Understanding our measurements
10 Mar 2011
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The aim of these pages is to help someone who doesn't have a comprehensive knowledge of all our measurement techniques to gain a basic understanding and to see ways to use single or multiple data sets in their field of research.

 
Illustration of radar/lidar techniques

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A wide range of atmospheric science data are produced at the Chilbolton Observatory. Most researchers are not an expert in all of them, which can make it difficult for them to make the best use of the available information. 

Where to find the data

All of our main long-term data streams are archived by the British Atmospheric Data Centre and can be accessed via their website.

Data format

All our data are archived using the netCDF format. NetCDF is commonly used worldwide to store atmospheric data. It is known as a self-describing format. This means that all the information required to read the file is contained within it, hence avoiding the need for complex file format information to interpret complex proprietary formats. Routines to read netCDF exist within many common programming languages.

Datasets are categorised here into 4 main types: radar, lidar, microwave radiometer and meteorological measurements.

Radar and lidar

Radar and lidar share many similarities although there are many differences in technology and terminology between the 2 techniques. Both are active techniques, i.e. a beam is transmitted from the instrument, scattered from atmospheric constituents and finally detected at the instrument. In both techniques either continuous or pulsed transmitted beams can be used. Since all the systems operating at Chilbolton Observatory use pulsed beams, only such systems will be discussed here.

The amplitude of the scattered signal relates directly to the concentration of the scatterers, once differences in their cross-section for scattering are accounted for. Broadly speaking, the size of scatterer which can be detected decreases as the frequency of the radiation increases. Hence a lidar which transmits radiation at around optical wavelengths is sensitive to clouds, aerosols and even air molecules. Its beam is quickly absorbed by larger particles such as precipitation and water clouds. By contrast a radar is better suited to measuring clouds and precipitation but is insensitive to aerosols and molecules.

When a pulsed source is used, the distance to the scatterer can be directly determined from the time since the pulse was transmitted. The ability to accurately measuring scattering as a function of range in this way is one of the greatest benefits of radar and lidar. The short duration of pulses means that high range resolution, of the order of tens of metres, is possible.

The transmitted beam is commonly plane-polarised. This allows further information about the shape of the scatterers to be determined by observing the polarisation of the measured signal.

Some instruments allow the velocity of the scatterer along the direction from the source to be determined via the Doppler shift of the returned radiation. Light scatterers such as aerosols or small cloud droplets move at the same velocity as the local wind velocity.

Radar

Radars operating at 4 frequencies (1.275, 3, 35 and 94 GHz) are available at Chilbolton Observatory. Details of the individual systems can be found in the facilities section. The 1.275 GHz system is particularly aimed at observing clear air structures such as convection cells. The 3 GHz radar is well suited to precipitation measurements, but can also detect ice clouds (which have relatively large crystals). The 1.275 and 3 GHz system are mounted on the fully steerable 25 m dish. The 35 and 94 GHz radars are well suited to water and ice cloud measurements. They normally point vertically.

Further details of the quantities measured by the radars.

Lidar

Lidars operating at 355 nm, 905 nm and 1550 nm are available at Chilbolton Observatory and details are again found in the facilities section. All can measure the backscattering coefficient profile due to clouds and aerosols. The 355 nm systems are also able to detect scattering from air molecules due to their shorter transmitted wavelength. One of the 355 nm systems is a Raman scattering lidar which enables water vapour profiles to be measured. The other is a polarisation lidar capable of measuring the depolarisation of the transmitted beam. The 1550 nm HALO system measures the radial velocity of cloud droplets or particles and aerosols. All systems are normally pointed vertically or within a few degrees of vertical.

Further details of the quantities measured by the lidars.

Microwave radiometer

A Radiometrics MP1516A radiometer is located at Chilbolton Observatory. It points vertically upwards and operates continuously. In general microwave radiometers deduce atmospheric information by measuring the sky brightness temperature at a range of frequencies, in this case in the range 22 – 30 GHz. From these measurements values of the integrated liquid water and integrated liquid water at zenith are derived.

Profiles of the water vapour concentration as a function of height are also produced. Whilst they provide useful measurements, values derived from microwave radiometer measurements are always subject to greater uncertainties than those from other methods. This is a consequence of the complex relationship between measured brightness temperatures and the derived atmospheric quantities. In particular, the water vapour profiles have low resolution and so commonly smooth out sharp changes in water vapour concentration.

Measurements are also less reliable during or immediately after rain, while the instrument radome remains wet. Their strength is that they are passive rather than active instruments (i.e. they do not transmit radiation) and so are normally more reliable for continuous operation.

Meteorological measurements

The main aim of the wide range of meteorological instruments is to complement the other instruments, providing information about weather conditions during measurements. The more common measurements include air temperature, relative humidity, air pressure, wind speed and rainfall rate (via 3 types of rain gauge). There is a sky camera which can be used to give a qualitative view of cloud conditions and visible/near infrared and far infrared radiometers providing information on incoming (solar) short wave and re-emitted long wave radiation respectively. Measurements are continuous.

Synergistic use of instruments

Many options exist for synergistic use of the measurements at Chilbolton Observatory, some of which are yet to be discovered and exploited. At the simplest level meteorological instruments  can be used to provide surface measurements or cloud images at the time of profiling measurements from other instruments. It is also possible for experimenters to bring their own instruments to the site and further increase the range of available data.

As part of the EU-funded CLOUDNET project, lead by the University of Reading, a method was developed for the continuous simultaneous measurement of clouds using radars and lidars. Since each instrument type produces different returned signals depending on the particle size and type, more information can be deduced about the cloud droplets/crystals such as size and concentration than can be derived using a single instrument. This information is of use for weather forecasting and climate modelling.

Further information about CLOUDNET can be found in Illingworth, A. et. al., 'CLOUDNET ' Continuous Evaluation of Cloud Profiles in Seven Operational Models using Ground-Based Observations' - Bull. Am. Meteorol. Soc., 88, 883-898 (2007).​

Contact: Jeffery, Judith (STFC,RAL,RALSP)