Quantitative Interpretation of Laser Ceilometer Intensity Profiles

R. R. Rogers Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada

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M-F. Lamoureux Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada

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L. R. Bissonnette Defence Research Establishment Valcartier, Courcelette, Quebec, Canada

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R. M. Peters Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania

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Abstract

The authors have used a commercially available laser ceilometer to measure vertical profiles of the optical extinction in rain. This application requires special signal processing to correct the raw data for the effects of receiver noise, high-pass filtering, and the incomplete overlap of the transmitted beam with the receiver field of view at close range. The calibration constant of the ceilometer, denoted by C, is determined from the profile of the corrected returned power in conditions of moderate attenuation in which the power is completely extinguished over a distance on the order of 1 km. In this determination, the value of the backscatter-to-extinction ratio k of the scattering medium must be specified and an allowance made for the effects of multiple scattering. These requirements impose an uncertainty on C that can amount to ±50%. An alternative to determining the calibration constant is explained, which does not require specifying k, although it assumes that k is constant with height. Using this alternative approach, the authors have estimated many extinction profiles in rain and compared them with radar reflectivity profiles measured with a UHF boundary layer wind profiler. The values of the extinction coefficient in the examples shown in this paper range from about 2 to 12 km−1 and are generally larger than the values inferred from the radar reflectivity of the rain. The implication is that aerosol particles and cloud drops, which are not visible to the radar, are important in determining the optical extinction in rain in these examples.

Corresponding author address: R.R. Rogers, Atmospheric Sciences, McGill University, 805 Sherbrooke St. W., Montreal H3A 2K6 Canada.

Email: rogers@zephyr.meteo.mcgill.ca

Abstract

The authors have used a commercially available laser ceilometer to measure vertical profiles of the optical extinction in rain. This application requires special signal processing to correct the raw data for the effects of receiver noise, high-pass filtering, and the incomplete overlap of the transmitted beam with the receiver field of view at close range. The calibration constant of the ceilometer, denoted by C, is determined from the profile of the corrected returned power in conditions of moderate attenuation in which the power is completely extinguished over a distance on the order of 1 km. In this determination, the value of the backscatter-to-extinction ratio k of the scattering medium must be specified and an allowance made for the effects of multiple scattering. These requirements impose an uncertainty on C that can amount to ±50%. An alternative to determining the calibration constant is explained, which does not require specifying k, although it assumes that k is constant with height. Using this alternative approach, the authors have estimated many extinction profiles in rain and compared them with radar reflectivity profiles measured with a UHF boundary layer wind profiler. The values of the extinction coefficient in the examples shown in this paper range from about 2 to 12 km−1 and are generally larger than the values inferred from the radar reflectivity of the rain. The implication is that aerosol particles and cloud drops, which are not visible to the radar, are important in determining the optical extinction in rain in these examples.

Corresponding author address: R.R. Rogers, Atmospheric Sciences, McGill University, 805 Sherbrooke St. W., Montreal H3A 2K6 Canada.

Email: rogers@zephyr.meteo.mcgill.ca

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