Examining the extra-tropical upper troposphere and lower stratosphere using remote sensing limb-sounder measurements of unprecedented vertical resolution
Final Report Abstract
This report presents an examination of a two-dimensional snapshot of the chemical composition and structure of the extra-tropical upper troposphere and lower stratosphere (UT/LS) close to the subtropical jet. Using spatially highly resolved radiance measurements taken by the CRISTA-NF infrared limb-sounder on the 29th of July 2009, the first highly-resolved cross-sections of the complex layered structure of the extratropical transition layer and UT/LS could be produced. The evaluation of limb radiance measurements employs a novel cross-section retrieval that is able to dampen noise and produce vertically higher resolved results than conventional procedures. This allows the production of trace gas cross-sections with an unprecedented vertical resolution of down to 0.3 km with a more typical 0.5 km for most of the cross-sections and species. The spatial distribution of the retrieved trace gases O3, PAN, and HNO3 are discussed as they can be used best to identify the chemical association of air parcels with troposphere and stratosphere. The observations show an intricate filamentary structure in the discussed species with a vertical extent of less than 0.8 km and a horizontal extent of about 200 km across the subtropical jet. The relationship between PAN and O3 displays four distinct groups of chemically distinct air: polluted subtropical tropospheric air, clean tropical upper-tropospheric air, air from the lowermost stratosphere, and air from the overworld. These categories allow assignment of air parcels to dominantly tropospheric and stratospheric branches in tracer-tracer space. Air parcels not assigned to either branch are part of the two-way exchange between the troposphere and stratosphere and allow an estimation of the state and spatial distribution of mixing in the UT/LS. Some questions remain open. From the available data cannot be concluded, if the filamentary structure represents irreversible mixing. It is not clear, if the observed fine structure influences the speed of chemical processes in the UT/LS. Many global Chemistry-Climate models employ a vertical resolution insufficient for the representation of such structures. Would this therefore require changes to the gridding or the parameterisations of such models? Only further measurements of a larger spatial and temporal scale can answer such questions.
