Boundary-Layer Dynamics (mostly from an observational point of view)

1
Boundary-Layer Dynamics(mostly from an
observational point of view)

• Margaret (Peggy) LeMone
• EOL/ASP Colloquium
• 1 June 2009

2
REFERENCES Numerous field programs 2 types
Focus on PBL structure/dynamics/turbulence
(AHATS) PBL component of more
comprehensive experiment (GATE,
hurricanes) EARLY ONeill, Nebraska (Exploring
the Atmospheres First Mile, Lettau and Davidson
(1957) The Kansas Experiment (1968, SW
Kansas) MORE RECENT Puerto Rico (1972) AMTEX
(1975) GARP Atlantic Tropical Experiment (GATE, E
Tropical Atlantic Ocean, Summer 1974) STORM
Fronts Experiment Systems Test (NE Kansas,
Spring1992) CASES-97 (SE Kansas, Spring
1997) CASES-99 (SE Kansas, Fall 1999) ACE
(Atmospheric Chemistry Experiment (West of
Tasmania, Dec. 1995) IHOP_2002 (Southern Great
Plains, late Spring 2002) WKY-TV Tower (Oklahoma,
year round, until 1980s) CCOPE (Cooperative
Convective Precipitation Experiment, Montana,
1981) T-REX (Terrain-Induced Rotor Experiment,
Owens Valley, California, 2006) STAAARTE
(Switzerland, 1999) AND modeling studies as
well.
3

• Definition of Boundary Layer
• When you take off or land in an airplane, the air
  is bumpy near the ground but gets smooth higher
  up. The bumpy layer near the ground is the
  daytime planetary boundary layer (convective
  boundary layer)
• Which leads to the AMS Glossary of Meteorology
  Definition (paraphrased), the layer of air near
  the ground that is directly affected by friction
  from the ground and possibly by transport of heat
  from the surface.

4
Different Views of the Convective Boundary Layer
10 km horizontally
64 km along perimeter of 60-km circle Lidar
aerosol backscatter over the Pacific, west of
Tasmania (Donald Lenschow)
23 km N-S WCR radar reflectivity (insects) in dry
CBL 29 May 2002, OK, (Bart Geerts).
49 km E-W DIAL Lidar water vapor and vertical
velocity in dry CBL IHOP_2002, 7 June 2002,
OK (Chris Kiemle et al., 2007, JTech)
5
Vertical Distribution of Turbulence in
CBL Turbulence kinetic energy u2 v2 w2,
where the lower-case letters indicate a departure
from the mean, is elevated through the CBL.. u2
and v2 maximum near the surface w2 maximum
within PBL.
U, V horizontal wind W vertical wind, w is
a scaling velocity zi PBL depth.
Lenschow et al. (1980, AMTEX, JAS
6
Idealized PBL (1960s, pre-LES)
Force balance above CBL (Northern Hemisphere)
PGF
wind vector
H
Coriolis centrifugal
L
Force balance in CBL (Northern Hemisphere)
Friction
Coriolis centrifugal

• Wind at top of PBL along
• isobars (normal to pressure
• gradient).
• Wind at surface is
• slower,
• toward low pressure
• Slowdown by friction reduces Coriolis
• and centrifugal effects.

Wind hodograph in neutral PBL (Moeng and
Sullivan, JAS, 1994)
Potential temperature and mixing ratio well-mixed
7
Vertical Structure Idealized CBL(strong
convective heating from the bottom, supported by
LES)
Force balance above CBL (Northern Hemisphere)
PGF
Coriolis centrifugal
PGF
Force balance in CBL (Northern Hemisphere)
Friction
Coriolis centrifugal
Change in force balance with height, leads to
wind turning takes place in the entrainment
layer
8
Surface Layer (M-O theory, Paulson 1970, updated)
(bluelog profile) (redstability correction)
Stable
Neutral
Unstable
Figure from Fleagle and Businger, 1963, Adapted
from Lettau and Davidson, 1957, Exploring the
Atmospheres Lowest Mile)
9
Semi-Idealized Equations for Wind and Virtual
Potential Temperature
Virtual potential temperature Qv Horizontal wind
components U and V aligned such that
Overbars and capital letters indicate
averages Assuming horizontal heterogeneity and no
change in wind
10
CONVECTIVE BOUNDARY LAYER Assume horizontal
heterogeneity wind steady state
U, V and Qv well-mixed
vertically no sources/sinks for Qv
11
Convective Boundary Layer
Similarly
and
0
0
or
? Fluxes vary linearly with height.
C1, C2, and C3 are constants
12
TOP Idealized LEFT LES (shading) with
observations 10 Sept 1974 (GATE)1 RIGHT
Observed vertical flux of along- wind component
of momentum2
-0.04 0.0 0.08 0.16 0.24
1Nicholls et al. (GATE, 1982, QJRMS) 2Pennell
and LeMone (Puerto Rico,1974, JAS)
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For fair weather, light winds, wqv at h -0.2
wqv at surface
CAUTION The -0.2 rule applies to wq v not
wq v
h
Normalized virtual temperature flux for four
fair-weather days in GATE.2 Note that
mixing-ratio and humidity-flux profiles remain
linear, but with varying slopes.
10 Sept 1974 (Day 253, GATE) temperature-flux pro
file, tropical East Atlantic1
1Nicholls et al. (1982, QJRMS) 2Nicholls and
LeMone (1980, JAS)
14
Exception growing PBL with strong shear at PBL
top (Conzemius and Fedorovich, 2006)
15
How well does wind fit mixed-layer model?
OVER LAND (Oklahoma example) Less shear
daytime Low-shear occur local noon to early
afternoon OVER OCEAN (Tropical BL) Six-month
average 2 m/s increase with height 6 veering
with height (Gray 1972)
16
An exception rapidly-growing PBL
Nice mixed layer for 10 March, but not for 27
February. Horizontal advection and wind above
PBL similar. Shear on 27 Feb from rapid
engulfment of strong northerly momentum as PBL
grew in bottom example.
LeMone et al. (1998, BLM, STORM-FEST)
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IMPACT OF SURFACE HETEROGENEITYDATA SOURCE
50-km flight track surface array SE of Wichita,
Kansas
8
8
9
9
Winter Wheat (brown)




Grassland (tan, light green)
G
7
7


1 May 97 (CASES)
1 May 97 (CASES)
WW
7-9 on grassland
A
28 May 02 (IHOP)
16 May 02 (IHOP)
A
12 June 02 (IHOP)
14 June 02 (IHOP)
Winter Wheat Harvested 15 June
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Impact of Surface Heterogeneity IHOP_2002 (Summer)
A
Land-use map Red line flight track Oranges,
pinks crops Light green grassland Summer
(IHOP) Wheat dormant (warm) Grass green (cool) H
larger over/downstream of winter wheat Fluxes
are 4-km running averages plotted every
kilometer.
A
Longitude
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Impact of Surface Heterogeneity for CASES-97
(spring)
37.5
37.4
A
37.3
A
A and A green winter wheat (brown in
map) Green and Tan Grass (mixed dormant
and green)
H larger over/downstream of winter wheat Also
with super-adiabatic lapse rate, higher
elevations have higher temperatures than
surrounding air at same height.
20
Heterogeneous surface effect on horizontal winds
RWP 1-hour Consensus winds
Whitewater
Beaumont
COOL
WARM
Oxford
Wind (SSW 5-6 m s-1)
LeMone et al. BLM 2002
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IMPACT on BL STRUCTUREMesoscale Circulations in
CASES-97
Large-scale subsidence
Q for Eastern Track and Triangle Legs
Aircraft conv/div patterns
ABLE Radar wind profiles
Green fetch ? cool air
Dormant fetch elevated heat source (fetch along
ridge) ? Warm air
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Heterogeneity at the top of the PBL
Cloudsheterogeneous cloud distribution

• Wind stability conditions imply
• horizontal roll vortices over region to
• right
• Clouds streets visible only over land
• (LCL high enough for clouds to form).
• Similarly
• Over Ocean, clouds reveal islands
• Over land
• Differences in land cover affect cloud
• distribution1
• First clouds over harvested winter
• wheat field in Oklahoma
• Suppressed clouds over and around
• lakes
• Clouds/storms form preferentially over
• elevated terrain

1
Gemini image of cloud streets over Georgia coast.
23
Heterogeneity at the top of the PBL
Cloudsheterogeneous cloud distribution
1

• Over land
• Differences in land cover affect cloud
• distribution1
• First clouds over harvested winter
• wheat field in Oklahoma
• Suppressed clouds over and around
• lakes
• Clouds/storms form preferentially over
• elevated terrain

Cumulus clouds forming over foothills west of
Boulder there were no clouds anywhere else.
1Rabin et al (BAMS, 1990)
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Low shear Cumulus draw air from beneath via
buoyancy-generated pressure forces (solenoidal
circulation)
Large vertical shear at cloud base Pressure
forces generated by interaction of updraft with
shear (as well as buoyancy).
Buoyant
Where p p0sin(2p/L) w
-w0cos(2p/L) p and w are departures
from layer means.
Complete Equation (Rotunno and Klemp, MWR, 1982)
25

• Cumulus increase subcloud vertical-velocity
  variance
• (relative to clear-sky values)

AMTEX data and formula from Lenschow et al. (JAS)
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Modulation of PBL by waves (local origin)

1. Cu generate waves
2. Waves assume characteristics determined by
   lower-tropospheric environment
3. Waves modulate PBL behavior

Latitude (Degrees North)
9.4
Waves generated from other sources can also
modulate PBL motions.
8.4
Schematic Based on Clark et al. (1986) Data
LeMone and Meitin (1984)
23.4 23.0 22.6
Longitude (Degrees West)
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The Growing PBL (14 June 2002, Oklahoma Panhandle)
Figure 8, Bennett et al., to be submitted to MWR.
28
The Growing PBL
PBL top growth change
results from heating from below, entrainment of
air from above the boundary layer, represented
by entrainment velocity we
in response to buoyancy flux and
mechanical mixing BL grows against subsiding
air, represented by mean vertical velocity W
Surface virtual temperature flux
Bennett et al. (MWR, submitted, IHOP_2002)
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VERY idealized growth rate, for little shear at
PBL top, no advection
DQ
Where h is PBL depth
DzDh
g is the gradient above h
(no heat mixed in from above PBL, i.e., no
entrainment)
Thus, for constant flux, h t1/2. Note that
here there is no entrainment Growth by
encroachment
30
Entraining PBLs (still no shear) from Garratt
(1992) Start with two relationships
No entrainment (bDQv0) Obtain
Qvm
Qv
With entrainment
(same as previous slide)
Conzemius and Fedorovich (2006, JAS) discuss
importance of shear and note that a value less
than -0.2 for the ratio of buoyancy flux at h to
that at the surface is an indication of the
importance of shear.
31
400 800 1200 1600 2000
18 0 6 12 18
Signal-to-Noise Ratio
T.L.
h
stable layer top
super-adiabatic layer top
630
759
929
1229
1059
1400
1530
1700
1833
2022
2130
CASES-97
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Data CASES-99, from S. Burns
Nocturnal PBL
1200 LST
2000 LST
(Schematic from Garratt (1992)
At night, cooling due to IR radiation. Surface
cools most rapidly.
33
Nocturnal PBL Turbulence not necessarily decrease
with height upside-down BL z-less BL
Poulos et al. (BAMS, 2003, CASES-99)
34
Airflow at night can decouple from mean flow if
sufficiently stable, or sufficiently large.
Clear nights with light wind Air at low levels
decoupled from synoptic flow. Cooling ? negative
buoyancy and downhill flow. Air current flowing
downhill continues to cool, creating a linear
dependence of temperature with elevation in the
descending current. Windy Nights Near-surface
air coupled to synoptic flow (constant potential
temperature) Intermediate Near-surface
air intermittently decoupled from synoptic flow.
Top (Mahrt et al. BLM, 2001, CASES-99), Bottom
(LeMone et al. JAS, 2002, CASES-97). Also see
Acevedo and Fitzjarrald, JAS, 2001)
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Complex Terrain ABL affected by the presence of
terrain-forced and diurnal flows at many spatial
and temporal scales
Example of conceptual model of fair weather
evolution of ABL in mountains
NIGHT
DAY
Whiteman, 2000, after Fiedler, from de Wekker
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Daytime PBL Complex Terrain (aerosols as
tracers)
AL height
CBL height

• 1 mountain venting (elevated heat source)
• 2 cloud venting (clouds draw in air from below)
• 3 advection (local and from elsewhere)

(De Wekker)
De Wekker et al, 2004
37

• Outstanding Research Problems for PBL (only a
  subset)
• How to measure (at the surface) Surface energy
  budget, transfer of trace gases
• How to measure (at the PBL top) PBL top,
  entrainment rate, vertical velocity
• Interaction of PBL with cumulus and stratiform
  clouds
• Anything to do with nocturnal/stable PBLs
• Behavior of turbulence at small scales
• Surface energy budget in complex terrain (on a
  slope)
• Effects of surface heterogeneity
• (surface properties, terrain, ocean waves,
  cities, wind farms, solar farms)
• Dispersion of aerosols, trace gases (especially
  for complex terrain, stable conditions)
• Interaction of mesoscale phenomena (waves, PBL
  mesoscale circulations)
• with PBL turbulence and fluxes
• Effects of chemical reactions on PBL flux and
  concentration profiles
• Representation in models of
• surface layer (over land and ocean, especially in
  strong winds
• PBL
• Sub-grid turbulence
• The role of PBL in the evolution of precipitation
  convection

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References

• Carson, D.J., 1973 The development of a dry
  inversion-capped convectively unstable boundary
  layer. Q. J. Roy. Meteor. Soc., 99, 450-467.
• Conzemius, R.G., and E. Fedorovich, 2006
  Dynamics of sheared convective boundary layer
  entrainment, Part I Methodological background
  and large-eddy simulation. J. Atmos. Sci., 63,
  1151-1178.
• Garratt, J. The Atmospheric Boundary Layer,
  Cambridge University Press, 1992.
• And articles referred to on the individual pages.