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Chia Chou, Chao-An Chen, Pei-Hua Tan, and Kuan Ting Chen

Abstract

Global warming mechanisms that cause changes in frequency and intensity of precipitation in the tropics are examined in climate model simulations. Under global warming, tropical precipitation tends to be more frequent and intense for heavy precipitation but becomes less frequent and weaker for light precipitation. Changes in precipitation frequency and intensity are both controlled by thermodynamic and dynamic components. The thermodynamic component is induced by changes in atmospheric water vapor, while the dynamic component is associated with changes in vertical motion. A set of equations is derived to estimate both thermodynamic and dynamic contributions to changes in frequency and intensity of precipitation, especially for heavy precipitation. In the thermodynamic contribution, increased water vapor reduces the magnitude of the required vertical motion to generate the same strength of precipitation, so precipitation frequency increases. Increased water vapor also intensifies precipitation due to the enhancement of water vapor availability in the atmosphere. In the dynamic contribution, the more stable atmosphere tends to reduce the frequency and intensity of precipitation, except for the heaviest precipitation. The dynamic component strengthens the heaviest precipitation in most climate model simulations, possibly due to a positive convective feedback.

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Yen-Ting Hwang, Hung-Yi Tseng, Kuan-Chen Li, Sarah M. Kang, Yung-Jen Chen, and John C. H. Chiang

Abstract

This study investigates the transient responses of atmospheric energy and momentum fluxes to a time-invariant extratropical thermal heating in an atmospheric model coupled to an aquaplanet mixed layer ocean with the goal of understanding the mechanisms and time-scales governing the extratropical-to-tropical connection. Two distinct stages are observed in the teleconnection: (1) A decrease in the meridional temperature gradient in midlatitudes leads to a rapid weakening of the eddy momentum flux and a slight reduction of the Hadley cell strength in the forced hemisphere. (2) The subtropical trades in the forced hemisphere decrease and reduce evaporation. The resulting change to sea surface temperature leads to the development of a cross-equatorial Hadley cell, and the Intertropical Convergence Zone shifts to the warmer hemisphere. The Hadley cell weakening in the first stage is related to decreased eddy momentum flux divergence, and the response time-scale is independent of the mixed layer depth. In contrast, the time taken for the development of the cross-equatorial cell in the latter stage increase as the mixed layer depth increases. Once developed, the deep tropical cross-equatorial cell response is an order of magnitude stronger than the initial subtropical response and dominates the anomalous circulation. The analysis 31 combines the momentum and energetic perspectives on this extratropical-to-tropical teleconnection and moreover shows that the subtropical circulation changes associated with the momentum budget occur with a time-scale that is distinct from the deep tropical response determined by the thermal inertia of the tropical ocean.

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Yi-Hung Kuo, J. David Neelin, Chih-Chieh Chen, Wei-Ting Chen, Leo J. Donner, Andrew Gettelman, Xianan Jiang, Kuan-Ting Kuo, Eric Maloney, Carlos R. Mechoso, Yi Ming, Kathleen A. Schiro, Charles J. Seman, Chien-Ming Wu, and Ming Zhao

Abstract

To assess deep convective parameterizations in a variety of GCMs and examine the fast-time-scale convective transition, a set of statistics characterizing the pickup of precipitation as a function of column water vapor (CWV), PDFs and joint PDFs of CWV and precipitation, and the dependence of the moisture–precipitation relation on tropospheric temperature is evaluated using the hourly output of two versions of the GFDL Atmospheric Model, version 4 (AM4), NCAR CAM5 and superparameterized CAM (SPCAM). The 6-hourly output from the MJO Task Force (MJOTF)/GEWEX Atmospheric System Study (GASS) project is also analyzed. Contrasting statistics produced from individual models that primarily differ in representations of moist convection suggest that convective transition statistics can substantially distinguish differences in convective representation and its interaction with the large-scale flow, while models that differ only in spatial–temporal resolution, microphysics, or ocean–atmosphere coupling result in similar statistics. Most of the models simulate some version of the observed sharp increase in precipitation as CWV exceeds a critical value, as well as that convective onset occurs at higher CWV but at lower column RH as temperature increases. While some models quantitatively capture these observed features and associated probability distributions, considerable intermodel spread and departures from observations in various aspects of the precipitation–CWV relationship are noted. For instance, in many of the models, the transition from the low-CWV, nonprecipitating regime to the moist regime for CWV around and above critical is less abrupt than in observations. Additionally, some models overproduce drizzle at low CWV, and some require CWV higher than observed for strong precipitation. For many of the models, it is particularly challenging to simulate the probability distributions of CWV at high temperature.

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