87th AMS Annual Meeting

Wednesday, 17 January 2007
Howard University Raman Lidar results during WAVES 2006 field campaign
Exhibit Hall C (Henry B. Gonzalez Convention Center)
Mariana Adam, Howard University, Washington, DC; and R. Connell, D. Venable, D. N. Whiteman, B. B. Demoz, and E. Joseph
Poster PDF (259.7 kB)
The WAVES (Water Vapor Variability – Satellite/Sondes) 2006 field campaign took place at the Howard University Research Campus in Beltsville, MD from July 7 to August 10. The field campaign was intended to provide quality measurements of water vapor and ozone for comparison with AURA satellite retrievals and to quantify the air quality. The operations include intensive observations by multiple radiosonde / ozonesonde sensors and several lidar systems during overpasses of the AURA satellite. Lidar measurements are acquired by four lidar systems: NASA/GSFC Scanning Raman Lidar (SRL), NASA/GSFC Aerosol/Temperature Lidar (ATL), a Micropulse Lidar from Penn Sate and Howard University Raman Lidar (HRL). Coordinated lidar measurements took place as well at University of Maryland, Baltimore County (backscatter and Raman lidars) in order to provide information about the spatial variability of the aerosol and water vapor. In addition to the lidar / radiosondes operations, continuous measurements are taken by a 31m instrumented tower, various broad-band and spectral radiometers, microwave radiometer, Doppler C-band radar, MD Department of Environment instrumentation, wind profiler, sun photometer, and GPS system.

The HRL system operates at the third harmonic (typical operating power is 10 W) of an Nd:YAG laser and acquires data within three channels, at 354.7 nm (elastic backscatter and pure rotational Raman respectively), 386.7 nm and 407.5 nm (Raman scattering from nitrogen molecules and water vapor molecules). Eye-safety is accomplished by means of a 15X beam expander. The laser beam and telescope divergences are 50 µrad and 250 µrad respectively. The data acquisition is achieved with Licel Transient Recorders which allow both photon counting and analog acquisition. The combination of both methods (“glue-ing”) gives maximum dynamic range. For the data processing, the following corrections are applied: response time correction, dark-current and background subtraction, and noise reduction (data smoothing using a moving average: constant over time and variable in space). The first data of the HRL system were acquired in 2004 and the WAVES experiment is the first major participation within a field campaign aimed at intra-lidar comparison.

The present results show the temporal and spatial retrievals of the water vapor mixing ratio. The procedure to calculate the water vapor mixing ratio follows more or less the traditional techniques, as indicated below: - the differential transmission term is computed using radiosondes pressure and temperature measurements to account for the molecular attenuation and a constant aerosol extinction coefficient derived from the Aeronet aerosol optical depth and the length of the boundary layer height to account for aerosol attenuation. - the calibration constant is determined from comparison of the lidar integrated precipitable water (IPW) with the microwave radiometer (MWR) data; the lidar profile for water vapor mixing ratio is corrected for the region of incomplete overlap. - the water vapor mixing ratio profiles are compared with radiosondes profiles as well as with collocated SRL system.

Preliminary results show excellent agreement between HRL and SRL systems throughout the useful range of water vapor and aerosol profiles on one hand and between HRL and radiosondes profiles. Examples of these comparisons will be shown. Special attention will be given to the day time/night time measurements performances, including error analyses.

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