Introduction

Abstract

Atmospheric reanalyses - comprehensive records of atmospheric conditions synthesized from past meteorological observations using the systems employed today for numerical weather prediction - have found widespread application in many branches of meteorological and climatological research. Their use to help document and understand climatic trends and low-frequency variations requires caution, but can provide a valuable complement to direct study of the instrumental record.

The development of atmospheric reanalysis at ECMWF will be reviewed. Emphasis will be placed on how progress is being made in meeting the challenges posed by a changing in situ and space-based global observing system and by the presence of biases in observations, models and the way the data assimilation process brings them together. Recent results relating to low-frequency variability and trends in temperature and some aspects of the hydrological cycle will be presented. New capabilities related to atmospheric composition will also be noted.