Transcript Part 1

The Planetary Boundary Layer in
Complex Terrain
John Horel
NOAA Cooperative Institute for
Regional Prediction
Department of Meteorology
University of Utah
[email protected]
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 Issues
www.pnnl.gov/vtmx
VTMX 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.
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
Surface Wind and Vorticity Around Isolated Mountain:
Interaction with Large-scale flow
Chen, C.-C., D. Durran and G. Hakim
(2003) ICAM
Potential Temp, Vertical Velocity, and Turbulent
Mixing
Chen, C.-C., D. Durran and G. Hakim (2003) ICAM
Planetary
boundary
layer
1 km
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
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 down
The gully, and easterly flow likely drainage flow from Flint Hills. Numbers identify the
Sonic anemometers on the E-W transect. E is to the right and N into the paper.
Pollutant Transport in Valleys
Nighttime
Stable Layer
in Valley
After Breakup
of Nighttime
Stable Layer
in Valley
Savov et al. (2002; JAM)
Daytime vertical mixing processes
Jerome Fast
Diurnal mountain wind systems
Whiteman (2000)
Mountain-plain circulation, Rocky Mountains
US radar profiler network, 1991-1995, Jun-Aug, 500 m gate, max=3.5 m/s
Whiteman and Bian (1998)
Alpine pumping
Mountain-plain circulation in Alps (Vertikator)
Emissions within the area of Alpine Pumping are transported
into the Alps and mixed convectively to higher levels
Boundary of Alpine pumping
synoptic conditions
100 km
modify shape
Munich
Zürich
Graz
Innsbruck
Lyon
Milan
Turin
Lugauer et al. (2003)
Mountain venting, anti-slope flow
25 July 2001
1000
Vertical cross-section
of slope flow (upslope
to the right)
Height Above Ground (m)
900
800
CBL Height
600 from Lidar
700
500
400
300
3 m/s
200
100
0
0900
Local Time (PDT)
Reuten et al.( 2002
Valley cross sections
temperature
and wind
structure
layers at a
time midway
through the
transition
Whiteman (2000)
Whiteman (1980)
Channeling of synoptic/mesoscale winds
Forced Channeling
Whiteman (2000)
Pressure Driven Channeling
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 values
from 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
Along valley
2.65±4.50
2.00±3.80
1.20±1.24
6.23±18.11
2.19
2.13
1.55
5.88
D. Lenschow
West DEM Grid Points vs. MesoWest Stations
60
% of Total
50
Green-West
Blue-MesoWest
40
30
20
10
0
-12 -10 -8 -6 -4 -2
Valley
0
Flat
2
4
6
8 10 12
Mountain
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
400
300
* Sites located
disproportionately
in urban areas and
near water
resources.
200
100
0
e
Ba
r
an
Ur
b
ub
Sh
r
Op
e
n
dl
e
.N
ee
er
Ev
g
at
W
. In
de
x
-100
Ve
g
% difference from West
500
Diurnal Temperature Range
A. Reinecke
Diurnal fair weather evolution of bl over a plain
Whiteman (2000)
free →
troposphere
mixed →
layer
surface →
layer
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)
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 country
0.03
Level country with low vegetation and isolated obstacles with
separations of at least 50 obstacle heights
0.10
Cultivated area with regular cover of low crops; moderately open
country with occasional obstacles with separations of at least 20
obstacle heights
0.25
Recently developed “young” landscape with high crops or crops of
varying height and scattered obstacles at relative distances of about
15 obstacle heights
0.50
Old cultivated landscape with many rather large obstacle groups
separated by open spaces of about 10 obstacle heights; low large
vegetation with with small interstices
1.0
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).
D. Lenschow
Effects of horizontal heterogeneity in
surface properties

Changes in surface roughness



Changes in surface energy fluxes



Rough to smooth
Smooth to rough
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