The influence of multi-dimensional radiative transfer on the evolution and radiative properties of tropical convective clouds

• Main Author: Allen, Clare
• Published: University of Leeds 2008
• Subjects: 551.5
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.490579

This study investigates the role of multi-dimensional (MD) solar radiation on the cloud radiative properties and cloud evolution for convective clouds using the UK Met Office Large Eddy Model (LEM) with an interactively coupled Monte Carlo (MC) radiation model. Simulations are performed with the MC scheme running in MD mode and in Independent Column Approximation (ICA) mode. These simulations have been studied to ascertain the impact and understand how MD radiation is influencing clouds differently from the lCA implementation. Since the difference between the ICA and MD radiation is greatest for clouds with large vertical extent, focus has been on warm phase shallow cumulus and mixed phase deep convective (DC) clouds. Using these two types of clouds, it . was possible to evaluate not only the effect of differing clo~d side extent but also the effect of microphysical phase on the evolution. For the cumulus case, the Small Cumulus Microphysics Study (SCMS) is used and for the DC cloud a new case study has been developed based on the Cirrus Regional Study of Tropical Anvils and Cirrus Layers - Florida Area Cirrus Experiment (CRYSTAL-FACE) observations. All simulations were performed in 2D. It is found that the MD radiation creates minor differences in the early phase of the first plume; however, these differences have an important role in affecting the secondary plumes for the warm phase cumulus SCMS caSe and cause significant changes in the mixed phase cloud properties (ice, snow, graupel) for the CRYSTAL-FACE case, altering them by as much as a factor of two in the cases studied. Detailed accounts of cloud evolution are presented and the process of how MD as opposed to ICA affects the evolution is presented in terms of radiation-cloud diminishing/enhancing effects that are developed in the work. Cloud radiative forcing (CRF) values were calculated to assess the effects of MD compared to lCA simulations for the upwelling fluxes at the TOA. A tool was developed that related the cloud geometry to the upwelling solar flux, called the effective cloud perimeter. Simulations were performed to calculate the total CRF as well as isolate effects due to geometry and cloud evolution differences. The CRF for geometry effects is up to 70Wm-2 for the SCMS case and 15vVm-2 for the CRYSTAL-FACE case. The impact of evolution is found to be up to 62\Vm-2 for the SCMS and 55\Vm-2 for the CRYSTAL-FACE case. The geometry effects are found to occur during the development and maturation stages and the evolution effects are most prominent in the later mature and dissipation stages. This research developed two tools which can be applied to other simulations. The first was the radiation-cloud diminishing/enhancing effect, which was used to help determine the impact of radiation modifying convective cloud evolutions. The second tool was the effective cloud perimeter, which was used to understand the impact of cloud features that modified the solar radiative properties. Both tools enabled a rigorous understanding of the interactive nature of radiation and cloud processes, and how these varied with ICA and MD solar radiation.