POST—A New Look at Stratocumulus

The POST (Physics of Stratocumulus Top) aircraft study of stratocumulus (Sc), involving a dozen organizations, is summarized (http://www.eol.ucar.edu/projects/post/. Participants: Hermann Gerber, Szymon Malinowski, Steven Krueger, Haflidi Jonsson, Samantha Balaich, Stuart Beaton, Anthony Bucholtz, Patrich Chuang, Dione Lee Rossiter, Carl Friehe, Wojciech Grabowski, Krzysztof Haman, Michael Hubbell, James Hudson, Djamal Khelif, Wojciech Kumula, William Thompson, Shouping Wang, Qing Wang, Takanobu Yamaguchi, Rob Wood, Arunas Kuciauskas).

In July and August 2008 the CIRPAS Naval Post Graduate School Twin Otter aircraft flew 17 quasi-Lagrangian POST flights in mostly unbroken stratocumuli off the West Coast of California. The principal motivations for this study are our historic inability of accurately measuring and modeling the physical processes associated with entrainment in Sc, and clarifying the relationship between mixing and microphysics near Sc top. The ultimate goal is to improve the parameterization of these processes for use in model predictions.

For the first time two very fast probes, the UFT (Ultra-Fast Temperature) probe and the PVM (Particle Volume Monitor; for LWC and effective radius) were located close to each other on an aircraft, with both capable of at least a 1000-hz data rate corresponding to 5-cm incloud resolution. These probes located near the gust probe on the Twin Otter's nose, as well as radiometers, particle and droplet spectrometers, other meterological probes, and satellite data formed a full set of measurements capable of looking at the finest to the largest Sc scales. The aircraft flight strategy was primarily vertical porpoising +/- 100 m centered on cloud top for focusing on near cloud-top processes, with additional profiles above and below cloud top, and with horizontal flux legs near the ocean surface and just below cloud base. About half the flights were split between day and night.

A few initial observation of the POST Sc are also described: The increased variability and complexity of the POST Sc, e.g., in comparison to DYCOMS II Sc that resemble more Bernard-type cells, is a surprise with the former being affected often by wind shear near cloud top, cloud roll structures, and “frizzy” cloud tops. Shear causes turbulence within the cloud as well as increasing the vertical extend of the EIL (entrainment interface layer) that appears to reduce the entrainment velocity into cloud top. Night time entrainment is larger than day time entrainment, but not in all cases. Entrainment causes minimal changes in the droplet spectra, but again not in all cases. Temperature and LWC changes in entrained parcels are correlated, although temperature in similar-sized cooled parcels also exist without changes in LWC suggesting that cooling due to LWC evaporation near cloud top is minimal. (Support provided by NSF Grant ATM-0735121)