Radiative sensitivities of tropical anvils to small ice crystals

Stratiform anvils in the upper tropical troposphere were simulated to determine the sensitivities of their radiative properties to the presence of small ice crystals. Cloud evolution was modelled in a one-dimensional (vertical) framework incorporating an updraft, deposition, sublimation, sedimentation, nucleation, and radiation. The sensitivities of cloud radiative forcing, albedo, emissivity, and heating rate were derived from a test that included then excluded the presence of numerous small crystals. These crystals sizes (3 < L < 20 μm) have been measured in recent observations but are smaller than the detection limit of most past observations. The shortwave forcing and albedo were very sensitive to the presence of the small crystals, even though these crystals accounted for less than 2% of total cloud mass. For optically thick anvils the longwave forcing and emissivity were in general much less sensitive to the small ice crystals than their shortwave counterparts. Radiative treatments assuming a hexagonal crystal habit yielded the same sensitivities as the spherical habit. The results agreed with previous studies in that the increased backscatter from hexagonal crystals enhanced the planetary albedo by ∼ 10–15%. The heating rate sensitivity to the small crystals depended on vertical location within the cloud and showed cancellation between the longwave and shortwave heating perturbations. The small crystals changed heating rates by up to 50% at cloud top and base.