Three-dimensional radiative transfer over inhomogeneous terrain

BERNHARD MAYER

Deutsches Zentrum fuer Luft- und Raumfahrt (DLR)
Oberpfaffenhofen, 82234 Wessling, Germany

ARVE KYLLING

Norwegian Institute for Air Research (NILU)
P.O. Box 100, N-2027, Kjeller, Norway      

At many places, the Earth's surface is complex and
structured. Inhomogeneous surface albedo and orography have a direct 
impact on the radiance observed by satellite instruments. But also the
radiation flux at the the surface is influenced, mainly by multiple
reflections between surface and atmosphere. A radiative transfer model
(MYSTIC, Monte Carlo Code for the physically correct tracing of photons
in cloudy atmospheres) has been developed to simulate the atmospheric 
radiation transport over inhomogeneous surfaces including
orography. Example simulations are shown for the location of Tromsoe,
Norway where the presence of snow causes enhanced levels of ultraviolet
radiation through several months of the year. Here, a clear boundary
between high-albedo and low-albedo surfaces is drawn by the complex
coastline which separates the snow-free ocean from the snow-covered 
land. The radiation field has been studied for the snowline moving
upward from sealevel towards 1000m in the course of spring and
summer. Simulations of the nadir radiance at the top of the atmosphere
show the influence of the aerosol smoothing out the surface features
by multiple scattering. These examples clearly demonstrate the
importance of a correct treatment of the inhomogeneous surface, for
radiative flux calculations as well as for remote sensing applications.