Jochen Wagner

Comparison of surface UV irradiance in mountainous regions derived from satellite observations and model calculations with ground-based measurements

Several UV data products derived from satellite measurements, 1-D and 3-D radiative transfer modeling are compared with high-quality ground-based measurements. Data products include the UV index, erythemally weighted daily dose and spectrally resolved UV irradiances at 305, 310, 324 and 380 nm. The study focuses on the UV radiation climate in mountainous terrain under cloud-free conditions. The results show, that overall the 3-D- and the 1-D-model agree best with the measurements (average ratio 1.10 and 1.13, range 0.88-1.6). It is also found that snow and local topography have a rather minor impact on ground UV-irradiance, while altitude plays a significant role >5 %). Satellite-retrieved values significantly underestimate irradiance for most of our stations due to erroneous cloud correction (average ratio 0.89, range 0.6-1.35). However, if one compares the uncorrected (cloud-free) satellite-retrieved values to the measurements, the ratios are only slightly larger (average ratio 1.14, range 0.8 - 1.6) than for the 1-D- and 3-D-model. The main deficiencies arise in determining the correct surface height and albedo within the satellite-retrieval algorithm.

Investigation of the effect of contrails on direct and diffuse irradiance

In the present study we investigate the effect of contrails on global shortwave radiation. This investigation is performed using continuous hemispherical fish eye photographs of the sky, diffuse and direct shortwave measurements. These measurements have been performed at the solar observatory Kanzelhohe (1540 m.a.s.l) located in the southern part of Austria during a period of one and half year. The time resolution of the measurements is one minute, which allows to accurately follow the formation-eventually the disappearance or the movement - of the contrails in the sky. Using the fish eye photographs we identified clear sky days with a high contrail persistence. We especially look at situations where the contrails were obstructing the sun. First results show that contrails moving between sun and observer/sensor may reduce the global radiation by up to 68%. In general we however observe that during days with a high contrail persistence the diffuse irradiance is slightly increased. Finally a statistic of th...

Short range forecast of convective rain using satellite data

The potential use of Meteosat Second Generation (MSG) clear air products in forecasts of convective rain over Germany in summer is examined. The three quantities Lifted Index (LI), Layer Precipitable Water (LPW ML ) of the Middle Layer and the trend of the Lifted Index (dLI/dt) are combined in a heuristic approach to gain a probabilistic forecast of convective rain with a lead time of three to six hours. Data from 33 days with a typical convective weather situation in summer 2005 and summer 2006 were used to derive the data pool necessary for the probabilistic forecast. The input parameter LI, LPW ML and dLI/dt were divided into 5, 4 and 5 classes, respectively. This leads to 100 possible combinations of the input parameters. For each set of input parameter, the information about rain three to six hours later is assigned. DWD radar data are used to detect the rain pattern three to six hours later. For typical summer convective days in Germany, a valuable probability forecast of convective rain is calculated for cloud-free areas.