Upper-Tropospheric Cloud-Radiative Heating Exhibits Strong Ice Microphysical and Optical Sensitivities

Ice clouds in the upper troposphere absorb and reemit outgoing longwave radiation from Earth’s surface and scatter and absorb incoming solar radiation. Although a relatively small component of the overall atmospheric heating budget, this upper-tropospheric cloud-radiative heating (CRH) has an important influence on circulation, both in current and future climates. Over the past few years, we have performed a series of modeling studies on the sensitivities of upper-tropospheric CRH to ice microphysical and optical schemes. Both in the tropics and midlatitudes, and for grid spacings sufficient to resolve deep convection, we show that ice microphysical formulations can alter CRH both quantitatively and qualitatively. Idealized offline radiative transfer simulations indicate that the same is true for three different ice optical schemes over a wide range of ice cloud states. We identify a few factors that play into these strong microscale sensitivities, including unconstrained heterogeneous ice nucleation efficiencies, inaccurate coupling of microphysics to radiation or convective schemes, and the use of thermodynamic versus microphysical dependences for ice optical parameters.