Inter-comparison of raingauges in a sub-tropical environment Tam Kwong-hung

1. WMO Technical Conference on Meteorological and Environmental Instruments and Methods of ObservationTECO-2012Brussels, Belgium, 16 – 18 October 2012 Inter-comparison of raingauges in a sub-tropical environment Tam Kwong-hung Senior Scientific Officer Hong Kong Observatory, Hong Kong, China 16 October 2012

2. Background

3. Background (continued) HKO = 46 GEO = 86 DSD = 20 Total = 152 All are 0.5 mm resolution raingauges

4. Background (cont.) • Started inter-comparison in 2000 • Formal comparison in 2011 (with revised recording method and test bed) • Gathered results so far from 2011 – 2012 (latest results of 2012 included in this presentation) • Target: • evaluate 0.1 mm resolution raingauge models meeting WMO +/-5 % accuracy requirement • suitable for use in the sub-tropical environment with rainfall intensity exceeding 300 mm/hr • Raingauge robust enough to be deployed in the field

5. Background (cont.) Hong Kong International Airport (HKIA) King’s Park (KP)

6. (no correction)

7. Location of raingauges at the King’s Park Meteorological Station test bed

8. Spatial distribution of various rain gauges used for the field inter-comparison at King’s Park

10. Location of raingauges at the Hong Kong International Airport Meteorological Station test bed.

11. Equipment set-up for in-house calibration of raingauges High precision dispensing pump to provide constant water flow rate in order to determine the accuracy of the raingauge under different simulated rainfall intensities Water bottle to act as water reservoir Electronic balance for determining the amount of water pumped out of the bottle Raingauge under calibration

12. Error curves (compared with WMO’s + 5% uncertainty limits) obtained in the laboratory under different simulated rainfall rates for those raingauges at the King’s Park inter-comparison site Note: second order polynomial functions are used for curve fitting

13. Error curves (compared with WMO’s + 5% uncertainty limits) obtained in the laboratory under different simulated rainfall rates for those raingauges at the HKIA inter-comparison site Note: Second order polynomial functions are used for curve fitting. Ogawa raingauges were checked up to 100 mm/hr as their errors exceed 5 % for rainfall intensity reaching above 100 mm/hr (Chan et.al, 2004).

14. Summary of rainfall comparison results at King’s Park (Data period : 1 April 2011 to 17 September 2012. )

15. Summary of rainfall comparison results at the Met Garden in AMO (Data period : 19 March 2011 to 13 September 2012 )

17. Time series of rainfall intensities recorded by various 0.1/0.2-mm resolution raingauges at the King’s Park on the morning of 17 June 2011.

18. Time series of rainfall intensities recorded by various 0.1 mm resolution raingauges at the HKIA on the morning of 28 June 2011.

19. Results and Discussions • Total number of rain episodes (24 hr rainfall >=10mm) recorded in 2011-2012: 75 (KP) and 62 (HKIA) • Highest rainfall rate recorded: ~ 150 mm/hr (at KP by R102_ETG) and ~ 300 mm/hr (at HKIA by Ogawa) • Observations from KP Test Bed: • Very small spatial variability of rainfall during test period (< 1mm difference between 2 manual raingauges; mean absolute percentage difference of same model raingauges ~1-2%) • Mean absolute percentage differences (MAPD) of most (except R102_ETG and one Logotronic) raingauges < 5 % • SL3-1 performs the best among 0.1 mm raingauges • MAPD of Casella 0.5 mm < 3% (good for use though coarser resolution) • Observations from HKIA Test Bed: • MAPD of MeteroSevis < 4% (meeting WMO requirement) On the other hand, the Ogawa, Logotronic, R102_ETG and MeteoServis raingauges which were equipped with sophisticated electronics would be vulnerable to the impact of lightning. The former two raingauges also required considerable amount of manpower to maintain their operations. Hence, the Working Group considered that it would be more appropriate to deploy them at manned stations rather than automatic weather stations.

20. Results and Discussions (cont.) • SL3-1 is a good choice to replace existing Casella 0.5mm resolution raingauges （simple design, relatively low equipment cost (～1:3) and ease of maintenance. • Ogawa, Logotronic, R102_ETG and MeteoServis raingauges more appropriate to be deployed at manned stations rather than AWS: • equipped with sophisticated electronics • vulnerable to the impact of lightning, • required considerable amount of manpower to maintain operations On the other hand, the Ogawa, Logotronic, R102_ETG and MeteoServis raingauges which were equipped with sophisticated electronics would be vulnerable to the impact of lightning. The former two raingauges also required considerable amount of manpower to maintain their operations. Hence, the Working Group considered that it would be more appropriate to deploy them at manned stations rather than automatic weather stations.

21. The design of the 0.1mm resolution SL3-1 raingauge. A two-layer tipping mechanism is used to provide a larger buffering capacity to cope with higher intensity rainfall events.

22. Way Forward • Extend the field inter-comparison to 2013 • Start a new phase of testing – deploy SL3-1 at some existing AWS remote sites to test its robustness and durability and compare its performance with Casella 0.5mm resolution raingauges • Start rainfall intensity measurement and comparison

23. Thank you !