A Survey of Dominant Conditions in the Formation of Southern High-Latitude Stratiform Cloud-Voids

Observations returned from the National Science Foundation/National Center for Atmospheric Research (NSF/NCAR) Gulfstream-V High-Performance Instrumented Airborne Platform for Environmental Research (HAIPER-GV) Aircraft during the Southern Ocean Clouds, Radiation, and Aerosols Transport Experimental Study (SOCRATES) Campaign are discussed. Following accounts of in-cloud voids discussed in previous aircraft-based microphysical research, an anecdotal hypothesis for these voids was created, suggesting that subsiding, warming parcels are responsible for the presented phenomena. To test this potential cause of formation, a filtering scheme was designed to identify these regions within stratiform masses and analyze w-vector wind velocities present. Specifically, three filtering steps exist in order to process these data. Initially, to determine that the region in question was in fact subsaturated, visual based cloud droplet probes were used. Due to concern of usable size range, two probes were utilized, CDP (measures cloud particles from 2 to 50 µm range) and 2DC (25 to 1600 µm) probes were sorted based on returns of zero or greater than zero, where returns of zero suggest that no droplets are present; this created a basic criteria for in-cloud and out-of-cloud observations. To improve resolution, a second scheme based on Vertical-Cavity Surface-Emitting Laser (VCSEL) observed water vapor volume mixing-ratio (ppmv) was created. Relative humidities for ice and liquid within the regions in question were calculated to verify that regions where CDP and 2DC probes returned values of zero, did in fact have subsaturated conditions based on saturation vapor pressure. Finally, a temporal filter was applied to determine the cloud-void regions in question and separate them from simple gaps in between clouds that potentially could skew values of w-vector velocities. A cut-off of 50 seconds duration was decided on to provide an illustration of the distribution of duration for these voids. In final discussion, these data suggest that positive vertical velocities are more dominant than negative, alluding to a different regime leading to their formation rather than the initial suggestion that negative velocities would be more present. Future study is required to understand these phenomena as a single research campaign data set does not show complete understanding of all possible causes.