On the nature of the atmospheric cloud radiative effect and its impact on tropical convection
Tropical high clouds have been shown to converge radiant energy within the atmosphere. We term this phenomenon the Atmospheric Cloud Radiative Effect (ACRE). The addition of energy unequally in space and time has profound effects on tropical convection. In this thesis, we show that the extra heating by clouds serves to enhance the divergence of energy transport out of the atmospheric column. Additionally, the water vapor lofted into the upper troposphere by convection behaves in a similar fashion to the clouds, heating the atmosphere and enhancing energy export. This atmospheric moisture radiative effect accounts for as much as a fifth of the total radiative heating owing to convection. Principal component analysis of satellite-retrieved cloud data reveal offsetting changes in cloud amount, cloud optical thickness, and cloud top height that give rise to an insensitivity in the top-of-atmosphere cloud radiative effect to changes in sea surface temperature. While increasing vertical motion makes the cloud radiative effect more negative, that decrease is offset by reductions in outgoing longwave radiation owing to increases in water vapor. The absorption of radiant energy by the clouds warms the upper tropical troposphere compared to simulations where ACRE is artificially removed. We show this increase in stability requires greater surface moist static energy to initiate convection, and hence, contracts the intertropical convergence zone (ITCZ) equatorward where sea surface temperatures are at a maximum. The meridional gradient in ACRE requires greater poleward energy transport. Thus, despite the increase in stability, the mean meridional circulation intensifies to export more energy out of the tropics. Finally, we show that the increase in stability owing to ACRE reduces cloud cover. ACRE influences the cloud cover in another way, however, and that is through destabilizing the cloud layer directly through absorption of longwave at cloud bottom and emission of longwave at cloud top. This cloud layer destabilization effect enhances the cloud areal coverage, offsetting some of the reduction from the tropospheric stability changes. The destabilization of the cloud layer also warms and thins the clouds, increasing the cloud radiative effect at the top of the atmosphere.
cloud; convection; precipitation; radiative transfer
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