Radiative Influences on the Glaciation Time-Scales of Mixed-Phase Clouds

Mixed-phase stratus clouds are dominant in the Arctic during much of the year. These clouds typically have liquid tops that precipitate ice. Time scales for the complete glaciation of such clouds (the Bergeron process) are typically computed using the classical mass growth equations for crystals and liquid drops with some studies using ice spheres. However, mixed phase arctic stratus have significant infrared cooling and solar heating (during the warm season) rates that can affect the growth of water drops and ice crystals, and possibly the strength of the Bergeron process. To examine the influence of radiative heating and cooling on the Bergeron process, we incorporate a radiation model into the mass growth equations for liquid and ice. Regardless of crystal habit and temperature, our results show that infrared radiative cooling reduces the glaciation time scale at cloud top, with lesser reductions at cloud base. Radiative reductions in glaciation time are greatest for plate crystals at T = -10C, where the reduction can be as large as 5 - 10 minutes. Solar radiative processes tend to off-set infrared processes. Moreover, the interior of the cloud is heated by solar radiation which substantially retards the Bergeron process.