We use our forward domain filling trajectory model to explore the impact of tropical convection on stratospheric water vapor (H2O) and tropical tropopause layer cloud fraction (TTLCF). Our model results are compared to winter 2008/2009 TTLCF derived from Cloud-Aerosol Lidar with Orthogonal Polarization and lower stratospheric H2O observations from the Microwave Limb Sounder. Convection alters the in situ water vapor by driving the air toward ice saturation relative humidity. If the air is subsaturated, then convection hydrates the air through the evaporation of ice, but if the air is supersaturated, then convective ice crystals grow and precipitate, dehydrating the air.
On average, there are a large number of both hydrating and dehydrating convective events in the upper troposphere, but hydrating events exceed dehydrating events. Explicitly adding convection produces a less than 2% increase in global stratospheric water vapor during the period analyzed here. Tropical tropopause temperature is the primary control of stratospheric water vapor, and unless convection extends above the tropopause, it has little direct impact. Less than 1% of the model parcels encounter convection above the analyzed cold-point tropopause. Convection, on the other hand, has a large impact on TTLCF. The model TTLCF doubles when convection is included, and this sensitivity has implications for the future climate-related changes, given that tropical convective frequency and convective altitudes may change.
Some previous analyses have indicated a possibly significant contribution from such convective overshooting, potentially leading to a positive climate feedback. This study argues against convective feedback being significant.
Technical Description of Figure:
A summary of the results from simulations adjusting various aspects of the model and its parameterization of gravity waves, convection, and microphysical processes. The colored symbols indicate the nucleation relative humidity (NRH) values used, with + and X to indicate the source of information used for convection. Experiments incorporating gravity wave measurements from the Google Loon balloon project have embedded small green circles. The arrows indicate the general tendency of the impacts of each of these processes water vapor and cloud fraction.
Scientific significance, societal relevance, and relationships to future missions:
The tropical tropopause is the gateway through which most air entering the stratosphere passes, and it is also the coldest part of the atmosphere. Accordingly, air entering the stratosphere via this route is freeze dried, and its humidity is a measure of the lowest temperature it experienced en route. This study uses a model to quantify various contributions to this temperature and the humidity it determines from gravity wave, microphysical, and convective processes. The study also indicates that direct injection of ice by overshooting deep convection (transport that bypasses the “cold trap”) is not a significant contributor. Long-term changes in stratospheric water vapor have a significant impact on surface climate. For example, a significant drop in stratospheric water vapor in 2000 is estimated to have reduced surface warming in the subsequent decade by 25%. A strong contribution to stratospheric humidity from overshooting convection would lead to a significant positive feedback on climate, given the expectation that such convection will increase in a warming climate, but this study implies that such convection does not contribute significantly. Aura MLS is one of only two remaining spaceborne instruments making daily near-global observations of water vapor.
Water vapor observations are from v4.23 of the dataset from the Microwave Limb Sounder (MLS) instrument on NASA’s Aura mission. Cloud fraction observations are taken from v4.1 of the dataset from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument on NASA’s Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) mission. The process model described in the paper uses meteorological fields from the NASA Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) dataset, with information on gravity waves and convection taken from a range of other sources as described in the paper.
References: Schoeberl, M., E. Jensen, L. Pfister, R. Ueyama, M. Avery, and A. Dessler, “Convective Hydration of the Upper Troposphere and Lower Stratosphere,” Journal of Geophysical Research: Atmospheres, doi:10.1029/2018jd028286, 2018.