87th AMS Annual Meeting

Saturday, 13 January 2007
Radiative Influences on the Bergeron-Findeisen process in Arctic mixed-phase clouds
Zach Lebo, Penn State University, University Park, Pennsylvania; and N. Johnson and J. Y. Harrington

    The glaciation time-scale for a mixed phase Arctic stratus cloud as a result of the Bergeron-Findeisen process is computed via a box-model. The mass-growth equation for ice crystals includes a term accountable for longwave (LW) cooling and shortwave (SW) warming of the particles using a two-stream radiative transfer model to compute fluxes. Values are obtained for ice concentrations ranging from 0.1 L-1 to 1000 L-1 with no vertical velocity and at varying levels within an ideal mixed-phase Arctic stratus cloud. Results show a large enhancement in ice growth at cloud top due to strong LW cooling under nocturnal conditions and a slight suppression in growth leading to extended glaciation times under maximum solar heating conditions. Below cloud top, the magnitude of LW fluxes drops rapidly allowing for a region to exist in which the cloud undergoes net warming. Within this region, glaciation times are greatly increased, so much so, that at temperatures closer to 0 C, crystal growth stops prior to incorporating all the initial liquid water. The effect of radiation on plate and column crystal habits reflects that of the spherical case qualitatively, however, some quantitative differences do arise. Further verification of this study was performed using a cloud parcel model to account for changes in supersaturation with respect to liquid and ice, temperature, pressure, and density as well as both liquid droplet and ice crystal distributions. The liquid distribution is initiated with a distribution of aerosol particles while a a distribution of ice crystals is activated only once an specified liquid water content is obtained. Preliminary results produce strikingly similar values for cases with and without radiative fluxes. Possibly more important, after the activation of the ice crystal distribution small droplets evaporate as expected, however, large drops continue to grow. This prolonged growth is produced by the solute effect on the small droplets which allows the supersturation to remain high enough for large cyrstal growth to occur. Both the radiative effect on glaciation time scales, as well as the effect of aerosol particles in large droplets are crucial in our understanding of the formation and longevity of mixed-phase Arctic stratus. Moreover, the effect of radiation may in part be the cause for the observed increase in cloud cover during spring and summer months over the Arctic.

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