Convective impact on the stratospheric-entry water vapor through the tropical UTLS in boreal winter and summer

Abstract:

Stratospheric humidity is primarily controlled by freeze drying (i.e., growth and sedimentation of ice crystals in cirrus clouds) of air ascending across the cold tropical tropopause. To what extent convection impacts cirrus in the upper troposphere and lower stratosphere (UTLS) and consequently the stratospheric water vapor budget remains unclear.

The role of convection on the humidity of air entering the stratosphere through the tropical UTLS is investigated in simulations of cirrus clouds along trajectories launched from potential temperature level surfaces above the tropopause. The one-dimensional (vertical) cloud model tracks individual ice crystals through their lifecycles, beginning with nucleation or detrainment from convection, followed by deposition growth, sedimentation and sublimation. Convective influence of the parcels is diagnosed by tracing the trajectories through time-dependent fields of convective cloud-top height adjusted to match the CloudSAT and CALIPSO statistics. Model simulations of UTLS water vapor and cloud fields are evaluated and constrained by comparison with MLS and CALIPSO measurements.

About 70% of parcels in the tropical lower stratosphere in boreal winter and summer have been influenced by convection sometime in the past 60 days. The majority of these convectively influenced parcels are subsequently dehydrated before arriving in the lower stratosphere. The final dehydration locations, as indicated by the locations of final in situ cirrus cloud formation along the trajectories, occur over the western Pacific and northwestern Australia in boreal winter, and over the Asian summer monsoon region in boreal summer. About half of the total convective impact on the lower stratosphere is due to convectively influenced parcels that are not subsequently dehydrated by in situ cloud formation. Globally, the overall impact of convection on the stratospheric-entry water vapor is approximately 10% (0.2 and 0.6 ppmv in boreal winter and summer, respectively).