The Impacts of Bias in Cloud-Radiation-Dynamics Interactions on Central-Pacific Seasonal, Interannual and El Nino Simulations in Contemporary GCMs

Most CMIP5 climate models do not include atmospheric falling snow in their radiative transfer calculations. Over the convective regions of the Pacific there are commonly large falling ice particles near cloud tops, and ignoring them results in a lower effective emission height and overly aggressive cloud top radiative cooling.

Using controlled simulations with the CESM1-CAM5 model, we show that the exclusion of precipitating ice radiative effects does indeed generate excessive upper-level radiative cooling which leads to overly vigorous convection with anomalous low-level outflows. These outflows weaken the easterly trades, reducing upper-ocean mixing and leading to a positive mean SST bias in the trade wind regions on the flanks of the ITCZ. In CP-El Nino events, the anomalous outflow from the modeled convection in the central Pacific allows unrealistic eastward propagation of warm SST anomalies following the peak in CP-El Nino activity. Including the radiative effects of precipitating ice reduces these model biases and improves the simulated life cycle of the CP-El Nino.

Our results suggest that the inclusion of the contribution of the precipitating ice and its radiative effects in models is important to improve the simulation of the Pacific mean state, seasonal cycle, and CP-El Niño.