Comments on “Tropical Convection and the Energy Balance at the Top of the Atmosphere”

Analyses of the Earth Radiation Budget Experiment (ERBE; Barkstrom 1984) data show that the shortwave (SW) and longwave (LW) cloud radiative forcing (CRF) at the top of the atmosphere (TOA) in the tropical deep convective regions have a similar magnitude but opposite signs. The CRF is defined as the difference between the full-sky and clear-sky radiative fluxes. Hartmann et al. (2001, hereafter HMF) computed the CRF for individual cloud types using a radiative transfer model with the cloud information taken from the International Satellite Cloud Climate Project (ISCCP; Rossow and Schiffer 1991) data archive. They found that individually each cloud type had a large impact on the radiation at the TOA, but collectively the effect was small. From the analyses of the ERBE data, they claimed that the net CRF at the TOA is small not only over deep convective regions but also over the nonconvective regions. The perceived small CRF led them to conclude that the contrast in the net TOA radiation between the convective and nonconvective regions must also be small. HMF used a simple model to investigate the relation between convection and radiation in the Tropics. They hypothesized that the small contrast in the TOA radiation between the convective and nonconvective regions was due to the high sensitivity of circulation to the sea surface temperature (SST) gradient and the high sensitivity of cloud albedo to vertical velocity.