Impact of convectively lofted ice on the seasonal cycle of tropical lower stratospheric water vapor

Abstract. We use a forward Lagrangian trajectory model to diagnose mechanisms that produce the tropical lower stratospheric (LS) 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 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 LS water vapor between 10N40N than observed by MLS. We show that including evaporation of convectively lofted ice in the trajectory model increases the simulated maximum value in the 10N40N water vapor seasonal cycle by 1.9ppmv (47%) and increases the seasonal amplitude by 1.26ppmv (123%), which improves the prediction of LS water vapor annual cycle. We conclude that the moistening effect from convective ice evaporation in the TTL plays a key role regulating and maintaining the tropical LS water vapor seasonal cycle. Most of the convective moistening in the 10N40N range comes from convective ice evaporation occurring at the same latitudes. A small contribution to the moistening comes from convective ice evaporation occurring between 10S10N. Within 10N40N, the Asian monsoon region is the most important region for convective ice evaporation and convective moistening during boreal summer and autumn.

Impact of convectively lofted ice on the seasonal cycle of tropical lower stratospheric water vapor Institutional Repository Document