Formation and Maintenance of Thin Tropical Cirrus and Its Role in Troposphere-Stratosphere Exchange

University Of Washington, Seattle WA

Investigators

Abstract

Thin cirrus clouds located in the tropical tropopause layer (15 to 17 km altitude) occur more than 30% of the time over large areas of the tropical western Pacific Ocean. The formation and maintenance of these clouds and the role they play in troposphere-stratosphere exchange is uncertain. Questions regarding the origin and maintenance of tropical cirrus were posed more than twenty years ago. While transport from stratosphere to troposphere is relatively well understood, the processes that move air from the troposphere into the stratosphere in the tropics remain enigmatic. Thin cirrus may well play a role in this process but the exact nature of how that occurs is not known. Previous investigations have identified dynamics, microphysics and radiative transfer as all playing some role, but no studies to date have combined all three into a single coupled model. This project will finalize development of a high-resolution process model for thin cirrus that combines dynamics, microphysics and radiative transfer. The model will be used to investigate the role that these three processes play in maintaining thin cirrus layers and the role that Kelvin wave activity may play in forming the layers. In addition, model results will shed light on the transport of small ice crystals and the potential dehydration of air processed through the thin cirrus layers from the troposphere into the stratosphere. Intellectual merit: The result of this research will be an assessment of the relative importance of these three processes in maintaining thin cirrus and the way in which they may couple to Kelvin wave dynamics to generate thin cirrus. The main hypothesis is that Kelvin wave activity generates thin cirrus, which then grows and maintains itself through dynamical motions driven by radiative heating. Cloud ice water content is maintained by a balance between crystal growth during vertical lifting and sedimentation. The coupled model will provide insight on how air is processed through these thin layers by radiative heating and dynamics and whether the circulation can produce dehydrated air that can subsequently be injected into the stratosphere. Broader impact: Large scale models of the tropical atmosphere struggle to capture the structure of the upper troposphere and lower stratosphere. Part of this problem is due to a lack of adequate vertical resolution but lack of knowledge about the physical processes also plays a large role. The results of this research will permit an assessment of the magnitude and importance of diabatic heating in the tropical tropopause layer and the role that thin cirrus play in the heating. By combining model results with new satellite observations, the areal extent of the heating can be evaluated. Similarly an assessment can be made of the magnitude of induced vertical transport. These results will both constrain existing model results and be used to parameterize thin cirrus properties and impacts in the next generation of models.