Regional Radiative Forcing by Line-Shaped Contrails Derived from Satellite Data

Kurzfassung

The influence of contrails on the radiation budget depends mainly on their coverage and optical depth. This study derives these parameters from data of the advanced very high resolution radiometer sensor onboard the NOAA 14 satellite for a region over western Europe within 40°–56°N and 10°W–23°E during 2 years starting March 1995. The data are classified by an operational contrail detection algorithm with a new evaluation of the detection efficiency. From the classification results, daytime and nighttime coverage by line-shaped contrails over western Europe are derived. The daytime contrail coverage for the analyzed region is 0.7% in the annual mean, 1.0% during winter, and 0.4% during summer. The daytime contrail cover is 3 times higher than the night-time contrail cover. An effective optical depth of the observed contrails is derived from the radiation contrasts in the 11 μm channel for contrails and adjacent pixels. The optical depth in the visible spectral range is computed from these results assuming a previously measured size spectrum of ice crystals. The mean visible optical depth is found to be about 0.1, much smaller than that derived from previous case studies. The average radiative forcing at the top of the atmosphere is computed for the derived contrail cloud cover and optical depth. Here contrails mostly cause positive radiative forcing (heating). Larger heating effects during night and summer get partly compensated through more contrail cover during day and winter. The radiative forcing by contrails in the examined region is ∼0.03 W m-2, which is much smaller than was estimated earlier.