First Climatology of Polar Mesospheric Clouds From GOMOS / ENVISAT Stellar Occultation Instrument
Kristell Pérot(1), Alain Hauchecorne(1), Franck Montmessin(1) and Jean-Loup Bertaux(1)
(1) LATMOS / IPSL / CNRS, Réduit de Verrières BP3 Route des Gâtines, 91371 Verrières le Buisson Cédex, France
Polar mesospheric clouds (PMC), also called noctilucent clouds, are the highest clouds in the Earth’s atmosphere. They can be observed during the summer months near the poles, where the mesopause region becomes the coldest place on Earth and when the Sun does not plunge too far below the horizon but can still illuminate these high-altitude clouds. The noctilucent clouds have been observed in increasing amounts and they have been becoming brighter since the 1960’s. This behavior suggests a possible link with long-term global change, why recently they have aroused a great interest. The clouds are formed of water ice crystals and often exhibit wavy structures. They have been recently studied from GOMOS on ENVISAT.
GOMOS is a stellar occultation instrument combining 4 spectrometers in the spectral range 250 to 950 nm (UV - visible - near IR) and 2 fast photometers (470-520 nm and 650-700 nm). On the day side, GOMOS measures also the solar light scattered by the atmospheric molecules. In the summer polar day, polar mesospheric clouds are clearly detected using the photometers signals. The sun-synchronous orbit of ENVISAT allows observing them in both hemispheres. The observation of PMC with the spectrometers provides the spectral dependence of the scattering by PMC particles from which it is possible to derive some information on particle size.
These clouds at the edge of space have been studied using four years of GOMOS data. The method of detection and the climatology of PMC obtained by GOMOS will be described, focusing on the seasonal and latitudinal coverage, represented by global maps and calculation of corresponding detection frequency. The asymmetry between hemispheres and the determination of main geometrical characteristics of the clouds (mean altitude, vertical thickness and geometric extent) will also be presented, as well as a first estimation of the particle size. A very high accuracy is possible thanks to the stellar occultation technique.