T. J. Duck

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.

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.

Evidence for critical level filtering of atmospheric gravity waves

Radiosonde measurements were used to investigate gravity wave activity within the lower stratosphere above the Canadian High Arctic. The sondes were launched from the weather station at Eureka (80°N, 86°W) on Ellesmere Island. Gravity waves were detected by the fluctuations they induced in temperature and the associated perturbation potential energy density was used to gauge the amount of wave activity. It was found that, over a 15 month period, there were isolated episodes of enhanced wave activity rather than any distinct seasonal cycle. Profiles of wind velocity were also measured and used here to show that periods of high wave activity occurred only when there would be no (or very little) critical level filtering of the stationary waves generated by flow over the rough terrain in the vicinity of Eureka. It was also found that the potential energy spectral density was enhanced at all resolvable scales during the periods of high wave activity.

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.

Enhanced Arctic stratospheric gravity wave activity above a tropospheric jet

Meteorological balloon measurements were applied to investigate gravity waves in the stratosphere above three weather stations in the Canadian Arctic. It was found that a distinct enhancement in the amount of wave activity occurred at each of the three stations simultaneously while a tropospheric jet stream crossed the Arctic during mid-November of 1996. An interpretation is proposed in which the enhanced wave activity occurs as the tropospheric jet provides conditions that are favourable for the upward propagation of mountain waves. Further propagation into the stratosphere is then facilitated during winter when the tropospheric and stratospheric jets coincide.

Seasonal transition in gravity wave activity during the springtime stratospheric vortex breakdown

Profiles of temperature and wind measured by meteorological balloons are used to examine the changes in gravity wave activity at high latitudes during a final springtime breakdown of the Arctic stratospheric vortex. The measurements show that gravity wave potential energy densities in the lower stratosphere decreased dramatically as the cyclonic wintertime vortex gave way to the weak anticyclonic summertime circulation. The reduction in gravity wave activity after the vortex breakdown is attributed primarily to increased critical level filtering of orographic waves and lowered stratospheric wind speeds.

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.