SOLWEIG 1.0 – Modelling spatial variations of 3D radiant fluxes and ...

712 Int J Biometeorol (2008) 52:697–713
resolution discussed above, will not affect the final outcome
of T mrt to any significant extent. Since the longwave fluxes
account for the bulk of the absorbed radiation, the
SOLWEIG model’s overall performance of Tmrt estimations
is good (Fig. 11).
Adjustment of clearness index
SOLWEIG 1.0 could also be used during non-clear weather
conditions. The model provides an objective method of
measuring cloudiness, where the ratio of observed solar
radiation to modelled clear-sky solar radiation is applied.
The clear-sky formulations presented by Crawford and
Duchon (1999) have some setbacks. Clear-sky solar
radiation is underestimated at very low sun elevations
during morning and evening, as well as during the winter
season, especially at high latitudes. This is accounted for in
SOLWEIG 1.0 by the established relationship illustrated in
Fig. 4 (Eq. 15). The data scattering in Fig. 4 could be
explained by the transmission coefficients for gases and
aerosols in the atmosphere that are used to assess clear-sky
solar irradiance. The actual amounts of gases and aerosols
are not taken into account in the formulations. However, the
formulas are advantageous, since they constitute an
objective method of determining cloudiness, as opposed
to using subjective cloudiness data which are usually
sparse. The more complex and variable radiation conditions
in semi-cloudy weather conditions result in higher scattering
in the data and the distance between the model domain
and the weather station used becomes more significant. In
this study, the distance between the two is only 1 km, but
large variations in shortwave radiation are still evident (e.g.
Fig. 9a at 1300hours LST).
Conclusions and future prospects
In this paper, the new computer model SOLWEIG 1.0 is
described. The model simulates spatial variations of 3D
radiation fluxes and mean radiant temperature based on
simple meteorological parameters and urban geometry
represented by building structures and ground topography.
In general, the correspondence between modelled and
measured values of T mrt are high (R 2 =0.94, p