Ice crystal shape effects on solar radiative properties of Arctic mixed-phase clouds—Dependence on microphysical properties

Abstract

Based on 1-year cloud measurements with radar and microwave radiometer broadband solar radiative transfer simulations were performed to quantify the impact of different ice crystal shapes of Arctic mixed-phase clouds on their radiative properties (reflectance, transmittance and absorptance). The ice crystal shape effects were investigated as a function of microphysical cloud properties (ice volume fraction f i, ice and liquid water content IWC and LWC, mean particle diameter D m I and D m W of ice/water particle number size distributions, NSDs). The required NSDs were statistically derived from radar data. The NSD was composed of a liquid and a solid mode defined by LWC, D m W (water mode) and IWC, D m I (ice mode). It was found that the ratio of D m I and D m W determines the magnitude of the shape effect. For mixed-phase clouds with D m I ≤ 27 μm a significant shape effect was obtained. The shape effect was almost insensitive with regard to the solar zenith angle, but highly sensitive to the ice volume fraction of the mixed-phase cloud. For mixed-phase clouds containing small ice crystals ( D m I ≤ 27 μm) and high ice volume fractions ( f i > 0.5) crystal shape is crucial. The largest shape effects were observed assuming aggregates and columns. If the IWC was conserved the shape effect reaches values up to 0.23 in cloud reflectance and transmittance. If the ice mode NSD was kept constant only a small shape effect was quantified (≤ 0.04).