The ALOMAR Rayleigh/Mie/Raman lidar: objectives, configuration, and performance

Abstract

We report on the development and current capabilities of the ALOMARRayleigh/Mie/Raman lidar. This instrument is one of the core instrumentsof the international ALOMAR facility, located near Andenes in Norway at69 degrees N and 16 degrees E. The major task of the instrument is toperform advanced studies of the Arctic middle atmosphere over altitudesbetween about 15 to 90 km on a climatological basis. These studiesaddress questions about the thermal structure of the Arctic middleatmosphere, the dynamical processes acting therein, and of aerosols inthe form of stratospheric background aerosol, polar stratosphericclouds, noctilucent clouds, and injected aerosols of volcanic oranthropogenic origin. Furthermore, the lidar is meant to work togetherwith other remote sensing instruments, both ground- and satellite-based,and with balloon- and rocket-borne instruments performing in situobservations. The instrument is basically a twin lidar, using twoindependent power lasers and two tiltable receiving telescopes. Thepower lasers are Nd:YAG lasers emitting at wavelengths 1064, 532, and355 nm and producing 30 pulses per second each. The power lasers arehighly stabilized in both their wavelengths and the directions of theirlaser beams. The laser beams are emitted into the atmosphere fullycoaxial with the line-of-sight of the receiving telescopes. The latteruse primary mirrors of 1.8 m diameter and are tiltable within 30 degreesoff zenith. Their fields-of-view have 180 mu rad angular diameter.Spectral separation, filtering, and detection of the received photonsare made on an optical bench which carries, among a multitude of otheroptical components, three double Fabry-Perot interferometers (two for532 and one for 355 nm) and one single Fabry-Perot interferometer (for1064 nm). A number of separate detector channels also allow registrationof photons which are produced by rotational-vibrational and rotationalRaman scatter on N-2 and N-2+O-2 molecules, respectively. Currently, upto 36 detector channels simultaneously record the photons collected bythe telescopes. The internal and external instrument operations areautomated so that this very complex instrument can be operated by asingle engineer. Currently the lidar is heavily used for measurements oftemperature profiles, of cloud particle properties such as theiraltitude, particle densities and size distributions, and ofstratospheric winds. Due to its very effective spectral and spatialfiltering, the lidar has unique capabilities to work in full sunlight.Under these conditions it can measure temperatures up to 65 km altitudeand determine particle size distributions of overhead noctilucentclouds. Due to its very high mechanical and optical stability, it canalso employed efficiently under marginal weather conditions when data onthe middle atmosphere can be collected only through small breaks in thetropospheric cloud layers.