New satellite mission providing global cloud and aerosol profiles

Climate models show that increasing levels of CO₂ and other greenhouse gases are driving changes in Earth's climate. But aerosols and clouds affect the flow of solar and thermal radiation in the atmosphere, and thus modify the warming effect of greenhouse gases. Atmospheric aerosols, such as desert dust and smoke, both reflect and absorb sunlight, cooling the Earth's surface. Clouds reflect sunlight, which has a cooling effect, but also trap outgoing thermal radiation, which has a warming effect. The balance between these two opposing cloud effects depends, among other things, on the altitude of the clouds.

Much of the current uncertainty about the effects of aerosols and clouds on Earth's climate is due to our limited ability to globally monitor aerosols and clouds from satellites. Aerosols over bright surfaces, such as deserts, are difficult to see from space, and the lifetimes and radiative effects of aerosols depend on their altitude. Clouds often occur in multiple layers, which strongly influences their radiative effects. Lidar can overcome these observational difficulties. It can provide vertical profiles of aerosols and multi-layer cloud structures, and can observe thin cloud and aerosol layers that passive instruments cannot detect.

The three-year Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission was developed as a collaboration between NASA and the French space agency Centre National d'Études Spatiales (CNES).¹ Launched on April 28, 2006, its primary objective is to provide the observations necessary to improve our understanding of the roles clouds and aerosols play in the climate system. The CALIPSO satellite carries a two-wavelength polarization lidar, the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP, pronounced like “calliope”). The payload also includes a three-channel imaging IR radiometer and a single-channel, wide-field visible imager. The satellite flies with the lidar pointing straight down, so that the instrument measures a vertically-resolved curtain of atmospheric data. The two passive sensors image a 60km swath centered on the lidar footprint (see Figure 1). CALIOP is the first satellite lidar optimized for atmospheric sensing, and the first lidar to orbit the Earth along with passive instruments.

The layout of the CALIPSO payload is shown in Figure 2. A diode-pumped Nd:YAG laser produces linearly-polarized pulses of light at 1064nm and 532nm. The atmospheric returns are measured to extract the backscattered intensity at 1064nm and the components of the 532nm return parallel and perpendicular to the transmitted beam's polarization. Expanders reduce the laser's angular divergence to produce a beam diameter of about 90m at the Earth's surface. The lidar sampling resolution is 30m in the vertical and 333m in the horizontal, values determined by the receiver's electrical bandwidth and the laser pulse repetition rate. Backscatter data will be acquired from the surface up to a height of 40km with the 30m vertical resolution. Additional technical detail on CALIOP is available in Winker et al.²

Aerosol and cloud layers are detected using an adaptive threshold technique.³ Aerosols are then discriminated from clouds using the return signal's magnitude and spectral behavior,⁴ and extinction profiles are retrieved using a linear iterative technique. These algorithms are implemented in a unique analysis scheme that employs a nested, multi-scale averaging approach designed to optimize tradeoffs between spatial resolution and signal-to-noise ratio.⁵

Data from the wide-field camera, operating at 670nm and developed by Ball Aerospace, and the three-wavelength IR imager, developed by CNES and the French company SODERN, complement the lidar observations. In particular, when combined with lidar data, the IR imager data help derive ice cloud microphysics and desert dust properties.

In addition to providing unique observations of clouds and aerosols to improve our understanding of their roles in Earth's climate system, CALIPSO also flies as part of the Afternoon constellation of satellites (A-train).⁶ Acquiring lidar observations that are simultaneous and coincident with observations from passive instruments on other A-train satellites will allow numerous measurement synergies.

CALIPSO data products should become publicly available by the end of 2006. Further details on this, and on the CALIPSO mission, are available on the CALIPSO website: http://www-calipso.larc.nasa.gov.