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Tropical and Extra-Tropical Influences on the Distribution of Free Tropospheric Humidity Over the Intertropical Belt

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Abstract

Free tropospheric humidity (FTH) is a key parameter of the radiation budget of the Earth. In particular, its distribution over the intertropical belt has been identified as an important contributor to the water vapour feedback. Idealized radiative transfer computations are performed to underscore the need to consider the whole probability distribution function (PDF) rather than the arithmetical mean of the FTH. The analysis confirmed the overwhelming role of the dry end of the PDF in the radiative perturbation of the top of atmosphere longwave budget. The physical and dynamical processes responsible for the maintenance of this dry part of the FTH distribution are reviewed, and the lateral mixing between the tropics and the extra-tropics is revealed as a major element of the dry air dynamics. The evolution of this lateral mixing in the framework of the global warming is discussed, and perspectives of work are listed as a mean of a conclusion.

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Notes

  1. And more recently for the stratospheric water vapour as well (Liu et al. 2010).

Abbreviations

AC:

Advection-condensation

AIRS:

Atmospheric Infrared Sounder

AR4:

Fourth Assessment Report

BLH:

Boundary Layer relative Humidity

BT:

Brightness temperature

DJF:

December, January and February

ECMWF:

European Centre for Medium-Range Weather Forecasts

ERA-Interim:

ECMWF ReAnalyses-interim

ERA-40:

ECMWF ReAnalyses-40

FIR:

Far infrared

FTH:

Free tropospheric humidity

IPSL:

Institut Pierre-Simon Laplace

ITCZ:

Inter Tropical Convergence Zone

JJA:

June, July and August

NCEP:

National Center for Environmental Prediction

OLR:

Outgoing Long wave Radiation

PDF:

probability distribution function

RH:

Relative Humidity

TS:

Temperature of the surface

TOA:

Top of atmosphere

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Roca, R., Guzman, R., Lemond, J. et al. Tropical and Extra-Tropical Influences on the Distribution of Free Tropospheric Humidity Over the Intertropical Belt. Surv Geophys 33, 565–583 (2012). https://doi.org/10.1007/s10712-011-9169-4

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