Geoscience Reference
In-Depth Information
box 4.1
significant
20th-c. advance
RADAR METEOROLOGY
Radio detection and ranging (
radar
), developed for aircraft detection during the Second World War, was swiftly applied
to tracking precipitation areas from the radar echoes. Radio waves transmitted by an antenna in the cm wavelength range
(typically 3 and 10 cm) are back-scattered by raindrops and ice particles, as well as by cloud droplets, particulates, insect
swarms and flocks of birds. The return signal and its time delay provide information on the objects in the path of the
beam and their direction, distance and altitude. The need to detect tropical rainstorms led to the first training programmes
in radar interpretation in 1944. In 1946 to 1947, the Thunderstorm Project led by H. R. Byers used radar to track the
growth and organization of thunderstorms in Florida and Ohio. Gradually, indicators of storm severity were devised based
on the shape and arrangement of echoes, their vertical extent and the strength of the back scatter measured in decibels
(dB). Much of this process is now automated. Specifically designed weather radars for the US Weather Bureau became
available only in 1957. In the 1970s the Doppler radar, which uses the frequency shift produced by a moving target to
determine the horizontal motion relative to the radar location, began to be used for research on hail and tornadoes.
Dual Doppler systems are used to calculate the horizontal wind vector. The Next Generation Weather Radar (NEXRAD)
deployed in the 1990s in the United States, and similar systems in Canada and European countries, are modern Doppler
instruments. The vertical profile of winds in the atmosphere can be determined with vertically pointing Doppler radar
operating in the VHF (30 MHz) to UHF (3 GHz) ranges. The wind velocity is calculated from variations in the clear air
refractive index caused by turbulence.
A major application of radar is in estimating precipitation intensity. R. Wexler and J.S. Marshall and colleagues first
established a relationship between radar reflectivity and rain rate in 1947. The reflectivity,
Z
, was found to depend on
the droplet concentration (N) times the sixth power of the diameter (D
6
). The basis of this relationship has recently been
questioned. Estimates are generally calibrated with reference to rain gauge measurements.
Reference
Rogers, R.R. and Smith, P.L. (1996) A short history of radar meteorology. In J.R. Fleming (ed.)
Historical Essays on Meteorology
1919-1995
. American Meteorological Society, Boston, MA, pp. 57-98.
c Frequency of rainstorms
maximum expected precipitation for storms of different
duration and frequency in the USA. The maxima are
along the Gulf Coast and in Florida.
It is useful to know the average time period within
which a rainfall of specified amount or intensity may be
expected to occur once. This is termed the
recurrence
interval
or
return period
. Figure 4.14 gives this type
of information for six contrasting stations. From this,
it would appear that on average, each twenty years, a
twenty-four-hour rainfall of at least 95 mm is likely
to occur at Cleveland and 216 mm at Lagos. However,
this
average
return period does not mean that such falls
necessarily occur in the twentieth year of a selected
period. Indeed, they might occur in the first or not at all!
These estimates require long periods of observational
data, but the approximately linear relationships shown
by such graphs are of great practical significance for the
design of flood-control systems.
Studies of rainstorm events have been carried
out in many different climatic areas. An example for
b Areal extent of a rainstorm
The rainfall totals received in a given time interval
depend on the size of the area that is considered. Rainfall
averages for a twenty-four-hour storm covering 100,00
km
2
may be only one-third to one-tenth of those for a
storm over a 25 km
2
area. The curvilinear relationship is
similar to that for rainfall duration and intensity. Figure
4.13 illustrates the relationship between rain area and
frequency of occurrence in Illinois, USA. Here a log-log
plot gives a straight line fit. For 100-year, or heavier falls,
the storm frequency in this region may be estimated from
0.0011 (area)
0.896
where the area is in km
2
.
