An assessment of upper troposphere and lower stratosphere water vapor in MERRA, MERRA2, and ECMWF reanalyses using Aura MLS observations

Abstract

Abstract Global water vapor (H 2 O) measurements from Microwave Limb Sounder (MLS) are used to evaluate upper tropospheric (UT) and lower stratospheric (LS) H 2 O products produced by NASA Modern‐Era Retrospective Analysis for Research and Applications (MERRA), its newest release MERRA2, and European Centre for Medium‐Range Weather Forecasts (ECMWF) Interim Reanalyses. Focusing on the H 2 O amount and transport from UT to LS, we show that all reanalyses overestimate annual global mean UT H 2 O by up to ~150% compared to MLS observations. Substantial differences in H 2 O transport are also found between the observations and reanalyses. Vertically, H 2 O transport across the tropical tropopause (16–20 km) in the reanalyses is faster by up to ~86% compared to MLS observations. In the tropical LS (21–25 km), the mean vertical transport from ECMWF is 168% faster than the MLS estimate, while MERRA and MERRA2 have vertical transport velocities within 10% of MLS values. Horizontally at 100 hPa, both observation and reanalyses show faster poleward transport in the Northern Hemisphere (NH) than in the Southern Hemisphere (SH). Compared to MLS observations, the H 2 O horizontal transport for both MERRA and MERRA2 is 106% faster in the NH but about 42–45% slower in the SH. ECMWF horizontal transport is 16% faster than MLS observations in both hemispheres. The ratio of northward to southward transport velocities for ECMWF is 1.4, which agrees with MLS observation, while the corresponding ratios for MERRA and MERRA2 are about 3.5 times larger.