Abstract
Abstract. We use a forward Lagrangian trajectory model to diagnose mechanisms that produce the water vapor seasonal cycle observed by the Microwave Limb Sounder (MLS) and reproduced by the Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM) in the tropical tropopause layer (TTL). We confirm in both the MLS and GEOSCCM that the seasonal cycle of water vapor entering the stratosphere is primarily determined by the seasonal cycle of TTL temperatures. However, we find that the seasonal cycle of temperature predicts a smaller seasonal cycle of TTL water vapor between 10 and 40∘ N than observed by MLS or simulated by the GEOSCCM. Our analysis of the GEOSCCM shows that including evaporation of convective ice in the trajectory model increases both the simulated maximum value of the 100 hPa 10–40∘ N water vapor seasonal cycle and the seasonal-cycle amplitude. We conclude that the moistening effect from convective ice evaporation in the TTL plays a key role in regulating and maintaining the seasonal cycle of water vapor in the TTL. Most of the convective moistening in the 10–40∘ N range comes from convective ice evaporation occurring at the same latitudes. A small contribution to the moistening comes from convective ice evaporation occurring between 10∘ S and 10∘ N. Within the 10–40∘ N band, the Asian monsoon region is the most important region for convective moistening by ice evaporation during boreal summer and autumn.
Highlights
Stratospheric water vapor is important for the radiative budget of the atmosphere and the regulation of stratospheric ozone (e.g., Solomon et al, 1986; Dvortsov and Solomon, 2001)
We investigated mechanisms that drive the seasonal cycle of water vapor in the tropical tropopause layer (TTL)
We use a Lagrangian trajectory model (Schoeberl and Dessler, 2011) to analyze the seasonal cycle in observations of water vapor made by the Microwave Limb Sounder (MLS) (Lambert et al, 2007; Livesey et al, 2017) as well as simulated fields from the Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM) (Rienecker et al, 2008; Molod et al, 2012; Pawson et al, 2008; Oman and Douglass, 2014)
Summary
Stratospheric water vapor is important for the radiative budget of the atmosphere and the regulation of stratospheric ozone (e.g., Solomon et al, 1986; Dvortsov and Solomon, 2001). Nielsen et al (2007) and Corti et al (2008) suggested that deep-penetrating convection deposits ice particles above the cold point tropopause, where ice may evaporate and cause a moistening effect. Analyses of observations have suggested that deep convection reaching the TTL may be important for regulating the amount of water vapor entering the stratosphere. This idea is supported by observations of enrichment of the deuterated isotopologue of water vapor (HDO) in the tropical LS (Moyer et al, 1996; Dessler et al, 2007; Steinwagner et al, 2010). The role of convective ice evaporation in the stratospheric entry water vapor has been addressed in several model studies. Schoeberl et al (2014, 2018, 2019) quantified the global impact of convective ice on winter 2008/2009 water vapor between 18 and 30 km and concluded that, for global average water vapor be-
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