Séminaire

Stratospheric temperature is a fingerprint of climate and a good proxy of both stratospheric composition and dynamic changes as well as of climate changes. A precise monitoring of the stratospheric temperature on a global scale is therefore essential and should take advantage of space and ground instruments. Apart from the now defunct rocket network, satellites and ground-based lidars are currently the two main sources of long-term data for mid and upper stratospheric temperature trends. However, several studies have pointed that lidar and satellite derived trends have discrepancies that are not fully understood, and this is true even for trends derived from lidar measurements from two relatively closeby stations, the Observatoire de Haute Provence (OHP, 43.8°N, 5.7°E) in France and the Hohenpeissenberg Observatory (HOH, 47.8°N, 11.0°E) in Germany, which present differences both in temperature and ozone. These differences emerge due to (a) temporal sampling, (b) instrument differences, and/or (c) different geographical location. Issue (a) arrives from the fact that lidar measurements are made only in nights without visible clouds, therefore measurements are not undertaken at the same dates at all stations.

In order to understand the relevance of these issues we present a cross-validation of lidar and satellite derived trends, based on data from OHP, HOH, and the Advanced Microwave Sounding Unit (AMSU) satellite data. Module A of AMSU has 15 channels giving temperature information from near-surface to the mid-strastosphere, and has replaced the Stratospheric Sounding Unit as the sentinel instrument to monitor stratospheric temperature since the early 2000’s. We found, based on OHP and AMSU data for the period 2001-2007, that lidar and AMSU are well correlated, but temporal sampling can lead to differences in monthly mean temperature calculation and subsequently on temperature trend estimation. Further analyses comparing OHP, HOH and AMSU (for the period 2001-2008) suggest that temperature trend estimates at OHP and HOH present dissimilarities that reflect distinct geophysical dynamics rather than instrumental
differences. These results have important implications for example for strategies of satellite data use for stratospheric temperature monitoring, and it also suggests that stratospheric models should be able to reproduce differences at relatively fine spatial scales.