1. Rainfall amount and intensity measured by the THIES Laser Precipitation Monitor
Eckhard Lanzinger
Manfred Theel
Herbert Windolph
My name is Eckhard Lanzinger and I would like to mention first the people who have contributed to this work, first of all my colleague Manfred Theel and also Herbert Windolph of the manufacturing company THIES for his informations on the calibration of the instrument.
The THIES Laser Precipitation Monitor is originally designed to serve as a Present Weather Sensor and around 120 devices are being introduced into the network of the German Weather Service (DWD).
Besides the PW information it also measures rainfall amount and intensity and this investigation is intended to check how well it performs and if this information could eventually be used operationally.
Deutscher Wetterdienst (DWD)
TI23 b : Measuring Systems
Frahmredder 95
D Hamburg
Germany
Adolf Thies GmbH & Co. KG
Hauptstraße 76
D Göttingen
Germany

2. Principle of operation
light sheet 20 x 228 x 0.75 mm3
receiver
light source
Since it can measure the volume of a single droplet the LPM has a low detection threshold of mm/h
aspherical lense t T
Amplitude Diameter
Duration Velocity
receiver signal

3. Measurement uncertainty for volume measurement: 2.2%
Factory Calibration
Optical Alignment
Beam Analyser automatically checks width, thickness and alignment of laser light sheet
Volume Calibration
automated calibration bench
precision dispensing pump
droplet generator (accuracy of diameters 2%)
x-y positioning system: 15 positions used
2-step process: adjustment and final test
Measurement uncertainty for volume measurement: 2.2%

4. Experiment LPM OTT Pluvio DFIR GPCC gauges Pit Gauge Wasserkuppe
( Elevation 950 m )
Manned Weather Station 24h operation
Reference site for Global Precipitation Climate Centre
Test site for sensor intercomparisons
Pit Gauge
DFIR
GPCC gauges
LPM OTT Pluvio
For rainfall amount we could use the data of the GPCC pit gauge as reference.
For rainfall intensity we used the weighin rain gauge OTT Pluvio within our test field section as reference
We have a dedicated DAQ system for the test field which provides synchronous time stamps for all sensors recorded.

5. Rainfall amount Deviation of LPM from pit gauge [mm]
Reference rainfall amount (daily sums) [mm]
outlier
+20.2%
+15%
+5.3% 1 10
100
0.1 -5 5 15 20 25 30 35 40 45
In this half-logarithmic picture the deviations of all 3 LPMs are plotted over the daily rainfall amounts. In the right upper corner you can see outliers that could not yet be explained. For a daily sum of around 55 mm rainfall the LPMs accumulated on average 35 mm more than the reference.
All other values agree better with the reference but still give too high measures. A linear plot

6. Rainfall amount Reference rainfall amount (daily sums) [mm]
Deviation of LPM from pit gauge [mm]
0.5 1
1.5 2 -1
- 0.5
Differences of daily accumulation of the LPMs from the reference pit gauge and linear fit curves. The picture gives a zoomed view of rainfall amounts less than 2 mm.

7. Rainfall intensity +19.2 % +31.2 % +37.2 %
LPM rainfall intensity [mm/h]
Rainfall intensity of the LPMs against reference rainfall intensity. The slope of the fit curve gives the relative deviation from the reference
Reference rainfall intensity [mm/h]

8. Rainfall intensity Overshoots in peaks Typical time series
Rainfall intensity [mm/h]
Overshoots in peaks
Rainfall intensity
Typical time series
Coloured bars represent rainfall intensities of the LPMs, the black curve shows the reference intensities of the Pluvio weighing rain gauge.
There is no delay and the LPMs follow the reference curve very well.
In the intensity peaks the LPMs tend to overshoot.
At first we could not find an explanation for this behaviour.

9. Edge Correction Laser light sheet
A statistical correction has to increase the measured rainfall amount
Laser light sheet

10. LPM 3
LPM 8
LPM 9
Reference
Rainfall intensity
thin lines = with correction
thick lines = without corr.
Edge effect is over corrected. Re-Calibration should reduce differencies
What about the differencies between the LPMs?

11. Conclusions
The LPMs in this test generally measured too high rainfall amounts and intensities
The deviations increased with increasing intensity (overshoot)
LPM has excellent time response
LPM has very low sensitivity threshold (0.001 mm/h) and thus complements catchment type rain gauges in the low intensity range
Potential to significantly improve the measurements by
improved edge correction
improved calibration

12. Thank you for your attention!