Beam Propagation modelling and blockage correction in the Nordic Weather Radar Network (NORDRAD) Uta Gjertsen, met.no, Norway Günther Haase, SMHI, Sweden Joan Bech, Meteocat, Spain Heikki Pohjola, FMI, Finland ERAD, Barcelona, 18.9.- 22.9.2006

Structure The NORDMET/NORA Beam Propagation Project Examples from Korpoo, Hemse, Røst Anaprop modelling Beam blockage correction for standard conditions, example Bømlo Summary blockage correction Conclusions

NORDMET NORA Beam Propagation Project Background: Issues concerning the beam propagation are of great importance for the quality of radar products. Objectives: Coordinate the work carried out in the Nordic countries to define common algorithms. Address typical “cold climate challenges” like beam propagation in inversion situations, shallow precipitation, snow … Deliverables: beam blockage correction, anaprop information

Radars Hemse, Sweden and Korpoo, Finland 4.7.2006

Radar Røst, 6 July 2005 00 UTC Simulated height of the lower beam limit and beam blockage HIRLAM11 analysis at the radarsite Simulated height of the lower beam limit and beam blockage radiosonde Bodo

Questions Refractivity: What is normal beam propagation in the Nordic countries? Is the BPM a useful tool? And what are the limitations? Radiosonde or NWP model data for refractivity profiles? Should anaprop be considered when correcting precipitation estimates for beam blockages? When and where is anaprop most likely? Refractivity:

ANAPROP variability Data from 2003 and 2004 Norwegian Radio Soundings Shows only surface ducts

Summary ANAPROP BPM produces reasonable output with radiosondes and HIRLAM Radiosondes are not always representative for the conditions at the radar site, HIRLAM might be better HIRLAM 6-hour forecast is good for the cases investigated Potential for forecasting sea clutter, 1-dim. HIRLAM Refractivity shows a latitude-dependent pattern

Correction field for precipitation (BB=beam blockage, y=distance center beam - topography, a=radius radar beam cross section) BB 50% bcorr 1.54 60% 1.77 70% 2.12 (bcorr=correction factor for precipitation, b=Marshall-Palmer b coefficient)

= * - = Radar Bømlo Correction filed Corrected precipitation Uncorrected precipitation - = Corrected precipitation Uncorrected precipitation Corrected areas

Blockage correction and VPR G/R bias as a function of distance for 3 winter months at radar Bømlo

Summary blockage correction The blockage correction reduces bias and scatter Average bias reduction is 2.6 dB at ranges 40 to 160 km and blockage between 50-70% Correction fields for standard conditions are good enough Considering a decrease of reflectivity with height within the measurement volume improves the results

Conclusions The BPM gives realistic output for normal conditions and in cases with anaprop What is the most realistic profile of temperature, humidity and pressure at the radar site? Correction fields produced by BPM reduce the gauge/radar bias and scatter The improvement is dependent on the distance from the radar and the degree of blockage -> consider to correct for inhomogenous beam filling? The blockage correction for standard conditions should be implemented in the Nordic countries There is also a potential for sea clutter forecasting and radar site selection assessment (presentation by G. Haase) We could use a set of VPRs in the future. The quality information will be very useful in the future, also for hydrological applications of radar data.

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