HR: 15:25h
AN: A53B-08    [Abstracts]
TI: Entrainment and Microphysics in DYCOMS-II Stratocumulus
AU: * Gerber, H E
EM: hgerber6@comcast.net
AF: Gerber Scientific, 1643 Bentana Way, Reston, VA 20190 United States
AU: Frick, G
A53B-08 AF: Gerber Scientific, 1643 Bentana Way, Reston, VA 20190 United States
AU: Malinowski, S
A53B-08 AF: Warsaw University, ul. Pasteura 7, Warsaw, 02-093 Poland
AU: Burnet, F
A53B-08 AF: Meteo-France, CNRM, Tolouse, 31057 France
AU: Brenguier, J
A53B-08 AF: Meteo-France, CNRM, Tolouse, 31057 France
AB: During the nine DYCOMS-II flights through stratocumulus (Sc) off the California Coast with the NCAR C-130 research aircraft measurements of thermodynamics and microphysics were made with unprecedented resolution by three co-located probes. The UFT (ultra-fast temperature probe; U. of Warsaw) and the PVM (LWC and effective radius; Gerber Scientific) measured incloud with a resolution of 10 cm, and the FFSSP (fast FSSP; Meteo-France) measured with a resolution of 2 m. Our measurements and their analyses have led to an improved understanding of the physical processes associated with entrainment and its affect on Sc microphysics. We describe our results including the following: Cloud-top interacts with the warm and dry free atmosphere above the Sc to create the EIL (entrainment interface layer) several tens of m thick on the average. Further cloud detrains and mixes with the EIL to generate cloud-free moisture and temperature conditions ranging between cloud-top and free-atmosphere conditions. Buoyancy sorting occurs in the EIL with some parcels approaching the buoyancy at cloud-top. At that point these parcels enter cloud in a near thermodynamically-neutral fashion as shown by comparing UFT measurements in entrainment features ("cloud holes") with unaffected adjacent cloud, and in a mixing manner that resembles inhomogeneous mixing by diluting droplet number but not reducing their size as shown by the PVM and the FFSSP. This behavior occurs despite nearly all the Sc showing strong CTEI (cloud-top entrainment instability). Thus cooling due to the evaporation of cloud water contributes a negligible amount to buoyancy production at cloud top, it simply contributes a small amount to conditioning the EIL. Further, supersaturation transients caused by mixing of saturated parcels at different temperature near cloud top are not present. We find that the holes with LWC reduced by entrainment and embedded in down-welling regions caused by radiative cooling can reach their SEL (sinking evaporation level; above the LCL) where all LWC in the holes has evaporated. Here we find evidence of CIMI (cloud-interior mixing instability) where the holes can, by descending further, generate buoyancy by evaporating more liquid water. We describe areas where further progress is needed in the understanding of entrainment and microphysics in Sc: The significance of buoyancy production and effects on microphysics by CIMI need to be established, the mechanism by which the conditioned EIL parcels ultimately enter cloud top remains unknown, and the different frequency and size of the holes and their horizontal inhomogeneity found for Sc on different days needs explanation. We also attempt to explain the average factor of three difference in the measured entrainment velocities in the Sc as determined on the C-130 by using the high-resolution data and conditional sampling of the holes, and by using three other methods based on the "flux-jump" approach.
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0312 Air/sea constituent fluxes (3339, 4504)
DE: 0320 Cloud physics and chemistry
DE: 0394 Instruments and techniques
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005