Validation of water vapour profiles from the Atmospheric Chemistry Experiment (ACE)

Abstract

The Atmospheric Chemistry Experiment (ACE) mission was launched in August 2003 to sound the atmosphere by solar occultation. Water vapour (H 2 O), one of the most important molecules for climate and atmospheric chemistry, is one of the key species provided by the two principal instruments, the infrared Fourier Transform Spectrometer 5 (ACE-FTS) and the MAESTRO UV-Visible spectrometer (ACE-MAESTRO). The first instrument performs measurements on several lines in the 1362–2137 cm −1 range, from which vertically resolved H 2 O concentration profiles are retrieved, from 7 to 90 km altitude. ACE-MAESTRO measures profiles using the water absorption band in the near infrared part of the spectrum at 926.0–969.7 nm. This paper presents a com-10 prehensive validation of the ACE-FTS profiles. We have compared the H 2 O volume mixing ratio profiles with space-borne (SAGE II, HALOE, POAM III, MIPAS, SMR) ob-servations and measurements from balloon-borne frostpoint hygrometers and a ground based lidar. We show that the ACE-FTS measurements provide H 2 O profiles with small retrieval uncertainties in the stratosphere (better than 5% from 15 to 70 km, gradually 15 increasing above). The situation is unclear in the upper troposphere, due mainly to the high variability of the water vapour volume mixing ratio in this region. A new wa-ter vapour data product from the ACE-MAESTRO (Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation) is also presented and initial comparisons with ACE-FTS are discussed.