Space observations of clouds
Clouds cover about 70 % of the Earth's surface and play a dominant role in the energy and water cycle of our planet. Only satellite observations provide a continuous survey of the state of the atmosphere over the whole globe, and their record length exceeds now more than 35-40 years. Most current satellite instruments are radiometers, measuring reflected, scattered and emitted radiation from the earth's surface, atmosphere and clouds. To convert the measured radiances into cloud properties, clouds have first to be distinguished from clear sky situations and then their properties have to be determined using inverse radiative transfer models.
During the last years joint efforts have been undertaken to improve the quality of the sensor data (e.g., calibrated radiances) and to produce and reanalyze climate records of geophysical variables, like cloud, aerosol and surface properties. Within the framework of WCRP and GEWEX, assessments have been established to evaluate the quality of these datasets. In particular, the GEWEX cloud assessment is joined so far by about ten international teams, and intercomparisons have highlighted the different sensitivities of various instruments and retrieval algorithms to thin cirrus and to low clouds. Considering these different sensitivities, the seasonal cycles of regional cloud properties agree well enough to be used for the evaluation of climate models Stubenrauch et al. (2009).
Cloud properties from IR Sounders
CO2 sensitive channels of infrared (IR) vertical sounders such as the TIROS-N Operational Vertical Sounder (TOVS) onboard the NOAA polar satellites since 1979, the Atmospheric InfraRed Sounder (AIRS) onboard Aqua since 2002 and the IR Atmospheric Sounding Interferometer (IASI) onboard METOP since 2006, allow to determine cloud height and cloud emissivity (of the uppermost cloud layer in the case of multi-layer cloud scenes). Cloud property retrievals using these data rely on the principle that radiances measured from near the centre of a CO2 absorption band are only sensitive to the upper atmosphere while radiances from the wings of the band (away from the band centre) see successively lower levels of the atmosphere. The high spectral resolution of IR vertical sounders provides especially reliable cirrus (semi-transparent ice cloud) properties, day and night Stubenrauch et al. (1999), Stubenrauch et al. (2006), Stubenrauch et al. (2008) and Stubenrauch et al. (2010).
Geographical maps of high cloud amount (cloud pressure smaller than 440 hPa) for January (left) and for July (right), above: averaged over 8 years (1987-1995, observation time: 7h30/19h30 local timeTOVS Path-B cloud climatology) middle: averaged over 6 year (2003-2008, observation time 1h30/13h30 local time AIRS-LMD cloud climatology) and below: first results from IASI for 2008. (Click the figure to enlarge it or click here)
Cirrus microphysical properties from IR Sounders
Cirrus clouds cover about 30% of the globe and play an important role in the climate system. Due to their complex microphysical characteristics (consisting of non-spherical ice crystals of various shapes and sizes), however, their physical and radiative properties are still not completely understood. These properties are expected to depend on the formation process and on the atmospheric environment. Within the framework of the European project CIRAMOSA, microphysical bulk properties such as ice water path and effective ice crystal diameters have been retrieved Rädel et al. (2003) , and their behaviour has been studied in correlation to atmospheric properties Stubenrauch et al. (2004). An analysis combining TOVS and Scanner for Radiation Budget (ScaRaB) data with flux computations using the MetOffice radiative transfer model has shown that pristine crystals seem to be plausible only for cirrus with IWP less than 30 gm-2. For larger IWP, ice crystal aggregates lead to cirrus SW albedos in better agreement with the observations. The data also indicate that climate models should let increase the cirrus effective ice crystal diameter (De) with IWP, especially in the range up to 30 gm-2 Stubenrauch et al. (2007), PhD Thesis by R. Holtz (2001) (ARA/LMD/IPSL), PhD Thesis by F. Eddounia (2004) (ARA/LMD/IPSL).
Upper tropospheric humidity and cirrus
To study the formation and dissipation of cirrus is fundamental for the understanding of interactions between cirrus and climate. The increase in air traffic during the 20th century has led to numerous theoretical and observational investigations of its impact on the formation of cirrus. Condensation trails (contrails) form when the hot and humid exhaust gases from the combustion mix with the ambient, cold atmosphere. Their persistence depends on upper tropospheric relative humidity. Our analysis of eight years of TOVS Path-B satellite data has detected a significant increase of cirrus in areas with very high air traffic when the meteorological situation is favourable for contrail formation Stubenrauch and Schumann (2005).
Propagation of contrails over Paris in the morning (photo M. Virchaux).
Since summer 2006 data are available from two active instruments aboard the A-Train: the lidar of the CALIPSO mission and the CloudSat radar. The synergy with the Atmospheric InfraRed Sounder (AIRS) aboard the EOS Aqua satellite provides an even more detailed insight into cirrus, especially into their relationship with upper tropospheric humidity Lamquin et al. (2008), Lamquin et al. (2009), PhD Thesis by N. Lamquin(2009) (ARA/LMD/IPSL).