MID-LATITUDE CIRRUS PROPERTIES DERIVED FROM LIDAR MEASUREMENTS

Cirrus clouds play an important role in the Earth’s climate system. They consist of ice particles with different number densities and sizes and are typically situated in the upper troposphere, where they influence the water vapor budget as well as the radiative properties of the atmosphere. Cirrus clouds generally have a global warming effect on the Earth’s climate as they are able to absorb the Earth’s outgoing long wave radiation. However, low cirrus as well as optically thick cirrus may have a cooling effect, as they scatter incoming solar radiation back to space. As the climate changes, the occurrence and properties of cloud types may also change, leading to shifts in the Earth’s radiation budget. To understand the possible future effect of clouds on climate, it is essential to understand the current clouds and their effect on the climate system. In this thesis, the focus is on mid-latitude cirrus clouds. We performed ground-based LIDAR (LIght Detection And Ranging) measurements of cirrus clouds at our institute roof in Zürich and on the high-alpine Research Station Jungfraujoch, using an ALS 450 cloud lidar from Leosphere. We collaborate with colleagues in Jülich, who used the same type of lidar at their institute. In this study, we developed the Fast LIdar Cirrus Algorithm ”FLICA”, which automatically evaluates lidar measurements, enabling the rapid analysis of large lidar data sets. FLICA combines a cloud detection scheme with classical lidar evaluation. Using FLICA, we present a mid-latitude cirrus climatology based on 13000 hours of lidar measurements. We find similar cirrus cloud properties at our measurement sites, with thinner clouds being observed at Jungfraujoch than at the other measurement sites. Differences may be due to the higher measurement site enabling the detection of thinner clouds, or due to real differences in cloud properties, e.g. because of the special orography above the mountainous measurement site. Using the retrieved optical depths and top temperatures of the cirrus clouds, we use a radiation model to estimate their radiative effect. The estimated warming due to the cirrus clouds of approximately 1 Wm−2 is in agreement with the findings of previous studies. We have also assessed the sensitivity of Lagrangian cirrus modeling to uncertainties in the input data. We used a microphysical boxmodel to simulate a cirrus cloud measured by our lidar over Jungfraujoch. The assessed uncertainties include the temporal resolution of input wind fields, the temporal resolution of the trajectories, the representation of smallscale temperature fluctuations that cannot be resolved by the underlying numerical weather prediction (NWP), the initial humidity provided by the NWP model, and the number density of heterogeneous ice nuclei. The main focus of the study is the sensitivity of the temporal resolution of NWP and trajectory data on the simulated cirrus profile. We used trajectories calculated from the NWP model COSMO-2 using data with temporal resolutions between 20 s and one hour. The accuracy of the simulations improves significantly when data with a higher temporal resolution are used. For this case study, trajectories with a temporal resolution of 20 s were able to reproduce the measured cirrus cloud without superimposing small-scale temperature fluctuations. For coarser temporal resolutions, small-scale temperature fluctuations need to be superimposed onto the trajectories. The small-scale temperature fluctuations are obtained by scaling the power spectral densities (PSDs) of temperature, measured by aircrafts, to the COSMO-2 intensity at lower frequencies. For future Lagrangian modeling studies, if using NWP data with a spatial resolution of 2.2 km, we suggest using trajectories based on NWP data with a temporal