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Relationships between wind speed, humidity and precipitating shallow cumulus convection

Investigating the connection between wind speed, humidity, and precipitating shallow cumulus convection. Large Eddy Simulation and bulk analysis reveal complex interactions influencing cloud behavior.

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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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