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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 cm2.
•Particle diameters recorded using 22 bins
•Fall speeds recorded using 20 bins 1 7
Modern Optical DisdrometersThies Disdrometer
The Parsivel Optical Disdrometer has a similar operation
The test site: EskdalemuirUndertheseclouds!
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
Disdrometer Pole
Height of disdrometer
above ground
Orientation(pole end
given first)
T1 A 2m North-South
T2 B 1.75m North-South
T3 B 2m West-East
Parsivel C 2m West-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 rate T1 T2 T3 Parsivel
< 1 mm/hr 286 286 284 212
> 1 mm/hr 886 898 897 719
Comparison of Thies and Parsivel drop count distributions
TECO-2008 St Petersburg, November 29th, 2008 Graham Upton
1. Log scale on vertical axis2. Parsivel’s consistently higher (though small) counts of larger diameter drops3. 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
Time T1 T2 T312:03 1618 1645 1290
: : : :12:11 2222 2279 1879Total 18090 18049 15428
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
Time T1 T2 T312:03 1618 1645 1290
: : : :12:11 2222 2279 1879Total 18090 18049 15428
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
Winddirection
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. ofminutes
77 196 126 213 178 123 226 204 132 65
These are statistically significant variations from 50%
Shielding: the effect of wind direction
TECO-2008 St Petersburg, November 29th, 2008 Graham Upton
Winddirection
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. ofminutes
77 196 126 213 178 123 226 204 132 65
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
Winddirection
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. ofminutes
77 196 126 213 178 123 226 204 132 65
The two disdrometers are at the same height above groundT1 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
mij = Median(Counti / countj)where counti denotes the one-minute drop count for
disdrometer iOnly 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
m23
Wind from the Westm32
0.125- 0.81 0.85
0.25- 0.86 0.96
0.375- 0.99 0.98
0.5- 1.00 0.98
0.75- 0.94 0.96
1- 0.98 0.96T2 is oriented North-South (T3 is not)
Test statistic is m23 = Median(count2 / count3)
Shielding and drop size
TECO-2008 St Petersburg, November 29th, 2008 Graham Upton
Drop diameter(mm)
Wind from the South
m23
Wind from the Westm32
0.125- 0.81 0.85
0.25- 0.86 0.96
0.375- 0.99 0.98
0.5- 1.00 0.98
0.75- 0.94 0.96
1- 0.98 0.96T3 is oriented West-East (T2 is not)
Test statistic is m32 = Median(count3 / count2)
Shielding and drop size
TECO-2008 St Petersburg, November 29th, 2008 Graham Upton
Drop diameter(mm)
Wind from the South
m23
Wind from the Westm32
0.125- 0.81 0.85
0.25- 0.86 0.96
0.375- 0.99 0.98
0.5- 1.00 0.98
0.75- 0.94 0.96
1- 0.98 0.96
Smallest drops
Greatestloss
Test statistics are m23 = Median(count2 / count3) and m32 = Median(count3 / count2)
Shielding and wind speed
TECO-2008 St Petersburg, November 29th, 2008 Graham Upton
Wind speed(m/s)
Wind from the South
m23
Wind from the Westm32
0- 1.00 1.00
1- 0.97 0.97
2- 0.92 0.99
3- 0.88 0.91
4- 0.89 0.92
5- 0.86 0.78
Very light wind
No problem
Test statistics are m23 = Median(count2 / count3) and m32 = Median(count3 / count2)
Shielding and wind speed
TECO-2008 St Petersburg, November 29th, 2008 Graham Upton
Wind speed(m/s)
Wind from the South
m23
Wind from the Westm32
0- 1.00 1.00
1- 0.97 0.97
2- 0.92 0.99
3- 0.88 0.91
4- 0.89 0.92
5- 0.86 0.78Strongwind
Serious problem
Test statistics are m23 = Median(count2 / count3) and m32 = Median(count3 / count2)
The combined effectof 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 > 3m/s
< 0.25 mm 0.87 0.80
> 0.25 mm 0.98 0.93
Entries are m23 ratio
Fall-speed calibration
Values of m21 = Median(count2 / count1) Values of m23 = Median(count2 / count3)
TECO-2008 St Petersburg, November 29th, 2008 Graham Upton
Drop diameter
(mm)
Fall speed (m/s)
< 1 > 1 All
< 0.25 1.23 0.93 0.99
> 0.25 0.95 1.04 1.03
All 1.16 1.00 1.01
Drop diameter
(mm)
Fall speed (m/s)
< 1 > 1 All
< 0.25 1.55 0.87 0.95
> 0.25 0.95 1.02 1.01
All 1.33 0.97 0.99First result: All three Thies disdrometers are recording
roughly equal numbers of both small drops and large drops
Fall-speed calibration
Values of m21 = Median(count2 / count1) Values of m23 = Median(count2 / count3)
TECO-2008 St Petersburg, November 29th, 2008 Graham Upton
Drop diameter
(mm)
Fall speed (m/s)
< 1 > 1 All
< 0.25 1.23 0.93 0.99
> 0.25 0.95 1.04 1.03
All 1.16 1.00 1.01
Drop diameter
(mm)
Fall speed (m/s)
< 1 > 1 All
< 0.25 1.55 0.87 0.95
> 0.25 0.95 1.02 1.01
All 1.33 0.97 0.99Second result: All three Thies disdrometers are recording
roughly equal numbers of the faster falling drops
Fall-speed calibration
Values of m21 = Median(count2 / count1) Values of m23 = Median(count2 / count3)
TECO-2008 St Petersburg, November 29th, 2008 Graham Upton
Drop diameter
(mm)
Fall speed (m/s)
< 1 > 1 All
< 0.25 1.23 0.93 0.99
> 0.25 0.95 1.04 1.03
All 1.16 1.00 1.01
Drop diameter
(mm)
Fall speed (m/s)
< 1 > 1 All
< 0.25 1.55 0.87 0.95
> 0.25 0.95 1.02 1.01
All 1.33 0.97 0.99Third 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 m21 = Median(count2 / count1) Values of m23 = Median(count2 / count3)
TECO-2008 St Petersburg, November 29th, 2008 Graham Upton
Drop diameter
(mm)
Fall speed (m/s)
< 1 > 1 All
< 0.25 1.23 0.93 0.99
> 0.25 0.95 1.04 1.03
All 1.16 1.00 1.01
Drop diameter
(mm)
Fall speed (m/s)
< 1 > 1 All
< 0.25 1.55 0.87 0.95
> 0.25 0.95 1.02 1.01
All 1.33 0.97 0.99Fourth result: T2 assigns more drops to the slow-falling speeds
than either T1 or, particularly, T3
Fall-speed calibration
Values of m21 = Median(count2 / count1) Values of m23 = Median(count2 / count3)
TECO-2008 St Petersburg, November 29th, 2008 Graham Upton
Drop diameter
(mm)
Fall speed (m/s)
< 1 > 1 All
< 0.25 1.23 0.93 0.99
> 0.25 0.95 1.04 1.03
All 1.16 1.00 1.01
Drop diameter
(mm)
Fall speed (m/s)
< 1 > 1 All
< 0.25 1.55 0.87 0.95
> 0.25 0.95 1.02 1.01
All 1.33 0.97 0.99Fifth 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?