Using HIRLAM data to forecast weather radar propagation conditions
Joan BECH1, Uta GJERTSEN2, Günther HAASE3
1Meteorological Service of Catalonia, Spain
2Norwegian Meteorological Institute, Norway
3Swedish Meteorological and Hydrological Institute, Sweden
Abstract
One of the sources of uncertainty in weather radar observations collected in hilly areas is the blockage of the radar beam due to screening effects caused by surrounding mountains. Under these circumstances -independently of detection, removal and substitution of clutter echoes- quantitative applications of radar data such as precipitation estimates, assimilation in NWP models or radar-based nowcasting systems require correction of beam blockage to recover the original reflectivity field. To address this problem a number of beam-blockage correction procedures have been proposed in the last decades.
On the other hand, to calculate the trajectory of the radar beam using ray-tracing procedures, the so-called standard or normal propagation conditions are commonly assumed. However, this hypothesis is not valid under anomalous propagation of the radar beam, which may occur quite frequently depending on each region. This fact is well illustrated with more realistic simulations of radar trajectories, such as those obtained with the Radar Simulation Model (RSM) fed with mesoscale NWP model output.
Taking into account both radar beam-blockage and radio-propagation variability, a Beam Propagation Model (BPM) was developed to quantify the interaction with the topography and to calculate correction factors dynamically. This presentation covers the application of BPM to a number of radars of the Nordic Weather Radar Network (NORDRAD), using radiosonde observations and HIRLAM analysis and forecasts to simulate radar beams and topograpical blockages realistically. The study includes an assesment on the variability of propagation conditions and a verification of the radar-corrected data with raingauge observations.