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

Margaret (Peggy) LeMoneEOL/ASP Colloquium

1 June 2009

REFERENCES: Numerous field programs

2 types: Focus on PBL structure/dynamics/turbulence (AHATS) PBL component of more comprehensive experiment (GATE, hurricanes)

EARLY:O’Neill, Nebraska (“Exploring the Atmosphere’s First Mile”, Lettau and Davidson (1957)The “Kansas Experiment” (1968, SW Kansas)

MORE RECENTPuerto 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.

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.

49 km E-WDIAL Lidar water vapor and vertical velocity in dry CBLIHOP_2002, 7 June 2002, OK(Chris Kiemle et al., 2007, JTech)

23 km N-SWCR radar reflectivity(insects) in dry CBL29 May 2002, OK, (Bart Geerts).

64 km along perimeter of 60-km circleLidar aerosol backscatter over the Pacific, west of Tasmania(Donald Lenschow)

Different “Views” of the Convective Boundary Layer= 10 km horizontally

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.

1/3

*i v

v

gz ww

Lenschow et al. (1980, AMTEX, JAS

Idealized PBL (1960s, pre-LES)

Force balance above CBL(Northern Hemisphere)

PGF

Coriolis + centrifugal

wind vector

Force balance in CBL(Northern Hemisphere) Coriolis + centrifugal

Friction

Wind at top of PBL alongisobars (normal to pressuregradient).

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)

H

L

Potential temperature and mixing ratio well-mixed

Vertical Structure – Idealized CBL(strong convective heating from the bottom, supported by LES)

Change in force balance with height, leads to wind turning takes place in the entrainment layer

Force balance above CBL(Northern Hemisphere)

PGF

Coriolis + centrifugal

Force balance in CBL(Northern Hemisphere)

PGF

Coriolis + centrifugalFriction

Surface Layer (M-O theory, Paulson 1970, updated)

*

0

21

ln ,

1 12ln ln 2 tan

2 2 2

hh sh

sh

u zS

k z

x xx

*

2

ln ,

12ln

2

hh th

t

th

T zT

k z

x

1

41

zx

L

3*

' 'v

v

u TL

kg w T

Figure from Fleagle and Businger, 1963,Adapted from Lettau and Davidson, 1957,Exploring the Atmosphere’s Lowest Mile)

Stab

le

Neu

tral

Unsta

ble

(blue=log profile)(red=stability correction)

Semi-Idealized Equations for Wind and Virtual Potential Temperature

' '1

' '1

' 'vv

v

u wU PfV V U

t x z

v wV PfU V V

t y z

wV S

t z

��������������

��������������

��������������

', .u U u etc

Overbars and capital letters indicate averagesAssuming horizontal heterogeneity and no change in wind…

Virtual potential temperature v

Horizontal wind components U and V aligned such that 0P

x

' '1

' '1

' 'vv

v

u wU PfV V U

t x z

v wV PfU V V

t y z

wV S

t z

��������������

��������������

��������������

CONVECTIVE BOUNDARY LAYERAssume horizontal heterogeneity wind steady state U, V and v well-mixed vertically

no sources/sinks for v

Convective Boundary Layer

2

2

' ' 1

' ' 1

u w PfV

z x

u w P Vf

z z x z

≈ 0 = 0

2

2

' '0

u w

z

or1

' 'u wC

z

Similarly

2

' 'v wC

z

and

'

3

'vwC

z

Fluxes vary linearly with height.C1, C2, and C3 are constants

-0.04 0.0 0.08 0.16 0.24

TOP: Idealized

LEFT: LES (shading)with observations10 Sept 1974 (GATE)1

RIGHT: Observedvertical flux of along-wind component of momentum2

1Nicholls et al. (GATE, 1982, QJRMS); 2Pennell and LeMone (Puerto Rico,1974, JAS)

10 Sept 1974 (Day 253, GATE) temperature-fluxprofile, tropical East Atlantic1

h

Normalized virtual temperature fluxfor four fair-weather days in GATE.2

Note that mixing-ratio and humidity-fluxprofiles remain linear, but with varyingslopes.

1Nicholls et al. (1982, QJRMS)2Nicholls and LeMone (1980, JAS)

For fair weather, light winds, w’v‘ at h ≈-0.2 w’v‘ at surface

CAUTION:The “-0.2” ruleapplies tow’ v’ not w’ v’

Exception: growing PBL with strong shear at PBL top (Conzemius and Fedorovich, 2006)

0

' '

' 'v

v

w T

w T

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 height6° veering with height (Gray 1972)

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 strongnortherly momentum as PBL grew in bottom example.

LeMone et al. (1998, BLM, STORM-FEST)

IMPACT OF SURFACE HETEROGENEITY:

DATA SOURCE: 50-km flight track + surface array SE of Wichita, Kansas

28 May 02 (IHOP)

12 June 02 (IHOP)

16 May 02 (IHOP)

14 June 02 (IHOP)

Grassland(tan, light green)

Winter Wheat (brown)

WW

G7

89

+

+ +

A

A’

7-9 on grassland

1 May 97 (CASES) 1 May 97 (CASES)

Winter Wheat Harvested ~ 15 June

7+

++8

9

Impact of Surface Heterogeneity IHOP_2002 (Summer)

Land-use mapRed line = flight trackOranges, pinks: cropsLight green: grassland

Summer (IHOP)Wheat dormant (warm)Grass green (cool)

H larger over/downstream ofwinter wheat

Fluxes are 4-km running averagesplotted every kilometer.

Longitude

A

A’

Impact of Surface Heterogeneity for CASES-97 (spring)37.5

37.4

37.3 AA’

A and A’ – green winter wheat (brown in map)Green and Tan – Grass (mixed dormant and green)

H larger over/downstream of winter wheatAlso – with super-adiabatic lapse rate, higher elevations havehigher temperatures than surrounding air at same height.

Heterogeneous surface effect on horizontal winds

Whitewater

Oxford

Beaumont

Wind (SSW 5-6 m s-1)

LeMone et al. BLM 2002

WA

RM

CO

OL

RWP 1-hour “Consensus” winds

IMPACT on BL STRUCTURE:Mesoscale Circulations in CASES-97

Large-scale subsidenceLarge-scale subsidence

Aircraft conv/div patterns

ABLERadar windprofiles

for Eastern Trackand “Triangle Legs”

Green fetch cool air

Dormant fetch +elevated heat source (fetch along ridge) Warm air

Heterogeneity at the top of the PBL – Cloudsheterogeneous cloud distribution

Wind + stability conditions implyhorizontal roll vortices over region toright

Clouds streets visible only over land(LCL high enough for clouds to form).

SimilarlyOver Ocean, clouds reveal islands

Over landDifferences 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 overelevated terrain

1Rabin et al (BAMS, 1990)Gemini image of cloud streets over Georgiacoast.

1

Heterogeneity at the top of the PBL – Cloudsheterogeneous cloud distribution

Over landDifferences 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 overelevated terrain

Cumulus clouds forming over foothills west of Boulder – there were no clouds anywhere else.

1

1Rabin et al (BAMS, 1990)

Low shearCumulus draw air from beneath via buoyancy-generated pressure forces (solenoidal circulation)

Large vertical shear at cloud basePressure forces generated by interaction of updraft with shear (as well as buoyancy).

2

2

p B

z z

00

eLw dUp

dz

Where

p’ = p0sin(2L)w’ = -w0cos(2L)

p’ and w’ are departures fromlayer means.

22 22

0

'2 2 2

p u v w u v u w v w B

p x y z y x z x z y z

Complete Equation:(Rotunno and Klemp, MWR, 1982)

Buo

yant

AMTEX data and formula from Lenschow et al. (JAS)

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

a. Cu generate wavesb. Waves assume characteristics

determined by lower-tropospheric environment

c. Waves modulate PBL behavior

Schematic: Based on Clark et al. (1986)Data: LeMone and Meitin (1984)

Waves generated from other sourcescan also modulate PBL motions.

Modulation of PBL by waves (local origin)

9.4

8.4

Latit

ude

(Deg

rees

Nor

th)

23.4 23.0 22.6

Longitude (Degrees West)

The Growing PBL (14 June 2002, Oklahoma Panhandle)

Figure 8, Bennett et al., to be submitted to MWR.

The Growing PBL

PBL top growth change:

ie

zw W

t

' 'e

w sw

s

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, representedby mean vertical velocity W

Surface virtual temperature fluxBennett et al. (MWR, submitted, IHOP_2002)

VERY idealized growth rate, for little shear at PBL top, no advection

' ' ' 'v vv v h hw wh

t h t h

Where

h is PBL depth

v

h

≡ is the gradient above h

2 0 0' ' ' ' ' '1

2v v vh

w w whh h

t t

Thus, for constant flux, h ~ t1/2.

Note that here there is no entrainment Growth by “encroachment”

z=h

(no heat mixed in fromabove PBL, i.e., no entrainment)

Entraining PBLs (still no shear) from Garratt (1992)

Start with two relationships:

0' ' ' 'v vvm h

mh

w w

t hh

wt t t

vm v' 'v hw 0' 'vw

0(1 ) ' 'vv hh

whw

t t h

No entrainment (=v=0)Obtain:

2 0' '

2vw

ht

(same as previous slide)

2 0' '

(1 2 )2

vwh

t

With entrainment:

Conzemius and Fedorovich (2006, JAS) discuss importance of shear; andnote 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.

4:00 8:00 12:00 16:00 20:0018 0 6 12 18

Signal-to-Noise Ratio

6:30 7:59 9:29 12:2910:59 14:00 15:30 17:00 18:33 20:22 21:30

hT.L.

super-adiabatic layer topstable layer top

CASES-97

1200 LST

2000 LST

Nocturnal PBL

(Schematic from Garratt (1992)

Data: CASES-99, from S. Burns

At night, cooling due toIR radiation. Surface coolsmost rapidly.

Nocturnal PBL:Turbulence not necessarilydecrease with height

“upside-down” BL“z-less” BL

Poulos et al. (BAMS, 2003, CASES-99)

Clear nights with light wind: Air at low levels decoupled from synopticflow. Cooling negative buoyancyand downhill flow.

Air current flowing downhillcontinues to cool, creating a linear dependence of temperaturewith elevation in the descending current.

Windy NightsNear-surface air coupled to synoptic flow(constant potential temperature)

Intermediate: Near-surface airintermittently decoupled from synopticflow.

Top (Mahrt et al. BLM, 2001, CASES-99), Bottom (LeMone et al. JAS, 2002, CASES-97). Also see Acevedo and Fitzjarrald, JAS, 2001)

Airflow at night can decouple from mean flow if sufficiently stable, orsufficiently large.

2

v

gzRiUz

Complex Terrain: ABL affected by the presence of terrain-forced and diurnal flows at many spatial and temporal scales

Whiteman, 2000, after Fiedler, from de Wekker35

Example of conceptual model of fair weather evolution of ABL in mountains:

NIGHT

DAY

1: mountain venting (elevated heat source)

2: cloud venting (clouds draw in air from below)

3: advection (local and from elsewhere)

AL height

CBL height

36De Wekker et al, 2004

Daytime PBL – Complex Terrain (aerosols as tracers)

(De Wekker)

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 (onset of convection through “recovery” of boundary layer)

•Behavior of the PBL during the “evening transition”

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.