Relationships between wind speed, humidity and precipitating shallow cumulus convection
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Relationships between wind speed, humidity and precipitating shallow cumulus convection Louise Nuijens and Bjorn Stevens* UCLA - Department of Atmospheric Sciences *Max Planck Institute for Meteorology. Outline Motivation, main idea and questions

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Relationships between wind speed, humidity and precipitating shallow cumulus convection

Louise Nuijens and Bjorn Stevens*

UCLA - Department of Atmospheric Sciences

*Max Planck Institute for Meteorology


  • Outline shallow cumulus convection

    • Motivation, main idea and questions

      what is the nature of the observed relationship between winds, humidity and precipitation?

    • Large Eddy Simulation - preliminary results

    • Bulk analysis

    • Summary and thoughts


RICO observations shallow cumulus convection

Nuijens, Stevens and Siebesma (2009)


Ideas and questions shallow cumulus convection

  • A column of air moving at a greater speed:

  • enhanced upward transport of moisture, deeper clouds, hence more rain?

Betts and Ridgway ('89), Bellon and Stevens ('05), Stevens ('06)

  • is the relationship between wind speed and humidity purely one

    reflecting enhanced surface fluxes and moisture transport into the

    cloud layer?

  • how does the cumulus cloud ensemble change with wind speed?

  • in equilibrium, can similar surface fluxes be maintained at different

    wind speeds?

  • analogy to precipitating deep convection?

Back and Bretherton (2005), Raymond (2003, 2005)


Large Eddy Simulation shallow cumulus convection

  • Initial profiles and forcings GCSS RICO Intercomparison

  • 12.8 x 12.8 x 5 km domain, 50 x 50 x 40 m resolution

  • Interactive surface fluxes, shifted geostrophic wind profiles


Time series and profiles shallow cumulus convection

BL depth h

cloud fraction

after 60 hrs:


Flux behavior shallow cumulus convection


Sensitivity to wind speed? shallow cumulus convection

  • Stronger winds lead to enhanced evaporation, more humid and deeper cloud layers

  • Surface fluxes show a different behavior than expected

  • The surface buoyancy flux and sub-cloud layer depth for different wind speeds are very similar

  • Entrainment fluxes of temperature and humidity are larger for stronger winds

  • A first approach: bulk analysis

  • 1) what constrains the buoyancy flux and sub-cloud layer depth?

  • 2) what is the influence of cloud layer air (via entrainment)?


  • Bulk analysis (1) shallow cumulus convection


    Bulk analysis (2) shallow cumulus convection

    Dq fixed, Dq varies to keep B constant

    Dqv

    q

    q

    SH

    LH

    B

    hLCL

    we

    M


    Summary and questions shallow cumulus convection

    • Wind speed may considerably affect cloud and boundary layer properties

    • Understanding its impact seems interesting and challenging enough

    • (and we have not even considered precipitation) …

    • More evaporation, deeper clouds, more mass flux, more drier downdrafts?

    • Is a change in the jump in virtual potential temperature across the transition layer necessary to explain the behavior?

    • How about shear? How about relations between wind speed, updraft speed, and precipitation?

    • Can we generalize our results? (how specific is the RICO case?)


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