1. What controls the climatological PBL depth? Brian Medeiros Alex Hall Bjorn Stevens UCLA Department of Atmospheric & Oceanic Sciences 16th Symposium on Boundary Layers and Turbulence, Portland, Maine

2. Quick Outline Climatological PBL depth The mass budget approach Finding PBL regimes Summary

3. Annual-mean PBL Depth Mean PBL depth from 10-year UCLA GCM simulation. SST and sea ice concentration are prescribed to climatological values. Coarse resolution is used (4º×5º×15levels). UCLA PBL PARAMETERIZATION (Suarez et al. 1983) Modified vertical coordinate Mixed layer assumption Simplified stratocumulus prediction

4. Control Simulation No Diurnal Cycle Effects of the diurnal cycle

5. leading mode of spatial variability for PBL depth. dominated by diurnal effects over land w/o diurnal cycle, contrast is diminished and PBL depth over land is much deeper a simple CBL model shows the same behavior Representation of the residual layer created by the nightly PBL collapse is essential for getting a “reasonable” mean PBL depth over land. ~63 hPa ~104 hPa ~115 hPa ~127 hPa Sahara NE Pacific CBL Model Land-sea contrast Land & ocean should be treated separately.

6. PBL Mass Budget GCM obeys such a mass budget. Steady state gives simple estimate of PBL depth. Entrainment is proportional to buoyancy fluxes and stability. A few references: Stage & Businger (1981), Lewellen & Lewellen (1998), Stevens (2002) Now how does h depend on B, σ, M B...

8. Deep Convection

9. Deep Convection STBL SBL

12. SBL & STBL CBL

13. Regime Buoyancy Flux Static Stability PBL depth SBL (Cold surfaces) LowHighLow STBL (subtropics) Mid/High (cloud-top cooling) MidHigh CBL (dry land) High (sensible) Low Deep Conv. (tropical ocean) Low (latent) LowLow/Mid Summary of PBL regimes’ characteristics.

14. NE Pac Sc NW Pac SH Sea Ice Desert N Asia Seasonal changes in PBL depth

15. How well does this do? A linear multiple regression gives Accounts for 91% and 83% of geographic variability for ocean and land, respectively.

16. Conclusions... A few PBL regimes dominate the distribution (in this model). PBL depth is controlled by the interplay of buoyancy, stability, and cumulus convection Mean PBL depth is strongly affected by seasonal shifts in PBL regime and by diurnal variation over land. A similar analysis of global observations could provide useful insights into PBL processes and model validation