1. The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology Characterizing Radar Raingauge Errors for NWP Assimilation Sandy Dance 27/4/2012 www.cawcr.gov.au

2. The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology The Intention: radar rainfall variance Radar assimilation into NWP: this requires that rainfall data be accompanied by the variance value for that data. Adjust ZR relationship based on feedback from gauges: requires variance to control weights for Kalman filter. Variance gives weights when generating mosaic products.

3. The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology Rainfields, our QPE system Bayesian Clutter Partial Occultation Vertical Profile Convective/Stratiform ZR Real-time gauge adjustment

4. The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology Simulate gauge data from rainfields output

5. The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology The data we used in this study consisted of 2 years of rain over suitable radars in Australia (18), and about 1500 raingauges, resulting in about 380,000 non-zero record per year. Both networks are heterogeneous.

6. The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology Overall statistics of radar gauge errors Clearly logerrors (expressed as dB) is the correct measure for further studies.

7. The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology We wanted to predict what variance a given radar rainfall datum has given factors such as: altitude, terrain roughness, beam width, radar range, radar band and season (ie, convective vs stratiform rain). One non-radar factor that might influence radar / gauge error variance is gauge bucket size. Below we show results from looking at these factors.

8. The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology Mean and variance vs radar range Typical charts showing no clear pattern for the variance vs range, mean is low close in due to clutter suppression, and far out due to beam overshooting echo tops.

9. The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology Altitude and RMSE We expected increasing variance with altitude due to orographic enhancement, clutter suppression and beam blocking. However we found no significant trend overall.

10. The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology Terrain roughness Roughness was defined as the SD of altitude around a point. We included this for completeness. We expected to find increasing variance with roughness due to clutter suppression. Again no strong relationship was found.

11. Radar beam width vs band vs range vs season Hot season Cold season The only strong relationship found in the study was hot season S2 radar variance Increased with range. This was due to one older radar dominating the data.

12. The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology Conclusion Overall, we have not been able to find a complex relationship between the variance of logerror (radar/gauge) and various possible factors. One problem is that it takes 2000 datapoints for.95% confidence that we have the variance within 5% of the true value. As the figures show, it can take 3 years or more for this in our arid land. This can exceed the time between hardware or software changes in the raingauge and radar networks.

13. The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology It is possible that the variance may depend upon multiple variables in complex ways, but in general, adding a new factor roughly doubles the amount of data required to verify the relationship. Data we dont have.

14. The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology Sandy Dance Phone: +613 9669 4395 Email: s.dance@bom.gov.au Web: www.cawcr.gov.au Thank you www.cawcr.gov.au