download pdf - Institut für Umweltphysik - Ruprecht-Karls-Universität ...

2.3. RADIATIVE TRANSFER 49
2.3.2 Oxygen A-band measurements for solar photon path length distribution
studies
Participating scientists Scholl, T., and K. Pfeilsticker
Abstract High resolution spectroscopy of the oxygen A-band (760-780 nm) in zenith scattered
skylight is a powerful tool to infer path length distributions (PDF) of solar photons transmitted to
the ground. The relation between the different moments of the PDF (〈L〉, 〈L 2 〉) and the rescaled
cloud optical depth τ ∗ is a strong indicator for 3D-effects on the radiative transfer (RT).
Height [m]
10000
8000
6000
4000
2000
0 :0
12:
:5 :10 :15 :20 :25 :30 :35 :40 :45 :50 :55
Time [UTC]
-60 -50 -40 -30 -20 -10 0 10 20
Reflectivity [dBZ]
Figure 2.21: left panel: Measured radar reflectivities from KNMI 35 GHz Radar for May 22, 2003 UT 12:00-
13:00, right panel: Mean cloud photon paths 〈Lc〉 as a function of effective cloud optical depth τ ∗ c . The black
lines are predictions for different values of the Lévy exponent α ≤ 2. The 3 data clusters (color-coded blue,
red and green) correspond to the 3 different probed cloud situations.
Background Modelling RT in cloudy skies is
one of the most challenging tasks in climate modelling.
Photon PDF is commonly a hidden property
of standard RT models, controlled by the
spatial distribution of scattering and absorption.
The distribution in the near infrared region is
very representative for the shortwave region as
a whole. The principle of equivalence allows to
draw conclusions about radiative properties of
the atmosphere from a photon PDF measured in
one wavelength band, since the scattering properties
of clouds and aerosols vary slowly and predictably
with wavelength. Here the first two moments
of the photon PDF of solar photons transmitted
through cloudy skies to the ground are
investigated. Combining the spectroscopic measurements
with other cloud properties measured
simultaneously by in-situ techniques during two
campaigns allows to test the theory of anomalous
photon diffusion through clouds.
Methods and Results The multiple of different
line strengths offered by the oxygen A-band
implicitly provide direct information on the PDF
of the photons transmitted to the ground. The
spectral retrieval is solved by forward modelling
the measured spectra with a prescribed photon
PDF (usually a Γ function) at high spectral resolution.
The free parameters of the PDF are
then iteratively calculated by using a Nonlinear
Least Square Fit leading to the searched quantities
〈L〉 and 〈L 2 〉. Davis & Marshak [2002]
inferred for the first moment of photon PDF:
〈Lc〉/∆H = (1/2 · [1 + C(ɛ)] · τ ∗ ) α−1 . α is the so
called Lévy index which ranges between 1 and 2.
In particular, the value α = 2 is attained for a homogenous
slab and for α < 2 for inhomogeneous
slab. In Figure 2.21 the enhancement of the mean
photon path length in the cloud (as a function of
τ ∗ = τ · (1 − g)) and the received Lévy index is
shown.
The findings confirm the theory of anomalous
photon diffusion through clouds and the predictions
for the values in the path length to optical
depth relations. It also provides further evidence
that cloudy sky photon path length require consideration
of non-classical photon transport theory.
Funding comes through the AFO-2000 4D
Clouds project (BMBF-07ATF24).
Outlook Photon PDFs are a central concept of
cloud RT modelling. For further improvements
imaging DOAS spectroscopy at high temporal and
spatial resolution is required.
Main publication Scholl, T., Photon path
length distributions for cloudy skies: Their first
and second-order moments inferred from high resolution
oxygen A-Band spectroscopy, PhD-Thesis,
Universität Heidelberg, Heidelberg, in preparation.