© Earth Observatory / NASANASA Earth Observatory image by Jesse Allen, using CALIPSO data provided by the Langley Atmospheric Science Data Center, with meteorological analyses by Andreas Dörnbrack, Institute of Atmospheric Physics, DLR Oberpfaffenhofen, Germany.
© Earth Observatory / NASA
Clouds do not usually form in the stratosphere because of the dry conditions. But in the polar regions, often near mountain ranges, atmospheric gravity waves in the lower atmosphere (troposphere) can push just enough moisture into the high altitudes. The extremely low temperatures of the stratosphere condense ice and nitric acid into clouds that play an important role in depletion of stratospheric ozone.
The top image was assembled from data from CALIPSO's Light Detection and Ranging instrument, or lidar, which sends pulses of laser light into Earth's atmosphere. The light bounces off particles in the air and reflects back to a receiver that can measure the distance to and thickness of the particle- and air masses below. The data was acquired between 4:30 and 4:44 Universal Time on January 4, 2011, as the satellite flew 1120 kilometers (695 miles) from north to south over the Greenland Sea and Denmark Strait, as depicted in the map above.
CALIPSO has observed stratospheric clouds before, but never one this high, says Mike Pitts, an atmospheric scientist at NASA's Langley Research Center. This cloud reached an altitude of more than 30 kilometers (19 miles).
The cloud was the result of mountain waves in the atmosphere, which form when stable air masses pass over mountains or high ice sheets, providing vertical lift. Pitts said such stratospheric ice clouds are rare because they only form when the jet stream in the Arctic is properly aligned with the edge of the polar vortex, a large air pressure system over the poles. The circulating air in the vortex needs to align with the jet stream to create enough vertical motion and propagate the waves to the upper atmosphere. The January 4 cloud was formed when those winds aligned and sent an air mass up over the high ice sheet and mountains of Greenland.
I'm not sure if this is going to help, but here's what I've found so far that has links too not only the cooling of the polar regions due to climate change, but also links climate cooling to the contrails, and supports the idea of a cooling polar vortex region extending down southwards over recent years due to extreme tropospheric weather systems, which goes in hand with the lowering of the jet stream, and bringing the jet stream and polar vortex interaction further south.
Nacreous clouds are wave clouds. They are often found downwind of mountain ranges which induce gravity waves in the lower stratosphere. Their sheet-like forms slowly undulate and stretch as the waves evolve. The clouds can also be associated with very high surface winds which may indicate the presence of, or induce, winds and waves in the stratosphere. Nacreous clouds, sometimes called mother-of-pearl clouds, are rare but once seen are never forgotten. They are mostly visible within two hours after sunset or before dawn when they blaze unbelievably bright with vivid and slowly shifting iridescent colours. They are filmy sheets slowly curling and uncurling, stretching and contracting in the semi-dark sky. Compared with dark scudding low altitude clouds that might be present, nacreous clouds stand majestically in almost the same place - an indicator of their great height.
They need the very frigid regions of the lower stratosphere some 15 - 25 km (9 -16 mile) high and well above tropospheric clouds. They are so bright after sunset and before dawn because at those heights they are still sunlit. They form at temperatures of around minus 85ºC, colder than average lower stratosphere temperatures, and are comprised of ice particles ~10µm across. The clouds must be composed of similar sized crystals to produce the characteristic bright iridescent colours by diffraction and interference.
They are seen mostly during winter at high latitudes like Scandinavia, Iceland, Alaska and Northern Canada. Sometimes, however, they occur as far south as England! They can be less rare downwind of mountain ranges. Elsewhere their appearance is often associated with SEVERE TROPOSHPERIC WINDS AND STORMS.
Nacreous clouds do occur at lower latitudes, they have been visible over England at least twice since 1996. A gale was blowing and the sun was setting on January 29th 2000 as this nacreous cloud glowed like an electric discharge. It was likely 15-25 km high and far above the apparently close by creamy coloured cirrus cloud and the dark storm clouds.
Nacreous clouds far outshine and have much more vivid colours than ordinary iridescent clouds which are very much poor relations and seen frequently all over the world.
Type II Nacreous clouds composed of ice crystals with temperatures of ~minus 85ºC.
Type I - Less spectacular than nacreous clouds, more diffuse and less bright colours. Sometimes nacreous clouds are embedded in them. Type I clouds are slightly warmer (~ minus 78ºC) than Type II and are composed of exotic solids or liquid droplets.
Type Ia - Crystalline compounds of water and nitric acid - especially NAT, nitric acid trihydrate HNO3.3H2O
Type Ib - Small spherical droplets of a solution of nitric and sulphuric acids.
Type Ic - Small non spherical particles of a metastable nitric acid - water phase
Type I clouds are now known as sites of harmful destruction of stratospheric ozone over the Antarctic and Arctic. Their surfaces act as catalysts which convert more benign forms of man-made chlorine into active free radicals (for example ClO, chlorine monoxide). During the return of Spring sunlight these radicals destroy many ozone molecules in a series of chain reactions. Cloud formation is doubly harmful because it also removes gaseous nitric acid from the stratosphere which would otherwise combine with ClO to form less reactive forms of chlorine.
Denoxification (removal of gaseous Nitrogen Oxides NOx) occurs on PSCs. NO2 is in gas-phase equilibrium with N2O5:
2 N2O54 NO2 + O2
N2O5 is removed from the gas-phase by the following reactions, catalysed by PSC particles:
N2O5 + H20->2 HNO3
N2O5 + HCl->ClNO2 + HNO3
The overall effect is a net removal of NO2. This is significant because ClO is an important catalyst in the destruction of Ozone, but is itself removed by the reaction:
ClO + NO2 + M->ClONO2 + M
(where M is any air molecule)
Thus a decrease in the levels of NO2 helps maintain large levels of ozone-destroying ClO. As the clouds grow, they begin to settle out of the stratosphere, taking the Nitric Acid with them. This removal of nitrogen compunds is termed ''denitrification''. It further encourages denoxification.
A major European campaign, the European Arctic Stratospheric Ozone Experiment (EASOE) was organised to study the polar regions during the winter of 1991/92. Link and free CD’s of data here
[Link]European Arctic Stratospheric Ozone Experiment (EASOE)
[Link]According to the latests reports above
“The Arctic polar stratosphere cooled down as usual in November / December 2007 as the Arctic vortex grew in strength. By mid-December temperatures at 10 hPa were below those necessary for Polar Stratospheric Clouds to form, an unusual occurrence at these altitudes - the observed temperatures were among the lowest in the 50 year record. At lower altitudes the temperatures were not so unusual though they were slightly below average. These cold conditions have, however, been sustained: the PSC formation temperature was reached at the start of December and the minimum temperatures inside the vortex have remained below it ever since indicating the potential for PSC existence for all of the last two months. PSCs have indeed been observed above several ground stations in the Arctic. Despite some transient warming going in recent days, according to the meteorological forecasts the vortex will remain cold and stable for at least the next week or so.
The accumulated volume of PSCs to date this winter is already now unusually large and it
may grow further if the cold conditions are sustained. This current winter is generally consistent
with the tendency of the cold winters becoming colder in recent years, although overall the average stratospheric temperatures over the Arctic have remained the same or even warmed. In other words it appears that the range (variability) of the observed temperatures has increased. The reasons for this are not yet clear, though SCOUT-O3 scientists are investigating the possibility of a link to CLIMATE CHANGE.
The volume of PSCs shows a tight, empirical relation to the amount of ozone loss in the Arctic
vortex if it survives until the end of March. Assuming that this relation holds for the current winter
and that the vortex is stable for the next few weeks, there is clearly the potential for large ozone
losses in the Arctic vortex this winter. In these circumstances, the volume of PSCs observed to date indicates the potential for an overall loss of more than 20% in the column amount of ozone by the end of March. The longer the vortex remains cold enough for PSCs to be present, the larger that loss is likely to become.
The latest European research campaign is called THESEO (Third European Stratospheric Experiment on Ozone) which takes places from 1997-1999. Scientists from many European countries, including some of this site, are collaborating on a wide range of experiments to determine the processes responsible for depleting ozone in the lower stratosphere but at mid-latitudes over the northern hemisphere.
Whilst wandering around here [Link]I found this press release from Brussels, May 28, 1998 “Rapid increase of aircraft emissions could affect atmospheric ozone and climate in the future”. The main points of which are:
The 20-50% increase in the NOx abundance caused by aircraft traffic in the vicinity of their cruising altitude (10-12 km) has produced a 4-8% increase in the ozone concentration of the upper troposphere (maximum value during summertime) where ozone is a strong greenhouse gas. The warming effect associated with this ozone increase is comparable to the warming effect of CO2 emitted by aircraft (about 2-3% of all anthropogenic CO2 emissions).
Climate perturbations could also have resulted from the formation of persistent contrails and high-level cirrus clouds produced in the busiest flight corridors. Additional effects on the radiative balance of the atmosphere could have been generated by the soot and sulphur particles released by aircraft engines. The warming effect of the changes in cloudiness is more difficult to assess but appears to be also of the same magnitude as the warming effect of CO2 emitted by aircraft.
The total climate forcing caused by the present fleet of commercial aircraft (about 0.1 Wm-2) is a small contribution to the total forcing (2.4 Wm-2) associated with past industrial development. However, with air traffic in the next 20 years expected to grow faster than the global economy, the relative contribution of aviation to environmental changes (pollution, stratospheric ozone, climate) could become more significant, unless new, less-polluting engines and more fuel-efficient aircraft technologies are introduced.
The development of a fleet of supersonic aircraft flying at high altitudes (17-20 km) could perturb the ozone layer in the stratosphere. Current models indicate, however that changes in the ozone column produced by a hypothetical fleet of 500 aircraft would be just a few percent and should lead to changes in the UV-B level at the Earth's surface of less than 2%. However, more research on the complex and poorly understood processes that affect the chemistry of ozone in the lower stratosphere is required before a conclusive assessment can be produced.
The assessment also identified a number of areas where improved knowledge could advance our understanding of how aircraft perturb the atmosphere. It stresses that the region where aircraft fly, straddling the tropospheric and stratospheric boundary at around 12km, is not sufficiently understood. A better understanding of the background ('natural') state of this region is required if the perturbations arising from aircraft emissions are to be known with confidence. For instance, the natural production of NOx from lightning needs to be better quantified before the impact of aircraft-induced NOx can be determined with confidence. In addition, the effect of aircraft emissions on the abundance of particles that provide the surface for complex heterogeneous reactions, needs to be carefully studied. The large uncertainty and the large potential for climatic impact due to possible changes in cloudiness induced by aircraft emissions, requires more emphasis on this topic than before. Finally, the relative importance of aircraft emissions may evolve in the course of future changes e.g. in tropospheric and stratospheric temperature, in water vapour concentration and in the residence time of other greenhouse gases like methane.
Very interesting! I hope this helps some :o)