1. Investigations into the improvement of automated precipitation type observations at KNMI
Marijn de Haij
Wiel Wauben
KNMI
R&D Information and Observation Technology
TECO-2010, Helsinki | 31 August 2010

2. Contents The main issues Investigation of new sensors
Conclusions and outlook
TECO-2010, Helsinki | 31 August 2010

3. Precipitation type observation
Visual observations in SYNOP/METAR issued by KNMI fully automated using Vaisala FD12P scatterometers, with exception of 2 airports
Combines optical (~size) and DRD12 detector (~water content) signals
Differences with human observer analyzed and reported to users (e.g. Wauben, 2002)
Most important issues:
Discrimination of mixed/solid precipitation
Classification of light precipitation events
Detection of hail
Precipitation detection in fog (MOR<400m)
FD12P De Bilt Test
TECO-2010, Helsinki | 31 August 2010

4. Comparison with human observer
Overlapping hourly observations at 6 KNMI stations in
Correction rules and averaging applied on 1-min sensor data
Poor skill scores found for freezing and solid precipitation
Additional rules based on RH, TA, MOR evaluated with reference set
Further improvement not likely -> test with ‘new’ instruments
TECO-2010, Helsinki | 31 August 2010

5. Investigation of new sensors (2008-2010)
KNMI selected four commercially available sensors:
- with the potential to improve the observation (combined w/ FD12P)
- which are suitable for use at AWS at an affordable price tag
Ott Parsivel, Thies LPM, Lufft R2S, Vaisala WXT520
Setup:
Field test in De Bilt September 2008-March 2010
Additional data: FD12P (2x), rain gauge, wind, PTU, …
Assessment of possibilities for indoor check
Reference:
Evaluation by data validation specialists (10-min) and meteorologists (hourly) in a web tool
Only precipitation type is used – wawa without intensity indication
TECO-2010, Helsinki | 31 August 2010

6. Sensors under test Ott Parsivel Optical disdrometer 51cm2 sheet, 650nm
Extinction-> D,v
8 types: L,LR,R,LRS,S,SG,SP,A
Thies LPM
Optical disdrometer
46cm2 sheet, 785nm
Extinction-> D,v
9 types: P,L,LR,R,LRS,S,IP,SG,A
Lufft R2S
24 GHz Doppler radar
Frequency shift-> v
4 types: R,LRS,S,A
Vaisala WXT520
RAINCAP Ø94mm
Drop impact-> volume
Distinction rain/hail: R,A
TECO-2010, Helsinki | 31 August 2010

7. Example 16 January 2010: wintry precipitation
Transition from liquid to solid precipitation around 19UT
Captured well by disdrometers, 2 FD12P sensors show difference
R2S: mixture reported due to temperature threshold 4˚C
Meteorologist confirms light drizzle detections of LPM
TECO-2010, Helsinki | 31 August 2010

8. Example 16 January 2010: wintry precipitation (2)
First report LRS/S
R2S 1615
PAR 1842
LPM FD
TECO-2010, Helsinki | 31 August 2010

9. Example 15 December 2008: dense fog
Dense fog event identified in the evening (MOR<200 m), just above 0˚C
Both FD12Ps report snow and snow grains at max mm/h
Other sensors do not report precipitation, as confirmed by meteorologist
TECO-2010, Helsinki | 31 August 2010

10. Results: evaluation
Hourly evaluation performed by meteorologist beside normal duties
Selection of events where disagreement with FD12P was indicated
Results (≠ skill scores):
Hourly observations
# obs.
# OK / N / NOK
LPM
141
56 / 0 / 85
Parsivel
31 / 0 / 110
FD12oper
22 / 0 / 119
10-min observations
# obs.
# OK / N / NOK
LPM
269
232 / 7 / 30
Parsivel
184 / 0 / 85
FD12oper
107 / 21 / 141
TECO-2010, Helsinki | 31 August 2010

11. Results: general impression
Technically OK for 18 months without maintenance
Frequency distribution (10-min)
LPM: UP due to spiders, some
added value for hail and classification
of light events, no detection in fog
Parsivel: high FAR for hail types,
insensitive to L/SG, solid “spider”
reports (no T included)
WXT520: no hail events reported,
although 3 confirmed cases
R2S: high FAR for LRS, insect
detections, threshold D≥0.3mm
TECO-2010, Helsinki | 31 August 2010

12. Conclusions and outlook
None of the automated systems has perfect performance
Thies LPM is able to partially solve the issues encountered with the precipitation type observation by the FD12P
Analysis of the improvement limited due to availability of reference
Winter :
Second test of LPM disdrometer at airports Schiphol and Rotterdam
Entry of PW changes on a 1-minute basis by human observer
Optimization of combination FD12P/LPM for precipitation type
LPM issues that still need to be addressed:
Contribution of false reports by spider(web)s
Sensitivity/threshold
Wind effect on the determination of the precipitation type
TECO-2010, Helsinki | 31 August 2010

13. Thanks for your attention! See paper 3(2) for further details
TECO-2010, Helsinki | 31 August 2010

14. TECO-2010, Helsinki | 31 August 2010

15. TECO-2010, Helsinki | 31 August 2010

16. Indoor check
Setup of test for homogeneity and reproduceability of disdrometers
Prior to field test and after 1 year
Problem: accurate positioning of drops in the light sheet!
Peristaltic pump
Droplet plate
Sensor
Scale
Good agreement with Thies factory calibration
TECO-2010, Helsinki | 31 August 2010

17. Contingency table
TECO-2010, Helsinki | 31 August 2010

18. Amplitude  Diameter Duration  Velocity
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19. Classification FD12P vs disdrometer
Vaisala FD12P
Disdrometer (bv. Ott/Thies)
Optisch/DRD12 =
“grootte”/”waterinhoud”
+ temperatuur
+ max. deeltjesgrootte
+ evt. temperatuur
TECO-2010, Helsinki | 31 August 2010

20. Intermezzo scores event other method yes no reference hit miss false
none
Probability of detection
POD = hit / (hit+miss)
False alarm rate
FAR = false / (hit+false)
Critical succes index
CSI = hit / (hit+miss+false)
TECO-2010, Helsinki | 31 August 2010

21. Example 26 May 2009: hail event
Parsivel and LPM report hail between 0215 and 0225UT
Temperature drops 5˚C, radar summer hail chance >90%
But unfortunately no evaluation
Other sensors report heavy rain, including both FD12Ps
TECO-2010, Helsinki | 31 August 2010

22. Overview Precipitation type PW code NWS METAR No precipitation 00 C -
Unknown precipitation 40 P UP
Drizzle 50 L DZ
Freezing drizzle 55 ZL
FZDZ
Drizzle and rain 57 LR
DZRA
Rain 60 R RA
Freezing rain 65 ZR
FZRA
Drizzle/rain and snow 67
LRS
RASN
Snow 70 S SN
Ice pellets 75 IP PL
Snow grains 77 SG
Ice crystals 78 IC
Snow pellets 87 SP GS
Hail 89 A GR
TECO-2010, Helsinki | 31 August 2010