by Pilar Gumà-Claramunt, Stefanos Samaras, Lukas Pfitzenmaier, Veronika Wolf, and Elisa Adirosi

The goal of the project was to separate clear sky and cloudy sky conditions during June 2014 above Juelich. The used instrument was the InfraRed Thermometer (IRT), which measures downwelling radiance at 11 µm. Data of the Atmospheric Emitted Radiance Interferometer (AERI) measuring at the same wavelength were also used.

Fig. 1: Time series of infrared brightness temperatures at 11 µm over the full month of June 2014. Symbols as explained above.

Three different methods were applied (Fig. 1):

- A fixed brightness temperature (TB) threshold defined according to the TB frequency distribution on a clear day (blue lines for clouds and red dots for clear sky in Fig. 1).

- A combined threshold of TB and standard deviation of TB over a 10 minutes time window (green dots for clear sky in Fig. 1).

- A threshold of maximum TB solely due to water vapour presence, calculated with a radiative transfer (RT) model considering the surface temperature and assuming specific temperature and relative humidity profiles (black dots for clear sky in Fig. 1).

The first method appears to be a simplistic approach, not taking into account changes in water vapour and possibly underestimating the cloud fraction. The second and the third method make use of more sophisticated criteria and should in theory offer a more realistic retrieval, but there is still room for improvements, e.g. more advanced statistical tools, real humidity profiles etc. In summary, the IRT is a convenient passive remote sensing instrument for distinguishing clear and cloudy conditions and thus estimating the cloud fraction.