Closure (atmospheric science)
Encyclopedia
A closure experiment in atmospheric science is a combination of different measurement techniques to describe the current state of the investigated system as fully as possible, and to find inaccuracies in one or some of the methods involved. The comparison of different types of measurement often involves model calculations, which may also be tested in this process.
A common example for closure experiments are aerosol
studies. Aerosols can be studied from space, from aircraft, and from the surface. The different properties of aerosols (chemical composition, particle size, particle number concentration, optical absorptivity, thermal emissivity, index of refraction, pattern of light scattering, etc.) require very different kinds of measurement techniques. Different instruments have been developed to implement each technique, and researchers are usually specialized to one or a few of them. Due to the logistic efforts required to bring different groups of researchers with different measurement techniques together, such experiments stand out among routine experiments that are more focussed on one or a few techniques and have fewer participants.
An aerosol closure experiment might look like this:
Additionally, measurements can be performed within the plume or outside, by means of remote sensing
, from the surface, from aircraft, and from satellites in space. Different viewing geometries again introduce complexity, which can be assessed in closure experiments.
Algorithms exist, for example, to predict the optical properties from a given particle shape, size distribution, and index of refraction. Closure is achieved if the predictions of such an algorithm agree (within the limits of measurement uncertainty) with the optical properties that have been derived from the independent radiation measurements; if the lidar information about the vertical distribution of the aerosol sums up to the total optical depth determined by the sun photometer; etc.
A common example for closure experiments are aerosol
Aerosol
Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can...
studies. Aerosols can be studied from space, from aircraft, and from the surface. The different properties of aerosols (chemical composition, particle size, particle number concentration, optical absorptivity, thermal emissivity, index of refraction, pattern of light scattering, etc.) require very different kinds of measurement techniques. Different instruments have been developed to implement each technique, and researchers are usually specialized to one or a few of them. Due to the logistic efforts required to bring different groups of researchers with different measurement techniques together, such experiments stand out among routine experiments that are more focussed on one or a few techniques and have fewer participants.
An aerosol closure experiment might look like this:
- Individual aerosol particles are sampled, and their size and shape is determined under an electron microscope;
- A lidarLIDARLIDAR is an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light, often using pulses from a laser...
emits a laser beam into the aerosol plume and detects the backscatter signal; - An aerosol flow is led through optical detectors that record the size distribution of the particles;
- the sunlight attenuated by the aerosol is measured by a sun photometerSun photometerA sun photometer is a type of photometer conceived in such a way that it points at the Sun.Recent sun photometers are automated instruments incorporating a Sun-tracking unit, an appropriate opticalsystem, a spectrally filtering device, a photodetector, and a...
, yielding the optical depthOptical depthOptical depth, or optical thickness, is a measure of transparency. Optical depth is defined by the negative logarithm of the fraction of radiation that is not scattered or absorbed on a path...
of the aerosol layer; - the spectral absorptionAbsorption (electromagnetic radiation)In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
of sunlight, as well as the scatteringScatteringScattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
properties of the aerosol are measured by spectrometers; - the extent and total reflectivity of the aerosol plume is observed by a satellite radiometer.
Additionally, measurements can be performed within the plume or outside, by means of remote sensing
Remote sensing
Remote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...
, from the surface, from aircraft, and from satellites in space. Different viewing geometries again introduce complexity, which can be assessed in closure experiments.
Algorithms exist, for example, to predict the optical properties from a given particle shape, size distribution, and index of refraction. Closure is achieved if the predictions of such an algorithm agree (within the limits of measurement uncertainty) with the optical properties that have been derived from the independent radiation measurements; if the lidar information about the vertical distribution of the aerosol sums up to the total optical depth determined by the sun photometer; etc.