Sensitivity of Tropical Cloud Radiative Heating and Precipitation to Ice Optical Properties

Ice clouds regulate the atmospheric radiation budget, both by reflecting shortwave radiation and absorbing-emitting longwave radiation. Cirrus and top anvil clouds in the tropics are no exception. Ice cloud radiative properties depend on the size, shape, and complexity of in-cloud ice crystals. The parameters used by the ice optical schemes included in radiative transfer (RT) calculations and numerical weather prediction (NWP) models can play an important role also. Here, we propose both a series of idealized single-column radiative transfer calculations and storm-resolving simulations to study how ice cloud radiative heating (CRH) rates respond when ice crystal complexity is considered. We use the ecRad RT scheme for the idealized simulations, together with ICON v2.6.4 for a more realistic approach. We evaluate three ice optical schemes for a range of ice cloud formation temperatures or altitudes, geometrical depths, ice water paths (IWP), and ice crystal effective radii. Additionally, we present a heating rate sensitivity matrix as a condensed visualization of the CRH response across a broad parameter space. We find that including ice complexity in geometrically thin and high-altitude clouds with high IWP, can diminish the net in-cloud heating and cloud-top cooling by 2.5 K d^-1 and 15 K d^-1, respectively. Furthermore, while temperature-based schemes can have similar behaviors to other schemes at low altitudes, the net CRH at cloud bottom can be as high as 12 K d^-1 more than size-dependent schemes in high-IWP clouds next to the tropical tropopause layer. Moreover, preliminary results when running ICON over the Asian monsoon area show that switching to a more sophisticated ice optical scheme can diminish net CRH by roughly 0.1 K d^-1 at the altitude where ice mixing ratio q_i peaks. For this more sophisticated scheme, the simulated precipitation peak over the area and period under study increases by 7%.