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Željka Fuchs and David J. Raymond

Abstract

A minimal model of a moist equatorial atmosphere is presented in which the precipitation rate is assumed to depend on just the vertically averaged saturation deficit and the convective available potential energy. When wind-induced surface heat exchange (WISHE) and cloud–radiation interactions are turned off, there are no growing modes. Gravity waves with wavenumbers smaller than a certain limit respond to a reduced static stability due to latent heat release, and therefore propagate more slowly than dry modes, while those with larger wavenumbers respond to the normal dry static stability. In addition, there exists a stationary mode that decays slowly with time. For realistic parameter values, the effect of reduced static stability on gravity waves is limited to wavelengths greater than the circumference of the earth. WISHE and cloud–radiation interactions both destabilize the stationary mode, but not the gravity waves.

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Zeljka Fuchs and David J. Raymond

Abstract

A highly simplified parameterization of diabatic processes is applied to linearized equations on a equatorial beta plane. The diabatic processes include moist convection, cloud–radiation interactions (CRI), and wind-induced surface heat exchange (WISHE). The precipitation rate is assumed to increase linearly as the vertically averaged saturation deficit decreases.

The modeled modes are Matsuno’s normal modes, that is, Kelvin waves, mixed Rossby–gravity waves, Rossby waves, and inertio–gravity waves, and an additional mode called here a slow moisture mode. All of the Matsuno modes are damped and remain stable even when CRI and WISHE are turned on. Their phase speeds do not vary much from Matsuno’s adiabatic values except for very long wavelength Kelvin and Rossby modes, for which the phase speeds are reduced compared to the adiabatic values. The slow moisture modes are stationary and unstable under CRI, while WISHE causes them to propagate. Under CRI and WISHE together the slow moisture modes are unstable and eastward propagating for long wavelengths and slowly moving relative to the mean flow for short wavelengths. The dispersion relations of the slow moisture modes are one of nearly constant or decreasing frequency with increasing wavenumber. The most important model parameter is the tropospheric moisture relaxation time scale, which is chosen to be 1 day.

The model failed to explain the observed phase speeds of convectively coupled Matsuno modes. Following Mapes, the authors suggest that other dynamics, more realistic than the one including only the first baroclinic mode, may be responsible for these modes.

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David J. Raymond and Željka Fuchs

Abstract

Moisture mode instability is thought to occur in the tropical oceanic atmosphere when precipitation is a strongly increasing function of saturation fraction (precipitable water divided by saturated precipitable water) and when convection acts to increase the saturation fraction. A highly simplified model of the interaction between convection and large-scale flows in the tropics suggests that there are two types of convectively coupled disturbances: the moisture mode instability described above and another unstable mode dependent on fluctuations in the convective inhibition. The latter is associated with rapidly moving disturbances such as the equatorially coupled Kelvin wave.

A toy aquaplanet beta-plane model with realistic sea surface temperatures produces a robust Madden–Julian oscillation–like disturbance that resembles the observed phenomenon in many ways. Convection in this model exhibits a strong dependence of precipitation on saturation fraction and does indeed act to increase this parameter in situations of weak environmental ventilation of disturbances, thus satisfying the criteria for moisture mode instability. In contrast, NCEP’s closely related Global Forecast System (GFS) and Climate Forecast System (CFS) models do not produce a realistic MJO. Investigation of moist entropy transport in NCEP’s final analysis (FNL), the data assimilation system feeding the GFS, indicates that convection tends to decrease the saturation fraction in these models, precluding moisture mode instability in most circumstances. Thus, evidence from a variety of sources suggests that the MJO is driven at least in part by moisture mode instability.

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Željka Fuchs, Saska Gjorgjievska, and David J. Raymond

Abstract

The analytical model of convectively coupled gravity waves and moisture modes of Raymond and Fuchs is extended to the case of top-heavy and bottom-heavy convective heating profiles. Top-heavy heating profiles favor gravity waves, while bottom-heavy profiles support moisture modes. The latter behavior results from the sensitivity of moisture modes to the gross moist stability, which is more negative with bottom-heavy heating.

A numerical implementation of the analytical model allows calculations in the two-dimensional nonrotating case as well as on a three-dimensional equatorial beta plane. In the two-dimensional case the analytical and numerical models are mostly in agreement, although minor discrepancies occur. In three dimensions the gravity modes become equatorial Kelvin waves whereas the moisture modes are more complex and require further investigation.

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Željka Fuchs, Michael J. Herman, and David J. Raymond

Abstract

Cross-isobaric flow and Ekman pumping are investigated in the frictional spindown of an initially barotropic vortex in a stratified atmosphere. Consistent with early work by Holton and others, it is found that the stratification limits the vertical penetration of the secondary circulation driven by friction, resulting in more rapid spindown than in the unstratified case. As a result, the cross-isobaric flow and Ekman pumping are weaker and shallower than classical calculations ignoring the stratification would lead one to believe. The effect of stability becomes stronger as the vortex becomes smaller for fixed boundary layer depth. For weak geostrophic vortices with horizontal scales of several hundred kilometers or less, such as tropical easterly waves, the reduction is particularly pronounced, which raises questions about the efficacy of Ekman pumping in forcing convection in such vortices. These results suggest a revised conceptual model for the role of Ekman pumping in the atmosphere. The theory as it stands is limited to weak, linear vortices in which geostrophic balance holds approximately, corresponding to small Rossby number, though extensions of the analytical theory to stronger vortices may be possible.

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Michael J. Herman, Zeljka Fuchs, David J. Raymond, and Peter Bechtold

Abstract

The authors analyze composite structures of tropical convectively coupled Kelvin waves (CCKWs) in terms of the theory of Raymond and Fuchs using radiosonde data, 3D analysis and reanalysis model output, and annual integrations with the ECMWF model on the full planet and on an aquaplanet. Precipitation anomalies are estimated using the NOAA interpolated OLR and TRMM 3B42 datasets, as well as using model OLR and rainfall diagnostics. Derived variables from these datasets are used to examine assumptions of the theory. Large-scale characteristics of wave phenomena are robust in all datasets and models where Kelvin wave variance is large. Indices from the theory representing column moisture and convective inhibition are also robust. The results suggest that the CCKW is highly dependent on convective inhibition, while column moisture does not play an important role.

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David J. Raymond, G. B. Raga, Christopher S. Bretherton, John Molinari, Carlos López-Carrillo, and Željka Fuchs

Abstract

One of the goals of the East Pacific Investigation of Climate, year 2001 process study (EPIC2001), was to understand the mechanisms controlling the forcing of deep atmospheric convection over the tropical eastern Pacific. An intensive study was made of convection in a 4° × 4° square centered on 10°N, 95°W in September and October of 2001. This is called the intertropical convergence zone (ITCZ) study region because it encompasses the eastern Pacific intertropical convergence zone. Starting from an analysis of the theoretical possibilities and a plethora of dropsonde, in situ, radar, and satellite data, it is found that newly developing convection occurs where a deep layer of air (of order 1 km deep or deeper) is conditionally unstable with only weak convective inhibition. Shallower conditionally unstable layers are associated with numerous small clouds, but do not seem to produce deep convection.

The occurrence of deep convection over the ITCZ study region is presumably related to the propensity of the environment to produce areas of weak convective inhibition over such a deep layer. Three theoretically possible factors control the formation of such convectively unstable areas: 1) the strength of the total surface heat (or moist entropy) fluxes; 2) the advection of moisture into the region; and 3) temperature anomalies caused by dry adiabatic ascent of the inhibition layer, which lies typically between 700 and 850 mb. The areal fraction covered by such instability is small even on highly convective days.

In the tropical eastern Pacific, it is found that the total surface entropy flux is the most significant of these factors, with a warm layer in the 700–850-mb range, resulting presumably from subsidence, playing an important suppressive role in certain cases. These two factors account for approximately two-thirds of the variance in satellite infrared brightness temperature averaged over the study region. Moisture (or moist entropy) advection appears to be of less importance. Tropical disturbances such as easterly waves, Kelvin waves, and the Madden–Julian oscillation presumably control convection primarily via these two mechanisms during their passage through this region.

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Yolande L. Serra, Jennifer S. Haase, David K. Adams, Qiang Fu, Thomas P. Ackerman, M. Joan Alexander, Avelino Arellano, Larissa Back, Shu-Hua Chen, Kerry Emanuel, Zeljka Fuchs, Zhiming Kuang, Benjamin R Lintner, Brian Mapes, David Neelin, David Raymond, Adam H. Sobel, Paul W. Staten, Aneesh Subramanian, David W. J. Thompson, Gabriel Vecchi, Robert Wood, and Paquita Zuidema
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