Products for reliable rainfall monitoring
Reliable rainfall monitoring is key for any
water system analysis and for operational water-system control. Traditionally
rainfall is measured using rain gauges which are manually read or which
automatically send real time data to a telemetry network using wired or
wireless communication. It is a proven technology which works fine for
monitoring when many rain gauges are available in the area to monitor.
Radar monitoring
However, heavy rainfall events often occur
very locally, and with the use of rain gauges alone, there is a high
probability that a rainfall event is missed by the rain gauges. To be able to
detect such events, continuous spatial monitoring such as rainfall radar
monitoring is a better alternative. Radars measure circularly and via an antenna
send out radio signals which may or may not hit raindrops falling from clouds.
When the signal hits such raindrops it reflects and part of the reflected
signal is then detected by the radar dish. The travel time of the radio signal
and the diffusion of the signal are measures for distance and intensity of
rainfall monitored by the radar.
The embedded software of the radar system
converts the signal into a rainfall intensity on a certain place and converts
it from dBZ to mm of rainfall per minute. C-band radars which are most often
applied for this purpose measure once every 5 minutes up to a distance of
100-200 km. Specific software such as SCOUT form hydro&meteo is used to
combine the measurements of several radars and to transfer the radial
measurements into a grid which can be used for water management analytics and
operations.
The monitoring principle has a weakness in
that radio signal is lost due to the collision with raindrops. This attenuates
or lowers the intensity of the signal behind a rain front and consequently the
radar underestimates rainfall intensity at such places.
The solution is to continually correct the
radar signal using a correction factor field, which is determined by means of
differences of the raw radar measurement with online point measurements of rain
gauges.
By doing so, best of both worlds can be
used: accuracy of local rainfall measurements with rain gauges and; spatial
measurement of the radar. Practice shows that quite accurate spatial rainfall
monitoring can be made available by combining both products. An example is the
corrected HydroNET rainfall product, available for the Netherlands and parts of
Germany, South-Africa and Australia.
Satellite rainfall monitoring
Apart from rainfall radar, satellite
information can be used for rainfall monitoring. These types of measurements
are available through polar and geo-stationary satellites. Polar satellites
continually scan the atmosphere and the earth surface. The advantage of polar
satellites is that the distance to the clouds monitored is small compared to
geostationary satellites. Therefore, polar satellites monitor with the highest
spatial resolution; however, the time resolution is usually sparse. Blended
products of geostationary and fine polar satellites are available by NASA and
ESA. New products appear at high frequency and some of them have more accurate
results in certain regions of the world than others.
The obvious advantage of satellite rainfall
monitoring is that it is available over large parts of the world, which is
advantageous in sparsely monitored regions. The downside of satellite rainfall
monitoring is that it is indirect and sometimes inaccurate. Clouds are
monitored basically and coverage and temperature at specific heights are
drivers for the computed rainfall estimates.
Atmospheric models
Weather monitoring organizations operating
globally such as ECMWF and NOAA continually run weather models which simulate
the entire atmosphere, using physically based dynamics equations. The models
are assimilated with monitoring data e.g. from ground stations and weather
balloons and in this way they can be regarded as smart interpolators of
monitored and unmonitored weather variables. The power of these models in
monitoring is that they can also be a source of the blended products, described
above.
