1. The PBL, Part 2: Structure as observed and modeled by WRF (with in-class demonstration)
ATM 419/563
Spring 2017
Fovell

2. Outline Observations of PBL diurnal cycle
Visualization of diurnal cycle using WRF single-column model simulations

3. …collapses after sunset
PBL height
Daytime:
Deep vigorous mixing,
unstable near surface
Height (m)
…collapses after sunset
surface inversion
PBL height computed vs observed
Local time
Yamada and Mellor (1975)

4. A strongly-heated afternoon PBL
• Temporally averaged vertical profiles of virtual potential temperature (v), vapor mixing ratio (q) and horizontal momentum (M)
• Strong surface heating, combined with surface friction that impedes vertical mixing, causes v to decrease with height near the ground in the surface layer (superadiabatic)
• Farther above, convective turbulence (eddies) efficiently mixes the atmosphere and its conserved properties in the mixed layer
• For a dry adiabatic process, v and q are conserved. If inviscid and unaccelerated, M is also conserved.
Hartmann’s text Fig. 4.6
Virt PT because vapor content influences density via R
Surface layer is absolutely unstable but its shallowness and frictional drag makes it hard for eddies to form and “get a grip”
Virtual potential temperature contains a moisture influence:
q = water vapor mixing ratio (kg of water per kg of air)

5. A strongly-heated afternoon PBL
• Vertical mixing of conserved quantities tends to reduce vertical gradients (i.e., temperature decreases with height but potential temperature does not)
• The entrainment zone (h0 to h2) separates the mixed layer from the free troposphere. Mixing between the PBL and free troposphere occurs intermittently there. Note that it is more stable there than the free troposphere above or the mixed layer below due to that mixing.
[entrain: French, to drag away]
• The effect of surface drag can be seen in the M profile
What does the T-z profile look like?
Why does q decrease w/ z in mixed layer? I think it’s due to entrainment from above, which is larger closer to the free atmosphere (i.e., near PBL top). In case of potential temp, entrain from above brings high theta down, which helps maintain the neutral profile, but with vapor, it’s dry, so entrain erodes moisture at PBL top.
Also consider the countergradient term and what even theta looks like without it.

6. A strongly-heated afternoon PBL
• Here, M is being compared to Mg, the geostrophic momentum
• The straight-line large-scale wind comes into geostrophic balance, a stalemate between the pressure gradient force (which drives the wind) and the Coriolis force (proxy for Earth’s rotation). [geostrophic: Greek = Earth turns]
• The geostrophically balanced wind is slowed by surface friction. Upward mixing of slower air causes the wind to be subgeostrophic in mixed layer… during the day, anyway….
Why does Mg increase with z? If for no other reason, mean density is decreasing with height.

7. Diurnal variation of wind speed and shear with height
noon
midnight
So when is vertical shear largest? Plot can be deceptive. Largest at MIDNIGHT.
Actually, convective mixing during the DAY is mixing out the momentum, decreasing the wind at 350m and shear over the sfc-350m layer. But there’s another factor (next 2 slides)
Vertical wind shear varies through day owing to diurnally-driven mixing.
Recall ARLFRD example…
This is a much deeper layer.
Hartmann’s text, p. 99

8. …collapses after sunset
PBL height
Daytime:
Deep vigorous mixing,
unstable near surface
Height (m)
Residual layer
…collapses after sunset
surface inversion
PBL height computed vs observed
Local time
Yamada and Mellor (1975)

9. Contrast day vs. night Daytime Nighttime q M supergeostrophic
Less vertical shear in
surface layer (SL) q
Because of diurnal variation of surface heating and stability, frictional drag felt on the wind above surface varies. This causes an oscillation in the force balance that governs the wind, creating a pendulum-like oscillation. Like a pendulum, the system oscillates around a stable point, which can include swinging past it. Here, the swing past results in temporary supergeostrophic speeds. This is important to severe weather in the Great Plains because these nocturnal jets advect moisture and storms are influenced by low level shear.
M supergeostrophic
above surface
RL = residual layer
SBL = stable BL
Stull text (2000) q
G - geostrophic

10. Nocturnal low level jet (LLJ)
Height in kilometers
LLJs influence moisture
advection and low-level
wind shear, both very
important to severe weather.
Midnight and 6AM local
Noon and 6PM local
Bonner (1968)

11. Eddy mixing for momentum (Km)
Max values ~ 100 m2/s
Max in afternoon
Yamada and Mellor (1975)

12. Eddy mixing for momentum (Km)
Vertical profile at time
of maximum mixing
Yamada and Mellor (1975)

13. Visualizing the PBL diurnal cycle
Demonstration with
WRF single column model, or SCM

14. WRF SCM 1-D (vertical) single column
Initialization employs input_sounding and input_soil
Many different PBL schemes in WRF, but two basic types: non-local and local schemes
Local schemes predict turbulent kinetic energy (TKE) directly and use it to get Km, Kh as functions of height
Non-local schemes estimate the PBL layer depth and impose a vertical profile of Km, Kh in layer
Popular non-local scheme: YSU PBL
Popular local schemes: MYJ and MYNN2 PBL

15. theta (k)
u and v
(m/s)
z(m)
qv (g/g)
sfc p (Pa)
input_sounding
Initial wind is westerly at 18 m/s
at all height levels supplied

16. Km, Kh profile with height
YSU (non-local scheme) imposes this
MYJ (local scheme) tries to develop it
Hong and Pan (1996)

17. • Non-local schemes tend to do a better job of developing an adiabatic
(obs)
non-local
local
obs
Hong and Pan Fig. 3
• Non-local schemes tend to do a
better job of developing an adiabatic
(constant q or qv) mixed layer than
local schemes
• Shown are two different times during
the daytime heated boundary layer
obs
Hong and Pan (1996)

18. Hong and Pan Fig. 4
• PBL schemes can differ with respect
to how well mixed water vapor gets.
This can influence CAPE and likelihood
of convective activity.
• Here, note non-local scheme is
drier near surface, more moist near
PBL top (too much mixing of vapor
likely)
• PBL scheme variability can be large
(see next slide)
obs
obs
Hong and Pan (1996)

19. • Local schemes don’t always
do better with water vapor…
Grey shade: model PBL depth
Arrow: observed PBL depth
Stensrud’s text, p. 177; after Bright and Mullen (2002)

20. LANDUSE.TBL Log wind profile Surface roughness z0, in centimeters
USGS SUMMER
ALBD SLMO SFEM SFZ0 THERIN SCFX SFHC ’
1, , .10, .88, 80., 3., 1.67, 18.9e5,'Urban and Built-Up Land'
2, , .30, .985, 15., 4., 2.71, 25.0e5,'Dryland Cropland and Pasture'
3, , .50, .985, 10., 4., 2.20, 25.0e5,'Irrigated Cropland and Pasture'
4, , .25, .985, 15., 4., 2.56, 25.0e5,'Mixed Dryland/Irrigated Cropland and Pasture'
5, , .25, .98, 14., 4., 2.56, 25.0e5,'Cropland/Grassland Mosaic'
6, , .35, .985, 20., 4., 3.19, 25.0e5,'Cropland/Woodland Mosaic'
7, , .15, .96, 12., 3., 2.37, 20.8e5,'Grassland'
8, , .10, .93, 5., 3., 1.56, 20.8e5,'Shrubland'
9, , .15, .95, 6., 3., 2.14, 20.8e5,'Mixed Shrubland/Grassland'
10, , .15, .92, 15., 3., 2.00, 25.0e5,'Savanna'
11, , .30, .93, 50., 4., 2.63, 25.0e5,'Deciduous Broadleaf Forest'
12, , .30, .94, 50., 4., 2.86, 25.0e5,'Deciduous Needleleaf Forest'
13, , .50, .95, 50., 5., 1.67, 29.2e5,'Evergreen Broadleaf Forest'
14, , .30, .95, 50., 4., 3.33, 29.2e5,'Evergreen Needleleaf Forest'
15, , .30, .97, 50., 4., 2.11, 41.8e5,'Mixed Forest'
16, , 1.0, .98, 0.01, 6., 0., 9.0e25,'Water Bodies'
These z0 are starting points;
Some schemes modify them

21. Finding the 10-m wind speed
• Very often, the lowest model
level is about 27 m above ground
level (AGL). (This is WRF default.)
• Many (not all!) wind observations
are taken at 10 m AGL.
• How do you compare model
winds to observations?
• Standard practice: employ log
wind profile

22. Finding the 10-m wind speed
• Recall the log profile assumes
neutral conditions, and requires
adjustment when not neutral
• An unstable surface layer has
less vertical shear, so log profile
would underestimate 10-m wind
• A stable layer has more shear, so
the unadjusted 10-m wind is too
large
• As wind speed increases, surface
layer often presumed neutral

23. Finding the 10-m wind speed
If the log wind profile is valid at both the first model
level (z = Za) and at 10 m, then the 10-m wind (V10) in
terms of the first model level wind Va is:
Stability corrections
computed at 10m and Za
Zero when neutral
(divided two log wind profiles
for first model level and 10-m level)

24. Stability function y • In WRF, you call PBL & surface layer
schemes separately (and a
land surface scheme also)
• A function similar to this is
employed in the surface layer
scheme
• L is computed. Then y is computed
at lowest model U level Za and at
10 m level
• This is applied to log profile to get
10-m wind from the Za wind
Stensrud’s text, p. 38

25. Nomenclature
Surface layer scheme handles interactions with the surface, computes friction velocity, log profile stability adjustment, 2-m T and qv and 10-m winds. sf_sfclay_physics
PBL scheme uses surface layer scheme information and computes PBL height, mixing Km and Kh. bl_pbl_physics
Land surface model (LSM) uses surface layer scheme information to compute surface fluxes of heat and moisture, interaction with soil, and effect of vegetation canopy. sf_surface_physics
PBL and surface layer schemes often paired, and LSM selected independently, but not always. But, they’re all tied together, and strongly influence each other.

26. Do PBL_demonstration script