Inter-Annual Variability in Tropical Cirrus Extent Simulated with a Global Chemistry Transport Model

Abstract:

The extent and variability of cirrus coverage can play an important role for the transport of trace constituents through the tropical tropopause layer into the stratosphere. This is due to direct radiative effects from cirrus clouds, their impact on local water vapour abundances in the UTLS region and due to chemical processes on the surface of ice crystals. We investigate the variability of tropical cirrus cloud coverage over a period of 10 years (2004–2014) and its correlation with the Southern Oscillation Index. We use a global 3D offline chemistry transport model (SLIMCAT-Cirrus), with a parameterization of cirrus clouds formed by homogeneous nucleation. We compare the model's ability to reproduce the inter-annual variability in tropical cirrus extent with remote sensing data from satellites as well as with in-situ observations from the NASA Global Hawk in the Pacific region as part of the ATTREX campaign. Impacts from cirrus on the vertical transport from the troposphere to the stratosphere in the tropics and its inter-annual variability can be diagnosed from the model results.