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

Thanks TECO-2008 St Petersburg, November 29th, 2008 Graham Upton 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!)

The Thies disdrometer TECO-2008 St Petersburg, November 29th, 2008 Graham Upton Uses a 780 nm infrared parallel light-beam with a cross-section of 45.6 cm 2. Particle diameters recorded using 22 bins Fall speeds recorded using 20 bins

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

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

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

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

Arrangement of the four disdrometers TECO-2008 St Petersburg, November 29th, 2008 Graham Upton DisdrometerPole Height of disdrometer above ground Orientation (pole end given first) T1A2mNorth-South T2B1.75mNorth-South T3B2mWest-East ParsivelC2mWest-East

Restrictions on comparisons TECO-2008 St Petersburg, November 29th, 2008 Graham Upton 1.Counts in lowest Thies size bin ignored (to permit comparison with Parsivel: diameters considered are therefore between 0.25mm and 8mm) 2.Use only hours with complete data (there were some computer problems: this restriction ensures that individual minutes are correctly matched) 3.Use only minutes in which the average Thies count was 50 drops (to eliminate records not caused by rain)

Average drop counts (per minute) TECO-2008 St Petersburg, November 29th, 2008 Graham Upton 1.Excellent agreement between the Thies 2.Number of drops recorded by the Parsivel is only about 75% of that recorded by a Thies Rain rateT1T2T3Parsivel < 1 mm/hr > 1 mm/hr

Comparison of Thies and Parsivel drop count distributions TECO-2008 St Petersburg, November 29th, 2008 Graham Upton 1.Log scale on vertical axis 2.Parsivel’s consistently higher (though small) counts of larger diameter drops 3.Thies’s much greater count of the smaller drops Notes:

Overall equality of counts between the three Thies does not mean that they agree all the time! TECO-2008 St Petersburg, November 29th, 2008 Graham Upton TimeT1T2T3 12: :::: 12: Total Nine minutes of heavy rain during a storm at midday on June 30th, 2007

Overall equality of counts between the three Thies does not mean that they agree all the time! TECO-2008 St Petersburg, November 29th, 2008 Graham Upton TimeT1T2T3 12: :::: 12: Total North-South North-South West-East Pole A Pole B Pole B 2m 1.75m 2m

Shielding: the effect of wind direction TECO-2008 St Petersburg, November 29th, 2008 Graham Upton Wind direction 110°-130°-150°-170°-190°-210°-230°-250°-270°-290°- T1 > T3 (% of minutes with wind in given direction) 85%64%39%27%26%31%48%56%61%50% No. of minutes These are statistically significant variations from 50%

Shielding: the effect of wind direction TECO-2008 St Petersburg, November 29th, 2008 Graham Upton Wind direction 110°-130°-150°-170°-190°-210°-230°-250°-270°-290°- T1 > T3 (% of minutes with wind in given direction) 85%64%39%27%26%31%48%56%61%50% No. of minutes The two disdrometers are at the same height above ground

Shielding: the effect of wind direction TECO-2008 St Petersburg, November 29th, 2008 Graham Upton Wind direction 110°-130°-150°-170°-190°-210°-230°-250°-270°-290°- With wind in given direction, % of minutes T1 > T3 85%64%39%27%26%31%48%56%61%50% No. of minutes The two disdrometers are at the same height above ground T1 is oriented North-South T3 is oriented West-East

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

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

Shielding and drop size TECO-2008 St Petersburg, November 29th, 2008 Graham Upton Drop diameter (mm) Wind from the South m 23 Wind from the West m T2 is oriented North-South (T3 is not) Test statistic is m 23 = Median(count 2 / count 3 )

Shielding and drop size TECO-2008 St Petersburg, November 29th, 2008 Graham Upton Drop diameter (mm) Wind from the South m 23 Wind from the West m T3 is oriented West-East (T2 is not) Test statistic is m 32 = Median(count 3 / count 2 )

Shielding and drop size TECO-2008 St Petersburg, November 29th, 2008 Graham Upton Drop diameter (mm) Wind from the South m 23 Wind from the West m Smallest drops Greatest loss Test statistics are m 23 = Median(count 2 / count 3 ) and m 32 = Median(count 3 / count 2 )

Shielding and wind speed TECO-2008 St Petersburg, November 29th, 2008 Graham Upton Wind speed (m/s) Wind from the South m 23 Wind from the West m Very light wind No problem Test statistics are m 23 = Median(count 2 / count 3 ) and m 32 = Median(count 3 / count 2 )

Shielding and wind speed TECO-2008 St Petersburg, November 29th, 2008 Graham Upton Wind speed (m/s) Wind from the South m 23 Wind from the West m Strong wind Serious problem Test statistics are m 23 = Median(count 2 / count 3 ) and m 32 = Median(count 3 / count 2 )

The combined effect of drop diameter and wind speed TECO-2008 St Petersburg, November 29th, 2008 Graham Upton The worst case occurs with small drops in strong winds Drop diameter (mm) Wind speed 3m/s < 0.25 mm > 0.25 mm Entries are m 23 ratio

Fall-speed calibration Values of m 21 = Median(count 2 / count 1 ) Values of m 23 = Median(count 2 / count 3 ) TECO-2008 St Petersburg, November 29th, 2008 Graham Upton Drop diameter (mm) Fall speed (m/s) < 1> 1All < > All Drop diameter (mm) Fall speed (m/s) < 1> 1All < > All First result: All three Thies disdrometers are recording roughly equal numbers of both small drops and large drops

Fall-speed calibration Values of m 21 = Median(count 2 / count 1 ) Values of m 23 = Median(count 2 / count 3 ) TECO-2008 St Petersburg, November 29th, 2008 Graham Upton Drop diameter (mm) Fall speed (m/s) < 1> 1All < > All Drop diameter (mm) Fall speed (m/s) < 1> 1All < > All Second result: All three Thies disdrometers are recording roughly equal numbers of the faster falling drops

Fall-speed calibration Values of m 21 = Median(count 2 / count 1 ) Values of m 23 = Median(count 2 / count 3 ) TECO-2008 St Petersburg, November 29th, 2008 Graham Upton Drop diameter (mm) Fall speed (m/s) < 1> 1All < > All Drop diameter (mm) Fall speed (m/s) < 1> 1All < > All Third result: T2 assigns fewer of the fast-falling drops to the smaller drop-size categories than do either T1 or, particularly, T3

Fall-speed calibration Values of m 21 = Median(count 2 / count 1 ) Values of m 23 = Median(count 2 / count 3 ) TECO-2008 St Petersburg, November 29th, 2008 Graham Upton Drop diameter (mm) Fall speed (m/s) < 1> 1All < > All Drop diameter (mm) Fall speed (m/s) < 1> 1All < > All Fourth result: T2 assigns more drops to the slow-falling speeds than either T1 or, particularly, T3

Fall-speed calibration Values of m 21 = Median(count 2 / count 1 ) Values of m 23 = Median(count 2 / count 3 ) TECO-2008 St Petersburg, November 29th, 2008 Graham Upton Drop diameter (mm) Fall speed (m/s) < 1> 1All < > All Drop diameter (mm) Fall speed (m/s) < 1> 1All < > All 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.

Summary of results TECO-2008 St Petersburg, November 29th, 2008 Graham Upton 1.Overall counts agree well from one Thies to another 2.Thies counts are appreciably greater than those from a Parsivel 3.Drop size distributions agree well from one Thies to another 4.The drop size distributions seen by Thies and Parsivel are markedly different (as are the water volumes) 5.Thies suffer from shielding with counts in the smallest drop sizes reduced by as much as 20% in strong winds 6.Fall speeds can vary appreciably from one Thies to another --- particularly in the case of the smaller drops

Summary of results TECO-2008 St Petersburg, November 29th, 2008 Graham Upton 1.Overall counts agree well from one Thies to another 2.Thies counts are appreciably greater than those from a Parsivel 3.Drop size distributions agree well from one Thies to another 4.The drop size distributions seen by Thies and Parsivel are markedly different (as are the water volumes) 5.Thies suffer from shielding with counts in the smallest drop sizes reduced by as much as 20% in strong winds 6.Fall speeds can vary appreciably from one Thies to another --- particularly in the case of the smaller drops

Summary of results TECO-2008 St Petersburg, November 29th, 2008 Graham Upton 1.Overall counts agree well from one Thies to another 2.Thies counts are appreciably greater than those from a Parsivel 3.Drop size distributions agree well from one Thies to another 4.The drop size distributions seen by Thies and Parsivel are markedly different (as are the water volumes) 5.Thies suffer from shielding with counts in the smallest drop sizes reduced by as much as 20% in strong winds 6.Fall speeds can vary appreciably from one Thies to another --- particularly in the case of the smaller drops

Summary of results TECO-2008 St Petersburg, November 29th, 2008 Graham Upton 1.Overall counts agree well from one Thies to another 2.Thies counts are appreciably greater than those from a Parsivel 3.Drop size distributions agree well from one Thies to another 4.The drop size distributions seen by Thies and Parsivel are markedly different (as are the water volumes) 5.Thies suffer from shielding with counts in the smallest drop sizes reduced by as much as 20% in strong winds 6.Fall speeds can vary appreciably from one Thies to another --- particularly in the case of the smaller drops

Summary of results TECO-2008 St Petersburg, November 29th, 2008 Graham Upton 1.Overall counts agree well from one Thies to another 2.Thies counts are appreciably greater than those from a Parsivel 3.Drop size distributions agree well from one Thies to another 4.The drop size distributions seen by Thies and Parsivel are markedly different (as are the water volumes) 5.Thies suffer from shielding with counts in the smallest drop sizes reduced by as much as 20% in strong winds 6.Fall speeds can vary appreciably from one Thies to another --- particularly in the case of the smaller drops

Summary of results TECO-2008 St Petersburg, November 29th, 2008 Graham Upton 1.Overall counts agree well from one Thies to another 2.Thies counts are appreciably greater than those from a Parsivel 3.Drop size distributions agree well from one Thies to another 4.The drop size distributions seen by Thies and Parsivel are markedly different (as are the water volumes) 5.Thies suffer from shielding with counts in the smallest drop sizes reduced by as much as 20% in strong winds 6.Fall speeds can vary appreciably from one Thies to another --- particularly in the case of the smaller drops

Summary of results TECO-2008 St Petersburg, November 29th, 2008 Graham Upton 1.Overall counts agree well from one Thies to another 2.Thies counts are appreciably greater than those from a Parsivel 3.Drop size distributions agree well from one Thies to another 4.The drop size distributions seen by Thies and Parsivel are markedly different (as are the water volumes) 5.Thies suffer from shielding with counts in the smallest drop sizes reduced by as much as 20% in strong winds 6.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?