Weather Watch Radar
In this mode of operation, radar is used to detect the presence of
rain and hail (referred to as "precipitation") and to estimate its
intensity from the strength of the received echoes. The information
is presented to forecasters on computer displays in the form of maps
which show the location of the rain in relation to local features such
as the coastline, with different colours used to depict intensity.
For example, dark blue may represent light drizzle, with red used to
depict very heavy rain possibly containing hailstones.
When displayed in this way, it is often found that particular weather
situations give rise to radar echoes with distinctive patterns or shapes
which help forecasters to identify them. For example, tropical cyclones
are well known for their spiral bands of rain, cold fronts for their line
structure and tornadoes for their small hook like shapes. Time lapse
sequences of Weather Watch radar images obtained at regular intervals are
of great value to forecasters in following the movement of such weather
systems and estimating when particular events will occur, such as a cyclone
crossing the coast or a cold front reaching a populated area.
Sample radar display screen: Thunderstorms near Coffs Harbour
Because of radar's ability to estimate the intensity of rainfall over
wide areas, it has a useful role to play in monitoring weather situations
which might result in serious flooding. Also, by replaying a series
of recorded images after the event, radar can often provide useful
evidence or insights into the meteorological situation at the time of
incidents such as storm damage or aircraft accidents.
All capital cities and many major centres in Australia have Weather
Watch radar coverage, with particular attention being paid to the
northern coastal areas which are subject to destructive tropical cyclones.
Radar is able to depict light rain to distances beyond about 100 km
from the radar and to indicate the possibility of severe storms to 250 km
or more, limited in the main by earth's curvature. Wherever possible,
Weather Watch radars are located on high ground in order to clear local
obstructions and give the best possible coverage of the surrounding area.
Sample radar display screen: Tropical Cyclone Bobby off WA coast,
near Dampier
Windfinding Radar
In this application, radar is used to track the flight of a gas-filled
balloon as it ascends through the atmosphere from the ground up to a
height of about 30 km. The balloon is blown along by the wind, and by
monitoring changes in the balloon's position at regular intervals,
typically every minute, it is possible to determine a profile of wind
speed and direction at different heights in the atmosphere.
Wind profiles are of great importance for the safe and economical
operation of aircraft and in addition provide a most important source
of data for the Bureau's general forecasting system. Windfinding
stations are located on a roughly regular grid every few hundred
kilometers and typically perform four upper wind soundings each day,
wherever possible at the same time. In this way, three dimensional
"pictures" of the wind field over the whole country are obtained at
regular intervals, giving vital information about the
atmosphere's behaviour.
The gas-filled balloons are themselves invisible to radar, but carry a
lightweight reflecting radar "target" made of aluminium foil and foam
plastic which allows them to be tracked to great distances, often in
excess of 200 km in very strong winds, the so-called "jet streams".
Their rate of climb through the atmosphere is generally about 300
metres per minute before they burst, which means that a typical wind
sounding can take about one or two hours. At some stations a
radiosonde transmitter is attached to the same balloon to send back
information regarding temperature and humidity at different heights.
Balloon launch at Gove Airport (NT) Meteorological Office
The balloon carries a metallic foil radar reflecting target.
An interesting and relatively recent development in radar
windfinding is the Wind Profiler, where a powerful vertically pointing
radar is able to obtain echoes directly from turbulent eddies within the
atmosphere itself, without the need for a balloon-borne reflecting
target. Profilers can provide completely automatic continuous operation,
giving new data every few minutes rather than every few hours, as with
balloon-based windfinding. The technology is still evolving and the Bureau
has an active program of further develpment in collaboration with other
organisations, both local and overseas. Present experience indicates
that Profilers are unable to achieve the same altitude coverage as
balloon-based systems, but can provide valuable data to assist with
short-term forecasting in major population centres, or for specific
purposes such as monitoring wind patterns in the vicinity of airports or
near enterprises which emit pollutants into the atmosphere and need to
time their discharges for winds which will carry the emissions away from
sensitive areas.
Sample wind profiler display: Wind Profile at Sydney Airport
"Zonal" is the west to east component of the wind, and "meridional"
is south to north.
"Range" refers to height above ground in km.
Meteorological Radar