Tropospheric water vapour observations by ground-based lidar

Ground-based lidars can provide continuous observations of tropospheric humidity profiles using the Raman-scattering of light by water vapour and nitrogen molecules. Profiles obtained since the beginning of year 2000 at the Koldewey Station (Ny-Alesund, Spitsbergen) will be presented. Under nighttime conditions the observations cover a range from about 200 m altitude up to the upper troposphere, while daylight limits the observations to the lower troposphere, depending on water vapour content of the atmosphere. Lidar soundings are limited to clear-sky and high-cloud conditions. The usage of additional, weather-independent methods like radiosonde or GPS-based observations will be discussed. Simultaneous observations of humidity and aerosol extinction during the advection of aerosol rich air masses from the Kara Sea show some delay of the extinction increase compared with humidity increase. By another case study, the influence of the mean wind direction and the orography on the water vapour concentration near the ground and in the free troposphere will be discussed. E.g. during wintertime often a humidity inversion up to about 1.5 km altitude with drier air near the ground has been found, if wind comes from the south-east. Such local effects and small-scale structures observed by stationary lidar mostly cannot be resolved by satellite soundings or atmospheric models used e.g. for meteorological analyses or regional climate investigations. Introduction Water vapour causes about two third of the natural greenhouse effect of the Earth’s atmosphere and is for this reason the most important greenhouse gas. Several climate models show that an increase in atmospheric humidity by 12-25 % will have the same global mean radiative effect than doubling the CO2 concentration (Harries 1997). But in contrast to the homogenous distribution of the long-lived carbon dioxide is the water vapour distribution highly variable in space and time. Additionally, beside its (direct) radiative effect water vapour acts indirectly by interaction with aerosols, clouds, and precipitation (Hegg et al. 1996; Ramanathan et al. 2001). This indirect effect of surface cooling provides one of the largest uncertainties in the understanding of the radiative balance of the Earth’s atmosphere (IPCC 2001). To improve the understanding of the role of water vapour in the atmosphere, extensive water vapour soundings with high spatiotemporal resolution are necessary. Up to know, radiosondes provide the most valuable humidity data set, e.g. for numerical weather prediction (NWP) models. But today’s standard radiosondes are of limited accuracy under the dry and cold conditions (Elliot & Gaffen 1991; Miloshevich et al. 2001) typical for the Arctic. Process studies of the hydrological cycle and aerosol-water vapour interaction require time series of humidity profiles, typically not performed by free ascending radiosondes. But this continuous water vapour soundings can be provided by optical lidar. Water vapour observations by detection of the Raman-scattering of laser light have been described first by Melfi et al. (1969). A short laser pulse is emitted into the atmosphere. Besides elastic Rayleigh scattering with the air molecules, inelastic Raman scattering occurs, producing light with a wavelength shift characteristic for the scattering molecule. Water vapour Raman lidars detect the light backscattered by nitrogen and water vapour molecules. The ratio of the photons scattered by water vapour and nitrogen is proportional to the water vapour mass mixing ratio. Water vapour lidars dispread past the end of the 80ies, when more powerful laser system elude the problems of small Raman backscatter cross sections (e.g. Whiteman et al. 1992). The Koldewey Aerosol Raman Lidar (KARL) at Ny-Alesund (78.9°N, 11.9°E) was build up in the end of the 90ies (Schumacher et al. 2001) and started regular water vapour sounding in the beginning of year 2000. It emits light at three different wavelengths (UV, vis, IR), and detects the backscattered light at seven different wavelengths from different height regimes in the troposphere and lower stratosphere. Table 1 summarises the system parameters. The KARL water vapour channels cover an altitude range between about 200 m and 6 km at nighttime conditions, while daylight limits the range to the lower troposphere, depending on water vapour content and skylight conditions. Typically, integration times of 30-60 minutes, altitude resolutions of 60 m and additional running averages of 180 m to 300 m are applied for water vapour profiles. The resolution in time can be increased for time series.