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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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slide1

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

slide2

Outline

    • 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
slide3

RICO observations

Nuijens, Stevens and Siebesma (2009)

slide4

Ideas and questions

  • 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)

slide5

Large Eddy Simulation

  • 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
slide6

Time series and profiles

BL depth h

cloud fraction

after 60 hrs:

slide8

Sensitivity to wind speed?

    • 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)?
slide10

Bulk analysis (2)

Dq fixed, Dq varies to keep B constant

Dqv

q

q

SH

LH

B

hLCL

we

M

slide11

Summary and questions

  • 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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