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   9th International Conference on Urban Drainage Modelling Belgrade 2012 1   Weather radar and heavy rainfall - how to estimate the real amount of precipitation? Thomas Einfalt 1 1  hydro & meteo GmbH & Co. KG, Breite Str. 6-8, D-23552 Lübeck, Germany (  einfalt@hydrometeo.de    )   ABSTRACT Extreme precipitation often occurs on relatively small areas which are not sufficiently equipped with rain gauges to completely observe the occurred event. Therefore, radar data are extremely helpful to localise the most intense parts of the  precipitation. However, radar data are prone to errors under extreme rainfall and  bear higher uncertainties at higher rainfall intensities due to the non-linearity of the relationship between radar reflectivity and rainfall intensity and due to the unknown drop-size distribution of the rain cells. A case study illustrates a practical approach to test several assumptions on the drop-size distribution and important radar data quality considerations for high intensity precipitation. KEYWORDS Extreme events, radar rainfall, rain gauges, sparse network 1 INTRODUCTION Local heavy precipitation is usually not captured by traditional point rainfall gauges. Weather radar, although less precise in rainfall volume at a point, permits a detailed view into spatial structures of  precipitation. Therefore, weather radar plays an increasingly important role for the a-posteriori analysis of such events, in particular in presence of damage. For online data processing, different  procedures are required. Crucial points with radar are the potential measurement errors (Michelson et al., 2005) and the unknown drop size distribution required for a good estimation of rainfall amounts (Collier, 1989). For  practical work with radar data in the urban context, a number of quality controls and data corrections need to be performed before a reliable result can be achieved (Einfalt et al., 2004). 2 RADAR DATA QUALITY CONTROL As mentioned by Michelson et al. (2005), “there are several sources of error which affect the ability of radars to measure precipitation and which influence the accuracy of the measurements.” These errors    2   are not always present in the radar measurement, but many of them appear only under certain meteorological conditions:    Attenuation: reduction of the measured reflectivity due to heavy precipitation (see 2.2),    Bright band: high values in the melting layer of the atmosphere when snow melts to rain,    Anomalous propagation: measurement of ground targets due to atmospheric conditions  preventing a straight propagation of the radar beam. Among (mostly) stationary influences on a correct radar measurement, ground clutter (see 2.1) and (partial) blockage of the radar beam by buildings or topography are the most frequent ones. Readers interested in more details on error sources and their effects on radar measurements should refer to the document of Michelson et al. (2005). 2.1 Ground clutter Ground clutter is usually strong due to the relative radar cross-section of the ground being much greater than that from meteorological targets. Ground clutter can be minimized through intelligent radar siting, Doppler suppression, and through the use of post-processing methods such as static clutter maps. 2.2 Attenuation of the radar signal Heavy rain, graupel and hail can attenuate energy, leading to strong underestimation of precipitation intensities. Especially in hail the scattered energy can be attenuated to the point of virtual extinction of the signal. Shorter wavelengths (X and C bands) are more seriously affected. Attenuation can be detected by a close look into measurements from rain gauges and radar (figure 1) and by visual inspection of the radar images (e.g. figure 2). The time series analysis shows during which time interval radar has seen considerably less precipitation than the rain gauge (in the red ellipse): the thin line shows the rain gauge measurement and the two bold lines radar measurement at two neighbouring points. It becomes obvious that radar has seen much less precipitation during the short time interval just before 18:00 hours. Figure 1: Attenuation of the radar signal: detection through time series comparison to rain gauges    3   Figure 2: Attenuation to the NW of the radar 2.3 Attenuation at the radar site (radome attenuation) In heavy rain, a thin film of water will cover the radome, causing signal attenuation. In cold conditions, snow and ice may build up on top of the radome, also causing attenuation and limiting the quantitative use of reflectivity measurements. Radome attenuation can best be observed in a sequence of radar images (figure 3 - left) where the decrease and later increase of the radar signal over the whole radar range can be observed. A counter measure is extremely difficult because the attenuation is not necessarily uniform over the different viewing directions of the radar. Else, a simple correction factor over the complete radar scope could sometimes save the data – this manual method in some cases nevertheless improves the data (figure 3 - right).    4   Figure 3: Uncorrected radar measurement with radome attenuation (left) and correction attempt (right)
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