Development of Rainfall Area-Intensity-Duration-Frequency Curves from Weather Radar Data

Intensity-duration-frequency (IDF) curves are frequently used for practical derivation of design storms. These curves are valid for extreme rainfall at local points. For the estimation of design flows in larger catchments, the point rainfall leads to overestimation. Hence, an adjustment is often carried out based on areal reduction factors. Radar data are available with a high resolution in space and could be used for a direct derivation of areal design storms. The objective of this study is to estimate area-intensity-duration-frequency curves (AIDF) directly from weather radar data and to analyze its spatial and temporal variability. First, radar data are adjusted with rainfall observations from the daily station network. Thereafter, AIDF curves are calculated for various circular areas centered at different locations. In order to assess the spatial and temporal variability, reduction factors are calculated for different durations, return periods and areas. A partial validation of the AIDF curves is carried out for the duration of one day using the dense network of non-recording stations. Also the radar based results are compared with the German standard areal reduction factors. The data used for this study cover 13 years of observations from the radar device located near Hannover in Northern Germany as well as 46 recording rain gauges and 512 non-recording rain gauges. AIDF curves are analyzed for rainfall durations from 5 minutes to 24 hours and return periods from 1 year to 30 years for summer, winter and the total year. It is hypothesized, that the results will show significant differences to the standard reduction factors and a substantial spatial variability of the AIDF curves. Moreover, a decrease of the influence of radar data error with increasing area is assumed.