Comparison of lower stratospheric tropical mean vertical velocities

Abstract

We have analyzed 13 years (1993–2005) of tropical stratospheric water vapor data from the Halogen Occultation Experiment and over 3 years of data (October 2004 through November 2007) from the Aura Microwave Limb Sounder. By correlating the phase lag of the water vapor “tape recorder” signal between levels we estimate the time mean vertical velocity. Our estimated vertical velocity compares well with calculations from the Goddard Earth Observing System (GEOS) chemistry‐climate model (CCM) and from the GEOS data assimilation system. Between 18 and 26 km both the GEOS CCM simulations and water vapor observations agree that the vertical velocity is below 0.04 cm/s, with a minimum near 20 km of 0.03 cm/s. Vertical velocities deduced from water vapor observations are higher than those from the GEOS CCM in the region 16–18 km (0.04 cm/s) and above 26–30 km (up to 0.07 cm/s). These estimates are close to earlier estimates from a shorter water vapor record and radiative transfer models. No evidence is found for velocities as high as 0.15 cm/s as was recently estimated from aircraft CO2 measurements in the upper troposphere/lower stratosphere. Further diagnosis of the aircraft CO2 data and model simulations of CO2 show that while the CO2 data give an apparent upward transport velocity of ∼0.06 cm/s, about half of this is due to vertical and horizontal eddy transport. Accounting for the eddy terms gives a CO2‐based estimate of the vertical velocity of ∼0.03 cm/s, in much closer agreement with that estimated from water vapor.