Microwave Limb Sounder - Stratosphere / Troposphere Interaction Research

STRATOSPHERE/TROPOSPHERE INTERACTION

Understanding dynamical, transport and chemical processes in the extra-tropical (ET) upper troposphere and lower stratosphere (UTLS) is critical to advancing climate change and pollution transport studies. For example, water vapor and ozone are both key greenhouse gases in the UTLS, and their distributions are strongly affected by stratosphere-troposphere exchange processes. In the lowermost stratosphere (the portion of the extratropics where some part of the lower latitude region is in the troposphere), there is a complex interplay of stratospheric (e.g., chemical ozone loss) and tropospheric (e.g., pollution generation and transport, fire-related convection) processes, and intrusions of stratospheric air into the troposphere and vice versa are common.

The coupled chemistry and climate of the UTLS respond to both tropospheric climate change [e.g. Santer et al., 2004; WMO, 2007] and to O3 depletion in the lower stratosphere (link to polar processes section) [e.g., Seidel and Randel, 2006]; the complexity of causes of tropopause-level changes makes our ability to accurately detail UTLS variability especially critical. Both radiative forcing and surface temperature are most sensitive to O3 changes near the tropopause [e.g., Forster and Shine, 1999]. Radiative effects of water vapor near the ET tropopause have been shown to be instrumental in determining the thermal structure in the UTLS [e.g., Randel 2007]. Upper tropospheric winds (the jet streams whose tops extend into the lowermost stratosphere) and tropopause characteristics have been shown to be sensitive to climate change [e.g., Lorenz and DeWeaver, 2007; Archer and Caldeira, 2008]. The Antarctic O3 hole has caused significant temperature changes in the lower stratosphere; a significant part of the ozone depletion occurs in the sub-vortex (the lowest part of the polar vortex that extends into the lowermost stratosphere), where it can be more efficiently transported to midlatitudes and possibly into the troposphere. Changes in the Brewer-Dobson Circulation (the large-scale seasonally varying circulation of the stratosphere), synoptic eddies (storms) in the upper troposphere, and midlatitude convection are expected to alter the extent and consequences of ET stratosphere-troposphere exchange. Pollution products transported from the troposphere up into the lower stratosphere influence ozone chemistry there [e.g. Hegglin et al., 2006; WMO, 2007]; conversely, stratospheric O3 transported down also influences tropospheric chemistry [e.g., WMO, 2007; Hsu and Prather, 2009].

Until recent years, most satellite measurements did not extend down into the ET UTLS, and studies of the ET UTLS were primarily limited to analysis of sporadic and sparse (albeit very high spatial and temporal resolution) aircraft data. With the launch of the Aura satellite, MLS provides measurements of O3, HNO3, CO and H2O, as well as temperature, covering the lowermost stratosphere with global daily coverage; other measurements (e.g., HCl and ClO) extend into the upper part of this region. Meteorological analyses from advanced data assimilation systems (DAS) provide comprehensive information on the jet streams (the winds that control transport and mixing of trace gases), tropopause variations, and temperature structure in the UTLS, that is, the dynamical factors that control trace gas transport and influence chemical processes. Studies using MLS data, in conjunction with DAS analyses and high resolution but sparse aircraft data, can contribute to advancing our understanding of the ET UTLS and its role in climate change processes.

STRATOSPHERE/TROPOSPHERE INTERACTION

Extra-Tropical Strat-Trop Exchange