Modeling Urban SurfaceAtmosphere

1
Modeling Urban Surface-Atmosphere Sensible Heat
Exchanges
Sarah M. Roberts1, T.R. Oke1, A. Lemonsu2,
C.S.B. Grimmond3 and P. Jackson1
1Department of Geography, University of British
Columbia, Vancouver, B.C., Canada 2Centre de
Recherches Météorologiques, Météo-France,
Toulouse, France 3Atmospheric Sciences Program,
Indiana University, Bloomington, IN, USA
2

• Overview
• Methods
• Field site description
• Measurement approach
• Modeling approach

• Simulation Results
• Surface Energy Balance
• Sensitivity to Wind Speed
• Sensitivity to Urban Geometry
• Sensitivity to Surface Radiative Properties
• Sensitivity to Surface Thermal Properties

• Concluding Remarks

3
Field Site Characteristic Mediterranean City

• Marseille is an ideal locale in which to study
  urban surface-atmosphere sensible heat exchanges
• Warm, dry summer climate
• Massive urban development
• Very little vegetation cover

High-density buildings (H/W 2.0) with clay tile
roofs and thick stone walls (0.5 - 1 m) provide
a large thermal mass for heat storage
4
Measurement Approach
Observations performed in the constant flux layer
between 35-44 m above street level. Two levels
of eddy correlation instrumentation directly
measured radiometric (Q), turbulent sensible
(QH), and latent heat (QE) fluxes. Net energy
storage flux (DQS) is calculated as the residual
in the measured surface energy balance.
L1
L2
5
Modeling Approach
Massons Town Energy Balance model (TEB, 2000)
couples the micro- and meso-scales and is widely
used to represent the urban energy budget in
atmospheric circulation models.

• Forced with atmospheric and radiation data
  recorded above-roof level.
• Incorporates detailed representations of the
  urban surface to simulate individual energy
  balances for walls, roads, and roofs.
• TEB performs well at highly-urbanized sites
  (central Mexico City, Vancouver light industrial
  area, and Marseille city center).

6
Simulated Surface Energy Balance

• TEBs good performance at this site and others
  suggests that the model can be reasonably
  employed to gain more insight into responses to
  various surface and meteorological forcing
  parameters at the Marseille site.
• Thirty-minute TEB energy balance output are
  converted to hourly ensemble averages over an
  eight-day period in July 2001. Results are
  expressed in terms of DQS biases ( DQSREF -
  DQSSIM).

7
Simulated Sensitivity to Wind Speed

• Flow consistently weaker/stronger than the
  reference (measured) flow generates expected
  behavior
• Weaker daytime flow facilitates greater energy
  conduction whereas higher daytime flow favors
  turbulent surface-atmosphere exchanges.
• At night, weaker flow results in more DQS
  release. A consequence of strong daytime flow is
  a reduced energy source for nocturnal release.

8
Simulated Sensitivity to Urban Geometry (I)
Canyon Aspect Ratio (reference H/W 2.0)

• Canyon H/W is altered by adjusting the average
  building height and keeping canyon width constant
  between model runs.
• These modifications do little to impact the
  amount of energy taken up or released from
  storage (note magnitude of biases).

9
Simulated Sensitivity to Urban Geometry (II)
Building plan area (reference 56 Bldg.)

• In these simulations, the relative proportion
  of impervious ground and buildings is adjusted
  (plan area of water and vegetation are held
  constant).
• More building volumes (less canyons)
  greater local-scale
  energy uptake.

10
Simulated Sensitivity to Surface Radiative
Properties
Built surface albedo

• Largest biases occur in the daytime and with
  higher surface albedo values.
• Changes to surface emissivity (not shown) do
  little to impact the local-scale energy balance
  at this site (biases lt 3 W m-2).

11
Simulated Sensitivity to Surface Thermal
Properties

• Thinner road surfaces are more responsive later
  in the day, when the oblique solar path allows
  for lower canyon exposure.
• Modifying wall thickness results in little
  diurnal bias.
• Greater roof volume, while impacting the hourly
  behavior of energy uptake/release, results in
  little (1 W m-2) overall diurnal bias.

12
Concluding Remarks
Numerical simulations performed with a
local-scale urban surface-atmosphere model
demonstrate the relative impact of various
meteorological and surface forcing parameters at
a site in the city center of Marseille, France

• Wind speed at this highly urbanized site plays
  the most significant role in surface-atmosphere
  coupling and subsequent sensible heat exchange.
• Sensible heat flux partitioning is impacted to a
  lesser extent by changes to urban geometry, with
  the most bias associated with plan area of
  buildings.
• Increased built surface albedo and modifications
  to surface thermal parameters result in
  appreciable differences in diurnal ?QS
  partitioning.