Radiative properties of mid-latitude cirrus clouds derived by automatic evaluation of lidar data from Zürich, Jülich and the high alpine research station Jungfraujoch

Cirrus clouds influence the earth's radiation budget. They warm the planet while absorbing outgoing longwave radiation or cool the earth as they reflect incoming solar radiation back to space. These processes strongly depend on the microphysical properties of the clouds such as ice crystal number density, size of the ice crystals and ice water content.

In order to quantity the optical properties of cirrus clouds, we perform lidar measurements of cirrus clouds on Jungfraujoch since November 2011 using a commercial Lidar manufactured by Leosphere. Jungfraujoch is located at 3580 m asl in the Swiss Alps. This high-altitude location enables lidar measurements of a much higher quality than at lower elevation due to an enhanced transparency of the planetary boundary layer at Jungfraujoch along with closer vicinity to the measured clouds. We also performed measurements in Zürich, and analysed data using the same Lidar model from Jülich as well (provided by Christian Rolf and Martina Krämer). We expect being able to measure thinner clouds from Jungfraujoch than from Zürich and Jülich.

To date, 73 days of measurement data on Jungfraujoch, 83 days in Zürich and 37 days from Jülich are available. This data are analysed using an automatic evaluation algorithm, which detects the measured clouds with a pixel-based scheme. Properties such as extinction, cloud top, cloud base and optical depths are automatically calculated. Categorizing the retrieved clouds we indeed find a larger fraction of subvisible clouds on Jungfraujoch than in Zürich or Jülich.

A lidar measurement provides no information about the ice crystal number density nor about the size distribution of the particles. Therefore we use a radiation model of Corti and Peter (2009) requiring temperature and optical depth as input to assess the radiative properties of the measured clouds. The optical depths are retrieved in our algorithm and the temperatures extracted from the weather prediction model COSMO. We find an all-sky net radiative forcing of 0.7 Wm-2 (Jungfraujoch) to 1.5 Wm-2 (Jülich) that match well with the value of 1.3 Wm-2 observed by Chen et al. (1999) using satellite data combined with NASA/GISS-data. Considering cloud coverage as well, we calculate values of between 0.02 Wm-2 (Jülich) and 0.1 (Zürich) Wm-2. This highlights the importance of cloud radiative forcing as these values are at the same order of magnitude as the radiative forcing of stratospheric water vapour from CH4 in the latest IPCC-report and thus having a substantial effect on the earth's climate.