ELISIR · Elucidating the role of ice crystal number and ice crystal size for high cloud feedbacks

High clouds play an important role in modulating Earth’s radiative balance by (i) reflecting shortwave radiation and (ii) preventing longwave radiation from escaping to space. These interactions with radiation also influence atmospheric circulation at all scales, from global to local. The temperature and height response of high clouds is well understood, but changes in their extent, optical depth and ice properties including ice water content, ice crystal number, and ice crystal size remain uncertain. Ice properties are important because they determine cloud radiative effects and lifetime. However, many climate models cannot interactively simulate ice crystal number and size because they use simple 1-moment microphysical schemes with ice crystal mass as the only prognostic cloud ice variable. The proposed project ELISIR will study the role of ice crystal size and number for high cloud feedbacks and dynamical responses and compare results from 1-moment with more advanced 2-moment cloud microphysical schemes, in which also ice crystal number is a prognostic variable. We hypothesize that a direct link exists between the upper tropospheric stability and the number of nucleated ice crystals, which may lead to changes in cloud optical properties and lifetime, with a significant impact on climate. The research aims will be achieved with the help of global climate model simulations and high resolution limited-domain simulations in a tropical domain, whereby each simulation will be performed once with a 1-moment and once with a 2-moment microphysical scheme. Furthermore, the CMIP6 archive will be analysed to test whether there are systematic differences in simulated cloud radiative effects between models using 1-moment and models using 2-moment microphysical schemes. This will help to better understand the drivers of high cloud feedbacks and therefore to narrow estimates of Earth’s climate sensitivity and uncertainties in the high cloud impact on the atmospheric circulation.