On the signature of the cirrus twilight zone

Cirrus clouds are known to play a key role in the climate system, but their overall effect on Earth's radiation budget is not yet fully quantified. The uncertainties are, in part, due to ambiguities in cirrus extent or coverage. Here we show that despite careful filtering of cloudy pixels, cirrus clouds have a clear statistical signature. This signature can be estimated by the proximity to detectable cirrus clouds. Such a residual signature can affect retrievals that rely on a cloud-free atmosphere, such as aerosol optical depth (AOD) or sea surface temperature. Analyzing MODIS raw-data and products, we show a clear increase in the reflectance when approaching detectable cirrus clouds. We estimated a mean increase in AOD of 0.03 +/- 0.01 and a decrease in the Angstrom-exponent of -0.22 +/- 0.20 in the first kilometer around detectable cirrus. The effect decays tenfold at a typical distance of 5.5 +/- 1.8 km. Such trends confirm the contribution of large particles that are likely to be ice crystals to the so-called cloud-free atmosphere near detectable cirrus clouds.