Transcript Slide 1

WXT as a disdrometer?
Heikki Pohjola1) and Leila Konkola2)
1)
2)
Vaisala Oyj
University of Helsinki
The opportunity
• Disdrometers are expensive and thus rare
• Global distribution of DSD is badly known
• Climatological research and radar measurements could benefit of
reasonably priced disdrometers
For example NASA´s
Global Precipitation Measurement (GPM) Program:
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Satellite will be launched 2013 ?
Understanding of clouds and rainfall processes
Make frequent rainfall measurements on a global basis.
FMI involved in ground validation with HelsinkiTestbed and radar
network
Radar applications
• Set one or more WXTs to
blocked sectors of radar
– combine dBZ values
for each image
• Gapfilling between radars
• Attenuation correction
– between the radar and the
target, some microwave power
is lost.
(Barnes, S.L., 1964: A technique for maximising details in
numerical weather map analysis. Jour_.
AppI_Meteor., _3,396-409)
Radar´s measurement volume, ~ km3
~0,5 km
~0,5...1 km
Radar´s measurement volume, ~ km3
~0,5 km
Raingauge e.g WXT,
diameter ~ 10 cm!!
~0,5...1 km
Radar´s measurement volume, ~ km3
~0,5 km
Raingauge,
diameter ~ 10 cm!
~0,5...1 km
Z from WXT measurement
Raincap sensor of WXT works as a disdrometer, and
measures N and D in time unit.
Thus, we can calculate reflectivity on ground
Zg = NiDi6
and avoid all fuzzy Z/R conversions
2006-07-22 10:31 - 11:31
800
600
N 400
200
0
1.00
1.25
1.60
2.00
2.50
Drop diameter [mm]
3.20
4.00
5.00
Data
• Drop size distribution data from WXT, disdrometer (RD69) and POSS from Järvenpää, Sorto
• WXT_Z: 21.8. ->
• RD-69: 1.8. ->
• POSS: All Testbed campaings
DSD data from WXT_Z
Date
Time
D1.00
D1.25
D1.60
D2.00
D2.50
D3.20
D4.00
D5.00
2006-09-03
23:15:00
10.9
57.6
23.2
9.4
3.1
0
1
1.3
2006-09-03
23:20:00
21
44
27
3.6
2.9
0
0
2.3
2006-09-03
23:25:00
24.9
44.6
27.6
13
1
0.7
0
0
2006-09-03
23:30:00
16.3
41.5
18.1
2.7
1.1
0
0
0
2006-09-03
23:35:00
4.9
15.9
7.7
6.8
1.7
0
0
0
2006-09-03
23:40:00
15.7
49.1
63
20.7
4.6
0.7
0
0
2006-09-03
23:45:00
25.4
57.7
34.7
11.5
1.8
0
0
0
2006-09-03
23:50:00
3.6
6
1
0
0
0
0
0
What next?
• Comparisons between disdrometer, WXT (and POSS)
DSD data.
• If promising, first comparisons with radar reflectivity
data.
Challenges:
• WXT data 5 min intervals, accuracy 5 %
• RD-69 data 1 min intervals, accuracy 1,4 %!!!!
• Different drop diameter classes
back up slides
Defaults for WXT510 Disdrometer test units
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DSD test units transmit automatically in every minute one message (so called auto
composite message) which includes wind, PTU and disdrometer parameters in one
string.
Disdrometer counters are automatically reset after any output including disdrometer
parameters.
Example:
0R0,Dm=270D,Sm=0.9M,Ta=22.4C,Ua=35.8P,Pa=1009.7H,1.00=10.41n,1.25=11.01
n,1.60=6.45n,2.00=3.44n,2.50=2.98n,3.20=2.46n,4.00=0.51n,5.00=0.00n
Dm=270D,Sm=0.9M
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Ta=22.4C,Ua=35.8P,Pa=1009.7H
1.00=10.41n
1 minute average of wind direction [deg] and speed [m/s] updated once a minute
Air temperature [Celsius], relative humidity [%RH] and barometric pressure [hPa] updated
once a minute
Composite number [pcs] of 1.00 mm raindrops in bin range of [0.800 *) - 1.122] mm
1.25=11.01n
Composite number [pcs] of 1.25 mm raindrops in bin range of [1.122 - 1.403] mm
1.60=6.45n
Composite number [pcs] of 1.60 mm raindrops in bin range of [1.403 - 1.795] mm
2.00=3.44n
Composite number [pcs] of 2.00 mm raindrops in bin range of [1.795 - 2.244] mm
2.50=2.98n
Composite number [pcs] of 2.50 mm raindrops in bin range of [ 2.244 - 2.895] mm
3.20=2.46n
Composite number [pcs] of 3.20 mm raindrops in bin range of [2.896 - 3.591] mm
4.00=0.51n
Composite number [pcs] of 4.00 mm raindrops in bin range of [3.591 - 4.489] mm
5.00=0.00n
Composite number [pcs] of 5.00 mm raindrops in bin range over 4.489 mm
*) appr. minimum size of detectable raindrop