Boundary Layer of shallow clouds and deep convection

1. Boundary Layer of shallow clouds and deep convection

2. Review of last lecture • Vertical structure of the atmosphere and definition of the boundary layer • Vertical structure of the boundary layer • Definition of turbulence and forcings generating turbulence • Static stability and vertical profile of virtual potential temperature: 3 cases. Richardson number • Boundary layer over ocean • Boundary layer over land: diurnal variation

3. Shallow clouds • Include stratus, stratocumulus and cumulus clouds • Cloud top height generally below 4 km • Sometimes associated with light rain, sometimes not

4. Global distribution of shallow clouds • Do we notice any patterns? Klein and Hartman, 1993

5. Vertical structure of shallow clouds • Intense longwave radiative cooling at cloud top drives eddies in BL • Eddies pick up moisture and maintain cloud • Eddies also entrain warm, dry air from above the inversion • Entrainment lifts the cloud, subsidence lowers it Bretherton et al. 2004

6. Boundary layer of shallow clouds: well-defined (From Barnes 1982)

7. Deep convection • Cloud top height above 9 km • Generally associated with rain • Sometimes organized into mesoscale convective systems

8. Global distribution of deep convection

9. Vertical structure of deep convection: Four components Convective updrafts (controlled by lower troposphere moisture) High θe Mesoscale updrafts Low θe Mesoscale downdrafts High θe Convective downdrafts Zipser (1977), modified by Houze (1993)

10. Boundary layer of deep convection: Not well-defined

11. Differences between shallow clouds and deep convection: Boundary layer T and q

12. Differences between shallow clouds and deep convection: Change of boundary layer T Shallow/midtop convection: T decreases Deep convection: T decreases

13. Differences between shallow clouds and deep convection: Change of boundary layer q Shallow/midtop convection: q increases Deep convection: q decreases

14. Differences between shallow clouds and deep convection: Change of boundary layer moist static energy h Moist static energy h = Cp T + L q + g z Shallow/midtop convection: h keeps constant Deep convection: h decreases

15. Self-suppression processes in deep convection Missing physics III, IV Convective updrafts (controlled by lower troposphere temperature and moisture) High θe Mesoscale updrafts Low θe Missing physics II Mesoscale downdrafts High θe Convective downdrafts Missing physics I Zipser (1977), modified by Houze (1993)

16. Over-stabilized state: A state far awayfrom quasi-equilibrium Model atmosphere fluctuates around the quasi-equilibrium state (always drizzle) The observed atmosphere swings far away from the quasi-equilibrium state (switch between heavy shower and clear sky)

17. Summary • Global distribution and vertical structure of shallow clouds • Global distribution and vertical structure of deep convection. Four components: convective updraft, convective downdraft, mesoscale updraft, mesoscale downdraft • Differences between shallow clouds and deep convection: change of T, q and h in the boundary layer • Self-suppression processes in deep convection: Overly stabilized state after deep convection • Problems in current global climate models: lack of self-suppression processes