Polar Stratospheric Cloud observations at Davis

Full description

This indicator is no longer maintained, and is considered OBSOLETE.

INDICATOR DEFINITION
Lidar observations at Davis Station, Antarctica, are used to determine daily values of average percentage occurrence and upper and lower altitude limits for Polar Stratospheric Clouds. The occurrence values are evaluated from the total time clouds were detected divided by total observing time, expressed as a percentage. The altitude limits are the maximum and minimum altitudes at which clouds were detected during the time interval.

TYPE OF INDICATOR
There are three types of indicators used in this report:
1.Describes the CONDITION of important elements of a system;
2.Show the extent of the major PRESSURES exerted on a system;
3.Determine RESPONSES to either condition or changes in the condition of a system.

This indicator is one of: CONDITION

RATIONALE FOR INDICATOR SELECTION
The formation of stratospheric clouds in the polar regions requires special conditions relating to temperature and chemistry. The clouds play a critical role in the depletion of ozone at high latitudes by initiating heterogeneous chemical reactions that convert chlorine and bromine compounds to reactive forms. A general cooling of the stratosphere is predicted to accompany anthropogenically enhanced greenhouse-gas warming of the lower atmosphere. This may provide conditions more favourable for the formation of stratospheric clouds. However, perturbations to stratospheric thermodynamics and chemistry can be caused by volcanic events. Long-term quantification of stratospheric cloud properties will aid in understanding and predicting natural and anthropogenic variability.

DESIGN AND STRATEGY FOR INDICATOR MONITORING PROGRAM
Spatial scale: Davis Station, Antarctica.

Temporal scale: Values for every day of lidar operation.

Measurement technique: Lidar (532nm wavelength). The raw data used in the analysis of this indicator are obtained from observations by the Davis Lidar in Rayleigh and Doppler modes. In Rayleigh mode, the raw data consist of profiles of the intensity of 532nm laser backscatter versus altitude. In Doppler mode, spectral scans of 532nm laser backscatter versus altitude are obtained with a high-resolution Fabry-Perot spectrometer. These measurements are used to obtain the total backscatter versus altitude by collapsing the wavelength data down to one bin.

RESEARCH ISSUES
Changes in the extent and occurrence of stratospheric clouds has implications for the chemistry, structure and dynamics of the lower stratosphere. To obtain a more comprehensive picture of the spatial extent of these changes, the underlying physics, and future implications, the Davis observations should be assimilated into research incorporating a diverse body of observations and model predictions. Important additional data will come from in-situ and remote-sensing profiling of trace gas species (importantly, ozone, and compounds of nitrogen, chlorine, bromine and sulphur) and the thermal and dynamical behaviour of the atmosphere.

Lineage

Progress Code: completed

Statement: This indicator is now OBSOLETE. These data are no longer archived in this location, and are therefore not up-to-date. The raw data used in the analysis of this indicator are obtained from observations by the Davis Lidar in Rayleigh and Doppler modes. In Rayleigh mode, the raw data consist of profiles of the intensity of 532nm laser backscatter versus altitude. The backscatter profiles are converted to profiles of absolute total molecular density using in-situ or extrapolated density measurements for the altitude range 27-28km obtained from balloon-borne radiosonde measurements. In producing the density profiles, account is made for molecular extinction, including ozone, and background signals due to sky light and instrument noise. In Doppler mode, spectral scans of 532nm laser backscatter versus altitude are obtained with a high-resolution Fabry-Perot spectrometer. These measurements are used to obtain the total backscatter versus altitude by collapsing the wavelength data down to one bin. Profiles of absolute total molecular density are then obtained using the analysis scheme used for the Rayleigh mode data. Profiles of Rayleigh scattering ratio are generated with 1 hour time binning and 100 metre altitude binning. The scattering ratio is defined as the ratio of the lidar-derived density profile divided by the total molecular density profile obtained from the MSISE-90 atmospheric model. The scattering ratio profile is generally greater than unity below about 27km where scattering by stratospheric aerosols contaminates the lidar-derived density profile. The 'background' scattering ratio profile due to the normal (i.e. cloud-free) loading of stratospheric aerosols (averaged over the month preceeding the first occurrence of stratospheric clouds for the winter season - typically May) is removed. Enhancements in the residual scattering ratio with greater than 2-standard deviation statistical significance are classed as cloud features.