What controls the climatological PBL depth? Brian Medeiros Alex Hall Bjorn Stevens UCLA Department...
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Transcript of What controls the climatological PBL depth? Brian Medeiros Alex Hall Bjorn Stevens UCLA Department...
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