Environmental Engineering Reference
In-Depth Information
intervals around Eastleigh (Hampshire) at 11.45 UTC, 29
March 1986. Location of Eastleigh is shown by E.
Source: Pike (1992).
MEASURING PRECIPITATION FROM SPACE
new developments
There are many parts of the world where it is extremely difficult to measure precipitation.
Uninhabited areas presented problems before automatic weather stations becames
available, but ice-covered areas, mountains and, most important, the oceans were almost
impossible to monitor adequately. Now that satellites provide continuous surveillance of
the globe, techniques have been developed to allow us to estimate precipitation over most
parts of the world. Admittedly the results may not compare precisely with existing
surface instruments, but where these are not available the satellite estimates are
invaluable.
Methods of estimation can be classified into two approaches: direct and indirect
methods. The direct methods utilize microwave techniques that have been available since
the 1970s but have become available for precipitation estimation only recently as the
physics of the interactions became better understood. Microwave radiation can be
absorbed and scattered by precipitation particles in the atmosphere. The rate of rainfall
can then be estimated from the degree of absorption by the droplets, using a variety of
assumptions.
Indirect methods of estimation have a longer history, based on the types of clouds
imaged by both satellite types. Using infra-red and visible wavebands (during daylight),
cloud types are identified. The probable precipitation rate and duration can then be
estimated from cloud characteristics such as the thickness, areal extent and cloud top
temperature for any given cloud type. Some cloud types such as cirrus, on their own, are
unlikely to give any precipitation at all. Cumulonimbus clouds are almost certain to
produce precipitation, the amount being influenced by the lifespan of the cloud, its depth,
and surface temperatures.
and 7.9. In parts of the world, especially over the oceans where there are few or no rain-
gauge measurements, satellites have been used to estimate precipitation totals on the
basis of the frequency of occurrence of the types of cloud expected to produce rainfall
(see box). Even the levels of outgoing long-wave radiation and microwaves have been
used in satellite estimation of surface precipitation. Although there are many problems it
is possible to obtain an estimate of probable precipitation totals in previously ungauged
parts of the world.
SNOW
In some countries the water equivalent of snowfall is found by melting the snow which
has accumulated in the gauge. Clearly this is not very accurate, especially during heavy
snowfall, when a low gauge may be totally covered. In the United States the tall gauge
prevents this happening, but the gauge tends to underestimate the amount of snow
reaching the ground. In Canada and Russia separate snow gauges are used. Recently there
