Tropical Stratosphere-Troposphere Exchange (STE) strongly influences the composition and climate of the global stratosphere. STE also interacts with tropical thin cirrus clouds and has direct relevance for their climate forcing, and how such forcing may change over time, which affects tropospheric climate (through radiation). Although it is well known that air enters the stratosphere preferentially through upwelling in the tropics, the exact mechanisms driving tropical upwelling are still uncertain. Proposed mechanisms have been found either to be too slow or to be insufficient in mass flux to supply for the stratospheric Brewer-Dobson circulation. We hypothesize that the radiative driven upwelling associated with cirrus clouds in the tropical tropopause layer may explain a significant portion of the mass flux of the Brewer-Dobson circulation. In addition we will examine the mass flux minimum in the lower stratosphere indicated by recent studies.
The overall goal of this proposed project is to improve our understanding of the mechanisms responsible for tropical troposphere to stratosphere transport, to be able to predict the future evolution of STE. The specific scientific issues which we will address include (1) testing the cirrus-radiative-heating driven hypothesis and quantifying the relative contributions of convective overshooting, clear-sky radiative heating, and cirrus radiative heating to the vertical transport in the tropical tropopause layer, (2) investigating the causes and implications of the mass flux minimum in the lower stratosphere, and (3) improving global model simulations of STE. These objectives will be achieved by integrated analyses of the Aqua and CALIPSO satellite data from various instruments including AIRS, MODIS, CERES, and CALIPSO Lidar observations. A global coupled chemistry-climate model with high vertical resolution in the upper troposphere and lower stratosphere will be used to help interpret the data. In turn, differences between the satellite data analyses and model analyses will be used to better constrain the model physics and improve simulations of STE.
The proposed research will lead to a much improved understanding of the mechanisms responsible for the vertical transport of mass and trace constituents in the tropical tropopause layer and their spatial and temporal dependences. It will directly contribute to the NASA strategic goal "Study Earth from space to advance scientific understanding and meet societal needs". This proposal belongs to the "Integrated Science Data Analysis Proposals".
