Impact of convectively lofted ice on the seasonal cycle of water vapor in the tropical tropopause layer

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.

Assessment of Observational Evidence for Direct Convective Hydration of the Lower Stratosphere

“…The same global-model convective ice product was used by Dessler et al (2016) to show that convective ice sublimation has an important contribution to model-predicted future trends in stratospheric humidity. The Wang et al (2019) analysis indicated that the convective hydration in their model was primarily occurring over the ASM, which contradicts the available observational evidence presented here and in previous studies. This inconsistency between the model results and observations attests to the importance of accurately representing the distribution of convective ice relative to the tropopause for estimation of lower stratospheric hydration in models.…”

“…This inconsistency between the model results and observations attests to the importance of accurately representing the distribution of convective ice relative to the tropopause for estimation of lower stratospheric hydration in models. Convective clouds in most global models, including the one used by Wang et al (2019), are generated by convective parameterizations, with large uncertainties for even moderately strong convection and even larger uncertainties for the extreme outlier systems reaching the tropopause and beyond.…”

Processes influencing lower stratospheric water vapour in monsoon anticyclones: insights from Lagrangian modeling

“…Compared to other processes, VMIX shows a weak increase of water vapour in the AMA related to the enhanced tropospheric mixing, including turbulence and convective updrafts. Apparently, this is not consistent with Ueyama et al (2018); Wang et al (2019) that highlight the importance of convection to reproduce the monsoonal maxima of water vapour. However, it is highly likely that the effect of convective updrafts is very limited by the filtering of air parcels below 250 hPa in our LTF set-up.…”

“…To create a common framework between previous studies focusing on water vapour simulation (Schoeberl et al, 2013;Zhang et al, 2016;Wang et al, 2019) and our CLaMS sensitivity experiments, we have implemented the forward domain-filling technique, here referred as Lagrangian Trajectory Filling (LTF), into CLaMS. This set-up, described by Schoeberl and Dessler (2011), has been widely used to study different properties of water vapour in the stratosphere and UTLS region (Schoeberl et al, 2012(Schoeberl et al, , 2013(Schoeberl et al, , 2014Dessler et al, 2014;Zhang et al, 2016;Ye et al, 2018;Schoeberl et al, 2018Schoeberl et al, , 2019.…”

“…This approach is based on the philosophy that LS water vapour depends on the processes acting in the upper troposphere and in the tropopause region and therefore assumes only a minor role of the lower to mid tropospheric water vapour distribution and the specific model set-up of air parcel release at locations close but below the tropopause. Although this approach has been successfully applied to answer many questions related to the water vapour distribution in the TTL (Schoeberl and Dessler, 2011;Schoeberl et al, 2012Schoeberl et al, , 2013Schoeberl et al, , 2014Zhang et al, 2016;Schoeberl et al, 2018Schoeberl et al, , 2019Wang et al, 2019), it has never been directly compared against other configurations that consider the whole troposphere as well. This implies another source of uncertainty that might affect the water vapour simulation.…”