Lower Stratospheric Transport Research Articles

Evaluating the Model Representation of Asian Summer Monsoon Upper Troposphere and Lower Stratosphere Transport and Composition Using Airborne In Situ Observations

AbstractChemistry Climate Models (CCMs) are essential tools for characterizing and predicting the role of atmospheric composition and chemistry in Earth's climate system. This study demonstrates the use of airborne in situ observations to diagnose the representation of chemical composition and transport by CCMs. Process‐based diagnostics using dynamical and chemical coordinates are presented which minimize the spatial and temporal sampling differences between airborne in situ measurements and CCM grid points. The chosen process is the chemical impact of the Asian summer monsoon (ASM), where deep convection serves as a rapid transport pathway for surface emissions to reach the upper troposphere and lower stratosphere (UTLS). We examine two CCM configurations for their representation of the ASM UTLS using a set of airborne observations from south Asia. The diagnostics reveal good model performance at representing tropospheric tracer distribution throughout the troposphere and lower stratosphere, and excellent representation of chemical aging in the lower stratosphere when chemical loss is dominated by photolysis. Identified model limitations include the use of zonally averaged mole fraction boundary conditions for species with sufficiently short tropospheric lifetimes, which may obscure enhanced regional emissions sources. Overall, the diagnostics underscore the skill of current‐generation models at representing pollution transport from the boundary layer to the stratosphere via the ASM mechanism, and demonstrate the strength of airborne in situ observations toward characterizing this representation.

Impact of Lagrangian transport on lower-stratospheric transport timescales in a climate model

Abstract. We investigate the impact of model trace gas transport schemes on the representation of transport processes in the upper troposphere and lower stratosphere. Towards this end, the Chemical Lagrangian Model of the Stratosphere (CLaMS) was coupled to the ECHAM/MESSy Atmospheric Chemistry (EMAC) model and results from the two transport schemes (Lagrangian critical Lyapunov scheme and flux-form semi-Lagrangian, respectively) were compared. Advection in CLaMS was driven by the EMAC simulation winds, and thereby the only differences in transport between the two sets of results were caused by differences in the transport schemes. To analyze the timescales of large-scale transport, multiple tropical-surface-emitted tracer pulses were performed to calculate age of air spectra, while smaller-scale transport was analyzed via idealized, radioactively decaying tracers emitted in smaller regions (nine grid cells) within the stratosphere. The results show that stratospheric transport barriers are significantly stronger for Lagrangian EMAC-CLaMS transport due to reduced numerical diffusion. In particular, stronger tracer gradients emerge around the polar vortex, at the subtropical jets, and at the edge of the tropical pipe. Inside the polar vortex, the more diffusive EMAC flux-form semi-Lagrangian transport scheme results in a substantially higher amount of air with ages from 0 to 2 years (up to a factor of 5 higher). In the lowermost stratosphere, mean age of air is much smaller in EMAC, owing to stronger diffusive cross-tropopause transport. Conversely, EMAC-CLaMS shows a summertime lowermost stratosphere age inversion – a layer of older air residing below younger air (an “eave”). This pattern is caused by strong poleward transport above the subtropical jet and is entirely blurred by diffusive cross-tropopause transport in EMAC. Potential consequences from the choice of the transport scheme on chemistry–climate and geoengineering simulations are discussed.

Understanding differences in chemistry climate model projections of stratospheric ozone

AbstractChemistry climate models (CCMs) are used to project future evolution of stratospheric ozone as concentrations of ozone‐depleting substances (ODSs) decrease and greenhouse gases increase, cooling the stratosphere. CCM projections exhibit not only many common features but also a broad range of values for quantities such as year of ozone return to 1980 and global ozone level at the end of the 21st century. Multiple linear regression is applied to each of 14 CCMs to separate ozone response to ODS concentration change from that due to climate change. We show that the sensitivity of lower stratospheric ozone to chlorine change ΔO3/ΔCly is a near‐linear function of partitioning of total inorganic chlorine (Cly) into its reservoirs; both Cly and its partitioning are largely controlled by lower stratospheric transport. CCMs with best performance on transport diagnostics agree with observations for chlorine reservoirs and produce similar ozone responses to chlorine change. After 2035, differences in ΔO3/ΔCly contribute little to the spread in CCM projections as the anthropogenic contribution to Cly becomes unimportant. Differences among upper stratospheric ozone increases due to temperature decreases are explained by differences in ozone sensitivity to temperature change ΔO3/ΔT due to different contributions from various ozone loss processes, each with its own temperature dependence. Ozone decrease in the tropical lower stratosphere caused by a projected speedup in the Brewer‐Dobson circulation may or may not be balanced by ozone increases in the middle‐ and high‐latitude lower stratosphere and upper troposphere. This balance, or lack thereof, contributes most to the spread in late 21st century projections.

Lidar and SAGE II observations of Shishaldin Volcano aerosols and lower stratospheric transport

During May 1999, data collected with two of University of L'Aquila lidar systems, Italy show an increase in the stratospheric aerosol content at altitudes between 15 and 18 km. Our initial hypothesis is that these aerosols originated from the Shishaldin (Alaska) volcanic eruption of 19 April 1999. SAGE II observations, taken between Julian day 100 and 180, at latitudes above 40°N, show several sightings that are good candidates for volcanic aerosols. A trajectory analysis shows that a few SAGE II measurements are in space and time compatible with the Shishaldin eruption, and the aerosol enhancements observed by L'Aquila lidars have been also sampled by SAGE II, later and in different locations.

Episodic total ozone minima and associated effects on heterogeneous chemistry and lower stratospheric transport

A description of the January 31, 1989, ozone minihole over Stavanger, Norway, is given on the basis of three‐dimensional model simulations. This minihole is typical (though of large magnitude) of many transient events in the lower stratosphere that arise because of cyclonic‐scale disturbances in the troposphere. The ozone reduction is a short‐lived reversible dynamical event. However, through heterogeneous chemical processes there can be a significant transfer of chlorine from reservoir molecules to active radicals. This chemically perturbed air is defined as processed air, and it is found that a single event can produce enough processed air to reduce the HCl in the entire polar vortex. Chemical processing on clouds associated with transient events is shown to be a major source of processed air in the polar vortex in December before background temperatures are cold enough for more uniform heterogeneous conversion. In the model, intense cyclonic scales propagating close to the vortex edge and large planetary wave events (especially stratospheric warmings) are the major mechanisms of extra‐vortex transport. Only a small amount of processed air is found outside of the polar vortex. The processed air is a strong function of longitude, and it is virtually excluded from the Pacific Basin.