The effects of ice crystal shape on the evolution of optically thin cirrus clouds in the tropics

Thin cirrus clouds in the tropical tropopause layer (TTL) play potentially important roles in Earth's radiation budget and in the transport of water into the stratosphere. Radiative heating of these clouds results in mesoscale circulations that maintain them against sedimentation and redistribute water vapor. In this study, the System for Atmospheric Modeling (SAM) cloud-resolving model is modified in order to calculate the fall speeds, growth rates, and radiative absorption coefficients of non-spherical ice crystals. This extended model is used in simulations that aim to constrain the effects of ice crystal shape on the time evolution of thin cirrus clouds and to identify the physical processes responsible. Model runs assuming spheroidal crystals result in a higher center of cloud ice mass than in the control, spherical case, which is roughly 60% due to a reduction in fall speeds and 40% due to stronger updrafts caused by stronger radiative heating. Other effects of ice crystal shape on the cloud evolution include faster growth and sublimation in supersaturated and subsaturated environments, respectively, and local temperature increases caused by diabatic heating. Effects of ice crystal shape on the total and mean ice crystal masses are within about 10% but do not appear to be entirely negligible. Comparisons of modeled ice crystal size distributions with recent airborne observations of TTL cirrus show that incorporating non-spherical shape has the potential to bring the model closer to observations. It is hoped that this work will eventually lead towards a more realistic physical representation of thin tropical cirrus in global climate models.