The Planetary Boundary Layer in Complex Terrain

John HorelNOAA Cooperative Institute for

Regional PredictionDepartment of Meteorology

University of Utahjhorel@met.utah.edu

Photo: J. Horel

What is CIRP? CIRP: NOAA Cooperative Institute for Regional Prediction

at the University of Utah Mission: Improve weather and climate prediction in regions

of complex terrain People:

Staff: John Horel, Jim Steenburgh, Mike Splitt, Judy Pechmann, Will Cheng, Bryan White, Brian Olsen

Students: Justin Cox, Jay Shafer, Ken Hart, Dave Myrick, Dan Zumpfe, Erik Crossman, Greg West

References •Barry, R., 1992: Mountain Weather and Climate. Rutledge•Blumen, W., 1990: Atmospheric Processes Over Complex Terrain. American Meteorological Society, Boston, MA.•Clements, C., D. Whiteman, J. Horel, 2003: Cold pool evolution and dynamics in a mountain basin. J. Appl. Meteor., 42, 752-768.•Garratt, J., 1992: The Atmospheric Boundary Layer. Cambridge•Horel, J., M. Splitt, L. Dunn, J. Pechmann, B. White, C. Ciliberti, S. Lazarus, J. Slemmer, D. Zaff, J. Burks, 2002: MesoWest: Cooperative Mesonets in the Western United States. Bull. Amer. Meteor. Soc., 83, 211-226. •Kalnay, E., 2003: Atmospheric Modeling, Data Assimilation and Predictability. Cambridge•Kossmann, M., and A. Sturman, 2003: Pressure-driven channeling effects in bent valleys. J. Appl. Meteor., 42, 151-1158.•Lazarus, S., C. Ciliberti, J. Horel, K. Brewster, 2002: Near-real-time Applications of a Mesoscale Analysis System to Complex Terrain. Wea. Forecasting. 17, 971-1000.•Stull, R. B., 1999: An Introduction to Boundary Layer Meteorology. Kluwer•Whiteman, C. D., 2000: Mountain Meteorology. Oxford•Zhong, S. and J. Fast, 2003: An evaluation of the MM5, RAWMS, and Meso-Eta Models at Subkilometer resolution using VTMX field campaign data in the Salt Lake Valley. Mon. Wea. Rev., 131, 1301-1322.•Notes: Summer School on Mountain Meteorology 2003. http://www.unitn.it/convegni/ssmm.htm

Outline

Part I- Characteristics/impacts of complex terrain Part II- Resources for observing surface weather Part III- Basin boundary layer Part IV- Mountain-valley and lake breezes

Field Programs CASES-99 Cooperative Atmosphere-Surface

Exchange Study. Kansas. Poulos et al., 2002: BAMS, 83, 555-581.

MAP Mesocale Alpine Program. Alps. Bougeault et al., 2002, BAMS, 82, 433-462.

VTMX Vertical Transport and Mixing Experiment. Salt Lake Valley. Doran et al. 2002, BAMS, 83, 537-551.

PBL IssuesVTMX Science Plan: Measurement and modeling of vertical transport and mixing processes in the

lowest few kilometers of the atmosphere are problems of fundamental importance for which a fully satisfactory treatment has yet to be achieved

Although a general theoretical understanding of many of the physical phenomena relevant to vertical transport and mixing processes exists, that understanding is incomplete, the representation of various phenomena in models is often poor, and the data needed to test those models are lacking.

The upward and downward movements of air parcels in stable and residual layers of the atmosphere and the interactions between adjacent layers are particularly difficult processes to characterize, and significant difficulties also exist in describing the behavior of the atmosphere during morning and evening transition periods.

Complications due to heterogeneous land surfaces and complex terrain further compromise our ability to treat vertical transport and mixing processes properly.

www.pnnl.gov/vtmx

VTMX Science Questions

What are the fundamental processes that control vertical transport for stable and transition boundary layers?

How can momentum, heat, and moisture fluxes be modeled and predicted in a stratified atmosphere with multiple layers?

What improvements in numerical simulations and forecasts of vertical transport and mixing during stable and transition periods are feasible and how can they be implemented?

What formulations are most appropriate for the description of vertical diffusion in stable air? For example, how rapidly will an elevated layer of pollutants mix towards the ground in a stable pool trapped within a basin, and how can that mixing be modeled?

What is the sensitivity of current local weather forecast and dispersion model predictions to variations in the treatment of vertical diffusivity and turbulence?

What limits our ability to forecast vertical transport in current numerical prediction models?

How do traveling weather systems remove stable stagnant air out of a basin, and under what conditions do these removal mechanisms fail?

What is the nature of the interaction of terrain-induced flows (e.g., drainage winds at night, upslope winds during the day, and waves) with cold air pools in basins, and how do such flows affect the formation and erosion of those pools and the dispersion of pollutants in them?

What are the effects of complex terrain?

Substantial modification of synoptic or meso scale weather systems by dynamical and thermodynamical processes through a considerable depth of the atmosphere

Recurrent generation of distinctive weather conditions, involving dynamically and thermally induced wind systems, cloudiness, and precipitation regimes

Slope and aspect variations on scales of 10-100 m form mosaic of local climates

(Barry 1992)

Effects of Complex Terrain

Carruthers and Hunt 1990

Billiard ball analogy “If the earth were greatly reduced in size while maintaining its shape, it

would be smoother than a billiard ball”. (Earth radius = 6371 km; Everest = 8.850 km)

Nonetheless, mountains have a large effect on weather. Why is this, if they are so insignificant in size?

Answer: the atmosphere, like the mountains, is also shallow (scale height 8.5 km) so mountains are a significant fraction of atmos depth.

But, this answer underestimates mountain effect for two reasons: Stability gives the atmosphere a resistance to vertical displacements The lower atmosphere is rich in water vapor so that slight adiabatic ascent

brings the air to saturation.

Example: flow around a 500-m mountain (<< 8.5 km) could include 1) broad horizontal excursions, 2) downslope windstorm on lee side, and 3) torrential orographic rain on windward side.

Smith (1979)

Distribution of mountains on the globe (Barry 1992)

Elevation range Mountains (106 km2) Plateau (106 km2) Mountains/Land Surface (%)

>3000 m 6 --- 4.0

2000-3000 m 4 6 2.7

1000-2000 m 5 19 3.4

0-1000 m 15 94 10.1

Total 30 119 20.2

Total land surface is about 149 million km2. Oceanic islands covering 2 million km2 are not included in the listed areas. Plateau & mountains are both included in the table’s 1st line.

Louis (1975)

Energetic Considerations

Since the atmosphere is heated mainly from the ground, cooling effect upon earth’s surface of latent and sensible heat fluxes is nearly double that of radiative fluxes

Since much of the land surface is hilly, thermally driven circulations play important role in global energy balance

F. Fiedler. Summer School Trento

Chen, C.-C., D. Durran and G. Hakim(2003) ICAM

Surface Wind and Vorticity Around Isolated Mountain:Interaction with Large-scale flow

Potential Temp, Vertical Velocity, and Turbulent Mixing

Chen, C.-C., D. Durran and G. Hakim (2003) ICAM

Energy and mass exchanges near ground---interactions among soil science, hydrological cycles (ground and air), ecosystems, and atmosphere.•Canopy •Terrain•Heterogeneous surfaces•Clouds/fog•Urban environment, air pollution

Heig

ht

(m)

Planetary boundary layer

1 km

D. Lenschow

Shallow Drainage Flows – Mahrt, Vickers, Nakamura, Soler, Sun,Burns, & Lenschow – BLM, 101, 2001.

Schematic cross-section of prevailing southerly synoptic flow, northerly surface flow downThe gully, and easterly flow likely drainage flow from Flint Hills. Numbers identify theSonic anemometers on the E-W transect. E is to the right and N into the paper.

Pollutant Transport in Valleys

Savov et al. (2002; JAM)

Nighttime Stable Layer in Valley

After Breakup of Nighttime Stable Layer in Valley

Daytime vertical mixing processes

Jerome Fast

Diurnal mountain wind systems

Whiteman (2000)Whiteman (2000)

Mountain-plain circulation, Rocky Mountains

2 m s-1

A A'

AZ

WS

MB

PV MC FBLT WC

a)

PV

MC

FBLT WC

A A'

b)

00 UTC/18 CST

00 UTC/18 CST

2 m s-1

0

1

2

3

4

5

6

US radar profiler network, 1991-1995, Jun-Aug, 500 m gate, max=3.5 m/s US radar profiler network, 1991-1995, Jun-Aug, 500 m gate, max=3.5 m/s

Whiteman and Bian (1998)Whiteman and Bian (1998)

Alpine pumping

Mountain-plain circulation in Alps (Vertikator)

Innsbruck

Boundary of Alpine pumpingsynoptic conditions modify shape 100 km

Munich

MilanTurin

Zürich

Graz

Emissions within the area of Alpine Pumping are transported

into the Alps and mixed convectively to higher levels

Lyon

Lugauer et al. (2003)

Mountain venting, anti-slope flow

Local Time (PDT)

0

100

200

300

400

500

600

700

800

900

1000

Heig

ht A

bove

Gro

und (

m)

0900

3 m/s

CBL Height from Lidar

Vertical cross-section of slope flow (upslope to the right)

25 July 200125 July 2001

ReutenReuten et al.( 2002) with Steyn et al.( 2002) with Steyn

Valley cross sections

Whiteman (2000)

Whiteman (1980)Whiteman (1980)

temperature temperature and wind and wind structure structure layers at a layers at a time midway time midway through the through the transitiontransition

Channeling of synoptic/mesoscale winds

Forced ChannelingForced Channeling

Pressure Driven ChannelingPressure Driven Channeling

Whiteman (2000)Whiteman (2000)

Bent valley with 45° changes in wind direction above valley

Kossmann & Sturman (2003)

Dynamic Channeling

Kossman and Sturman 2003

Western U.S. Terrain(high- dark;low-light)

High terrain (dark)

Flat (tan)Mtn. Valleys

(light)

A. Reinecke

Normalized surface-layer velocity standard deviations for near neutral conditions in the Adige Valley in the northern Italy alpine region. a is from Panofsky and Dutton, 1984; b the average valuesfrom MAP; e/u*

2 is the normalized turbulence kinetic energy (From de Franceschi, 2002).

σu /u* σv /u* σw /u* e /u*2

Flat uniform terrain

2.39 1.92 1.25 5.48

Rolling terrain

2.65±4.50 2.00±3.80 1.20±1.24 6.23±18.11

Along valley 2.19 2.13 1.55 5.88

D. Lenschow

West DEM Grid Points vs. MesoWest Stations

0

10

20

30

40

50

60

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

Valley Flat Mountain

% o

f T

otal

Green-WestBlue-MesoWest

Adding Physiographic Information to MesoWest

Land Data Assimilation Systems

(LDAS)

UMD Vegetation Types

Exposure? Forested? Nearby Water? Mountain/Valley? Urban? Slope? Aspect?

MesoWest land characterization

* Sites located disproportionately in urban areas and near water resources.

-100

0

100

200

300

400

500%

diff

eren

ce fr

om W

est

Diurnal Temperature Range

A. Reinecke

Diurnal fair weather evolution of bl over a plain

Whiteman (2000)Whiteman (2000)

surface →

layer

mixed →

layer

free →

troposphere

D. Lenschow

D. Lenschow

D. Lenschow

D. Lenschow

Diurnal evolution of the convective and stable boundary layers in response to surface heating (sunlight) and cooling.

D. Lenschow

Atmospheric structure evolution in valley terrain

Whiteman (2000)Whiteman (2000)

Roughness Effects

For well-mixed conditions (near neutral lapse rate) U2 = u1 ln (z2/zo)/ln(z1/z0)

Roughness length zo=.5 h A/S where h height of obstacle, A- silhouette area, S surface area A/S< .1

Zo- height where wind approaches 0

Roughness lengths zo for different natural surfaces (from M. de Franceschi, 2002, derived from Wieringa, 1993).

zo (m) Landscape Description________________________________________________________________0.0002 Open sea or lake, tidal flat, snow-covered plain, featureless desert, tarmac, concrete with a fetch of several km.0.005 Featureless land surface without any noticeable obstacles; snow covered or fallow open country0.03 Level country with low vegetation and isolated obstacles with separations of at least 50 obstacle heights0.10 Cultivated area with regular cover of low crops; moderately open country with occasional obstacles with separations of at least 20 obstacle heights0.25 Recently developed “young” landscape with high crops or crops of varying height and scattered obstacles at relative distances of about 15 obstacle heights0.50 Old cultivated landscape with many rather large obstacle groups separated by open spaces of about 10 obstacle heights; low large vegetation with with small interstices1.0 Landscape totally and regularly covered with similar sized obstacles with interstices comparable to the obstacle heights; e.g., homogeneous cities

Effects of irregular terrain on PBL structure

Flow over hills (horizontal scale a few km; vertical scale a few 10’s of m up to a fraction of PBL depth)

Flow over heterogeneous surfaces (small-scale variability with discontinuous changes in surface properties)

Inner layer – region where turbulent stresses affect changes in mean flow

Outer layer – height at which shear in upwind profile ceases to be important

(Kaimal & Finnigan, 1994).

(Kaimal & Finnigan, 1994).

(Kaimal & Finnigan, 1994).

D. Lenschow

Effects of horizontal heterogeneity in surface properties

Changes in surface roughness Rough to smooth Smooth to rough

Changes in surface energy fluxes Sensible heat flux Latent heat flux

Changes in incoming solar radiation Cloudiness Slope

Summary- Impacts of Complex Terrain

Terrain affects atmospheric circulation on local to planetary scales

Terrain induced eddies modify and contribute to the vertical and horizontal exchange of mass, temperature, and moisture in a much stronger manner than turbulent eddies over flat terrain

Photo: J. Horel

Problems and possible future directions

Most theoretical, modeling and observational results are applicable to a horizontally homogeneous PBL and underlying surface.

Non-uniform surfaces predominate over land. New tools are needed and are becoming available to

address PBL structure over heterogeneous terrain.

D. Lenschow