1. A UK Portrait of Wind-Induced Undercatch in Rainfall Measurement
EGU General Assembly 2017, Vienna
27th April, 2017
M. Pollock1,2, G. O’Donnell2, M. Colli3,4, A. Black5, M. Dutton1, P. Quinn2, M. Wilkinson6, M. Stagnaro3,4, L. Lanza3,4, E. Lewis2, C. Kilsby2 and E. O’Connell1,2
EML1, Newcastle University2, University of Genoa3, LC-PrIn4, University of Dundee5, James Hutton Institute6
I’ll start general, then I’ll gradually focus on specifics, and then I will sum up

2. Rainfall – what are we trying to measure?
Gauge Shape?
1.5m?
0.5m?
1.5m?
It’s easy to forget about the measurement, what are its strengths and limitations, and how might uncertainty affect a chosen application
“Methods for measuring precipitation should aim to obtain a sample which is representative of the true amount falling over the area which the measurement is intended to represent.” World Meteorological Organization (WMO), 2008
DOES IT REALLY MATTER????
“Methods for measuring precipitation should aim to obtain a sample which is representative of the true amount falling over the area which the measurement is intended to represent.” World Meteorological Organization (WMO), 2008
0.5m?
Picture sources, left to right:
0.0m

3. Why would it matter? Aerodynamics
“A Computational Fluid-Dynamics assessment of the improved performance of aerodynamic rain gauges”
(Colli et al., submitted to WRR for review)

4. Experimental design
Nafferton Farm

5. 1%

6. Catch Efficiency:0.95

7. 0.91

8. A Simple Linear Model
10mm to 75mm

9. Experimental design
Talla Water

11. 0.85

12. 0.76

13. 0.79

14. Desmond at Talla Water: Dec. 4th – 6th, 2015
Aero 0.5m:
0.86
Aero 1.5m:
0.83
Cylinder 0.5m:
0.82

15. Frank at Talla Water: Dec. 29th – 30th, 2015
Aero 0.5m:
0.90
Aero 1.5m:
0.87
Cylinder 0.5m
0.86

16. Largest event = 125mm / smallest = 25mm

17. What happens when we look at the wind speed?

18. Catch Efficiency vs Wind Speed at Talla Water
Storm Katie
Storm Desmond
Storm Abigail

19. Imposing some order! Sample: 20 largest events at Talla Water
Split into 4 hour intervals
Minimum rainfall accumulation per interval: 10mm

20. Catch efficiency vs Wind Speed at Talla Water

21. How to explain the variance…?
Only 42% of the variance explained by the wind speed directly, at a time aggregation interval of 4 hours.
Wind speed effect depends upon:
Drop size and distribution
Micro-physical site-specific characteristics
Turbulence
What have we learned?
Which explains the difficulty this problem has caused over the years.

22. ‘Real’ Rainfall Totals?
2015/2016 UK Winter Storms
‘Real’ Rainfall Totals?
10%
(mm)
20%
152
166
234
256
229
250 53 58 79 86
Left: Report published by UK Centre for Ecology and Hydrology. December 2nd, 2016
Right: Keswick’s glass flood defences before (above) and during overtopping (below).
Give some context: 50mm – 213mm recorded by one gauge in one day. For UK, this is extreme.
How would this affect flood forecasting? E.g. Floods aren’t predicted well in Carlisle because rain projections in the uplands are poor
Reference: Marsh, T.J.1 , Kirby, C.2 , Muchan, K.1 , Barker, L.1 , Henderson, E.2 and Hannaford, J The winter floods of 2015/2016 in the UK - a review. Centre for Ecology & Hydrology, Wallingford, UK. 37 pages.

23. DOES THIS REALLY MATTER…?
It’s easy to forget about the measurement, what are its strengths and limitations, and how might uncertainty affect a chosen application
“Methods for measuring precipitation should aim to obtain a sample which is representative of the true amount falling over the area which the measurement is intended to represent.” World Meteorological Organization (WMO), 2008
Picture sources, left to right:

24. Thanks for your attention!
Any questions?
A question for you: What are the implications of such rainfall underestimation on hydrological models…?