Tropical upper tropospheric humidity variations due to potential vorticity intrusions

M Sandhya,M Indira Devi,S Sridharan

doi:10.5194/angeo-33-1081-2015

M Sandhya, M Indira Devi + Show 1 more

Open Access

https://doi.org/10.5194/angeo-33-1081-2015

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Abstract

Abstract. Four cases (March 2009, May 2009, April 2010 and February 2012) are presented in which the ERA-interim relative humidity (RH) shows consistent increase by more than 50 % in the upper troposphere (200–250 hPa) over tropics at the eastward side of the potential vorticity (PV) intrusion region. The increase in RH is confirmed with the spaceborne microwave limb sounder observations and radiosonde observations over Gadanki (13.5° N, 79.2° E) and is observed irrespective of whether the PV intrusions are accompanied by deep convection or not. It is demonstrated that the increase in RH is due to poleward advection induced by the PV intrusions in their eastward side at the upper tropospheric heights. It is suggested that the low-latitude convection, which is not necessarily triggered by the PV intrusion, might have transported water vapour to the upper tropospheric heights.

Highlights

Upper tropospheric water vapour is an important greenhouse gas and can influence surface radiative forcing (Jones and Mitchell, 1991; Udelhofen and Hartmann, 1995)
We present four cases here, in which even tropical upper tropospheric relative humidity (UTRH) variations are shown to be dependent on the northward advection associated with the potential vorticity (PV) intrusions irrespective of whether the intrusions are accompanied by deep convection or not
In the case of 2010 event, the PV intrusion starts on 29 April and continues till 2 May over 63–72◦ E and it triggers convection, which can be inferred from low Outgoing Longwave Radiation (OLR) (< 200 W m−2) over 72.5–85◦ E

Summary

Introduction

Upper tropospheric water vapour is an important greenhouse gas and can influence surface radiative forcing (Jones and Mitchell, 1991; Udelhofen and Hartmann, 1995). Satellite measurements, namely, the Stratospheric Aerosol and Gas Experiment, the Television Infrared Observation Satellite (TIROS), operational vertical sounder, 6.7 μm brightness temperature from a single Geostationary Operational Environmental Satellite, Microwave Limb Sounder, and Atmospheric Infrared Sounder (AIRS) have contributed significantly to the retrieval of water vapour in the upper troposphere from the radiances and to the understanding of its variabilities (Rind et al, 1993; Soden and Bretherton, 1993; Soden and Fu, 1995; Liao and Rind, 1997; Gettelman et al, 2006). Rossby wave breaking at mid-latitudes leads to transport of stratospheric air with high PV in to tropical upper troposphere (De Bellevue et al, 2006) These PV intrusions can be identified from relatively high PV value (> 2 PVU) at 350 K isentropic level (∼ 200 hPa over tropics) (Waugh and Polvani, 2000).

ERA-interim data sets

MLS-UTRH data

Gadanki radiosonde observations

Results

Discussion and conclusions