Inferred Differences in Ice Crystal Nucleation Rates between Continental and Maritime Deep Convective Clouds

Abstract:

We present in situ and remotely sensed evidence for the following working hypothesis: Heterogeneous nucleation dominates during deep continental convection until ice nuclei in the updraft cannot prevent supersaturation from increasing. As it increases, homogeneous nucleation eventually occurs near cloud top (T < -60°C), with much faster ice crystal production rates. This is not the case in maritime anvil cirrus, where updrafts associated with deep convection are slower, promoting heterogeneous nucleation. We hypothesize that differences in updraft velocities and their effect on supersaturation might create a difference in the N/IWC ratios.

Based on In situ measurements of the ice particle size distribution (PSD) from two aircraft field campaigns (SPARTICUS & TC4) and MODIS satellite retrievals of the temperature dependence of the 12/11 μm effective absorption optical depth ratio or β_eff, ice crystal nucleation rates appear to be anomalously high near the tops of continental thunderstorms relative to maritime thunderstorms. The ice crystal nucleation rate, having units of g^-1 s^-1, is more related to the ratio of ice particle number concentration/ice water content (or N/IWC, with units of g^-1) than to N. A surprisingly tight relationship was discovered between β_eff and N/IWC, allowing N/IWC to be estimated from satellite retrievals of β_eff. These retrievals verified that deep convection during TC4 over water did not produce the much higher N/IWC ratios observed during SPARTICUS in continental anvil cirrus.

The imaging infrared radiometer (IIR) aboard CALIPSO has channels at 8, 10 and 12 μm and provides a data record of β_eff dating back to 2006, as well as vertical profiles of IWC, extinction, depolarization and 1064/532 nm backscatter ratio from the CALIOP lidar. We will compare the MODIS-derived β_eff and N/IWC relationship with that derived using the IIR data. We will also investigate the relationship between N/IWC, β_eff and the vertically-resolved lidar parameters to determine if nucleation type produces a measurable change in the vertical distribution of cloud ice. It appears promising that these relationships may be used to determine when and where homo- and heterogeneous nucleation dominate ice production in cirrus clouds as a function of season and latitude.