Why does BVG use the VAISLA WXT 520 with the RAINCAPR rain sensor?
The VAISALA WXT520 is a real time weather data collection and
transmission device that measures barometric pressure, humidity,
precipitation, temperature, wind speed and direction, which is accessible
on a real time bases. The VAISALA WXT520 is an accurate and stable,
power efficient device which is almost maintenance free as it has no
moving parts. Using the VAISALA’s RAINCAP sensor to measure rainfall
intensity and total precipitation this device does not require any human
interaction and eliminates most problems associated with traditional
rain gauges and measuring devices. Extensive laboratory and field testing
has proven that this as one of the most accurate devices available. Each
device is calibrated before shipment ensuring accuracy and stability in
most weather conditions.
How does the VAISALA WXT520 with RAINCAP rain sensor work?
The VAISALA rain sensor makes use of a stainless steel dome (Approx.
90mm in diameter). Beneath the stainless steel dome is a piezoelectric
detector which measures the acoustic signal of every rain drop that falls
onto the stainless steel dome. The sensor measures the size and
intensity of the rain drops and then determines the total rainfall. The
information is then relayed to a data logger which sends the information
in real time to the BVG weather database, from where it can be retrieved
and displayed. The device clearly distinguishes between hail and rain,
which is recorded separately.
Click here for more information.
The inaccuracy of traditional rain gauges.
The “under catchment” of traditional rain gauges was first observed in 1769, when it was
documented that the height of a traditional rain gauge showed a huge difference in catchment. This
phenomenon has since been well researched and documented. The effect of wind and turbulence
becoming the main focus point for researchers, as it became clear that the turbulence created by
the wind around the orifice of a rain gauge is the main reason for the under catchment of rain by
traditional rain gauges.
For a bit of history on the topic click here
The physics behind this is fairly complex, but it is something that we have all experienced at some
stage of our lives. When you sit on the back of a vehicle during a rain storm and it is moving at speed
you hardly know it is raining until the vehicle stops..... then you get soaking wet. This is what is
happening with your rain gauge when the wind blows.
As you can see from the pictures below is that the wind changes the angle in which the rain enters
the rain gauge and this change also reduces the area of the rain gauge’s opening quite dramatically.
If you combine the turbulence and the reduction in the catchment area due to the wind it is easy to
comprehend that a traditional rain gauge will under estimate the true rain fall by as much as 80% in
severe conditions. The height of the gauge above the ground also impacts on the turbulence and
also affects the amount of precipitation that the gauge measures. Scientists have tried to minimise
the effect of the wind by shielding the gauge. Shielding the gauge has improved the accuracy under
moderate winds, but has proven less so under strong winds.
N
o
w
i
n
d
Light wind
Strong Wind
A sketch of the effect wind has on rainfall trajectory.
For more information on the effect of wind on rain gauges click here.
What does an accurate rain gauge look like?
In recent studies on the accuracy of rain gauges, especially distrometers, scientists made use of a
tipping bucket build into a pit and covered by a non splash grid as a reference of true precipitation.
Building these pit gauges is extremely expensive and requires a lot of maintenance. The tipping
bucket system used in the pit, has its limitations.
A pit rain gauge (from http://www.docstoc.com/docs/28422197/Ground-Level-Rain-Gauge
The accuracy of the VAISAL WXT520 with the RAINCAP precipitation sensor.
VAISALA is a well-respected company that has developed and tested the RAINCAP system
extensively in the laboratories as well as in the field. Independent test have also shown that the
RAINCAP sensor is very stable and consistent in all types of conditions. The VAISALA WXT520 is also
extensively used to provide real time accurate information for disaster management and aviation.
The VAISALA RAINCAP sensor however does have its limitations. In very light rain it cannot measure
raindrops smaller than 0.3mm in diameter and in very heavy rain it does overestimate the rain
intensity. These shortcomings are however overshadowed by the ease of use of the VAISALA
WXT520 and the fact that in South Africa, where the VAISALA WXT520 will be used, it will hardly
ever be exposed to these extreme conditions.
The accuracy of the RAINCAP sensor has been proven to consistently be within 5% of the true
precipitation. Traditional gauges on the other hand are extremely inaccurate and inconsistent due to
the impact of wind and can easily deviate up to 80%.
To view a comparative study click here.
Conclusion.
At BVG we aim to bring only the best service and equipment to our customers. We realised the
inaccuracy of traditional rain gauges and the impact that it has on the planning and risk management
of farming in South Africa. The BVG Weather Station, which uses the VAISALA WXT520, is an
affordable, accurate, self-maintained unit, with real time data delivery to the central weather
database. It requires very little maintenance and the proven consistency and accuracy will help us
build a reliable data base of historical weather data. This data will be used to generate information,
which will assist our clients in the management of risks associated with farming in the years to come.
Bibliography
1. Analysis of highly accurate rain intensity measurements from a field test site
L. G. Lanza, E. Vuerich, and I. Gnecco
2. Assessing New Rainfall Estimation against Traditional Measurement Methods.
Michael Deering Pollock
3. The extend and implications of inaccuracies caused by wind induced undercatch on rain gauges in
the Eden catchment.
Michael Deering Pollock
4. Estimation of Wind-Induced Error of Rainfall Gauge Measurements Using a
Numerical Simulation
VLADISLAV NESˇPOR AND BORIS SEVRUK
5. Evaluation of Rainfall Measurements from the WXT510 Sensor for Use in
the Oklahoma City Micronet
Jeffrey B. Basara*, Bradley G. Illston, Thomas E. Winning Jr. and Christopher A. Fiebrich
6. Piezoelectric precipitation sensor from VAISALA.
Atte Salmi and Jouni Ikonen
7. Piezoelectric VAISALA RAINCAP rain sensor applied to drop size distribution monitoring.
Atte Salmi, Lasse Elomaa, Panu Kopsala and Emmi Laukkanen
8. Rain catch under wind and vegetal cover effects.
James C.Y. Guo, PhD and P.E.
9. Inadvertent Rain Gauge Inconsistencies and Their Effect on Hydrologic Analysis
David C. Curtis
10. Wilkinson, M. (2009). A Multiscale Nested Experiment for Understanding and Prediction of High
Rainfall and Flood Response Spatial Behaviour in the Eden Catchment, Cumbria, UK. Newcastle:
Newcastle University