Before a forecast can be made of what the weather
is likely to be in the future, a knowledge of the
present situation is essential. Therefore, regular,
reliable and accurate measurements are required. These
have to be rapidly sent around the world using a telecommunications
system dedicated to weather information.
The observations are fed into the computer and used
to analyse the weather patterns at a particular time.
Once the analysis has been carried out, the computer
produces a forecast of the weather for specified times
in the future. The forecaster uses the output from
the computer to produce weather forecasts that are
tailored to a wide range of customers.
Fig 1a: Analysis of mean sea-level pressure
(isobars) and weather fronts for 0000 GMT on 3
February 2006
Fig 1b: 48-hour forecast of isobars and
rain (round symbols indicate continuous rain and
the triangular symbols indicate showers) from
the computer model for 0000 GMT on 5 February 2006
Fig 1c: 48-hour forecast of isobars and
weather fronts prepared by a forecaster (based
on the output from the computer model) for 0000
GMT on 5 February 2006.
Observations
The many data sources used include ships, aircraft, oil
rigs, buoys and balloons, as well as manned land stations
around the world. Automation often assists or replaces
the human observer and can provide information from inhospitable
and remote areas. Information from remote-sensing equipment,
both on the ground and in space, increasingly supplements
and complements the conventional systems.
Traditionally, meteorologists have relied upon observations
taken near the Earth's surface using instruments (e.g.
barometers, thermometers, anemometers and rain gauges)
and visual observations (e.g. cloud and weather type).
These surface observations are made at approved sites
on land, and from ships at sea.
Standard types of instruments are used, with observations
usually made at least every three hours, and in many cases
hourly. Over land in the UK there are 33 key observing
stations which are needed to define the broad-scale weather
patterns. They are manned by professional meteorologists,
with 12 making observations every hour, both day and night.
The other 21 are manned during the daytime, thereafter
switching to an automatic system. An additional 29 sites
are manned by auxiliary observers such as coastguards,
and there are more than 100 fully automated sites.
For weather observations at sea, the Met Office is
indebted to the crews of 400 vessels of the UK Voluntary
Observing Fleet and to observers on about 30 offshore
drilling platforms. This is part of a much larger
scheme officially involving around 6,500 ships from
53 nations, although the real number is closer to
3,500 ships. To fill in some of the gaps, there is
a network of ocean buoys, most drifting, but some
moored.
Important sources of upper-air information are the
balloon-borne instruments (known as radiosondes) which
provide information about the pressure, temperature
and humidity through the atmosphere. Also, from the
track of the radiosonde, the wind can be deduced.
The radiosondes can reach a height of over 20 km (66,000
feet); they are released twice a day at the same time
(midday and midnight UTC) all over the world.
Within the global network, the Met Office maintains
six sites in the UK. Two of these are fully manned
while the remaining four sites are equipped with autosondes,
which are released remotely. There are also Met Office
radiosonde sites in Gibraltar, St Helena and the Falkland
Islands. Near the UK, there is one fully manned site
in the Irish Republic and a variety of different sites
in continental Europe. At sea, there are automatic
systems that release radiosondes from the decks of
merchant ships.
Aircraft reports (known as AMDARs) of wind and temperature
along their flight routes, including take-off and landing,
help boost the upper-air information.
A type of radar known as a Doppler radar is used to measure
the winds vertically through the atmosphere. When displayed
over a period of type, these Windprofiler data show the
vertical profile of wind above the site and how it changes
with time. At the time of writing, there are Windprofiler
observations made at six sites in the UK, two in the Irish
Republic and one on the Isle of Man, as well as in continental
Europe.
A system for measuring the amount of water vapour in
the atmosphere is being developed, which is known as the
Ground-based GPS Network. This uses information from Global
Positioning Satellites (GPS) and about 150 stations are
envisaged. The data have been shown to be of value in
numerical models.
Radar
As well as the Windprofiler radars, there is a network
of weather radars that provides a picture of the distribution
of rainfall. From the radar it is possible to work
out where it is raining and how heavy the rain is.
The network includes sites provided by the Republic
of Ireland and the States of Jersey and covers the
whole of the British Isles. Extensive radar information
from the continent is also available.
Radar pictures are often shown on television forecasts,
and are used by the Environment Agency for river management
and flood warnings.
Satellites
Since the first meteorological satellite was placed
in orbit in 1960, satellites have become essential
tools for weather forecasters. The satellites used
by meteorologists fall into two categories.
Polar-orbiting satellites pass around the earth from
pole to pole at a height of about 870 km. It takes
approximately 1 hour 42 minutes for the satellite to complete
its orbit, by which time the earth has rotated by about
25 degrees. Consequently, each pass provides information
about a different strip of the atmosphere.
The polar-orbiting satellites provide pictures of
clouds, and information about the temperature through
the atmosphere.
Geostationary satellites remain over the equator,
stationary with respect to the earth. This is achieved
by having the satellite in orbit at a height of about
36,000 km. At this height it takes exactly 24 hours
to complete one orbit, so it always views the same
part of the globe.
Meteosat, the name given to the European geostationary
satellites, like their US, Japanese and Indian counterparts,
give sequences of cloud images. From these, the development
and movement of weather systems can be followed and,
of particular importance, tropical storms can be tracked.
The motion of specified areas of cloud can also be
followed to calculate the wind at various levels in
the atmosphere.
Analysis
The Global Telecommunication System (GTS) has been
set up to transfer weather observations (and forecasts)
around the world. The international circuit comprises
a sequence of high-speed computer-to-computer links,
using communication satellites as well as land lines.
The Telecommunications Centre at Met Office Headquarters
in Exeter has the role of passing data between Washington
and continental Europe via Paris and Offenbach. It
also collects observations from the UK and transmits
them around the world via the GTS. A complete set
of observations from the UK is available about ten
minutes past the hour of observation.
The observations taken from the GTS are stored on
computer and are analysed in two different ways.
The observations at a specific time are plotted
on a chart and an analysis is produced by the computer.
This involves isobars (lines of constant pressure)
being drawn, which allows depressions and anticyclones
to be identified. The analysis may be modified by
the forecasters and fronts are added (with the aid
of satellite and radar information) in order to
understand what is going on in the atmosphere.
The observations are used to define the starting
conditions of the atmosphere for a computer forecast
which can go as far as six days ahead.
Forecast
The use of computers has played a key role in improving
the accuracy and detail of weather forecasts, and
in lengthening the period for which useful guidance
can be given. The calculations involved are both numerous
and complex and must be performed quickly so that
forecasts are available in good time. Consequently,
some of the most powerful computers in the world are
needed.
Weather forecasts are based on the solution of a
set of mathematical equations describing certain physical
processes in the atmosphere. To solve these complex
equations it is first necessary to divide the atmosphere
up into boxes, with a grid point in the centre of
each box. The properties of the atmosphere are then
represented by what is happening at each of the grid
points.
The array of grid points, the system of equations
and the method of solving the equations is referred
to as the model. In the present global model used
by the Met Office, there is a spacing of roughly 40
km between each grid point in the horizontal. The
grid points are also arranged in 50 vertical levels
through the atmosphere.
Fig 13: Some of the physical processes
represented in computer models used to forecast
the weather.
The observations taken at a particular time can be
used to compute values for each grid point of pressure,
temperature, humidity and wind. This set of values
(the computer analysis) then represents the atmosphere
at the start of the forecast. Using the mathematical
equations, a 15-minute forecast can be made of how
these basic elements change. Once all the new values
have been calculated, the process starts again with
another 15-minute forecast being made. By repeating
this procedure many times over, a forecast out to
six days can be built up. The supercomputer at the
Met Office only takes about an hour to produce a six-day
global forecast.
The computer model produces a global forecast twice
a day using the midnight and midday observations as
starting conditions. In order to provide more-detailed
forecast charts out to 48 hours for the UK and parts
of the Atlantic and Europe, the model is run again
at 0600 and 1800 daily.
For local forecasts, the Met Office has developed
a model which has an 11 km horizontal grid and covers
the British Isles and the near continent. This 'mesoscale
model' is especially good at taking into account the
local effect of ranges of hills and the contrast between
land and sea in its forecasts.
Despite greater computer power, improvements to the
computer models, and other technological advances,
there is still an important role for the forecaster.
For the general development of weather systems, the
model provides insight into how the atmosphere is
behaving and developing, but it is only a guide. Good
as it is, forecasters have to make allowances for
the model's known problem areas - the handling of
small-scale features, for example. The chief forecaster
on duty modifies the computer output to correct for
likely errors in the model output, such as removing
spurious areas of rainfall. Forecasters also have
to take into account any late observations and consult
the latest satellite and radar pictures.
In providing specific services to individual customers,
the local forecaster based at an airfield or regional
office will take the process even further. Experience
and local knowledge add the fine detail to the computer
forecast, so that the best advice for a specific location
(e.g. an oil rig) can be given. There is no doubt
that the combination of man and computer together
produces the best forecasting results.
