• Albrecht, B. A., R. S. Penc, and W. H. Schubert, 1985: An observational study of cloud-topped mixed layers. J. Atmos. Sci., 42 , 800822.

    • Search Google Scholar
    • Export Citation
  • Brenguier, J-L., T. Bourrianne, A. A. Coelho, J. Isbert, R. Peytavi, D. Trevarin, and P. Weschler, 1998: Improvements of droplet size distribution measurements with the Fast-FSSP. J. Atmos. Oceanic Technol., 15 , 10771090.

    • Search Google Scholar
    • Export Citation
  • Brenguier, J-L., H. Pawlowska, L. Schueller, R. Preusker, J. Fisher, and Y. Fouquart, 2000: Radiative properties of boundary layer clouds: Droplet effective radius versus number concentration. J. Atmos. Sci., 57 , 803821.

    • Search Google Scholar
    • Export Citation
  • Caughey, S. J., B. A. Crease, and W. T. Roach, 1982: A field study of nocturnal stratocumulus: II. Turbulence structure and entrainment. Quart. J. Roy. Meteor. Soc., 108 , 125144.

    • Search Google Scholar
    • Export Citation
  • Deardorff, P. G., 1980: Cloud-top entrainment instability. J. Atmos. Sci., 37 , 131147.

  • Duroure, C., and B. Guillemet, 1990: Analyse des heterogeneites spatiales des stratocumulus et cumulus. Atmos. Res., 25 , 331350.

  • Duynkerke, P. G., 1993: The stability of cloud top with respect to entrainment: Amendment of the theory of cloud top entrainment instability. J. Atmos. Sci., 50 , 495502.

    • Search Google Scholar
    • Export Citation
  • Gerber, H., 1996a: Microphysics and modeling of layer clouds. Proc. ETL/CSU Cloud Modeling and Measurement Workshop, Boulder, CO, National Oceanic and Atmospheric Administration, 175–187.

  • Gerber, H., 1996b: Microphysics of marine stratocumulus clouds with two drizzle modes. J. Atmos. Sci., 53 , 16491662.

  • Gerber, H., B. G. Arends, and A. S. Ackerman, 1994: A new microphysics sensor for aircraft use. Atmos. Res., 31 , 235252.

  • Gerber, H., S. P. Malinowski, J-L. Brenguier, and F. Burnet, 2002: On the entrainment process in stratocumulus clouds. Preprints, 11th Conf. on Cloud Physics, Ogden, UT, Amer. Meteor. Soc., CD-ROM, JP7.6.

  • Haman, K. E., S. P. Malinowski, B. Strus, R. Busen, and A. Stefko, 2001: Two new types of ultra-fast aircraft thermometer. J. Atmos. Oceanic Technol., 18 , 117134.

    • Search Google Scholar
    • Export Citation
  • Kawa, S. R., and R. Pearson Jr., 1989: An observational study of stratocumulus entrainment and thermodynamics. J. Atmos. Sci., 46 , 26492661.

    • Search Google Scholar
    • Export Citation
  • Khalsa, S. J. S., 1993: Direct sampling of entrainment events in a marine stratocumulus layer. J. Atmos. Sci., 50 , 17341750.

  • Korolev, A. V., and I. P. Mazin, 1993: Zones of increased and decreased droplet concentration in stratiform clouds. J. Appl. Meteor., 32 , 760773.

    • Search Google Scholar
    • Export Citation
  • Krueger, S., 1993: Linear eddy modeling of entrainment and mixing in stratus clouds. J. Atmos. Sci., 50 , 30783090.

  • Kuo, H. C., and W. H. Schubert, 1988: Stability of cloud-topped layers. Quart. J. Roy. Meteor. Soc., 114 , 887916.

  • Lenschow, D. H., M. Zhou, X. Zeng, L. Chen, and X. Xu, 2000: Measurements of fine-scale structure at the top of marine stratocumulus. Bound.-Layer Meteor., 97 , 331357.

    • Search Google Scholar
    • Export Citation
  • Lewellen, D., and W. Lewellen, 1998: Large-eddy boundary layer entrainment. J. Atmos. Sci., 55 , 26452665.

  • Lilly, D. K., 1968: Models of cloud-topped mixed layers under a strong inversion. Quart. J. Roy. Meteor. Soc., 94 , 292309.

  • MacVean, M. K., and P. J. Mason, 1990: Cloud-top entrainment instability through small-scale mixing and its parameterization. J. Atmos. Sci., 47 , 10121030.

    • Search Google Scholar
    • Export Citation
  • Moeng, C-H., 2000: Entrainment rate, cloud fraction, and liquid water path of PBL stratocumulus clouds. J. Atmos. Sci., 57 , 36273643.

    • Search Google Scholar
    • Export Citation
  • Moeng, C-H., and U. Schumann, 1991: Composite structure of plumes in stratus-topped boundary layers. J. Atmos. Sci., 48 , 22802291.

  • Nicholls, S., 1989: The structure of radiatively driven convection in stratocumulus. Quart. J. Roy. Meteor. Soc., 115 , 487511.

  • Nicholls, S., and J. R. Leighton, 1986: An observational study of the structure of stratiform cloud sheets. Part I. Mean structure. Quart. J. Roy. Meteor. Soc., 112 , 391426.

    • Search Google Scholar
    • Export Citation
  • Nicholls, S., and J. D. Turton, 1986: An observational study of the structure of stratiform cloud sheets. Part II. Entrainment. Quart. J. Roy. Meteor. Soc., 112 , 461480.

    • Search Google Scholar
    • Export Citation
  • Randall, D. A., 1980: Conditional instability of the first kind upside-down. J. Atmos. Sci., 37 , 125130.

  • Rogers, D. P., and J. W. Telford, 1986: Metastable stratus tops. Quart. J. Roy. Meteor. Soc., 112 , 481500.

  • Shy, S. S., and C. S. Bretherton, 1990: Laboratory experiments on the cloud-top entrainment instability. J. Fluid Mech., 214 , 115.

  • Siems, S. T., C. S. Bretherton, M. B. Baker, S. Shy, and R. T. Breidenthal, 1990: Buoyancy reversal and cloudtop entrainment instability. Quart. J. Roy. Meteor. Soc., 116 , 705739.

    • Search Google Scholar
    • Export Citation
  • Stevens, B., and Coauthors, 2003a: Dynamics and chemistry of marine stratocumulus—DYCOMS II. Bull. Amer. Meteor. Soc., 84 , 579593.

  • Stevens, B., and Coauthors, 2003b: On entrainment rates in nocturnal marine stratocumulus. Quart. J. Roy. Meteor. Soc., 129 , 34693493.

    • Search Google Scholar
    • Export Citation
  • Telford, J. W., and T. S. Keck, 1988: Atmospheric structure generated by entrainment into clouds. Atmos. Res., 22 , 191216.

  • Vali, G., R. D. Kelly, J. French, S. Haimov, D. Leon, R. E. McIntosh, and A. Pazmany, 1998: Finescale structure and microphysics of coastal stratus. J. Atmos. Sci., 55 , 35403564.

    • Search Google Scholar
    • Export Citation
  • VanZanten, M. C., and P. G. Duynkerke, 2002: Radiative and evaporative cooling in the entrainment zone of stratocumulus—The role of longwave radiative cooling above cloud top. Bound.-Layer Meteor., 102 , 253280.

    • Search Google Scholar
    • Export Citation
  • Wang, Q., and B. A. Albrecht, 1994: Observations of cloud-top entrainment in marine stratocumulus clouds. J. Atmos. Sci., 51 , 15301547.

    • Search Google Scholar
    • Export Citation
  • Wang, Q., and D. H. Lenschow, 1995: An observational study of the role of penetrating cumulus in a marine stratocumulus-topped boundary layer. J. Atmos. Sci., 52 , 27782787.

    • Search Google Scholar
    • Export Citation
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Holes and Entrainment in Stratocumulus

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  • 1 Gerber Scientific Inc., Reston, Virginia
  • | 2 Warsaw University, Warsaw, Poland
  • | 3 Météo-France, Toulouse, France
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Abstract

Aircraft flights through stratocumulus clouds (Sc) during the Dynamics and Chemistry of Marine Stratocumulus II (DYCOMS-II) study off the California coast found narrow in-cloud regions with less liquid water content (LWC) and cooler temperatures than average background values. The regions are named cloud holes and are assumed to be a result of water evaporated by the entrainment of dryer air from above the Sc. While such features have been noted previously, this study provided a unique opportunity to investigate in much greater detail the nature of the holes, as well as their relationship to the entrainment rate, because high-speed temperature and LWC probes with maximum spatial resolution of 10 cm were flown together for the first time. Nine long-duration flights were made through mostly unbroken Sc for which conditional sampling was used to identify the location and size of the holes. The holes are concentrated near cloud top, their average width near cloud top is about 5 m, their relative length distribution is nearly constant for all flights, and they can penetrate hundreds of meters deep into the Sc before being lost by mixing. Entrainment velocities at cloud top are estimated from measurements of fluxes of reduced LWC and vapor mixing ratios in holes, the fraction of cloud area covered by holes, and the total water jump between cloud top and the free atmosphere. Rates as large as 10 mm s−1 are found for nocturnal flights, and these rates are about 3 times larger than for daytime flight segments. The rates correlate best with the size of the buoyancy jump above the Sc; the present conditional-sampling approach for measuring the rates gives larger rates than the “flux jump” rates determined by others for the same flights by a factor of about 2. The stability criterion for all Sc predicts thinning and breakup of the Sc, which does not occur. The minimal amount of cloud-top evaporative cooling caused by entrainment contributes little to the top-down convection dominated by radiative cooling during nocturnal flights; however, evaporative cooling caused by the mixing of holes as they subduct with the large-scale eddy circulation in the Sc may contribute, but with an as-of-yet unknown amount.

Corresponding author address: Dr. H. Gerber, Gerber Scientific Inc., 1643 Bentana Way, Reston, VA 20190. Email: hgerber6@comcast.net

Abstract

Aircraft flights through stratocumulus clouds (Sc) during the Dynamics and Chemistry of Marine Stratocumulus II (DYCOMS-II) study off the California coast found narrow in-cloud regions with less liquid water content (LWC) and cooler temperatures than average background values. The regions are named cloud holes and are assumed to be a result of water evaporated by the entrainment of dryer air from above the Sc. While such features have been noted previously, this study provided a unique opportunity to investigate in much greater detail the nature of the holes, as well as their relationship to the entrainment rate, because high-speed temperature and LWC probes with maximum spatial resolution of 10 cm were flown together for the first time. Nine long-duration flights were made through mostly unbroken Sc for which conditional sampling was used to identify the location and size of the holes. The holes are concentrated near cloud top, their average width near cloud top is about 5 m, their relative length distribution is nearly constant for all flights, and they can penetrate hundreds of meters deep into the Sc before being lost by mixing. Entrainment velocities at cloud top are estimated from measurements of fluxes of reduced LWC and vapor mixing ratios in holes, the fraction of cloud area covered by holes, and the total water jump between cloud top and the free atmosphere. Rates as large as 10 mm s−1 are found for nocturnal flights, and these rates are about 3 times larger than for daytime flight segments. The rates correlate best with the size of the buoyancy jump above the Sc; the present conditional-sampling approach for measuring the rates gives larger rates than the “flux jump” rates determined by others for the same flights by a factor of about 2. The stability criterion for all Sc predicts thinning and breakup of the Sc, which does not occur. The minimal amount of cloud-top evaporative cooling caused by entrainment contributes little to the top-down convection dominated by radiative cooling during nocturnal flights; however, evaporative cooling caused by the mixing of holes as they subduct with the large-scale eddy circulation in the Sc may contribute, but with an as-of-yet unknown amount.

Corresponding author address: Dr. H. Gerber, Gerber Scientific Inc., 1643 Bentana Way, Reston, VA 20190. Email: hgerber6@comcast.net

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