Do Sc Detrain?

Much has been published about entrainment of warm and dry air from above the inversion into stratocumulus clouds (Sc), especially for the typical Sc off the western margins of continents. A popular and surviving concept has been CTEI (Cloud Top Entrainment Instability) that predicts dissipation of Sc for specific jumps over the inversion above Sc top. However, this prediction has not been supported by observations. A fair question to ask is does CTEI ever happen? We also ask do Sc detrain? We deal primarily with the detailed vertical structure near the EIL located above the cloud (Entrainment Interface Layer; see Malinowski et al., 2015: Atm. Chem. Phys.) The present observational study looks in high resolution at entrainment features ("cloud holes") found during the POST (Physics of Stratocumulus Top) aircraft field study of Sc off the West coast of U.S. Advantage is taken of closely spaced temperature, LWC, effective radius (Re), and vertical and horizontal velocity measurements on the CIRPAS Twin Otter, collected with a horizontal resolution of a few tenths of cm.

We show that the depleted LWC in the holes has to a large degree Re unchanged from the cloud unaffected by entrainment. Only a small fraction shows depleted LWC with reduced Re. The former suggests inhomogenous mixing as often noted in the literature, while the latter suggests partial droplet evaporation. All show that depleted LWC is uncorrelated to the reduced temperature in the holes. These observations suggest the following entrainment process: Air in the EIL has its source from unaffected air above the EIL and from cloudy air near cloud top. The nearly omnipresent horizontal wind shear near cloud top then mixes EIL and cloudy air causing a layer of fragmented and patchy evaporating cloud until the potential energy of the mixture is reduced to small enough values permitting entrainment into the cloud in a near isothermal manner. Thus, entrainment is mostly a dilution process rather than an inhomogeneous mixing process that occurs in the EIL. There are different definitions for a reference level for these processes that affect the entrainment velocity (e.g., see Schulz and Mellado, 2018: JAS). Here we chose the highest level of solid cloud where the entrainment holes first appear, and where the infrared cooling that causes the instability for entrainment has its strongest effect on Sc. If this level is considered as "cloud top", then the cloudy air mixing in the EIL has been detrained from this top. These factors differ from the simple expression originally formulated for CTEI, where now the role of wind shear and the behavior of the EIL should be considered. No POST Sc showed CTEI; however, very strong shear near Sc top as well as other inversion jump values need further investigation since they may hasten Sc dissipation, which occurs occasionally off the West coast.