Understanding Arctic Mixed-Phase Clouds: Use of Doppler Velocity Spectra
February 7, 2013
Hosted by Sue van den Heever
Observations reveal that long-lasting, mixed-phased stratiform clouds may be observed in all months of the year in the Arctic lower troposphere. These clouds have a large impact on radiative transfer through the atmosphere, and hence the surface energy budget. The ability of these clouds to persist for several days remains perplexing, because liquid-ice mixtures are inherently unstable and will glaciate. Modeling studies have suggested several processes operating on the local level that may contribute to their persistence. All these processes suggest close coupling between dynamical and microphysical processes. However, it is very difficult to obtain observations that simultaneously inform on the dynamics and microphysics. We use Doppler velocity spectra measured by millimeter wave profiling radars to evaluate the fidelity on cloud resolving model simulations of persistent mixed-phase stratus clouds. A Doppler spectrum contains information about the hydrometeor fall spectrum, the volume mean turbulence (sub-grid scale fluctuations) and vertical motion. The hydrometeor fall spectrum depends on the characteristics (size, phase, shape/habit, fall speeds) of the hydrometeors and their size distributions. Our work includes both forward modeling of observations from simulation output and decomposition of in-cloud Doppler spectra into liquid/ice hydrometeor classes.