An Assessment of Upper-troposphere and Lower-stratosphere Water Vapor in MERRA2 Reanalyses Using Aura MLS Observations

Global water vapor (H2O) measurements from Microwave Limb Sounder (MLS) are used to evaluate the upper troposphere (UT) and lower stratosphere (LS) H2O product produced by the second version Modern-Era Retrospective Analysis for Research and Applications (MERRA2) Focusing on quantifying the H2O amount and transport from UT to LS, we show that MERRA2 overestimate UT H2O by 50% compared to MLS observations. Both observation and reanalysis show that boreal summer convection has a dominant influence on UTLS H2O, resulting in moister air in northern hemisphere (NH) than in southern hemisphere (SH). However, substantial differences in H2O transports are found in different datasets. Vertically, the H2O transport across the tropical tropopause (16-19 km) simulated by the MEREA2 is 40& faster compared to MLS observations. In tropical LS (20-25 km), the MERRA2 have vertical transport velocities similar to the MLS value. Horizontally, both observation and MERRA2 analysis show faster poleward transport in the NH than in the SH; In the NH, the simulated 100hPa H2O horizontal transport velocity is ~ 100% faster than the MLS observed value; In the SH, the MERRA2 horizontal transport velocity is slower by 49% compared to MLS observations.