Divergence of sun-rays by atmospheric refraction at large solar zenith angles R. Uhl and T. Reddmann Institut für Meteorologie und Klimaforschung, Forschungszentrum Karlsruhe, Germany
Abstract. For the determination of photolysis rates at large zenith angles
it has been demonstrated
that refraction by the earth's atmosphere must be taken into account.
In fact, due to the modified optical path
the optical transmittance is thereby increased in most instances.
Here we show that in addition the divergence of sun-rays,
which is also caused by refraction
but which reduces the direct solar irradiance,
should not be neglected.
Our calculations are based on a spherically symmetric atmosphere
and include extinction
by Rayleigh scattering, ozone, and background aerosol.
For rays with 10km tangent altitude
the divergence yields a reduction
of about 10% to 40%
at solar zenith angles of 91° to 96°.
Moreover, we find that the divergence effect can completely cancel
the relative enhancement caused by the increase of transmittance.
Citation: Uhl, R. and Reddmann, T.: Divergence of sun-rays by atmospheric refraction at large solar zenith angles, Atmos. Chem. Phys., 4, 1399-1405, doi:10.5194/acp-4-1399-2004, 2004.