Solar radiative biases in deep convective regimes: Possible implications for dynamical feedback

Abstract

There is increasing evidence that interactions between radiation and convectively determined water vapour and cloud fields can have a significant dynamical impact on the tropical circulation. However, the use of a one-dimensional (1D) independent column method for the calculation of solar fluxes in cloud-resolving or mesoscale models could possibly misrepresent this feedback. This is investigated by calculating 1D and full three-dimensional (3D) solar fluxes through a 3D field of deep convective clouds generated using a high-resolution cloud-resolving model, and diagnosing the clear-sky subsidence velocities required to balance this if a state of radiative convective equilibrium were to exist. The mean clear-sky solar radiative heating rate is found to change by up to 15%. At low sun angles the shading of the clear-sky regions implies a reduction of net clear-sky heating rates. In contrast, at higher solar elevations the scattering effect dominates shading and the clear-sky heating rates are enhanced, implying an enhancement of the diurnal cycle. However, since the 1D/3D heating-rate differences in the clear sky increase smoothly from zero at the tropopause to their maximum value near the surface, the vertical gradient and thus the impact on the vertical profile of entrainment and detrainment into the convective regions is negligibly small. This implies that the strong radiative dynamical feedbacks so far documented in cloud-ensemble models cannot be discounted on the grounds of their use of an independent column calculation for the solar radiative transfer. That said, the substantial bias noted in surface downward solar fluxes could result in a substantial impact in convective organization if an interactive land surface were used as a lower boundary condition. Copyright © 2003 Royal Meteorological Society