On the comparisons of tropical relative humidity in the lower and middle troposphere among COSMIC radio occultations and MERRA and ECMWF data sets

P Vergados,C O Ao,H Su,A J Mannucci,J H Jiang

doi:10.5194/amt-8-1789-2015

Abstract

Abstract. The spatial variability of the tropical tropospheric relative humidity (RH) throughout the vertical extent of the troposphere is examined using Global Positioning System Radio Occultation (GPSRO) observations from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission. These high vertical resolution observations capture the detailed structure and moisture budget of the Hadley Cell circulation. We compare the COSMIC observations with the European Center for Medium-range Weather Forecast (ECMWF) Reanalysis Interim (ERA-Interim) and the Modern-Era Retrospective analysis for Research and Applications (MERRA) climatologies. Qualitatively, the spatial pattern of RH in all data sets matches up remarkably well, capturing distinct features of the general circulation. However, RH discrepancies exist between ERA-Interim and COSMIC data sets that are noticeable across the tropical boundary layer. Specifically, ERA-Interim shows a drier Intertropical Convergence Zone (ITCZ) by 15–20% compared to both COSMIC and MERRA data sets, but this difference decreases with altitude. Unlike ECMWF, MERRA shows an excellent agreement with the COSMIC observations except above 400 hPa, where GPSRO observations capture drier air by 5–10%. RH climatologies were also used to evaluate intraseasonal variability. The results indicate that the tropical middle troposphere at ±5–25° is most sensitive to seasonal variations. COSMIC and MERRA data sets capture the same magnitude of the seasonal variability, but ERA-Interim shows a weaker seasonal fluctuation up to 10% in the middle troposphere inside the dry air subsidence regions of the Hadley Cell. Over the ITCZ, RH varies by maximum 9% between winter and summer.

Highlights

Model simulations, reanalyses data sets, and satellite observations show large discrepancies of the global humidity climatology
All data sets display an upward current of moist air from the lower to the upper troposphere around the equatorial latitudes, which coincides with the Intertropical Convergence Zone (ITCZ) location
We identify regions of low relative humidity (RH) fields centered at ±20–25◦ between 600 and 500 hPa in both hemispheres, representing areas of dry air subsidence

Summary

Introduction

Reanalyses data sets, and satellite observations show large discrepancies of the global humidity climatology. Above 700 hPa MERRA shows a wetter environment than AIRS by more than 20 % These values are recorded over the Intertropical Convergence Zone (ITCZ) – a region characterized by deep convection and persistent cloud coverage. They reported that a composite of 16 climate models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) archive is 15 % drier than the AIRS observations below 600 hPa but 30 % wetter in the middle and upper troposphere. Chuang et al (2010) reported large differences in the interannual anomaly of the upper troposphere humidity among CMIP5 models, European Center for Mediumrange Weather Forecast (ECMWF) data sets, and AIRS observations over deep convective regions. Chuang et al (2010) reported large differences in the interannual anomaly of the upper troposphere humidity among CMIP5 models, European Center for Mediumrange Weather Forecast (ECMWF) data sets, and AIRS observations over deep convective regions. Chen et al (2008) showed disparities in the humidity field in ERA-40 and National Centers for Environmental Prediction (NCEP) reanalyses – documented by Huang et al (2005), who had found inconsistent interannual variabilities of the tropical humidity among the ERA-40 and the NCEP reanalyses with respect to the Geophysical Fluid Dynamics Labo-

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