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James A. Whiteway and Allan I. Carswell

Abstract

During February and March 1993, Rayleigh lidar observations of temperature structure and gravity wave activity were carried out in the high Canadian Arctic at Eureka, Northwest Territories (80°N, 86°W). A sudden warming was observed first in the upper stratosphere during late February and then at lower levels in early March. The warming appeared to be part of a disturbance of the entire middle atmosphere with temperature changes in the mesosphere and lower stratosphere being opposite in sign to those in the upper stratosphere. Shorter time and length scale temperature fluctuations, observed in the upper stratosphere, are interpreted as being a result of atmospheric gravity waves. The wave amplitudes are shown to be capable of inducing convective instability. The rms perturbation and available potential energy density show substantial vertical and day-to-day variability in regions of conservative and dissipative growth rates. Vertical growth of the potential energy spectral density is seen to cease at the broadband convective instability saturated limit. There appeared to be substantially greater dissipation of gravity wave energy within the upper-stratospheric warming in comparison with the preceding and following periods.

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S. R. Pal and A. I. Carswell

Abstract

The effects of ice crystals on the backscatter polarization in lidar returns have been measured. Scattering from ground-level snow of varying density, high-altitude cirrus clouds and ice pellet shower clouds has been studied using a linearly polarized ruby laser system with multiple receiver channels. The space and time dependence of the total intensity and the linear depolarization ratio δ of the lidar return have been investigated. It has been found that considerable variations occur in snow with δ varying between 0.2 and 0.8. In cirrus and ice pellet shower clouds δ is typically found to lie in the range 0.3 ± 0.1. The factors involved in the use of polarization data for ice-water discrimination in the atmosphere are presented and discussed.

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A. I. Carswell, A. Fong, S. R. Pal, and I. Pribluda

Abstract

This paper summarizes the results of a statistical analysis of lidar-determined cloud geometrical properties measured during the 1989 and 1991 campaigns of the Experimental Cloud Lidar Pilot Study. Useful lidar descriptors are introduced to specify the bottom-, top-, and midcloud altitudes. These are used to describe the behavior of cloud vertical location and vertical extent during several months of observations using a dual wavelength (1064 and 532 nm) Nd:YAG lidar at Toronto. Frequency distributions of cloud height and cloud thickness are presented and the relationship of the lidar descriptors to cloud properties are discussed. These data are compared with other information on cloud geometry available in the literature.

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S. R. Pal, A. I. Carswell, I. Gordon, and A. Fong

Abstract

This paper presents the Statistical properties of lidar-derived values of cloud extinction coefficients σ and optical depths τ. The data were collected at Toronto during two measurement phases (phase 1: September–October 1989; phase 2: June–July 1991) or the Experimental Cloud Lidar Pilot Study. Although the small dataset limits general application of the statistical trends observed, the measurements demonstrate the valuable potential of lidar data for improving cloud parameterization in general circulation models. The measurements show the frequent occurrence of optically thin clouds (σ¯≤0.2 km−1 and τ≤0.2), demonstrating the ability of lidars to detect these dilute clouds and the importance of including them in radiative transfer models.

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Thomas J. Duck, James A. Whiteway, and Allan I. Carswell

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Four hundred and twenty-two nights of stratospheric gravity wave observations were obtained with a Rayleigh lidar in the High Arctic at Eureka (80°N, 86°W) during six wintertime measurement campaigns between 1992/93 and 1997/98. The measurements are grouped in positions relative to the arctic stratospheric vortex for comparison. Low gravity wave activity is found in the vortex core, outside of the vortex altogether, and in the vortex jet before mid-December. High gravity wave activity is only found in the vortex jet after late December, and is related to strengthening of the jet and decreased critical-level filtering. Calculations suggest that the drag induced by the late-December gravity wave energy increases drives a warming already observed in the vortex core, thereby reducing vortex-jet wind speeds. The gravity waves provide a feedback mechanism that regulates the strength of the arctic stratospheric vortex.

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G. L. Stephens, R. G. Ellingson, J. Vitko Jr., W. Bolton, T. P. Tooman, F. P. J. Valero, P. Minnis, P. Pilewskie, G. S. Phipps, S. Sekelsky, J. R. Carswell, S. D. Miller, A. Benedetti, R. B. McCoy, R. F. McCoy Jr., A. Lederbuhr, and R. Bambha

The U.S. Department of Energy has established an unmanned aerospace vehicle (UAV) measurement program. The purpose of this paper is to describe the evolution of the program since its inception, review the progress of the program, summarize the measurement capabilities developed under the program, illustrate key results from the various UAV campaigns carried out to date, and provide a sense of the future direction of the program. The Atmospheric Radiation Measurement (ARM)–UAV program has demonstrated how measurements from unmanned aircraft platforms operating under the various constraints imposed by different science experiments can contribute to our understanding of cloud and radiative processes. The program was first introduced in 1991 and has evolved in the form of four phases of activity each culminating in one or more flight campaigns. A total of 8 flight campaigns produced over 140 h of science flights using three different UAV platforms. The UAV platforms and their capabilities are described as are the various phases of the program development. Examples of data collected from various campaigns highlight the powerful nature of the observing system developed under the auspices of the ARM–UAV program and confirm the viability of the UAV platform for the kinds of research of interest to ARM and the clouds and radiation community as a whole. The specific examples include applications of the data in the study of radiative transfer through clouds, the evaluation of cloud parameterizations, and the development and evaluation of cloud remote sensing methods. A number of notable and novel achievements of the program are also highlighted.

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C. M. Platt, S. A. Young, A. I. Carswell, S. R. Pal, M. P. McCormick, D. M. Winker, M. DelGuasta, L. Stefanutti, W. L. Eberhard, M. Hardesty, P. H. Flamant, R. Valentin, B. Forgan, G. G. Gimmestad, H. Jäger, S. S. Khmelevtsov, I. Kolev, B. Kaprieolev, Da-ren Lu, K. Sassen, V. S. Shamanaev, O. Uchino, Y. Mizuno, U. Wandinger, C. Weitkamp, A. Ansmann, and C. Wooldridge

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.

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