Model Urbanization Strategy: Summaries, Recommendations and Requirements - Meteorological and Air Quality Models for Urban Areas - page 145

very important variable because it exerts a strong control on boundary layer pro-
cesses and the intensity of the heat island. Such data would be very useful toward
diagnostic evaluation of urban model predictions. Of course, care is required to
address scale issues of observation and models. There are some critical assump-
tions in the derivation of the remotely sensed variables (e.g., emissivities, mixed
pixels). The comparison also is biased to conditions that the remotely sensed
data are operational (e.g., clear sky conditions for surface temperature, time of
overpass).
Also, because models have varying treatments for handling subgrid land use
and coverage, some of the resulting differences between observed and modeled
skin temperature may be result, in part, from these treatments.

• Scale-model measurements. Wind tunnels have the advantage that external con-

ditions can be controlled and are repeatable, but are limited by certain conditions
(e.g., Reynolds number may be a factor of 100 less than in the real world; no
concurrent radiative moisture forcing; typically treats only neutral stratification
cases). Numerical models allow for a wide range of conditions with real meteoro-
logical forcing to be compared (Chapter 5 of this volume). To date these models
remain simple in morphology and arrangement.

• CFD (large eddy simulation [LES] or Reynolds-averaged Navier-Stokes [RANS])

models. Such building resolving models can be run over a limited part of a city
to investigate flow properties to be used in UCP. Using CFD models, it is pos-
sible to derive the spatial averages required for UCP (Chapter 2 of this volume;
Chapter 4 of this volume). CFD-RANS lacks the accuracy for some complex con-
figurations. CFD-LES is more accurate but much more expensive in CPU time,
which, thereby, limits its use.

• Operational testing. Real-scale routine data from weather networks are used for

evaluation, most typically for weather forecasts (e.g., Chapter 3 of this volume).

15.6 Potential Community Activities

There is a wide range of activities that are needed to support the recent improve-
ments to the state of urbanization of models. These fall into a variety of categories.
To date, a systematic evaluation of urban land surface schemes has not taken place
as it has for vegetated environments. The model comparison outlined in Grimmond
et al. (Chapter 11 of this volume) takes some initial steps to address this. As they
note, it is anticipated that there will be need for further observations. There is a
clear need for both intensive and extensive observational data sets to allow the wide
range of variables to be evaluated over a wide range of synoptic conditions. The
development of urban testbeds and urban atmospheric observatories (e.g., Helsinki,
Shanghai, London, Paris, Hanover, Phoenix, Oklahoma City, Houston, New York
City, and Washington, DC) and long-term urban campaigns (e.g., CAPITOUL,
BUBBLE) enable these issues to be addressed. For example, studies evaluating the
Martilli scheme (Martilli et al., 2002) show that it is able to reproduce the genera-
tion of the urban heat island effect and to represent correctly most of the behavior