D. P. Donovan

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.

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.

Ozone differential absorption lidar algorithm intercomparison

An intercomparison of ozone differential absorption lidar algorithms was performed in 1996 within the framework of the Network for the Detection of Stratospheric Changes (NDSC) lidar working group. The objective of this research was mainly to test the differentiating techniques used by the various lidar teams involved in the NDSC for the calculation of the ozone number density from the lidar signals. The exercise consisted of processing synthetic lidar signals computed from simple Rayleigh scattering and three initial ozone profiles. Two of these profiles contained perturbations in the low and the high stratosphere to test the vertical resolution of the various algorithms. For the unperturbed profiles the results of the simulations show the correct behavior of the lidar processing methods in the low and the middle stratosphere with biases of less than 1% with respect to the initial profile to as high as 30 km in most cases. In the upper stratosphere, significant biases reaching 10% at 45 km for most of the algorithms are obtained. This bias is due to the decrease in the signal-to-noise ratio with altitude, which makes it necessary to increase the number of points of the derivative low-pass filter used for data processing. As a consequence the response of the various retrieval algorithms to perturbations in the ozone profile is much better in the lower stratosphere than in the higher range. These results show the necessity of limiting the vertical smoothing in the ozone lidar retrieval algorithm and questions the ability of current lidar systems to detect long-term ozone trends above 40 km. Otherwise the simulations show in general a correct estimation of the ozone profile random error and, as shown by the tests involving the perturbed ozone profiles, some inconsistency in the estimation of the vertical resolution among the lidar teams involved in this experiment.

Principal component analysis applied to multiwavelength lidar aerosol backscatter and extinction measurements.

The use of powerful Raman backscatter lidars enables one to measure the stratospheric aerosol extinction profile independently of the backscatter, thereby obtaining additional information to aid in retrieving the physical characteristics of the sampled aerosol. We used principal component analysis to construct a self-consistent method for the retrieval of aerosol bulk physical and optical properties from multiwavelength elastic and/or inelastic Raman backscatter lidar signals. The procedure is applied to synthetic and actual lidar signals. We found that aerosol surface area and volume can be usefully estimated and that the use of Raman-derived extinction data leads to a notable improvement in the accuracy of the estimations.

Ozone, column ClO, and PSC measurements made at the NDSC Eureka Observatory (80°N, 86°W) during the spring of 1997

During winter/spring 96/97 ozone levels over the Eureka NDSC observatory (80°N,86°W) were measured using a lidar, sondes, and a Brewer spectrophotometer. Column ClO measurements were also made using an FTIR system. Measurements show that lower stratospheric ozone mixing ratios decreased rapidly between mid-February and late-March though the ozone mixing ratio losses appear to have been less than for the 95/96 season. Elevated column amounts of ClO were found to be present over Eureka until late March.

Lidar observations of stratospheric ozone and aerosol above the Canadian high arctic during the 1994-95 winter

This letter reports on lidar observations of arctic stratospheric ozone and aerosol made from late December 1994 to mid-March 1995. These observations were conducted at Eureka (80°N,86.42°W) in the Canadian arctic. Based on NMC potential vorticity data and aerosol observations, the lower stratosphere over Eureka was seen to be clearly within the Polar Vortex for most of the observation period. The intravortex observations showed that in the stratosphere below the 500 K potential temperature level average ozone mixing ratios decreased on the order of 15% from early January to mid-February with an additional 15% decrease observed from mid February to mid-March. These trends are consistent with the Upper Atmospheric Research Satellite (UARS) Microwave Limb Sounder (MLS) measurements of ozone made during the same time periods.

Ozone and aerosol observed by Lidar in the Canadian Arctic during the winter of 1995/96

Lidar observations of stratospheric ozone made at Eureka (80.0oN,86.42oW) during the 95/96 winter show substantial declines in ozone mixing ra- tios. Reductions in ozone levels of up to 40 % between the 410 K and 580 K isentropic levels were observed be- tween mid-January and mid-March. The correlation of the ozone data with potential vorticity and concurrent lidar observations of stratospheric aerosol is consistent with the claim that significant chemical depletion did Occur.

Lidar measurements of the stratosphere at the Eureka and Toronto NDSC stations

Lidar observations of stratospheric ozone, aerosol and temperature have been carried out at Toronto (43.8 N, 79.5 W) since 1989 and during winter months at the Arctic Stratospheric Observatory (AStrO) at Eureka (80 N, 86 W) since 1992. The Raman DIAL (Differential Absorption Lidar) systems utilized at both observatories are briefly described and the measurements are discussed. The measurements of AStrO are discussed in relation to the dynamics of stratospheric polar vortex and the presence of polar stratospheric clouds (PSC). Results from the winters of 1994/95 and 1995/96 indicate very low polar stratospheric temperatures, capable of inducing PSCs and exhibit an appreciable ozone depletion.

Multiwavelength lidar aerosol measurements made at Eureka (80°N, 86°W) during early 1995

Principal Component Analysis (PCA) aerosol retrievals have been applied to multiwavelength lidar measurements made in early 1995 in the Canadian Arctic. The lidar data have been inverted to provide estimates of stratospheric aerosol volume, surface area, effective radius, and sulfate mass mixing ratio. Above the vortex lower boundary the aerosol parameters were relatively constant but changed notably in the sub-vortex region throughout the winter.

Steller Brewer, ozonesonde, and DIAL measurements of Arctic O3 column over Eureka, N.W.T. during 1996 winter/spring

Ozone total column amounts derived from differential UV absorption of starlight measurements made using the new Stellar Brewer instrument are compared with simultaneous measurements using a DIAL system and ECC ozonesondes. Good agreement within the 2% error estimate of the Stellar Brewer is shown between the various methods after correction is made for a calibration offset error for the Stellar Brewer. All three methods identify a generally decreasing trend of total ozone column between day 15 and 75 of the 1996 spring season.