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Relationships between wind speed, humidity and precipitating shallow cumulus convection

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

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  1. 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

  2. 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

  3. RICO observations Nuijens, Stevens and Siebesma (2009)

  4. 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)

  5. 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

  6. Time series and profiles BL depth h cloud fraction after 60 hrs:

  7. Flux behavior

  8. 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)?

  9. Bulk analysis (1)

  10. Bulk analysis (2) Dq fixed, Dq varies to keep B constant Dqv q q SH LH B hLCL we M

  11. 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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