1. The Evolution of the PBL
COMET Planetary Boundary Layer Symposium
Matt Haugland

2. Introduction Stages of PBL Evolution Diurnal Cycle at the Surface
Understand 4 stages of PBL evolution: morning, daytime, sunset, nighttime
Diurnal Cycle at the Surface
How the evolution of the PBL impacts conditions at the surface
Applications
Discuss examples of impact of PBL evolution on near-surface conditions

3. Morning Stage Driven by solar heating of the surface
Convection mixes out nocturnal boundary layer
Free
Atmos.
PBL
Potential
Temperature
Profile

4. Daytime Stage Sensible heating warms mixed layer
and increases depth of the boundary layer
Air freely transported through mixed layer
Air from surface heats top of PBL
Air from top of PBL cools surface, increases wind speed, decreases dewpoint, etc.

5. Entrainment Air from higher level transported to surface
During the day: stronger wind (less friction), lower temperature, lower dewpoint, etc. mixed to surface
Wind gust is smaller-scale example of this
Also horizontal entrainment into rising parcels
wind
cooler
drier
warmer

6. Sunset Stage Surface cooling begins before sunset
Near-surface layer becomes stable
Mixing virtually shuts off near surface
PBL decouples (detaches) from free atmosphere

7. Momentum – sfc winds and LLJ
Stable, uncoupled PBL + surface friction
Result is weak surface winds
Momentum from surface doesn’t disappear
Momentum transported above surface >> low-level “jet”
wind profile

8. Sunset – Surface Wind Speed Minimum
Light wind often observed at sunset (relative to day & night)
Near surface, sunset is most stable time of night – rapidly cooling surface but relatively warm daytime air above. Later, near-surface air becomes less stable
and strong wind shear creates turbulence
Sunset minimum only on mostly clear nights
wind profile
strong
wind shear

9. Sunset – Surface Wind Speed Minimum
9/12/2006-9/13/2006 Porter, OK
Nighttime wind stronger than at sunset but weaker than daytime
sunset
Time (local)

10. Nighttime Stage Stable boundary layer grows throughout night
Never gets as deep as mixed layer
Up to approx. 200 m deep

11. The Diurnal Cycle at the Surface
The sun “powers” the surface energy budget and PBL evolution
A diurnal cycle (or lack thereof) for any parameter indicates importance of land surface
When Rnet is near zero, fluxes (and thus, local variations) are minimal

12. The Diurnal Cycle at the Surface
How evolution of the PBL impacts various parameters measured near the surface
Examples from around the country

13. Dewpoint: Various land surfaces
May, New Orleans, LA
Jan., Minneapolis, MN
Large morning spike
Swamps, dense veg.
Jan., Barrow, AK
Snow cover – dewpoint simply a function of temperature
No sun = no diurnal cycle

14. Wind (speed & direction)
Typical Diurnal Cycle: Wichita, KS, August

15. Wind (speed & direction)
Almost always highest during afternoon – mixing of air from higher levels
Wide diurnal range in valleys
Late-afternoon peak near large oceans/lakes – sea-breeze
WIND DIRECTION
Generally more westerly during afternoon – mixing of air from higher levels
Often comes from direction of nearest body of water during afternoon
Channeled around terrain at night. Sharp changes at sunrise/sunset near mountains.

16. Wind – Various land surfaces
Jun., Arcata, CA
Jun., Jacksonville, FL
Strong sea breeze
Land breeze to easterly sea-breeze
Sep., Boise, ID
Jul., Fort Smith, AR
Up/down valley flows
Nighttime valley channeling
Gradual mixing of S flow

17. Precipitation STRATIFORM almost no diurnal cycle
Diurnal cycle (or lack thereof) indicates importance of land surface
STRATIFORM
almost no diurnal cycle
Land surface not very important
SURFACE-BASED CONVECTION
most precipitation during late afternoon
Land surface extremely important
MCS
most precipitation at night
Land surface important

18. Precipitation Stratiform Surface-Based Convection Nov., Seattle, WA
Jul., Jacksonville, FL
MCS
Jul., Albuquerque, NM
Jul., Goodland, KS

19. Discuss
What do the following diurnal cycles tell you about the land surface…
Think about:
terrain
vegetation
bodies of water
regional climate

20. Discuss
Discuss
July
Describe the land surface characteristics at and around this site

21. Discuss Discuss Boulder, CO A lot of D.A.T. – dry region
No evening spike - not much vegetation
Not much condensation
Sharp day/night transition, stronger directional signal at night - mountains nearby?
Mostly surface-based convective clouds/precip

22. Discuss
March
Describe the land surface characteristics at and around this site

23. Discuss San Diego, CA Small diurnal range - near coast?
Minimal dry air entrainment
Almost no sign of evapotranspiration – sparse vegetation?
Major body of water toward WNW
No channeling of wind at night
Mostly stratiform precipitation

24. Discuss
May
Describe the land surface characteristics at and around this site

25. Discuss Charleston, SC Minimal D.A.T. - moist region
Morning dewpoint spike – lots of evaporation/vegetation
Body of water toward the SE?
Convective precip.

26. Questions?
Questions?