Anomalous propagation (anaprop)

The use of meteorological satellites for rainfall estimation and monitoring was introduced as a way of augmenting conventional ground-based rainfall data for hydrological models and weather forecasting. Today the primary scope of satellite rainfall monitoring is to provide information on rainfall occurrence, amount and distribution over the globe for a number of applications such as meteorology at all scales, climatology, hydrology and environmental sciences. The uneven distribution of raingauges and weather radars and the lack of rainfall data over the oceans have always been a concern and until now the rainfall, as a prominent branch of the global hydrological cycle, is not well understood. In this sense the problem is not different from the determination of wind, pressure, temperature and humidity fields although precipitation is by far the most variable in space and time. Furthermore, unlike many other atmospheric phenomena, precipitation (or the lack of it) has a direct impact on human life (e.g. flash floods, Barrett & Michell, 1991). Therefore, satellite monitoring is used to address the key questions of spatial and temporal coverage, which cannot be achieved by other observing systems.Among the challenges that face the science and technology of satellite remote sensing the quantitative determination of rainfall from the variety of precipitating systems is one of the most difficult and is largely unsolved. Barrett & Martin (1981) and Kidder & Vonder Haar (1995) give excellent reviews of the methods available. Petty (1995) has examined the status of satellite rainfall estimation over land. A recent review by Levizzani (1998a) has covered results and future perspectives from the geostationary orbit. The perspecMeteorol. Appl. 8, 23-41 (2001) Precipitation estimations from geostationary orbit and prospects for METEOSAT Second Generation

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Ground clutter caused by anomalous propagation (anaprop) can affect seriously radar rain rate estimates, particularly in fully automatic radar processing systems, and, if not filtered, can produce frequent false alarms. A statistical study of anomalous propagation detected from two operational C-band radars in the northern Italian region of Emilia Romagna is discussed, paying particular attention to its diurnal and seasonal variability. The analysis shows a high incidence of anaprop in summer, mainly in the morning and evening, due to the humid and hot summer climate of the Po Valley, particularly in the coastal zone. Thereafter, a comparison between different techniques and datasets to retrieve the vertical profile of the refractive index gradient in the boundary layer is also presented. In particular, their capability to detect anomalous propagation conditions is compared. Furthermore, beam path trajectories are simulated using a multilayer ray-tracing model and the influence of the propagation conditions on the beam trajectory and shape is examined. High resolution radiosounding data are identified as the best available dataset to reproduce accurately the local propagation conditions, while lower resolution standard TEMP data suffers from interpolation degradation and Numerical Weather Prediction model data (Lokal Model) are able to retrieve a tendency to superrefraction but not to detect ducting conditions. Observing the ray tracing of the centre, lower and upper limits of the radar antenna 3-dB half-power main beam lobe it is concluded that ducting layers produce a change in the measured volume and in the power distribution that can lead to an additional error in the reflectivity estimate and, subsequently, in the estimated rainfall rate.