Tropical Convective Transition Statistics and Causality in the Water Vapor–Precipitation Relation

Journal Article · Thu Mar 09 00:00:00 EST 2017 · Journal of the Atmospheric Sciences

DOI:https://doi.org/10.1175/JAS-D-16-0182.1· OSTI ID:1346277

Kuo, Yi-Hung ^[1]; Neelin, J. David ^[1]; Mechoso, C. Roberto ^[1]

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1. Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, Los Angeles, California

Abstract Previous work by various authors has pointed to the role of lower-free-tropospheric humidity in affecting the onset of deep convection in the tropics. Empirical relationships between column water vapor (CWV) and precipitation have been inferred to result from these effects. Evidence from previous work has included deep convective conditional instability calculations for entraining plumes, in which the lower-free-tropospheric environment affects the onset of deep convection due to the differential impact on buoyancy of turbulent entrainment of dry versus moist air. The relationship between deep convection and water vapor is, however, a two-way interaction because convection also moistens the free troposphere. The present study adds an additional line of evidence toward fully establishing the causality of the precipitation–water vapor relationship. Parameter perturbation experiments using the coupled Community Earth System Model (CESM) with high-time-resolution output are analyzed for a set of statistics for the transition to deep convection, coordinated with observational diagnostics for the Green Ocean Amazon (GOAmazon) campaign and tropical western Pacific Atmospheric Radiation Measurement (ARM) sites. For low values of entrainment in the deep convective scheme, these statistics are radically altered and the observed pickup of precipitation with CWV is no longer seen. In addition to helping cement the dominant direction of causality in the fast-time-scale precipitation–CWV relationship, the results point to impacts of entrainment on the climatology. Because at low entrainment convection can fire before tropospheric moistening, the climatological values of relative humidity are lower than observed. These findings can be consequential to biases in simulated climate and to projections of climate change.

• Research Organization: Univ. of California, Los Angeles, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Biological and Environmental Research (BER)
• Grant/Contract Number: SC0011074
• OSTI ID: 1346277
• Alternate ID(s): OSTI ID: 1426863
• Journal Information: Journal of the Atmospheric Sciences, Journal Name: Journal of the Atmospheric Sciences Vol. 74 Journal Issue: 3; ISSN 0022-4928
• Publisher: American Meteorological Society
• Country of Publication: United States
• Language: English

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