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CloudSat and Drizzle: Insights into Subtropical MBL Dynamics

This study examines the prevalence and impact of drizzle on subtopical marine boundary layer (MBL) dynamics, using data from CloudSat and other satellite measurements. It investigates the factors controlling drizzle production and its influence on cloud macrostructure.

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CloudSat and Drizzle: Insights into Subtropical MBL Dynamics

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  1. Cloudsat and Drizzle: What can we learn?Robert Wood, University of Washington, Seattlecontributions from Chris Bretherton, Kim Comstock, Sandra Yuter, Peter Caldwell and Bjorn StevensCLOUDSAT Science Team Meeting, Laguna Beach, May 2004

  2. 0% 10% 20% 30% 40% 50% >50% Fraction of precipitation reports indicating “drizzle” Drizzle is prevalent form of precip. in MBL cloud regions

  3. Precipitation rates by dynamical regime

  4. Light precipitation constitutes sizeable fraction of the areal mean rain amount in the tropics and subtropics

  5. IMET Buoy (85W, 20S) measurements(data courtesy of Robert Weller, WHOI)

  6. 0 30 km 15 km 90 270 180 Scanning C-band radar(EPIC 2001)drizzle is intermittent with cellular structuremax reflectivity of 20-30 dBZ -24 -12 0 12 24 dBZ

  7. Can drizzle affect MBL dynamics?

  8. Cloud macrostructure Pockets Of Open Cells (POCs) are frequently observed in otherwise unbroken Sc.These may be formed as a cloud response to drizzle-induced decoupling POCs

  9. Microphysical-macrophysical links Figure courtesy Bjorn Stevens, UCLA 0 Tb 5 Low Tb indicative of large drops 11 - 3.75 m brightness temperature difference

  10. More drizzle in heterogeneous clouds MODIS brightness temperature difference, GOES thermal IR, scanning C-band radar

  11. What we know • Drizzle is common in the subtropical MBL • Drizzle has the potential to influence cloud dynamics and possibly macro-structure

  12. What we don’t know • Climatological distribution of drizzle • What factors control the production of drizzle (cloud thickness, LWP, cloud microphysics) • Quantitative assessment of the role of drizzle in modifying cloud dynamics/structure

  13. How Cloudsat/A-Train can help • Cloudsat will provide seasonal maps of drizzle in the subtropical MBL • Aqua (MODIS, AMSR-E) and CALIPSO will provide estimates of MBL forcing terms (cloud cover, LWP, microphysics, SST, MBL-depth)

  14. Fraction of areal mean precipitation observed

  15. How long do we need to average?

  16. Cloudsat Validation plans • IMET buoys in regions of light precipitation (SE Pacific, Atlantic trade-winds) – “surface drizzle indicator” • VOCALS 2007 – Ship/aircraft/island-based field campaign in SE Pacific stratus

  17. Suggested outline of approach (a) Produce quantitative seasonal mean maps of drizzle properties in the subtropical MBL using Cloudsat (b) Use Aqua/Calypso measurements and NCEP/ECMWF reanalysis to constrain MBL forcing terms (c) Begin holistic synthesis of satellite measurements using new Park/Leovy/Rozendaal MBL model as framework (d) Make quantitative assessment of the importance of precipitation in forcing the subtropical MBL

  18. Mean entrainment rates Entrainment rate [mm/s]◄ NE PacificSE Pacific ►Subsidence rate [mm/s]

  19. Can precipitation affect MBL dynamics? Mixed layer budget analysis using observationally-derived forcings (EPIC 2001) and entrainment, shows subcloud negative buoyancy fluxes ( decoupling) only when drizzle is included

  20. Frequency of precipitation occurrence (from Petty 1995) Fraction of reports indicating some form of precipitation (JJA 1958-1991) Regions dominated by MBL cloud do precipitate 0% 2% 4% 8% 16% 32%

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