Geoscience Reference
In-Depth Information
by the radar unit. The quicker the reflected wave
reaches back to ground the closer the cloud is to the
surface. The most difficult part of this technique is
in finding the best wavelength of electromagnetic
radiation to emit and detect. It is important that
the electromagnetic wave is reflected by liquid water
in the cloud, but not atmospheric gases and/or
changing densities of the atmosphere. A consider-
able amount of research effort has gone into trying
to find the best wavelengths for ground-based radar
to use. The solution appears to be that it is some-
where in the microwave band (commonly c-band),
but that the exact wavelength depends on the
individual situation being studied (Cluckie and
Collier, 1991).
Studies have shown a good correlation between
reflected electromagnetic waves and rainfall inten-
sity. Therefore, this can be thought of as a surrogate
measure for estimating rainfall. If an accurate
estimate of rainfall intensity is required then a rela-
tionship has to be derived using several calibrating
rain gauges. Herein lies a major problem: with this
type of technique: there is no universal relationship
that can be used to derive rainfall intensity from
cloud reflectivity. An individual calibration has to
be derived for each site and this may involve several
years of measuring point rainfall coincidentally with
cloud reflectivity. This is not a cheap option and the
cost prohibits its widespread usage, particularly in
areas with poor rain gauge coverage.
In Britain the UK Meteorological Office operates
a series of fifteen weather radar with a 5 km
resolution that provide images every 15 minutes.
This is a more intensive coverage than could be
expected in most countries. Although portable radar
can be used for rainfall estimation, their usage has
been limited by the high cost of purchase.
likely to produce rain has an extremely bright and
cold top. These are the characteristics that can be
observed from space by a satellite sensor. The most
common form of satellite sensor is passive (this
means it receives radiation from another source,
normally the sun, rather than emitting any itself the
way radar does) and detects radiation in the visible
and infrared wavebands.
LANDSAT
,
SPOT
and
AVHRR
are examples of satellite platforms of this
type. By sensing in the visible and infrared part
of the electromagnetic spectrum the cloud bright-
ness (visible) and temperature (thermal infrared)
can be detected. This so-called 'brightness tem-
perature' can then be related to rainfall intensity
via calibration with point rainfall measurements, in
a similar fashion to ground-based radar. One of the
problems with this approach is that it is sometimes
difficult to distinguish between snow reflecting
light on the ground and clouds reflecting light in
the atmosphere. They have similar brightness
temperature values but need to be differentiated so
that accurate rainfall assessment can be made.
Another form of satellite sensor that can be used
is passive microwave. The earth emits microwaves
(at a low level) that can be detected from space.
When there is liquid water between the earth's
surface and the satellite sensor (i.e. a cloud in the
atmosphere) some of the microwaves are absorbed
by the water. A satellite sensor can therefore
detect the presence of clouds (or other bodies of
water on the surface) as a lack of microwaves
reaching the sensor. An example of a study using a
satellite platform that can detect passive microwaves
(SSM/I) is in Todd and Bailey (1995), who used the
method to assess rainfall over the United Kingdom.
Although there was some success in the method it
is at a scale of little use to catchment scale hydrology
as the best resolution available is around 10
10 km grid sizes.
Satellite remote sensing provides an indirect
estimate of precipitation over an area but is still a
long way from operational use. Studies have shown
that it is an effective tool where there is poor rain
gauge coverage (e.g. Kidd
et al
., 1998), but in
countries with high rain gauge density it does not
Satellite remote sensing
The atmosphere-down approach of satellite remote
sensing is quite different from the ground-up
approach of radar - fundamentally because the
sensor is looking at the top of a cloud rather than
the bottom. It is well established that a cloud most
