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Variability of Water Vapor, Infrared Radiative Cooling, and Atmospheric Instability for Deep Convection in the Equatorial Western Pacific

Chidong ZhangDivision of Meteorology and Physical Oceanography, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida

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Ming-Dah ChouLaboratory for Atmospheres, NASA/Goddard Space Flight Center, Greenbelt, Maryland

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Print Publication:
01 Mar 1999
DOI:
https://doi.org/10.1175/1520-0469(1999)056<0711:VOWVIR>2.0.CO;2
Page(s):
711–723
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Abstract

In the troposphere of the equatorial western Pacific, the water vapor variability dominates the temperature variability in changing the clear-sky infrared (IR) cooling rate. The large water vapor variability, especially its bimodal distribution at certain levels of the upper troposphere, leads to distinct structures of the clear-sky IR radiative cooling rate. The IR cooling rate, its maximum normally in the upper troposphere (∼300 hPa) and minimum in the lower troposphere (∼650 hPa), tends to become vertically uniform when the upper troposphere is abnormally dry. A local, maximum IR cooling rate may occur in the boundary layer when the lower troposphere becomes extraordinarily dry. The changes in IR cooling due to the water vapor variability affect the rate of generation of convective available potential energy (CAPE) and the conditional instability for deep convection. Little or no mean rainfall over an area of roughly 3 × 105 km2 is observed when either the rate of generation of CAPE suffers from a reduction (magnitude of 50 J kg−1 day−1) or IR cooling decreases with height. The observed variability of water vapor results from both local vertical processes and the large-scale horizontal circulation. Horizontal advection accounts for a large fraction of the drying that is responsible for the changes in the IR cooling profile and in the atmospheric instability for deep convection. These results suggest that interactions among water vapor, radiation, and deep convection must be assessed by fully taking the large-scale circulation into consideration. This study is based on an analysis of upper-air soundings collected during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment Intensive Observing Period and calculations of a radiative transfer model.

Corresponding author address: Prof. Chidong Zhang, 4600 Rickenbacker Causeway, MPO, Miami, FL 33149-1098.

Email: czhang@rsmas.miami.edu

Abstract

In the troposphere of the equatorial western Pacific, the water vapor variability dominates the temperature variability in changing the clear-sky infrared (IR) cooling rate. The large water vapor variability, especially its bimodal distribution at certain levels of the upper troposphere, leads to distinct structures of the clear-sky IR radiative cooling rate. The IR cooling rate, its maximum normally in the upper troposphere (∼300 hPa) and minimum in the lower troposphere (∼650 hPa), tends to become vertically uniform when the upper troposphere is abnormally dry. A local, maximum IR cooling rate may occur in the boundary layer when the lower troposphere becomes extraordinarily dry. The changes in IR cooling due to the water vapor variability affect the rate of generation of convective available potential energy (CAPE) and the conditional instability for deep convection. Little or no mean rainfall over an area of roughly 3 × 105 km2 is observed when either the rate of generation of CAPE suffers from a reduction (magnitude of 50 J kg−1 day−1) or IR cooling decreases with height. The observed variability of water vapor results from both local vertical processes and the large-scale horizontal circulation. Horizontal advection accounts for a large fraction of the drying that is responsible for the changes in the IR cooling profile and in the atmospheric instability for deep convection. These results suggest that interactions among water vapor, radiation, and deep convection must be assessed by fully taking the large-scale circulation into consideration. This study is based on an analysis of upper-air soundings collected during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment Intensive Observing Period and calculations of a radiative transfer model.

Corresponding author address: Prof. Chidong Zhang, 4600 Rickenbacker Causeway, MPO, Miami, FL 33149-1098.

Email: czhang@rsmas.miami.edu

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