Verification and Case Studies for Urban Effects in HIRLAM Numerical Weather Forecasting - Meteorological and Air Quality Models for Urban Areas - page 131

parameterisation is to simulate the effect of buildings on meso-scale atmospheric
flow. It takes into account the main characteristics of the urban environment:
(i) vertical and horizontal surfaces (wall, canyon floor and roofs), (ii) shadowing and
radiative trapping effects of the buildings, (iii) anthropogenic heat fluxes through the
buildings wall and roof. In this parameterisation, the city is represented as a combi-
nation of several urban classes. Each class is characterised by an array of buildings
of the same width located at the same distance from each other (canyon width), but
with different heights (with a probability of particular heights). To simplify the for-
mulation we assume that the length of the street canyons is equal to the horizontal
grid size. The vertical urban structure is defined on a numerical grid.
In our study, the BEP urban module was used to simulate the effects of build-
ings on atmospheric urban flow taking into account a set of main characteristics
of the urban environment. Copenhagen is represented as a combination of several
urban classes. Each class is characterized by an array of buildings of the same width
located at the same distance from each other, but with different heights.
14.3 Results and Discussions: Sensitivity Tests and Verification
for Copenhagen Metropolitan Areas
The DMI-HIRLAM research models -U01 and -I01 (1.4 km resolution) were run for
the Copenhagen metropolitan area and surroundings (Fig. 14.1a). Independent runs
were performed for several specific cases and in a long-term mode. These included:
(1) control run with no modifications in the ISBA surface scheme; and (2) mod-
ified urbanised runs including (a) urban roughness and anthropogenic heat fluxes
modifications, and (b) building effect parameterization module.
The outputs for specific dates (+24 hour forecast) were evaluated for the Copen-
hagen metropolitan area. The meteorological fields' simulations were driven using
boundary conditions of the DMI-HIRLAM-S05 model. These conditions were used
as input for simulation of meteorological fields for the urbanized high resolu-
tion model. The diurnal cycle of meteorological variables and difference fields
(2D) for wind velocity (at 10 m), humidity and temperature (at 2 m), pressure as
well as fluxes were analyzed comparing outputs of the control run relative to the
urbanized runs.
14.3.1 Modified Urban Roughness and Anthropogenic Fluxes
Incorporating actual urban roughness and anthropogenic heat flux values modified
the structure of the surface layer wind and temperature fields over the urban and
suburban areas (Fig. 14.2). During daytime, the wind velocities became lower. With
roughness increased up to 2 m, this effect became more visible for suburban areas.
At night, this effect is smaller. The average differences in velocities can be up to
3ms − 1 . For temperature, the urban roughness effect did not contribute signifi-
cantly compared with wind. The differences also became well pronounced over the