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 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 Nuijens, Stevens and Siebesma (2009)

Ideas and questions  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) Betts and Ridgway ('89), Bellon and Stevens ('05), Stevens ('06) A column of air moving at a greater speed: enhanced upward transport of moisture, deeper clouds, hence more rain?

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

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

Flux behavior

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)?

Bulk analysis (1)

Bulk analysis (2)  fixed,  q varies to keep B constant  v h LCL SHLH B  q wewe M

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?)