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Carswell, A. I.

Publications and source records attributed to Carswell, A. I..

10 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

The Experimental Cloud Lidar Pilot Study (ECLIPS) for cloud-radiation research

The Experimental Cloud Lidar Pilot Study (ECLIPS) was initiated to obtain statistics on cloud-base height, extinction, optical depth, cloud brokenness, and surface fluxes. Two observational phases have taken place, in October-December 1989 and April-July 1991, with intensive 30-day periods being selected within the two time intervals. Data are being archived at NASA Langley Research Center and, once there, are readily available to the international scientific community. This article describes the scale of the study in terms of its international involvement and in the range of data being recorded. Lidar observations of cloud height and backscatter coefficient have been taken from a number of ground-based stations spread around the globe. Solar shortwave and infrared longwave fluxes and infrared beam radiance have been measured at the surface wherever possible. The observations have been tailored to occur around the overpass times of the NOAA weather satellites. This article describes in some detail the various retrieval methods used to obtain results on cloud-base height, extinction coefficient, and infrared emittance, paying particular attention to the uncertainties involved.

Platt, C. M.↗
NASA NTRS1992

Correcting for Interference of Mt. Pinatubo Aerosols on DIAL Measurements of Stratospheric Ozone

Since March 1991 our group has been routinely operating a Differential Absorption Lidar system in Toronto, Canada. The system is based on a XeCl Excimer laser and the main system parameters are reported. In all measurements after July 21, 1991 we have observed aerosol layers in the stratosphere, resulting from the Mt. Pinatubo eruptions around June 15, 1991. These aerosol layers have increased in size since they were first observed and backscatter ratios of more than 5 to 353 nm have been observed. They interfere significantly with any optical ozone measurement. In this paper we will describe our work to quantify the effects of the additional aerosol on differential absorption measurements and the attempt to correct for them.

Steinbrecht, W.↗
NASA NTRS1992

Two Wavelength Measurements of the Pinatubo Aerosol Above Toronto, Canada

Since March 1991 our group has been routinely operating a Differential Absorption Lidar system in Toronto, Canada. The system is based on a XeCl Excimer laser. We have also been running a Backscatter Lidar based on a NdYAG laser. In the period from March 1991 to March 1992 measurements have been taken during about 40 nights with the DIAL system and more than 90 nights with the NdYAG system. In all measurements after July 21, 1991 we have observed aerosol layers in the stratosphere, resulting from the Mt. Pinatubo eruptions around June 15, 1992. These aerosol layers interfere significantly with many optical remote sensing techniques, and we have been investigating their properties with our two Lidar systems. In this paper we present a summary of these measurements.

Steinbrecht, W.↗
NASA NTRS1992

Lidar Studies of Clouds at Toronto During the ECLIPS Program

The Experimental Cloud Lidar Pilot Study (ECLIPS) measurement program involves the measurement of cloud optical and physical parameters with vertically pointing ground-based lidars simultaneously with satellites observing the same cloud system from above. Our frequency doubled Nd:YAG lidar transmits about 500 mJ of linearly polarized radiation at each wavelength (532, 1064 nm) with a prf of 20 Hz. An overview of the statistical behavior of cloud physical parameters and optical parameters is presented.

Pal, S. R.↗
NASA NTRS1992

Correction for nonlinear photon counting effects in lidar systems

Photomultiplier tubes (PMT's) employed in the photon counting (PC) mode of operation are widely used as detectors in lidar systems. In our laboratory, we have developed a versatile Nd:YAG lidar which is used for measurement of both the middle atmosphere and the troposphere. With this system, we encounter a very wide range of signal levels ranging from the extremely weak signals from the top of the mesosphere to the very strong returns from low level clouds. Although the system is capable of operating the PMT's in either the analog detection or photon counting mode, we find that often when we use photon counting we have portions of our lidar return which contain very useful information but are not within the linear operating regime of the PC system. We report the results of our efforts to explore the extent to which such high intensity PC signals can be quantitatively analyzed. In particular, a useful model relating the mean 'true' count rate and the observed count rate is presented and it's application to our system demonstrated. This model takes into account the variation in height of the PMT output pulses and the effect of the pulse height discrimination threshold.

Donovan, D. P.↗
NASA NTRS1986

Inversion of 2 wavelength Lidar data for cloud properties

The inversion of the lidar equation to derive quantitative properties of the atmosphere has continued to present considerable difficulty. The results of a study in which Klett's procedure was utilized for the analysis of cloud backscatter measurements made simulataneously at two ruby lidar wavelengths (694nm,347nmm) are presented. With one lidar system a cloud is probed at the two wavelength and the backscatter measured simulataneously by separate receivers. As a result two sigma profiles which should differ only because the wavlength dependence of the scattering. Experimental data presented to demonstrate the effects and the implications of the applications of the inversion method will be discussed.

Pal, S. R.↗
NASA NTRS1986

Determination of cloud microphysical properties by laser backscattering and extinction measurements

The extinction and backscattering of 514 nm laser radiation in polydisperse water droplet clouds was studied in the laboratory. Three cloud size distributions with modal diameters of 0.02, 5.0, and 12.0 microns were investigated. The relationships between the cloud optical parameters (attentuation coefficient, sigma and volume backscattering coefficient, Beta (sub pi)) and the cloud water content, C, were measured for each size distribution. It was found that a linear relationship exists between sigma and C and between beta (sub pi) and C for cloud water content values up to 3gm/cubic m. The linear relationships obtained, however, have slopes which depend on the droplet size distribution. For a given water content both sigma and beta (sub pi) increase as the modal diameter decreases. The measured data are compared with existing theoretical analyses and discussed in terms of thie application to lidar measurements of atmospheric clouds. It is concluded that the empirical information obtained can serve as a basis for quantitative lidar measurements.

Dubinsky, R. N.↗
NASA NTRS1986

Laboratory measurements of forward and backward scattering of laser beams in water droplet clouds

Many aspects of the forward and backward scattering in dense water droplet clouds were studied using a laboratory scattering facility. This system is configured in a lidar geometry to facilitate comparison of the laboratory results to current lidar oriented theory and measurements. The backscatter measurements are supported with simultaneous measurements of the optical density, mass concentration, and droplet size distribution of the clouds. Measurements of the extinction and backscatter coefficients at several important laser wavelength have provided data on the relationship between these quantities for laboratory clouds at .633, 1.06, and 10.6 microns. The polarization characteristics of the backscatter of 1.06 microns were studied using several different types of clouds. More recently, the laboratory facility was modified to allow range-resolved backscatter measurements at 1.06 microns. Clouds made up of 3 layers, each with its own density, can be constructed. This allows the study of the effect of cloud inhomogeneity on the forward and backscatter.

Smith, R. B.↗
NASA NTRS1975

Underwater probing with laser radar

Recent advances in laser and electro optics technology have greatly enhanced the feasibility of active optical probing techniques aimed at the remote sensing of water parameters. This paper describes a LIDAR (laser radar) that has been designed and constructed for underwater probing. The influence of the optical properties of water on the general design parameters of a LIDAR system is considered. Discussion of the specific details in the choice of the constructed LIDAR is given. This system utilizes a cavity dumped argon ion laser transmitter capable of 50 watt peak powers, 10 nanosecond pulses and megahertz pulse repetition rates at 10 different wavelengths in the blue green region of the spectrum. The performance of the system, in proving various types of water, is demonstrated by summarizing the results of initial laboratory and field experiments.

Carswell, A. I.↗