Dominant Conditions in the Existence and Formation of Cloud-Voids in Southern High-Latitude Mixed-Phase Clouds

In-situ observations collected during the NSF Southern Ocean Clouds, Radiation, and Aerosols Transport Experimental Study (SOCRATES) Campaign associated with subsaturated cloud-voids within high latitude stratiforms are presented. Using the NSF/NCAR Gulfstream-V High-Performance Instrumented Airborne Platform for Environmental Research (GV HIAPER) these data were collected over the Southern Oceans during the southern hemispheric summer into fall. Initial survey of past literature suggest that these features are due to subsidence within the column reducing a parcel's relative humidity to subsaturation through adiabatic action. However, this understanding is challenged within the findings of this work. In analyzing these data, a criteria for determining the existence of a cloud void was created. Data were processed using a filtering scheme to collect areas of subsaturation within the flight path. Initially, CDP probe (measuring cloud particles from 2 to 50 µm range) and 2DC probe (25 to 1600 µm) returns were filtered to create arrays of both in-cloud (where 2DC or CDP returns are greater than zero count per liter, or count per cubic centimeter, respectively) and out-of-cloud (where CDP and 2DC returns are zero). Then using volume mixing ratio returns (ppmv, determined via VCSEL moisture number density), relative humidities for ice and liquid water were calculated to determine subsaturated conditions. Following these determinants, overall duration of these voids within the stratiforms were analyzed in order to create a finer criteria for these phenomena. Voids of time duration longer than 50 seconds were removed as likely these 'voids' are in fact sections of clear air between clouds. The remaining observations showed high probability of voids with durations of one to ten seconds, reducing as duration increases. Upon final discussion, data from these flights suggest positive vertical velocities occurred more frequently within the observed voids than the expected negative vertical velocities. These findings suggest a need for further study to understand these phenomena and attempt to observe the dominant physical processes that allow formation of these cloud voids.