Atmospheric water-vapor detection using laser remote sensing

Water vapor is a natural greenhouse gas. It plays a dominant role in a number of meteorological and climate processes by directly influencing the characteristics and balance of the radiation field. Accurate measurements of the water-vapor mixing ratio are of considerable importance for understanding atmospheric physics and climate change.

The metropolitan São Paulo (MSP) light detection and ranging (lidar) system,¹ set up in a densely populated suburban area, was recently upgraded to a Raman lidar operating on the basis of inelastic photon scattering in the atmosphere (see Figure 1). In its present three-channel configuration, it can monitor aerosol, nitrogen, and water-vapor concentrations up to an altitude of 6km in the troposphere. In addition to aerosol backscatter observations, measurements of atmospheric extinction coefficients, extinction-to-backscatter ratios, and the water-vapor mixing ratio at the planetary boundary layer can also be determined.

Figure 1. The metropolitan São Paulo (MSP) lidar system.

The coaxial lidar points vertically at zenith. Its light source is a 20Hz Quantel triple neodymium-doped yttrium-aluminum-garnet (Nd:YAG) laser. The pulse energy and length at a lasing wavelength of 355nm are about 40mJ and 4ns, respectively, offering a peak power of about 10MW. The output beam is expanded three times, resulting in less than 0.5mrad beam divergence. The scattered radiation is collected by a Newtonian telescope with a 300mm aperture. The incident optical signals are then separated by dichroic mirrors and directed to the Hamamatsu R7400 photomultiplier detectors. Narrowband interference filters with peak throughputs at 387nm (nitrogen channel, bandwidth 0.25nm), 408nm (water-vapor channel, bandwidth 0.25nm), and 355nm (Rayleigh scattering, bandwidth 1.0nm) are used to select the wavelength required for a given project. They also ensure efficient background reduction and improved signal-to-noise ratios. The receiving electronics can operate in both analog and photon-counting mode, recording data at 12-bit resolution and averaging every 1–3 minutes with a height resolution of 15–30m.

Very thin ‘subvisual’ cirrus clouds (SVCs), which can be detected with the MSP lidar but not using satellite imagery, may affect climate conditions by trapping heat,² although to what extent this occurs is unknown. Lidar measurements allow us to estimate their height, thickness, and optical depth, thus contributing crucial information to radiative-transfer models. Since 2004 we have obtained SVC measurements on a daily basis for four to eight hours, except when prevented by precipitation or other operational issues (see Figure 2).

Figure 2. Range-corrected signal (in arbitrary units) from the MSP lidar system observed on 26 April 2007. Cirrus clouds are detected at altitudes between 8 and 14km.

Figure 3. The water-vapor mixing ratio from the 408nm Raman channel and from radiosonde data at Sounding Base Marte Civil (SBMT).

Figure 3 represents typical Raman-lidar measurements, taken during nighttime on 5 March 2008. It shows the vertical profile of the water-vapor mixing ratio. We are collaborating with Howard University and NASA's Goddard Space Flight Center to develop independent calibration methodology³ aimed at obtaining the vertical distribution of the atmospheric water-vapor mixing ratio. In addition, we have taken the initiative to study cirrus clouds that extend or may follow wind patterns from South Africa to Brazil. Comparison of water-vapor and elastic-backscatter measurements will be done in collaboration with the Observatoire Haute-Provence (France), the Instituto de Pesquisas Meteorológicas (Brazil) and the Observatory of Atmospheric Physics of Reunion Island.

This project was supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico, the Fundação de Amparo à Pesquisa do Estado de São Paulo, the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, and the Comissão Nacional de Energia Nuclear (Brazil).