What controls the climatological PBL

depth?

Brian MedeirosAlex Hall

Bjorn Stevens

UCLA Department of Atmospheric &

Oceanic Sciences

16th Symposium on Boundary Layers and Turbulence, Portland, Maine

Quick Outline

•Climatological PBL depth

•The mass budget approach

•Finding PBL regimes

•Summary

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

Control Simulatio

n

No Diurnal Cycle

Effects of the diurnal cycle

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, MB...

DeepConvection

DeepConvection

STBL

SBL

SBLSBL & STBL

CBL

RegimeBuoyancyFlux

Static Stability

PBLdepth

SBL(Cold surfaces)

Low High Low

STBL(subtropics)

Mid/High(cloud-top cooling)

Mid High

CBL(dry land)

High(sensible)

Low Low

Deep Conv.(tropical ocean)

Low(latent)

Low Low/Mid

Summary of PBL regimes’ characteristics.

How well does this do?

A linear multiple regression gives

Accounts for 91% and 83% of geographic variability for ocean and

land, respectively.

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