Hybrid probabilistic forecast scheme for postfrontal shower precipitation
Final Report Abstract
The basic objective of the project was the development of a forecast scheme for convective shower precipitation behind cold fronts over Germany. In the early stage of the project more specific questions were raised: 1. Is it possible to describe the characteristics of a post-frontal precipitation field by a set of equations? 2. Measures like area size and rain rate are characteristics of the individual precipitation areas. How do these characteristics develop with time? 3. Which shape can be used as an assumption for an individual cluster? How is the orientation in relation to the wind field? 4. Is it possible to reproduce results of previous studies with a higher resolution radar composite? 5. Could we use the results of the analysis to develop a precipitation generator which produces precipitation fields with realistic rain rates and distributions? The final results of the work as published by THEUSNER (2007) and WEUSTHOFF (2008) provide a thorough description of the post-frontal convective precipitation field. Only a small number of parameters was found to be sufficient to describe the precipitation structure and its temporal development. This includes approximately 20 parameters for the geometrical structure and the diurnal cycles and another 25 for the temporal development of the individual clusters. The latter are composed of four parameters for the area development of each track type, one parameter for the rain rate development of each track type and altogether ten parameters for the life span distributions for the different track types. Ellipses with an ellipticity of 0,58 are good representatives for individual clusters. Clusters with an area greater than 100 km2 are preferably aligned in a direction about 20° relativ to the mean flow. Small-scale structures basically move with the mean wind in 925 hPa. As a main result it was found that in a stochastic way post-frontal shower clouds and there temporal development can be modeled. The respective equations and distribution functions were derived. Despite its characteristic appearance post-frontal showers can be described by a few parameters only. The principle life cycle of all individual shower clouds are self-similar, symmetric with time and scale all with lifetime. Due to reorganised time plan and additional extensive investigations which were identified to be necessary to fulfil the overall aim in a proper way, the final step, i.e. the scheme's development has to be left for subsequent studies. This is planned in collaboration with the Institute of Water Resource Management, Hydrology and Agricultural Hydraulic Engineering, Leibniz Universität Hannover. However, the overall results demonstrated clearly an improved understanding of post-frontal shower precipitation.
Publications
- 2007. An investigation on small scale precipitation structures. Dissertation, Institut für Meteorologie und Klimatologie, Hannover, 95 pp.
Theusner, M.
- 2008. First investigations on precipitation rates of convective showers using the RZ composite. Meteorol. Z., 17(6), 793-806
Weusthoff, T. and T. Hauf
- 2008. Studies on the characteristics of post-frontal precipitation structures in the mid-latitudes. Dissertation, Institut für Meteorologie und Klimatologie, Hannover, 117 pp.
Weusthoff, T.
- 2008. The life cycle of convective shower cells under post-frontal conditions. Quart. J. R. Meteorol. Soc, 134(633), 841-857
Weusthoff, T. and T. Hauf
- 2008. Universal functions for post-frontal showers - geometrical characteristics and rain rate development. International Conference on Cloud and Precipitation, ICCP 2008, 7. - 11. July 2008, Cancun, Mexico
Weusthoff, T. and T. Hauf
