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An indirect, negative radiative effect of water vapor in the tropics and its implications for regional surface heat stress
  • Jaya Khanna,
  • Karan karan
Jaya Khanna
National Institute of Science Education and Research

Corresponding Author:[email protected]

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Karan karan
National Institute of Science Education and Research
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We report an indirect, negative, cloud-mediated, surface radiative effect (RE) of water vapor (IWVE) in certain regions in the tropics, which may be consequential for day-to-day regional heat stress. Using reanalysis and satellite data we show that this effect is marked by a surprisingly dominant positive relationship of cloud RE with near surface and column humidity. These clouds are predominantly low level and altocumuli, previously reported to have a negative surface RE, possibly lending the net negative RE to water vapor. Also reported earlier, these clouds form in the mid-troposphere, as detrainment offshoots of deep convective towers and can be advected away to large distances, hence requiring no local convective triggering in the IWVE regions. Evidently, the IWVE are co-located with the horizontal branch of the Hadley cell, with the lowest vertical forcing in the tropics. Moreover, these are also the transition regions between the highly cloudy and the driest parts of the tropics, with a waning down occurrence of cirrus, deep convective and altostratus clouds, linked with positive RE, corroborating the hypothesis. IWVE regions also show a large temporal variability in humidity possibly providing opportunity for a large variability in cloud fractional coverage, however the mechanism controlling this covariability is not understood. The IWVE is tightly tied with the seasonal cycle of the ITCZ and hence is likely a dominant source of pre-monsoon surface temperature variability and heat stress in the current climate. The evolution of the IWVE under future climate warming needs further investigation.
21 Oct 2023Submitted to ESS Open Archive
27 Oct 2023Published in ESS Open Archive