By: Zahra Toofani Nejad
Abstract:
Obtaining reliable precipitation observations is important for hydrological simulations and weather forecasting. The most obvious way of measuring rainfall is using by rain gauges. This instrument has been used for many years and has been found to be very accurate at point scale. Knowing rainfall at one point can be useful, but knowing area averaged amounts is more useful. One way to achieve this is by using radar. Radar data can provide insight in the spatial variation of precipitation and can also detect large areas of rain and estimate rainfall rates. These rates can then be used to calculate total amounts of precipitation for a given area. Of course, the radar does not record precipitation directly; instead it processes a returned reflectivity from the precipitation droplets in the volume of the radar beam. The accuracy of radar estimation can be limited at times (Windsor, 2005). Also radar rainfall estimation can be prone to errors because of attenuation and ground clutter. Therefore, the main problem is how those errors can be detected and removed as much as possible from the radar observations (Borga and Fattorelli, 2002).
Weather radar systems nearly always operate in S-, C- or X-band. Since X-band systems require smaller antennas than those at C- or S- band, they are particularly suitable for monitoring small hydrological working (Matrosov et al. 2002, Rahimi et al., 2005).
This report discusses the potential of X-band radar systems for rainfall estimation over an urban area in the Netherlands. It focuses on removing ground clutter and attenuation from X-band radar measurements. In this study measurements from radar are compared against measurements from 4 tipping bucket rain gauges for a rainfall event.
In order to improve rainfall estimation using weather radar, firstly, errors caused by ground clutter and attenuation need to be removed. After that the corrected radar reflectivity value can be converted to rainfall rate via an adequate relationship. To assess the uncertainty in rainfall estimation, the results of estimation have been compared against rainfall rates recorded by rain gauges.
In general, the analysis demonstrated that the radar follows the general trend of the rain gauge measurements but the radar measurements need to be calibrated and corrected for errors. When corrections are applied, results are comparable to the rain gauge measurements. The advantage of radar data compared to rain gauge data is that the radar data can provide much more insight into the spatial variation of rainfall.