NOAA's efforts of recalibrating Stratospheric Sounding Unit Data towards Climate Data Records

The Stratospheric Sounding Unit (SSU) is a three channel radiometer designed to measure temperatures in the upper stratosphere. Carried on the series of NOAA polar orbiting satellites, the SSU instruments have made global stratospheric observations from 1979 to 2006 as the only instrument that made operational stratospheric temperature observations until Advanced Microwave Sounding Unit (AMSU). Due to its unique observation in the stratosphere, the SSU has been playing an important role in monitoring climate changes such as stratosphere cooling and ozone depletion. However, it is well-known that the SSU instruments suffered from a gas leak in the pressure modulated CO2 cells that caused the cell pressure to decrease over time. These cell pressure changes resulted in the channel weighting functions to peak at different layers at different observation time, making intersatellite brightness temperature differences on the order of 0.5 to 5 K. In addition, the long-term variation of the atmospheric CO2 concentration also affects the spectral absorption and weighting function of the instrument. Finally, diurnal sampling biases arise in the SSU satellite observations because some of the NOAA polar-orbiting satellites drift significantly from their original local observation time. These factors make the long-term SSU satellite observations hard for climate analysis and monitor.

This study presents the recent on-going efforts of recalibrating historic SSU datasets towards climate data records in the Center for Satellite Applications and Research (STAR) of NOAA supported by the NOAA Scientific Data Stewardship (SDS) program. A strategy has been developed to focus on the SSU unique issues with the aim of constructing a consistent, homogenous SSU datasets that can be used for related climate studies. In particular, we will demonstrate how we remove CO2 cell leaking effects and CO2 concentration effects from the SSU observations to make the corrected SSU brightness temperature corresponding to a stable and consistent weighting function. Second, we will evaluate and correct the diurnal sampling biases in the SSU satellite observations. After corrections, the SSU observations will be further merged based on the simultaneous observations and statistical methods. The new SSU time series will be presented and analyzed to evaluate the stratosphere cooling.