Exploring Doppler velocity spectra to characterize ice nucleation and microphysical processes for Arctic mixed-phase clouds

The Arctic is experiencing an increase of surface temperatures twice as large as the mid-latitude average. Mixed-phase clouds with super-cooled liquid droplets are known to contribute to the surface radiation budget significantly. But ice nucleation processes and riming processes that can alter the clouds’ lifetime and radiative properties are not fully understood. In this study, Doppler velocity spectra observations are analyzed to identify characteristics of mixed-phase clouds and sensitivities on microphysical processes.

Previously, using a habit prediction model called SHIPS (Spectral Ice Habit Prediction System, Hashino and Tripoli 2007), we analyzed the dependency of crystal habits on the immersion freezing process for a SHEBA case study. We found that large droplets (~30 micron) that are not yet activated contribute to the freezing rate significantly and modify crystal habits. In order to clarify the importance of such large droplets for mixed-phase clouds with warmer cloud-top temperatures and to investigate signatures of immersion freezing process on Doppler spectrum, we have conducted a set of 3D LES simulation for ISDAC April 8^th case, following Avramov et al (2011). Large-scale moisture advection as well as the freezing parameters were adjusted to obtain a realistic distribution of radar reflectivity, mean Doppler spectrum, and the spectrum width. In agreements with observations, the simulation generated mostly dendrites and aggregates of dendrites. For a control setting with the observed aerosol size distribution, large droplets did not form and the immersion freezing was dominated by 10 micron size droplets. When the initial aerosol size distribution was simply replaced with that of the SHEBA case study, large droplets (~30 micron) were found to play a role in the freezing. The distributions of simulated radar reflectivity and Doppler spectrum widths were clearly different from the control case. Further analysis of observed and simulated Doppler spectrum time series will be discussed along with impacts on the riming process.