Before a forecast
can be made of what the weather is likely t o 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.
