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Cloud Radiative Forcing in Pacific, African, and Atlantic Tropical Convective Regions

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  • 1 Blackett Laboratory, Imperial College, London, United Kingdom
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Abstract

The high degree of cancellation between longwave (LW) and shortwave (SW) cloud radiative forcing (CRF) observed in the Pacific warm pool region has generally been assumed to be a property of all convective regions in the Tropics. Analysis of the (Earth Radiation Budget Experiment) ERBE-like data from the Clouds and the Earth's Radiant Energy System (CERES) instrument on the Terra satellite reveals that a similar degree of cancellation occurs over the African convective region at monthly and longer time scales, but only in the area average. In the Atlantic intertropical convergence zone (ITCZ), the degree of cancellation is lower, particularly during the summer months, where the area-average SW forcing typically exceeds the LW forcing by more than 20 W m−2. This behavior is similar to that found previously for the eastern Pacific ITCZ region, which is consistent with the similarity in dynamics between these two regions.

Over Africa, substantial seasonal and spatial variations in net CRF occur, with significant departures from cancellation within the convective region. These are explained here by a combination of surface albedo and cloud effects. In particular the large negative values of net CRF found in the summer months result from the inclusion of the radiative effects of low cloud present during the course of the month in the monthly mean cloud forcings. This work highlights the limitations of monthly mean radiation budget data for studies of rapidly evolving processes such as convection, indicating the need for studies at a higher time resolution.

Corresponding author address: Mrs. J. M. Futyan, Space and Atmospheric Physics Group, Blackett Laboratory, Imperial College, London SW7 2BP, United Kingdom. Email: joanna.futyan@imperial.ac.uk

Abstract

The high degree of cancellation between longwave (LW) and shortwave (SW) cloud radiative forcing (CRF) observed in the Pacific warm pool region has generally been assumed to be a property of all convective regions in the Tropics. Analysis of the (Earth Radiation Budget Experiment) ERBE-like data from the Clouds and the Earth's Radiant Energy System (CERES) instrument on the Terra satellite reveals that a similar degree of cancellation occurs over the African convective region at monthly and longer time scales, but only in the area average. In the Atlantic intertropical convergence zone (ITCZ), the degree of cancellation is lower, particularly during the summer months, where the area-average SW forcing typically exceeds the LW forcing by more than 20 W m−2. This behavior is similar to that found previously for the eastern Pacific ITCZ region, which is consistent with the similarity in dynamics between these two regions.

Over Africa, substantial seasonal and spatial variations in net CRF occur, with significant departures from cancellation within the convective region. These are explained here by a combination of surface albedo and cloud effects. In particular the large negative values of net CRF found in the summer months result from the inclusion of the radiative effects of low cloud present during the course of the month in the monthly mean cloud forcings. This work highlights the limitations of monthly mean radiation budget data for studies of rapidly evolving processes such as convection, indicating the need for studies at a higher time resolution.

Corresponding author address: Mrs. J. M. Futyan, Space and Atmospheric Physics Group, Blackett Laboratory, Imperial College, London SW7 2BP, United Kingdom. Email: joanna.futyan@imperial.ac.uk

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