Validation of POAM III Water Vapor using HALOE and MOZAIC

The Polar Ozone and Aerosol Measurement (POAM III) has been successfully measuring stratospheric water vapor since March 1998. High resolution vertical profile measurements are made on a daily basis in the Arctic and Antarctic regions. In this paper, the water vapor retrievals are validated by comparison with measurements from the Halogen Occultation Experiment (HALOE), and with Measurements of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC). In the northern hemisphere, there are five periods of coincident measurements with HALOE, spanning 1998 to 2000. Quantitative comparisons show agreement to within 5% from 45 km down to 25 km. Below 25 km, POAM measurements are about 10% larger than HALOE. In the southern hemisphere, POAM water vapor is about 10% larger than HALOE from 15 km to 45 km. The airborne, in situ MOZAIC instruments provide a large number of measurements which can be used to validate POAM measurements in the highly spatially variable regions of the upper troposphere and lowermost stratosphere, where water vapor mixing ratios are much larger than those found throughout most of the stratosphere. The comparisons shows that there is no statistically significant difference in the response of the two instruments to changes in water vapor, and that, in the high water vapor mixing ratio regime where the MOZAIC measurements are most accurate, the POAM water vapor mixing ratios are systematically larger than MOZAIC by about 10%.