Variability of precipitation in complex terrain and the investigation of representativeness of a single point measurement for the Matre Hydro System, Western Norway.

Orography is strongly affecting precipitation. Especially over complex terrain, the precipitation fields can show high spatial variability even over very small scales. In cooperation between the power company BKK (Bergenshalvøens Kommunale Kraftselskap) and the Geophysical Institute, University of Bergen, 20 rain gauges and two weather stations have been deployed between 22 and 898 meters above sea level in the catchment area of the Matre Hydro System in Western Norway in the period April - October 2008. The derived high resolution data set in space and time was used to investigate the horizontal variability and the altitude dependence of precipitation in complex terrain under different synoptic conditions in this catchment area. An average vertical increase of precipitation amount with altitude of 5.2 % / 100 m was found. The precipitation amounts normalized to the altitude of the reference Station at Stordal allow for the investigation of the horizontal precipitation gradient in the area. This gradient is found to be southwest-northeast oriented, with approximately 115 % of the overall reference amount in the southwest and 85 % in the northeast. By this the representativeness of BKKs single point measurement at Stordal on the total precipitation amount of the whole catchment area has been addressed. Another purpose of the project was also to study the representativeness of a used single point ECMWF forecast for the reference station at Stordal. During the measurement period the model forecasted in total only about 5 % less then the observed amount, however it showed a smoothing tendency compared to the observations. The potential for improvement of precipitation prediction by use of operational model runs with higher horizontal resolution has also been addressed. Both tested model configurations MM5, with a horizontal resolution of 12 km, and HIRLAM4, with a horizontal resolution of 4 km, were not able to improve the point forecast quality for Stordal. An important part of the thesis was dedicated to a thorough characterization of the used HOBO rain gauges by laboratory tests and field comparisons. This included laboratory calibration, laboratory investigation of the effects of non-level mounting on sampling effciency and a field comparison with o±cial measurements that showed an accordance of better than 1 % for total precipitation amounts