Framework for Inclusion of

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Framework for Inclusion of
Urbanized Landscapes in a
Climate Model

AGU 12 Dec., 2003

Robert E. Dickinson

Georgia Institute of Technology

What is the Issue?


Science of Climate Change needs to be more
directed at issues of human welfare


Approximately half of humanity lives in cities


Hence need to scale projections of global
climate models to urban areas


Need to include potential feedbacks to
atmosphere and hydrological cycle

Specific Issues for Modeling of Urban Surfaces


Large fraction of impervious (mostly black)
surfaces that impact surface temperatures and
hydrology


Urban plants


either more or less than natural
background
-
may be irrigated


Turbulent transports generated by buildings
and vegetation at multiple heights


Modeling Issues Common to Other Surfaces


Radiative energy balance


exchanges both
within elements and with overlying atmosphere


Albedo


Emissivity


Geometry of objects



Bowen ratio

heat storage with a diurnal cycle
interacting with overlying boundary layer


Conditioning of air for moist convection
thunderstorms

Data Issues


What is urban?


Usual definition of lights seen from space not helpful
to modeling


Natural is to define in terms of a sufficiently large
fraction of urban concentrated surfaces such as road
density


Fractional areas needed?


Roads & and other impervious flat surfaces


Vegetation cover


trees or grass, etc.


buildings

Data Issues (continued)


Leaf and other plant areas (LAI & SAI)


Spatial scales of vegetation


homogeneous or
mixed?


Albedos (visible and Near

IR broadband)


Water fraction (lakes and pools)


Energy generated by electricity and fossil fuel
combustion


All above over seasonal cycle


Much of above available from MODIS and
other NASA instruments

Focus on turbulent transport from
multiple surfaces


Transport of heat, moisture, and other
constituents between surface and overlying
atmosphere


Need to do separate moisture and energy
budgets for different surfaces


These separate budgets require separate
formulations of transfer processes


usual
climate modeling assumption of separate
homogeneous surfaces probably fails


Mixing of transport from smooth and
rough surfaces


Assume an atmospheric mixed layer above the
canopy = the tallest trees or buildings


Transfer to mixed layer requires a
z
0

=
roughness length and
h
= displacement height
(centroid of momentum absorption).


Also need to formulate the transfer from the
(relatively) flat surfaces to canopy air space


same issue for grass and trees (savannah ) or
sparse vegetation in semi
-
arid regions

Complex Surface Parameters
-
Some
Background Literature


Earliest study


U Wisc. MS thesis of John Kutzback,
1961


studies effect on surface roughness of density of
objects that were bushel baskets placed on frozen Lake
Mendota


Mike Raupach1992,1994 formulates theoretically the
issue of partitioning of stress between smooth and such
roughness objects
-

relates roughness length and
displacement height to fractional area of obstacles
normal to wind direction


Linroth, 1993, identifies LAI as contributing to tree effect


MacDonald et al, 1998, formulates modified version for
buildings


Key Elements


U
h

= wind at the top of the canopy


Cd = drag coefficient ,such that the overlying
atmosphere loses
t

=
l

Cd U
h

2

of momentum to
the complex canopy


where
l

= effective frontal area density


But sqrt(
l

Cd ) =
0.4

/ log(( h


d)/ z
0
)
from the
simple relationship between logarithmic wind
profile and momentum transfer, which is inverted
to get
z
0
/ (h

d )


What’s not so easy!


Formulation of
l

Cd ?
Need vertical integral of
drag to scale as known coefficient only applies
at top of canopy


McDonald for buildings uses
a scaling factor of
b
(
1
-
d/h
) where
b

= 0.55 from
calibration


For vegetation, the dependence on LAI + SAI
-

this appears to be a scaling factor of

exp(
-

g

(LAI + SAI) )




Relating
d

to
z0 and
l:

Raupauch works
for trees, not buildings, McDonald for
buildings not trees.

Resolution of Roughness Issues


McDonald scaling likely ok if modified to include
reduction by leaf area factor for vegetation


Raupach expression for d/h should be modified
to allow for eddy sheltering (i.e. wake vortices of
building only interact over the area not covered
by building so that d should become h as
structures become a single building
-

however
for closed canopy, this is between 0.7 and 0.85
h depending on crown
-
aspect ratio

Partitioning of fluxes between surfaces


Raupach primarily interested in partioning of momentum


important for wind erosion issues.


All fluxes derivable through modeling of momentum
absorption and wind profile within the canopy


Derive from wind profile, the low
-
flat surface fluxes


that
from the trees or buildings then obtained by subtracting
this from the total


Has been shown to be a critical issue for semi
-
arid
vegetation; initial version of CLM2 released by NCAR
had skin temperatures over 10K too large. e.g.in
Arizona, because of an inappropriate treatment.


Directions


Incorporation into Community Land Model (NCAR)


Each atmospheric grid square currently is subdivided into
different plant functional types with prescribed properties
and with lakes a separate surface


Need to extend these data descriptions to include a global
data base for impervious and building fractional areas


improve lake sub
-
area data for urban locations


Estimate if most or all includes surfaces are mixed on a
scale of a few km or less, and if so, improve the energy
exchange formulations as outlined above.



Conclusions


Inclusion of urban surface types in global climate
models will facilitate assessment of climate
change of the environment in which lives half the
worlds population


Doing this requires the development of new data
sets for existing climate models


Doing this also requires further development of
the climate model treatments of complex
surfaces