Rotational Raman scattering in the Earth's atmosphere ... - SRON

Retrieval of cloud properties from NUV, VIS and NIR 103
prominent Fraunhofer lines and weaker molecular scattering in these spectral ranges.
Figure 5.2: Ring spectra in the wavelength range 388–400 nm for a clear-sky observation (gray curve;
f c =0, A s =0.05, surface pressure p s = 1013 hPa) and for two fully clouded observations (black curves;
f c =1, τ c =40) with different cloud top height. Increasing the cloud top height from 1.9 km (p c =800 hPa;
dotted line) to 5.6 km altitude (p c = 500 hPa; black solid line) leads to a reduction of the Ring effect
structures. We used the solar spectrum of Chance and Spurr [1997] as input for our simulations.
Finally, sensitivity to clouds in the measurements is caused by molecular absorption lines. The
most prominent absorption feature of O 2 is located in the NIR window. Another significant, but
much weaker absorption band is the O 2 -O 2 absorption band in the VIS window. Other absorption
features in the NUV, VIS and NIR windows are of minor importance for the sensitivity to clouds. A
large part of the sensitivity to clouds arises from the shielding effect of clouds; the vertical column
of O 2 and O 2 -O 2 below the cloud is shielded and hence the depth of the O 2 and O 2 -O 2 absorption
features are reduced, which can be used in a retrieval approach. At wavelengths with both strong and
weak atmospheric absorption such as in the O 2 A band additional sensitivity of the measurement is
obtained. The measured strong atmospheric absorption bands consist of the radiation that has been
scattered back to the satellite at high altitudes, because incident radiation cannot penetrate to lower
altitudes due to absorption. The situation is different for wavelengths with weak or no atmospheric
absorption. Here, the measured radiation travels all the way through the atmosphere, is reflected by
the cloud, and subsequently scattered by the air molecules into the satellite’s line of sight. Therefore
the measurement is sensitive to radiation from any altitude. This results in a characteristic sensitivity
of absorption bands such as the O 2 A band in reflectivity measurements to cloud properties which can
be used in a retrieval approach.