About this page

A brief summary of the data and data sources used in NWP (Numerical Weather Prediction).


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The need

Like medicine, a weather prognosis needs a diagnosis based on observation. In the case of weather this involves a complex system of measurements of the atmosphere from terrestrial and space based systems.

Since the early 1950s, technology has advanced greatly in computing and instrument design. In situ, and remotely sensed ,technologies have improved beyond recognition. This has led to a vast increase in the amount of data used in NWP models..

Terrestrial based observing

At the surface

Weather observations have for many years been made from networks of stations over land and from ships on passage. Increasingly, observing systems have been automated and objective measurements made of visibility, cloud base and amount. Even “weather” can be automated...

Over the land, gaps in networks can be filled by totally automated systems. Tethered and drifting data buoys are used over the oceans and seas to provide data in otherwise badly covered areas.

The ECMWF produces a list of data used in its predictions. The first diagram shows locations of weather reports from land ships at sea. "SYNOPS" refers to reports from lands station; "METARS" are abbreviated reports from airfields and "SHIPS" are reports from ships on passage.

The next diagram shows the coverage of data buoys...


Above the surface

The conventional method of measuring wind, temperature and humidity above ground level is the Radio-sonde. This is a cheap, use once, system that telemeters data back to the ground station. Winds used to measured using radar tracing but is now more often by using GPS.

There are some 600+ stations worldwide as shown here as “Temp.”

These ascents are made, usually, twice a day. At some locations, at intermediate hours, wind only ascents are made; Pilots. There are a few “Profilers” in use. These are vertical looking radars that can “see” clear air moving. The next diagram shows locations of Pilot and Profiler measurements.

In addition to these, measurements are also obtained automatically from aircraft en route and on the ascent/descent phases.

 

Space based systems

Sensors and orbits

There are two types of each. Sensors can be passive or active. Passive sensors “look” at the earth and atmosphere using visible, infrared and microwave frequencies. Active sensors transmit a signal and receive the return; these act lie radar.

Orbits can be geostationary or low earth, usually, near polar orbit at heights of 400 – 800 m Geostationary satellites are good at looking at cloud or water vapour and measuring movements to deduce winds at cloud levels. At 35, 000 km they are too high to be able to produce good measurements otherwise.

Low earth orbiters provide most of the temperature and humidity data using passive sensors. They use active sensors to measure winds at sea level. This is done by measuring the scattering of a radar beam using a “Scatterometer.”

Active sensors

Wind measurements are obtained in swathes as shown here.


Passive sensors from Geostationary orbit

This is an example of the area and numbers of observations of winds measured from a geostationary satellite. Visible and infra-red light is used. The latter can look at movements of clouds and areas of clear but moist air.


Passive sensors from low earth orbit

Infrared sensors measure temperature and water vapour. The two main systems are known as HIRS (High resolution Infrared Radiation Sounder) and AIRS (Advanced Infrared Radiation Sounder.)

This shows HIRS coverage.

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Micro-Wave sounders measure temperatures, water vapour and liquid Water and ice.

 

Data Volumes

The table will give some idea of the scale of the data handling involved.

Terrestrial based

Surface synoptic and ships

68,248

Data buoys, drifting and moored

7,650

Aircraft

98,826

Radio sonde

622

Balloon winds and profilers

3.304

Total terrestrial

178,650

Space based

Cloud motion winds

415976

Surface winds - Scatterometer

586695

Microwave

1.031.840

Infra-red

1.131,840

GPS etc

118339

Total space based

3,284,690

Total all data See note below

3.463,340


From what has been written earlier, it is clear that even the large amount of terrestrial based (in situ) observations are far from sufficient to describe the atmosphere adequately. The table shows the importance of space based data. There are some other forms of data not listed here still being developed.

For details on a daily basis on the number of observations used, see the ECMWF data monitoring pages.

NOTE.

An "observation" can be many bits of data. A radio-sonde ascent will have values of wind, temperature and humidity at many levels; satellite "soundings" can consist of radiances at several wavelengths. At a conservative estimate, the number of pieces of data available for use in a numerical weather prediction data analysis is probably over 30million.

Using the data

There are two major problems in handling data in NWP. One is the sheer volume of the data, the second is the variety of data available.

Satellite data

There are a number of problems in using satellite data.

First, is the sheer data volume; not all the data can be used effectively.

Secondly, the satellite passive sensors usually measure the effects of the atmosphere on radiation rather than the atmosphere itself.

Thirdly, the data refer to areas, often quite small areas, while conventional data are values at a point.

Fourth, measurements from low earth orbiters are at continuously varying time. Geostationary measurements are made at the same time as in situ surface based sensors.

Data analysis has to be done on a 4-dimensional basis. It is not surprising that roughly as much compute power as the NWP itself.


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