TTL-Xing

The Tropical Tropopause Layer (TTL) is the region of the atmosphere between 30S and 30N which is bounded by the convectively dominated tropical troposphere below and the layered motion in the stratosphere regulated by the Brewer Dobson circulation above. The TTL is the gateway which processes the air lofted from the surface by convection, including anthropogenic pollution, and controls the composition of the stratosphere. Due to this role and the radiative effects of its ubiquitous cirrus clouds, which are regulating the convection underneath, the TTL is a key component of the climate system. During summer, the TTL is mostly under the influence of the Asian monsoon where the upper layer circulation is dominated by a large anticyclone extending from east Asia to the Mediterranean sea. This circulation ventilates the convection and redistributes the lofted air. Since two decades, a new aerosol layer (ATAL=Asian Tropopause Aerosol Layer), recently discovered, has developed near 16 km altitude inside the monsoon anticyclone and is attributed to the growing anthropogenic emissions at the ground. The impact of the monsoon on the TTL and the stratosphere, the distribution of ground sources, the transport properties across the TTL and the processes related to aerosols and cirrus within the TTL, in particular the ATAL, are still poorly known due to a lack of available observations and limited modeling studies. However, a wealth of new observations is now being provided by a new generation of satellite instruments and a series of campaigns in Asia based on international collaboration. The EU funded StratoClim airborne campaign in 2017 will provide a large amount of unparalleled in situ observations within the Asian monsoon TTL and the ATAL. The overarching goal of our project is to understand how does climate change affect the features of the TTL (cirrus, radiative budget and composition of the air entering the stratosphere) during the Asian monsoon season and what is the feedback on the climate. The investigated questions in this proposal are (i) the description of high altitude clouds and of their injection of water and tracer compounds in the TTL, (ii) the processes from small to large scale which regulate transport, the distribution of cirrus and aerosol, (iii) the radiative impact of clouds, in particular the thin cirrus, and aerosols in the Monsoon region, (iv) the full exploitation of available data. An immediate urgent question is to understand the change in composition manifested by the ATAL, to determine its chemical and physical properties, to understand and monitor the sources and precursors and to determine its direct and indirect glaciation radiative forcing and the induced modulation of stratospheric composition. These questions will be answered by a coordinate set of tasks. Multi-satellite data will be analyzed to determine the properties of high altitude clouds and aerosols. The StratoClim campaign data will be exploited using Lagrangian modeling methods to link the measurements to the sources and the history of air parcels. We will a new set of observations by light weight innovative instruments flying under small balloons and ATAL impact will be monitored by the ground-based lidars of the NDACC network.Two complementary models with full representation of chemical and micro-physical processes will be used to reproduce the ATAL and compared with the campaign and satellite data. Climatologies of relevant ground sources and transport pathways will be established. Climate change projections will be estimated. Our project will be conducted by a team that gathers the highest expertise in atmospheric modeling, observation and data analysis and is already well trained to work together. Our project is a unique opportunity to make the best usage of the most recent observations and to produce major advances on a key issue in climate science.