Allan I. Carswell

Polarization properties of lidar backscattering from clouds.

The polarization properties of the backscattering of a lidar pulse from atmospheric clouds have been measured. A linearly polarized signal from a ruby laser at 694.3 nm is transmitted, and the scattering is simultaneously measured on a three-channel receiver that has polarizers oriented parallel, perpendicular, and at 45 degrees to the transmitted polarization. Substantial depolarizations (up to 0.5) are observed, and from the spatial variation of this depolarization the contribution of multiple scattering can be deduced.A wide variation in polarization properties is observed in different cloud types, and the results indicate that polarization signatures could be useful for cloud characterization and classification.

Rayleigh Lidar Observations of Thermal Structure and Gravity Wave Activity in the High Arctic during a Stratospheric Warming

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 ceas...

Automated method for lidar determination of cloud-base height and vertical extent

An algorithm for the automated analysis of lidar cloud returns has been developed as part of the Experimental Cloud Lidar Pilot Study program. This automated method determines the cloud-base and cloud-top heights as well as the altitude of the maximum return signal. A large body of cloud data that were obtained at 532 and 1064 nm with a Nd:YAG lidar have been evaluated and it has been found that the algorithm can handle the wide range of complex cloud situations encountered. The need for a more careful definition of the cloud-base height and cloud-top height is described and discussed in relation to the existing measurements with rotating beam and laser ceilometers.

Correction for nonlinear photon-counting effects in lidar systems

A useful analytic model describing the response of a photon-counting (PC) system has been developed. The model describes the nonlinear count loss and apparent count gain arising from the overlap of photomultiplier tube (PMT) pulses, taking into account the distribution in amplitude of the PMT output pulses and the effect of the pulse-height discrimination threshold. Comparisons between the model and Monte Carlo simulations show excellent agreement. The model has been applied to a PC lidar system with favorable results. Application of the model has permitted us to extend the linear operating range of the PC system and to quantify accurately the response of the system in its nonlinear operating regime, thus increasing the useful dynamic range of the system by 1 order of magnitude.

Lidar observations of gravity wave activity in the upper stratosphere over Toronto

The Rayleigh lidar technique has been applied to observe temperature fluctuations induced by gravity waves within the upper stratosphere. Observations were carried out on a routine basis for 1 year (130 clear nights) at the campus of York University near Toronto (44°N, 80°W). The waves were on occasion observed to induce marginal convective instability while exhibiting no substantial vertical amplitude growth. In general, the vertical variation in the amplitude of fractional temperature perturbations and associated available potential energy density implied the waves were strongly dissipated. Dramatic changes in the distribution of spectral energy with respect to vertical wave number were observed over the course of a few hours. The total resolved available potential energy in the gravity wave field varied considerably from day to day and seasonally with a winter maximum and summer minimum.

Measurements of gravity wave activity within and around the Arctic stratospheric vortex

Lidar measurements of gravity wave activity have been conducted at Eureka in the High Arctic since 1993. The waves are detected by the fluctuations they induce in temperature. It has been found that the amount of wave energy in the upper stratosphere is related to the position of the stratospheric polar vortex. In each of the four winters reported here, the wave activity was a maximum within the westerly jet at the edge of the vortex, a minimum inside the vortex near its centre and intermediate outside the vortex. The spectra of wave induced fluctuations show that it is at the longest resolved vertical wavelengths (8 to 15 km) that wave energy is being influenced by the background meteorological conditions. These findings are interpreted in terms of the Doppler shifting and critical level filtering that is imposed by the background wind profile.

Polarization anisotropy in lidar multiple scattering from atmospheric clouds.

Spatial variations in the polarization properties of multiple scattering have been observed in the lidar backscattering from atmospheric water droplet clouds. To detect these effects, the lidar receivers have been modified to incorporate spatial filters in the focal plane which block singly scattered radiation and transmit muliply backscattered radiation through sectors oriented at five azimuthal angles between 0 and 90° to the direction of the transmitted linear polarization. The parallel and perpendicular polarized components of the lidar multiple scattering have been measured as a function of pulse penetration depth for different cloud formations. The anisotropic distributions observed are found to resemble those previously recorded in our laboratory measurements on clouds of spherical scatterers. In this paper also, results of Mie scattering calculations are summarized which show that the observed polarization anisotropy originates directly from the polarization properties of the single scattering from spherical particles.

Mesospheric temperature inversions with overlying nearly adiabatic lapse rate: An Indication of a well‐mixed turbulent layer

The Rayleigh lidar technique was applied to study the thermal structure of the middle atmosphere. Observations were carried out on a routine basis for one year (130 clear nights) at the main campus of York University near Toronto (44°N,80°W). Mesospheric temperature inversions were generally found to occur below a height of 70 km during winter and above during summer. The most interesting aspect of our observations was that the inversions were often associated with an overlying nearly adiabatic lapse rate which extended for several kilometres. We interpret this as being an indication (or signature) of a well-mixed turbulent layer. A one-dimensional numerical model was applied to demonstrate that a well-defined turbulent layer within the mesosphere can bring about a thermal structure quite similar to that which was commonly observed—an inversion with overlying nearly adiabatic lapse.

Lidar observations of gravity wave activity and Arctic stratospheric vortex core warming

Measurements of stratospheric thermal structure and gravity wave activity have been obtained with a Rayleigh lidar in the Canadian High Arctic at Eureka (80°N, 86°W) during five recent winters. The observations reveal that an annual late December warming of the upper stratosphere occurred in the polar vortex core and was sustained through the winter. Increased gravity wave activity was detected in the vortex jet during the warming. That these two phenomena developed in parallel suggests they are related. It is proposed that increased gravity wave momentum deposition above the jet maximum forced flow into the vortex core where it descended and warmed adiabatically.

The Gravity Wave–Arctic Stratospheric Vortex Interaction

Abstract 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.