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Robert Wood, Atmospheric Sciences, University of Washington

The importance of precipitation in marine boundary layer cloud. Robert Wood, Atmospheric Sciences, University of Washington. Motivation. Marine boundary layer (MBL) clouds cover about 1/3 of the world’s oceans and have an enormous impact on

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Robert Wood, Atmospheric Sciences, University of Washington

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  1. The importance of precipitation in marine boundary layer cloud Robert Wood, Atmospheric Sciences, University of Washington

  2. Motivation • Marine boundary layer (MBL) clouds cover about 1/3 of the world’s oceans and have an enormous impact on • top-of-atmosphere (TOA) and surface radiation budgets • the general circulation • How clouds change remains one of the major uncertainties in future climate prediction • Until recently, precipitation in MBL clouds was assumed to be of secondary importance – this view is changing

  3. ERBE net cloud forcing SST anomaly from zonal mean

  4. ISCCP inferred St/Sc amount

  5. Tropical-subtropical general circulation from Randall et al., J. Atmos. Sci., 37, 125-130, 1980 warm SST cold SST

  6. Prescribed ISCCP clouds Climatology SST and wind stresscoupled ocean-atmosphere GCM Model clouds from Gordon et al. (2000)

  7. GFDL Clouds in climate models- change in low cloud amount for 2CO2 CCM model number from Stephens (2005)

  8. Precipitation in MBL clouds? • Pioneering study by Albrecht (1989) • importance of drizzle in cloud thermodynamics • suggestion of microphysical controls upon cloud coverage/lifetime • Early 1990s saw the development of sensitive radars that can detect even light drizzle (few tenths of a mm/day) • Petty (1995) highlighted prevalence of drizzle in volunteer ship observer reports

  9. 0% 10% 20% 30% 40% 50% >50% Fraction of precipitation reports indicating “drizzle” Drizzle is prevalent form of precip. in MBL cloud regions

  10. Field campaigns with focus on low clouds ISCCP stratus/stratocumulus cloud amount

  11. The southeast Pacific Low cloud amount (MODIS, Sep/Oct 2000)  Mean cloud fraction  Mean MBL depth

  12. The EPIC Stratocumulus study • Part of the East Pacific Investigation of Climate (EPIC) field program • Ship cruise (NOAA R/V Ronald H Brown,10-25 October 2001) under the stratocumulus sheet • Surface meteorological measurements, 3 hourly radiosondes, aerosols • Suite of remote sensors: scanning C-band radar, 35 GHz profiling radar (MMCR), lidar, ceilometer, microwave radiometer Bretherton et al. (2004), BAMS

  13. Drizzle challenges • What is the frequency and strength of drizzle over the subtropical oceans? • What are the structural properties of precipitating MBL cloud systems? • Can drizzle affect cloud dynamics, structure and coverage - how does it do so? • What controls drizzle production in MBL clouds?

  14. visible reflectance (MODIS) EPIC Sc. SST (TMI) & winds (Quikscat) Wood et al. (2004)

  15. Diurnal cycle and drizzle Surface-derived LCL Ceilometer cloud base

  16. Quantification of drizzle

  17. Quantifying drizzle Marshall-Palmer Z-R relationships derived using MMCR are then applied to the scanning C-band radar

  18. Quantifying drizzle

  19. Structural properties of precipitating stratocumulus

  20. u 20 km 10 km

  21. Mesoscale dynamics -10 -5 0 5 10 15 dBZ 23:09 UTC VRAD [m s-1] -3 -2 -1 0 1 2 3 1.5 km 23:18 UTC 0 10 20 30 [km]

  22. Animation of scanning C-band radar 30 km mean wind

  23. Echo Tracking Comstock et al. (2004)

  24. Average cell reflectivity (dBZ) 15 10 5 -1.5 -1 -0.5 0 0.5 1 1.5 Time to reflectivity peak (hours) Structure and evolution of drizzle cells • Drizzle cell lifetime 2+ hours • Time to rain out < ~ 30 minutes • Implies replenishing cloud water Comstock et al. (2004)

  25. Can drizzle affect MBL dynamics?

  26. What controls drizzle production?

  27. Summary of drizzle observations from previous field programs

  28. Closed Cells Open Cells Satellite Ship Radar

  29. Drizzle and cloud macrostructure MODIS brightness temperature difference (3.7-11 mm), GOES thermal IR, scanning C-band radar

  30. Summary • Precipitation is common in MBL clouds • The mean precipitation rates 1 mm day-1 are observed and can have significant thermodynamic impact upon the MBL • Precipitating MBL clouds display interesting mesoscale dynamics that may influence their macroscopic properties • Results suggest that drizzle is modulated by cloud LWP and by cloud droplet number

  31. Future directions • Broaden the scope of EPIC using a combination of satellite remote sensing, reanalysis, and buoy data (NSF funded, 2004-2007) • Plan and participate in a more extensive field program in the SE Pacific (VOCALS 2007) • Use Cloudsat (launch summer 2005) to begin to develop climatologies of precipitation in low cloud

  32. Fraction of areal mean precipitation observed How long do we need to average?

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