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Quantitative Interpretation of Laser Ceilometer Intensity Profiles

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Quantitative Interpretation of Laser Ceilometer Intensity Profiles 396 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 14 Quantitative Interpretation of Laser Ceilometer Intensity Pro®les R. R. ROGERS AND M.-F. LAMOUREUX Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada L. R. BISSONNETTE Defence Research Establishment Valcartier, Courcelette, Quebec, Canada R. M. PETERS Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania (Manuscript received 23 July 1996, in ®nal form 28 October 1996) ABSTRACT The authors have used a commercially available laser ceilometer to measure vertical pro®les 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 ®ltering, and the incomplete overlap of the transmitted beam with the receiver ®eld of view at close range. The calibration constant of the ceilometer, denoted by C, is determined from the pro®le of the corrected returned power in conditions of moderate attenuation in which the power is completely extin- guished 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 speci®ed and an allowance made for the effects of multiple scattering. These requirements impose an uncertainty on C that can amount to 650%. 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 pro®les in rain and compared them with radar re¯ectivity pro®les measured with a UHF boundary layer wind pro®ler. The values of the extinction coef®cient in the examples shown in this paper range from about 2 to 12 km21 and are generally larger than the values inferred from the radar re¯ectivity 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. 1. Introduction before being extinguished by snow or cloud at higher altitudes. Since November 1992 a laser ceilometer has been We have made many comparisons of ceilometer pro- used continuously as part of the atmospheric remote ®les in rain with pro®les of radar re¯ectivity measured sensing facilities at McGill University. It provides data simultaneously by a UHF wind pro®ler. The question on the height of cloud base as a function of time, and also records at a rate of twice a minute the complete initially was whether the observed extinction of the ceil- vertical pro®le of the power received by the ceilometer, ometer signal was consistent with the extinction that in the same sense as an intensity pro®le measured by a could be estimated from the radar re¯ectivity of the rain. lidar. Ceilometer ``signatures'' of rain, snow, fog, and To answer the question required quantitative interpre- haze are often recognizable by their different intensity tation of the ceilometer data using techniques developed pro®les and by the altitude reached before the signal for lidar data analysis. The conclusion we have reached diminishes to its noise level. The signal is typically is that the extinction in rain is determined more by fog, attenuated completely by clouds over a short distance, aerosols, and cloud droplets mixed in with the rain than but propagation distances through rain can be consid- by the raindrops themselvesÐno real surprise in ret- erable. Not uncommonly, the laser beam is able to pen- rospect. But along the way we have determined a sat- etrate a kilometer or two of rain in its upward course isfactory method of calibrating the ceilometer so that the measured intensity pro®les can be converted to pro- ®les of the atmospheric extinction coef®cient. Under appropriate conditions, a relatively inexpensive opera- Corresponding author address: R.R. Rogers, Atmospheric Sci- tional instrument can thus be used for quantitative es- ences, McGill University, 805 Sherbrooke St. W., Montreal H3A 2K6 Canada. timates of the atmospheric extinction and its variation E-mail: [email protected] with height and time. q1997 American Meteorological Society Unauthenticated | Downloaded 09/29/21 04:58 AM UTC JUNE 1997 ROGERS ET AL. 397 TABLE 1. Laser ceilometer (Vaisala model CT 12K). TABLE 2. McGill University±NOAA boundary layer pro®ler. Wavelength 0.904 mm Fixed parameters Peak power 40 W Frequency 915 MHz Pulse duration 0.135 ms Wavelength 32.8 cm Average power 5 mW Peak power 500 W Beamwidth 5 mrad Antenna aperture 1.8 m 3 1.8 m Vertical resolution 15.24 m (50 ft) Antenna type 64-element array Maximum range 4 km Number of beams 5 Pointing directions Vertical; 218 zenith angle at cardinal points This paper presents the background necessary for Beamwidth 98 quantitative analysis of ceilometer data and gives ex- Adjustable parameters and their typical values amples of results from one set of observations in rain. Pulse duration 0.7 ms These observations are typical of many others and are Interpulse period 61 ms Range resolution 105 m adequate to illustrate the principles of the analysis and Number of range samples 60 the results of the research. Number of coherent integrations 110 Number of spectral averages 50 Maximum radial velocity 6 11ms21 2. The ceilometer and the radar Number of spectral points 64 The ceilometer is the Vaisala model CT-12K, the main operating characteristics of which are given in Table 1. Its laser source is a gallium arsenide semicon- description may be found in Rogers et al. (1994). The ductor diode operated in pulsed mode with a pulse en- dwell time required for a complete set of spectral mea- ergy of 6.6 mJ and a pulse repetition frequency ranging surements in a given pointing direction is typically 35 from about 600 to 1100 Hz, automatically controlled to s. The radar is sensitive to scattering by precipitation maintain a constant average power output of 5 mW. The particles and by spatial ¯uctuations in the refractivity instrument is rigidly ®xed to the surface with the trans- of the optically clear air. At its long wavelength, atten- mitted beam pointed vertically. There is no provision uation by any atmospheric constituent is negligible and for pointing the beam in other directions. It is a bistatic the returned power can be converted to target re¯ectivity system with the transmitter and receiver separated by through the radar calibration factor. The radar sensitivity 31 cm. The transmitted beamwidth and the receiver ®eld is such that the minimum detectable signal at a range of view are both approximately 5 mrad (0.38). Although of 1 km corresponds approximately to a re¯ectivity fac- the ceilometer is an operational instrument, intended tor Z in rain of 215 dBZ or a refractivity structure 2 215 22/3 mainly to give a continuous indication of the height of parameter (Cn ) in clear air of 10 m . Because the cloud base, our unit is equipped with software that pro- re¯ectivity of a particle with diameter D is proportional vides a complete record of the returned signal strength to D6/l4 (a consequence of the Rayleigh scattering ap- as a function of height. Using as input what is called proximation) the radar is not able to detect clouds con- ``standard message #2'' in the technical manual for the sisting of droplets smaller than about 10 mm in diameter ceilometer (Vaisala 1989), the software integrates the or aerosols. received power for 30 s and generates data with an approximate range resolution of 15 m and a time res- 3. The lidar equation and inversion methods olution of 30 s. These data can be interpreted in terms of the vertical pro®les of the backscattering coef®cient The lidar equation for the ceilometer may be written and the extinction coef®cient using procedures devel- as (e.g., Zuev 1982) oped for lidar. Because the ceilometer runs continuously, zC22b(z)t(z) p(z)[P(z)5, (1) the data include pro®les through rain, snow, and haze, 22 as well as the sharp returns from cloud base. Although zz00 attenuation of the laser beam can be severe, penetration where p(z) is the power returned from altitude z, z0 is distances in rain of more than a kilometer are common. a ®xed reference altitude (1 km in our work), P(z)is The attenuation of snow is stronger, and that of cloud the range-adjusted power, b(z) the volume backscatter- still stronger. Scattering by aerosol particles is readily ing coef®cient, t2(z) the two-way transmittance, and C detectable in fair weather in conditions when the visi- the calibration constant of the ceilometer. The trans- bility is noticeably impaired, but is hard to detect on mittance is related to the extinction coef®cient g by clear days. z The radar is a 915-MHz (33-cm wavelength) wind 2 pro®ler with a ®ve-beam, phased array antenna, which t (z) 5 exp 22 E g(r) dr . (2) []0 transmits a peak power of 500 W and is normally op- erated with a range resolution of 105 m. Its operating Although (1) is applicable in general, it is sometimes characteristics are given in Table 2; a more complete modi®ed to indicate the separate contributions of dif- Unauthenticated | Downloaded 09/29/21 04:58 AM UTC 398 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 14 ferent coexisting atmospheric constituents (e.g., aerosol Strictly speaking, the backscattering coef®cient b in particles and cloud drops); then b is written as the sum (4) should be replaced by an effective coef®cient be- of the individual backscattering coef®cients and t2 as cause we are now including the effects of forward scat- the product of the individual transmittances.
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