown in Geller (1970) are hemispheric

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Andrew R. Jongeward, Zhanqing Li, Hao He, and Xiaoxiong Xiong

1. Introduction Aerosols contribute directly to atmospheric variability and to Earth’s radiative balance through scattering and absorption of solar radiation. Aerosols also contribute indirectly through complex aerosol–cloud interactions (ACI). The Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) indicates that while the mechanisms of aerosol direct effects are well known, the uncertainties in the estimates of aerosol direct and indirect effects are larger than any

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David M. Straus

physically reasonable way. The use of or thogonal functions to depict the seasonal cycle should prove especially helpful when the available data are limited (as in' GCM applications); in these cases the proposed treatment of the seasonal cycle provides a convenient way of quantifying its effects on the gen eral circulation. The discussion of interannual vari ability of Section 3 will hopefully also prove useful- in the discussion of long-time scale anomalies (such as blocking in midlatitudes and E1 Nifio

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Hailan Wang and Mingfang Ting

introduction, a brief description of the NCEP–NCAR reanalysis data and the linear model is given in section 2 . Sections 3 and 4 present the seasonal cycle of the derived three-dimensional diabatic heating field and the climatological stationary waves, respectively. Section 5 describes in detail the linear model simulations using zonal-mean basic state and the forcings, and the effects of the individual forcing mechanisms. Section 6 examines separately the effect of the seasonally varying zonal

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Barbara E. Carlson, John Caldwell, and Robert D. Cess

century. Global Effects of Environmental Pollution, S. F. Singer, Ed., Springer-Verlag, 139-155.Wiscombe, W. J., 1977: The delta-M method: Rapid yet accurate radiative flux calculations for strongly asymmetric phase functions. J. Atmos. Sci., 34, 1408-1422.A Model of Saturn's Seasonal Stratosphere at the Time of the Voyager Encounters BARBARA E. CARLSON, JOHN CALDWELL AND ROBERT D. CESS Laboratory for Planetary Atmospheres Research, State University of New York, Stony Brook 11794

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Amy C. Clement

model will be used to evaluate the effects of ocean heat transport on the seasonal Hadley circulation—one with and one without ocean heat transports. In the simulation in which ocean heat transports are included, the atmosphere model is forced with the observed climatological SSTs and run until the model climate is stable (referred to hereafter as “Control”). This simulation includes the effects of ocean processes, but only implicitly. A heat flux correction, commonly referred to as the Q-flux, can

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Steven C. Sherwood and Andrew E. Dessler

to perform dehydration, would produce too much vertical mixing to be consistent with the observed seasonal cycles. We utilize the model of SD01 . The usefulness of this model is that it has already proven able to explain vertical profiles of key constituents; we employ it here with minimal modifications, so that we can test the ability to explain the previous effects as well as seasonal variations simultaneously. a. The SD01 model Here we briefly summarize aspects of the SD01 model most

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