The most spectacular application of femtosecond pulses in the atmosphere is probably laser-induced condensation. The l;aser technique is believed to work over kilometers. These lasers can also be used to guide lightning and to remotely map electric fields. The lasers could be used to make it rain locally or even prevent flooding by reducing the amount of precipitation by causing less of a cloud to condense into rain.
True-colour image of laser-induced condensation in a cloud chamber, illuminated by a green auxiliary laser. The main laser features 20 mJ pulses in 60 fs pulse duration, at a repetition rate of 100 Hz, and is shot continuously over a few minutes.
Credit: IOP/ Laser-assisted water condensation in the atmosphere: a step towards modulating
precipitation?atmospheric conditions.
Most civilizations developed magic or religious practice aiming at controlling the weather, and in particular precipitation. The first scientific attempts rely on the observation that clouds convert only a few per cent to a few tens of per cent of the available humidity into precipitation, leaving a wide room for improving the process ‘yield’. The experiments date back to 1934, when the former Soviet Union launched programs of airborne cloud seeding using calcium chloride. After World War II, this activity developed further, with seeding by dry ice and silver iodide, both in the USSR and in the US with the well-known experiments from Langmuir and Schafer.
The European researcher say lasers might offer the opportunity to generate artificial CCN along their beam, with a relatively low local density that further reduces quickly due to dilution from the tiny volume of the laser filaments (typically in thecm3 range for a TW laser beam) into the open atmosphere. This density can, moreover, be finely tuned by adjusting the laser power and the duration of its operation. On the other hand, laser beams can be swept across the forming clouds, in which filaments are known to propagate with little perturbation besides the losses due to the scattering of the photon bath [49]. This could allow precisely defining the regions of the cloud where the artificial CCNs would be released. Alternatively, purposely injecting an excess of CCNs could provide a means to temporally reduce precipitation
Multiple filamentation of a 3 J, 30 fs laser pulse of 10 cm diameter, imaged in real colour on a screen.
Credit: IOP / Laser-assisted water condensation in the atmosphere: a step towards modulatingprecipitation?
Since then, several approaches have been proposed in order to enhance precipitation by cloud seeding. Among them, static cloud seeding is both the oldest one and still the most widespread today. It consists in injecting dry ice or silver iodide particles into cold clouds. Their molecular structure being close to that of ice helps in converting supercooled water droplets into ice, a state in which they are expected to grow faster. Alternatively, dynamic seeding aims at quickly converting supercooled water into ice particles in order to release latent heat that will in turn enhance convection and updrafts and finally allow the clouds to grow larger and harvest more water vapour from the atmosphere.
Finally, hygroscopic seeding of warm (e.g. tropical) clouds relies on the injection of artificial cloud condensation nuclei (CCN), i.e droplet embryos of sub-μm to a few μm size, made of water or hygroscopic salts to produce water droplets. For example, large-scale experiments are conducted in that direction in Colorado. The main challenge faced by this technique
resides in adequately choosing the right embryo size and dispersing them efficiently while keeping them inside the active cloud volume. The latter difficulty can be addressed at least in
part by using airborne pyrotechnic flares, which also have the advantage of reducing the required amount of necessary salt mass.
However, the efficiency of cloud seeding is still controversial. This controversy is fuelled by the intrinsic variability of the atmosphere in both time and space, raising the need for large samples in order to obtain statistically significant results. In particular, the atmospheric variability and its dynamics make it extremely difficult to identify which seeding technique can be efficient and under which conditions
Principle of Lidar. Laser pulses are emitted into the atmosphere. The backscattered light bears information about the size and number density of the encountered aerosol particles, via their Mie backscattering cross-section. (b) Principle of multispectral (or white-light) Lidar. Filaments generate a broad spectrum upstream of the region to be analysed, which is left unaffected by the laser but scatters back each wavelength to the detection device. (c) Principle of pump-probe Lidar. Particles are produced in situ by ultrashort laser filaments in the atmosphere, depending on the atmospheric conditions. These particles are probed after a predefined delay by a Lidar with a beam collinear with the filaments.
Credit: IOP / Laser-assisted water condensation in the atmosphere: a step towards modulatingprecipitation?
This question may soon get an even broader scope. Ultrashort laser pulses were observed to generate highly hygroscopic species, resulting in the nucleation of nanometric and micrometric particles. This raised the prospect for laser-induced cloud seeding.
Laser filaments can be generated at kilometre-range distances by adequately tailoring the laser pulses. The main parameters at hand are the beam diameter and focusing, as well as the pulse chirp, i.e. the temporal offset of its spectral components related to one another. If the initial pulse chirp is opposite to the atmospheric group-velocity dispersion, the pulse will tend to re-compress at a distance that depends only on the magnitude of the chirp, and can therefore be easily set on demand. From the technical point of view, this is realized easily in chirped pulse amplification (CPA) lasers by slightly varying the distance between the gratings of the
Furthermore, this long-distance capability can efficiently be exploited for atmospheric applications because of the unexpectedly high robustness of the dynamic balance at the
root of filamentation. Filaments have the unique capability to self-heal after being blocked by an obstacle like a water drop because they are replenished by the surrounding photon bath
(also known as energy reservoir
The last two years have seen the emergence of laser-induced water condensation in the atmosphere as a new potential field of application for ultrashort laser pulses, which might have a strong impact on cloud seeding. The observation of the phenomenon under various conditions by several groups is now well established, and some basic processes at play are
identified. However, understanding the full complexity of the different pathways contributing to this mechanism constitutes the most challenging question ahead in this field. Both aerosol
microphysics and plasma photochemistry have to provide a deeper view and understanding of laser-generated particles from their nucleation to their ultimate evolution.
The second challenge ahead regards the potential application of these results to the atmospheric seeding of clouds. Assessing its feasibility on a sufficiently large scale requires further knowledge on the efficiency of laser induced condensation under various atmospheric conditions, which would enable realistic modelling of the observed particle nucleation and growth, and allow the determination of cloud types, regions and conditions in which precipitation enhancement may be expected. Finally, the ability of the lasers to seed macroscopic cloud volumes has to be confirmed, so as to define realistic strategies for beam emission and sweeping. These strategies will then have to be tested in real scale during atmospheric campaigns.
Contacts and sources:
IOP
Citation: Laser-assisted water condensation in the atmosphere: a step towards modulating
Multiple filamentation of a 3 J, 30 fs laser pulse of 10 cm diameter, imaged in real colour on a screen.
Credit: IOP / Laser-assisted water condensation in the atmosphere: a step towards modulatingprecipitation?
Since then, several approaches have been proposed in order to enhance precipitation by cloud seeding. Among them, static cloud seeding is both the oldest one and still the most widespread today. It consists in injecting dry ice or silver iodide particles into cold clouds. Their molecular structure being close to that of ice helps in converting supercooled water droplets into ice, a state in which they are expected to grow faster. Alternatively, dynamic seeding aims at quickly converting supercooled water into ice particles in order to release latent heat that will in turn enhance convection and updrafts and finally allow the clouds to grow larger and harvest more water vapour from the atmosphere.
Finally, hygroscopic seeding of warm (e.g. tropical) clouds relies on the injection of artificial cloud condensation nuclei (CCN), i.e droplet embryos of sub-μm to a few μm size, made of water or hygroscopic salts to produce water droplets. For example, large-scale experiments are conducted in that direction in Colorado. The main challenge faced by this technique
resides in adequately choosing the right embryo size and dispersing them efficiently while keeping them inside the active cloud volume. The latter difficulty can be addressed at least in
part by using airborne pyrotechnic flares, which also have the advantage of reducing the required amount of necessary salt mass.
However, the efficiency of cloud seeding is still controversial. This controversy is fuelled by the intrinsic variability of the atmosphere in both time and space, raising the need for large samples in order to obtain statistically significant results. In particular, the atmospheric variability and its dynamics make it extremely difficult to identify which seeding technique can be efficient and under which conditions
Atmospheric applications have been investigated by several groups, with even real-scale outdoor experiments allowed by the mobile-terawatt laser Teramobile or the T&T lab.
Principle of Lidar. Laser pulses are emitted into the atmosphere. The backscattered light bears information about the size and number density of the encountered aerosol particles, via their Mie backscattering cross-section. (b) Principle of multispectral (or white-light) Lidar. Filaments generate a broad spectrum upstream of the region to be analysed, which is left unaffected by the laser but scatters back each wavelength to the detection device. (c) Principle of pump-probe Lidar. Particles are produced in situ by ultrashort laser filaments in the atmosphere, depending on the atmospheric conditions. These particles are probed after a predefined delay by a Lidar with a beam collinear with the filaments.
Credit: IOP / Laser-assisted water condensation in the atmosphere: a step towards modulatingprecipitation?
This question may soon get an even broader scope. Ultrashort laser pulses were observed to generate highly hygroscopic species, resulting in the nucleation of nanometric and micrometric particles. This raised the prospect for laser-induced cloud seeding.
Laser filaments can be generated at kilometre-range distances by adequately tailoring the laser pulses. The main parameters at hand are the beam diameter and focusing, as well as the pulse chirp, i.e. the temporal offset of its spectral components related to one another. If the initial pulse chirp is opposite to the atmospheric group-velocity dispersion, the pulse will tend to re-compress at a distance that depends only on the magnitude of the chirp, and can therefore be easily set on demand. From the technical point of view, this is realized easily in chirped pulse amplification (CPA) lasers by slightly varying the distance between the gratings of the
compressor.
root of filamentation. Filaments have the unique capability to self-heal after being blocked by an obstacle like a water drop because they are replenished by the surrounding photon bath
(also known as energy reservoir
identified. However, understanding the full complexity of the different pathways contributing to this mechanism constitutes the most challenging question ahead in this field. Both aerosol
microphysics and plasma photochemistry have to provide a deeper view and understanding of laser-generated particles from their nucleation to their ultimate evolution.
The second challenge ahead regards the potential application of these results to the atmospheric seeding of clouds. Assessing its feasibility on a sufficiently large scale requires further knowledge on the efficiency of laser induced condensation under various atmospheric conditions, which would enable realistic modelling of the observed particle nucleation and growth, and allow the determination of cloud types, regions and conditions in which precipitation enhancement may be expected. Finally, the ability of the lasers to seed macroscopic cloud volumes has to be confirmed, so as to define realistic strategies for beam emission and sweeping. These strategies will then have to be tested in real scale during atmospheric campaigns.
Contacts and sources:
IOP
Citation: Laser-assisted water condensation in the atmosphere: a step towards modulating
J Kasparian1, P Rohwetter2, L W¨oste2 and J-P Wolf1
1 GAP-Biophotonics, Universite de Geneve, Chemin de Pinchat 22, CH-1211 Gen`eve 4, Switzerland
2 Institut f¨ur Experimentalphysik, Freie Universit¨at Berlin, Arnimallee 14, D-14195 Berlin, Germany
E-mail: jerome.kasparian@unige.ch
Received 22 February 2012, in final form 29 May 2012
Published 4 July 2012
Online at stacks.iop.org/JPhysD/45/293001
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