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An investigation of factors affecting the accuracy of Thies disdrometers

An investigation of factors affecting the accuracy of Thies disdrometers. Graham Upton and Dan Brawn University of Essex, Colchester, UK. TECO-2008 St Petersburg, November 29th, 2008 Graham Upton. Thanks. To the UK Met Office (for providing the data)

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An investigation of factors affecting the accuracy of Thies disdrometers

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  1. An investigation of factors affecting the accuracy of Thies disdrometers Graham Upton and Dan Brawn University of Essex, Colchester, UK TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  2. Thanks • To the UK Met Office (for providing the data) • To the EPSRC (Grant:EP/P500214/1) (for the money) • To Dan Brawn (for letting me come to St. Petersburg!) TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  3. The Thies disdrometer • Uses a 780 nm infrared parallel light-beam with a cross-section of 45.6 cm2. • Particle diameters recorded using 22 bins • Fall speeds recorded using 20 bins TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  4. The test site: Eskdalemuir Under these clouds! TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  5. Overview of the instruments Looking North-West TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  6. The disdrometers (3 Thies & 1 Parsivel) Looking towards the North-West TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  7. Close up of the three Thies T3 T1 T2 Looking towards the West TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  8. Arrangement of the four disdrometers TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  9. Restrictions on comparisons Counts in lowest Thies size bin ignored (to permit comparison with Parsivel: diameters considered are therefore between 0.25mm and 8mm) Use only hours with complete data (there were some computer problems: this restriction ensures that individual minutes are correctly matched) Use only minutes in which the average Thies count was 50 drops (to eliminate records not caused by rain) TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  10. Average drop counts (per minute) Excellent agreement between the Thies Number of drops recorded by the Parsivel is only about 75% of that recorded by a Thies TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  11. Comparison of Thies and Parsivel drop count distributions Notes: Log scale on vertical axis Parsivel’s consistently higher (though small) counts of larger diameter drops Thies’s much greater count of the smaller drops TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  12. Overall equality of counts between the three Thies does not mean that they agree all the time! Nine minutes of heavy rain during a storm at midday on June 30th, 2007 TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  13. Overall equality of counts between the three Thies does not mean that they agree all the time! North-South North-South West-East Pole A Pole B Pole B 2m 1.75m 2m TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  14. Shielding: the effect of wind direction These are statistically significant variations from 50% TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  15. Shielding: the effect of wind direction The two disdrometers are at the same height above ground TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  16. Shielding: the effect of wind direction The two disdrometers are at the same height above ground T1 is oriented North-South T3 is oriented West-East TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  17. Shielding and drop size The next results compare counts for T2 and T3 (very close in space) T3 T2 TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  18. A useful comparison statistic mij = Median(Counti / countj) where counti denotes the one-minute drop count for disdrometer i Only minutes where both disdrometers count at least 50 drops are included TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  19. Shielding and drop size T2 is oriented North-South (T3 is not) Test statistic is m23 = Median(count2 / count3) TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  20. Shielding and drop size T3 is oriented West-East (T2 is not) Test statistic is m32 = Median(count3 / count2) TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  21. Shielding and drop size Smallest drops Greatest loss Test statistics are m23 = Median(count2 / count3) andm32 = Median(count3 / count2) TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  22. Shielding and wind speed Very light wind No problem Test statistics are m23 = Median(count2 / count3) andm32 = Median(count3 / count2) TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  23. Shielding and wind speed Serious problem Strong wind Test statistics are m23 = Median(count2 / count3) andm32 = Median(count3 / count2) TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  24. The combined effectof drop diameter and wind speed Entries are m23 ratio The worst case occurs with small drops in strong winds TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  25. Fall-speed calibration Values of m21 = Median(count2 / count1) Values of m23 =Median(count2 / count3) First result: All three Thies disdrometers are recording roughly equal numbers of both small drops and large drops TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  26. Fall-speed calibration Values of m21 = Median(count2 / count1) Values of m23 =Median(count2 / count3) Second result: All three Thies disdrometers are recording roughly equal numbers of the faster falling drops TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  27. Fall-speed calibration Values of m21 = Median(count2 / count1) Values of m23 =Median(count2 / count3) Third result: T2 assigns fewer of the fast-falling drops to the smaller drop-size categories than do either T1 or, particularly, T3 TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  28. Fall-speed calibration Values of m21 = Median(count2 / count1) Values of m23 =Median(count2 / count3) Fourth result: T2 assigns more drops to the slow-falling speeds than either T1 or, particularly, T3 TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  29. Fall-speed calibration Values of m21 = Median(count2 / count1) Values of m23 =Median(count2 / count3) Fifth result: T2 assigns more drops to the slow-falling speeds than either T1 or, particularly, T3. This is especially true of the smallest drops. TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  30. Summary of results Overall counts agree well from one Thies to another Thies counts are appreciably greater than those from a Parsivel Drop size distributions agree well from one Thies to another The drop size distributions seen by Thies and Parsivel are markedly different (as are the water volumes) Thies suffer from shielding with counts in the smallest drop sizes reduced by as much as 20% in strong winds Fall speeds can vary appreciably from one Thies to another --- particularly in the case of the smaller drops TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  31. Summary of results Overall counts agree well from one Thies to another Thies counts are appreciably greater than those from a Parsivel Drop size distributions agree well from one Thies to another The drop size distributions seen by Thies and Parsivel are markedly different (as are the water volumes) Thies suffer from shielding with counts in the smallest drop sizes reduced by as much as 20% in strong winds Fall speeds can vary appreciably from one Thies to another --- particularly in the case of the smaller drops TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  32. Summary of results Overall counts agree well from one Thies to another Thies counts are appreciably greater than those from a Parsivel Drop size distributions agree well from one Thies to another The drop size distributions seen by Thies and Parsivel are markedly different (as are the water volumes) Thies suffer from shielding with counts in the smallest drop sizes reduced by as much as 20% in strong winds Fall speeds can vary appreciably from one Thies to another --- particularly in the case of the smaller drops TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  33. Summary of results Overall counts agree well from one Thies to another Thies counts are appreciably greater than those from a Parsivel Drop size distributions agree well from one Thies to another The drop size distributions seen by Thies and Parsivel are markedly different (as are the water volumes) Thies suffer from shielding with counts in the smallest drop sizes reduced by as much as 20% in strong winds Fall speeds can vary appreciably from one Thies to another --- particularly in the case of the smaller drops TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  34. Summary of results Overall counts agree well from one Thies to another Thies counts are appreciably greater than those from a Parsivel Drop size distributions agree well from one Thies to another The drop size distributions seen by Thies and Parsivel are markedly different (as are the water volumes) Thies suffer from shielding with counts in the smallest drop sizes reduced by as much as 20% in strong winds Fall speeds can vary appreciably from one Thies to another --- particularly in the case of the smaller drops TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  35. Summary of results Overall counts agree well from one Thies to another Thies counts are appreciably greater than those from a Parsivel Drop size distributions agree well from one Thies to another The drop size distributions seen by Thies and Parsivel are markedly different (as are the water volumes) Thies suffer from shielding with counts in the smallest drop sizes reduced by as much as 20% in strong winds Fall speeds can vary appreciably from one Thies to another --- particularly in the case of the smaller drops TECO-2008 St Petersburg, November 29th, 2008Graham Upton

  36. Summary of results Overall counts agree well from one Thies to another Thies counts are appreciably greater than those from a Parsivel Drop size distributions agree well from one Thies to another The drop size distributions seen by Thies and Parsivel are markedly different (as are the water volumes) Thies suffer from shielding with counts in the smallest drop sizes reduced by as much as 20% in strong winds Fall speeds can vary appreciably from one Thies to another --- particularly in the case of the smaller drops Thank you for your attention. Any questions? TECO-2008 St Petersburg, November 29th, 2008Graham Upton

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