POST - A New Look at Stratocumulus

POST (Physics of Stratocumulus Top) is an aircraft field study conducted off the California Coast in July and August of 2008. A single aircraft, investigate unbroken stratocumulus clouds (Sc) near cloud top. The aircraft was instrumented with a full suite of probes including probes for measuring state parameters of the atmosphere, drop spectra, CCN concentrations and spectra, radiation irradiances, and wind velocity and turbulence. A motivation to conduct another Sc field study in that region was the desire to apply lessons learned from the earlier DYCOMS II study of CA Sc of which the measured scale of entrained parcels were smaller than the critical probe separation, and the lengthy distance of the gust probe from these probes was too large. The Twin Otter (TO) aircraft permitted the close co-location of these probes to the gust probe on the aircraft. The Ultra-Fast Temperature (UFT) probe provided by the U. of Warsaw (Kumula et al., 2010) and the PVM (liquid water content and effective radius; Gerber et al., 1994) provided high rate data to take advantage of the close separation (approximately 1 m) to the gust probe. Both probes produced 1000-hz data, and their averaged 50-hz data is consistent with that separation at the TO aircraft speed of approximately 50 m/s. The UFT and PVM as well as the whole suite of the other probes provided the first opportunity to look in detail at small scales that DYCOMS II taught us play a role in the Sc cloud-top physical processes. Unresolved questions include the role played by cooling at cloud-top caused by evaporation of cloud water due to the entrainment process. Is this cooling significant or negligible as proposed by Gerber et al. (2005)? What is the role of the Entrainment Interface Layer (EIL) in affecting entrainment? What affect does shear have near Sc top? How does the entrainment process affect microphysics? And, are such details required to understand the behavior and evolution of the Sc?
American Meteorological Society (AMS) Conference on Cloud Physics (13th), Conference on Atmospheric Radiation (13th), 28 June-2 July 2010, Portland, Oregon. Prepared in collaboration with Gerber Scientific, Inc, Reston, VA; Institute of Geophysics, University of Warsaw, Warsaw, Poland; and the Department of Atmospheric Sciences, University of Utah, Salt Lake City, UT. The original document contains color images.