1. The role of the mean relative humidity on the dynamics of shallow cumuli - LES Sensitivity experiments Stephan de Roode (KNMI/IMAU) Simon Axelsen (IMAU) Pier Siebesma (KNMI)

2. Large-eddy simulation -The BOMEX shallow cumulus case

3. LES sensitivity experiments - Set-up analogous to Derbyshire's EUROCS deep convection case

4. Results for cloud core : mass fluxAll averages taken over the 3rd hour

5. Results for cloud core : vertical velocity and core fraction

6. Mixed parcel diagram -Buoyancy reversal

7. Buoyancy reversal - Dependency on mean vertical gradients G 1.0 0.0

8.  and fractional entrainment rate.Diagnosis from LES results.

9. Kain & Fritsch parameterization- Assume flat PDF for ce = e02 (Bretherton et al. 2004)

10. Bowen ratio, convective velocity scale w* and mass-flux

11. Bowen ratio, convective velocity scale w* and mass-flux

12. Conclusions • Set I - Change relative humidity, but keep mean buoyance the same • Cloud fraction changes => • Mass flux changes • Set II - Change Bowen ratio, but keep surface buoyancy flux the same • Cloud fraction changes => • Mass flux changes (also at cloud base)

13. Results for cloud core : fractional entrainment and detrainment

14. c* from LES