Stratosphere-Troposphere Interactions

Breakout Session Summary DOE Atmospheric Chemistry Program Annual Meeting, Feb. 1998 Chaired by Michael McElroy, Harvard University Jerome Fast, Pacific Northwest National Laboratory

Participants included Cyndi Atherton (LLNL), John DeLuisi (NOAA), Jack Kaye (NASA), Irina Petropavloskikh (NOAA), Hans Schneider (Harvard University), and others.

Relevance

Research on stratosphere-troposphere interactions is important because the exchange between these two regions of the atmosphere can strongly affect atmospheric chemical concentrations and processes. Two types of issues are readily identified:

air-quality issues, which are episodic and regional.

The impact of energy use on climate change and air quality will not be completely assessed without a full understanding of stratosphere-troposphere interactions. Stratosphere-troposphere interactions have been examined primarily within the framework of global climate change studies, but it is now also critical to understand the relative contribution of stratospheric ozone to ozone concentrations in the lower troposphere. Because the new NAAQS for ozone is much closer to the ambient "background" concentrations, determining where and when stratospheric intrusions of ozone significantly affect near-surface concentrations has become more important. How stratospheric ozone affects near-surface concentrations is also of great interest to NARSTO, and ACP scientists should interact more closely with other scientists pursuing this type of research under NARSTO.

Scientific Issues

The processes associated the stratosphere-troposphere exchange have been identified. They include planetary waves, gravity waves, mesoscale processes (i.e. tropopause folds), small-scale mixing processes, and in situ chemical transformations. Mass flux exchanges averaged over a global scale seem to be well predicted, even by a relatively large-scale unsophisticated model. The times and locations at which stratospheric-troposphere exchange occurs, however, are not necessarily simulated well. This difficulty implies that global climate models might reasonably predict the overall exchange of trace-gas species, but exchange on a regional scale during specific episodes is not as well known and might be predicted incorrectly by models.

Scientific questions related to climate change include:

How well do we understand the processes associated with stratosphere-troposphere exchange?
How far does ozone come down into the troposphere?
How accurate are mass flux estimates and what is their variation in time (year-to-year, month-to-month)?
What are the feedback mechanisms associated with stratosphere-troposphere exchange forcing climate change?
How do upward exchange processes affect distribution of trace-gas species in the lower stratosphere and upper troposphere?
What are the causes for the differences between observed NO_x and modeled NO_x in the middle to upper troposphere?
Is stratosphere-troposphere exchange a steady process or is it due to a sum of episodic periods?
What is the resolution required by models to adequately represent the exchange of trace-gas species?

Some of the scientific questions related to air-quality include:

What are the combined meteorological processes responsible for ozone transported and mixed into the lower troposphere?
What are the relative contributions of stratosphere and tropospheric sources of ozone?
How frequently does stratospheric ozone increase ozone concentrations near the surface?
What regions are most likely to be affected by stratospheric ozone?

Needs

Scientists studying stratosphere-troposphere interactions have used both observational and modeling approaches. Important information includes large-scale meteorological fields, ozonesonde data, satellite observations, tracers of opportunity (e.g., ⁷Be), and aircraft trace-gas measurements . Nevertheless, observations made to date in the upper-troposphere and lower stratosphere are usually insufficient to adequately describe stratosphere-troposphere exchange.

Data from new measurement technologies

Some new measurement technologies are being developed to describe stratosphere-troposphere interactions. For example, NASA is developing ozone monitoring instrumentation that can be deployed on commercial aircraft. If this instrumentation were placed on a number of aircraft, ozone profiles could be obtained at many locations and times. This information would be extremely useful in determining the frequency of stratospheric intrusions of ozone throughout the U.S. NOAA's Forecast Systems Laboratory is developing a balloon-borne chemical monitoring package that could be deployed at elevations above the cruise altitudes of most commercial aircraft. The frequent and widespread use of such balloon-borne monitoring packages would describe spatial and temporal ozone variations in the upper troposphere and lower stratosphere at a better resolution than accomplished with current approaches. ACP scientists should collaborate with various groups developing instrumentation packages to better understand the behavior of trace-gas species in the upper troposphere.

Data from ACP field campaigns

Observations from ACP field campaigns could be used to examine issues related to stratosphere-troposphere interactions. Measurements from previous ACP field campaigns have usually been made in the lower troposphere, however, so that little information could be inferred for the upper troposphere and lower stratosphere. It would be useful for the research aircraft to make vertical profiles to detect ozone variations and other chemical and meteorological quantities in the upper troposphere. This data could be used in conjunction with near-surface observations to determine the influence of stratospheric ozone on near-surface concentrations. A series of such flights, once or twice a day for several consecutive days, would be necessary to adequately determine the exchange of ozone. Due to budget constraints and the need for aircraft that are capable of reaching sufficiently great altitudes, accomplishing such flights wholly within the ACP might be difficult. Surface measurements of ⁷Be and other trace-gas species might be a more cost-effective means to indicate the occurrence of stratospheric influences.

ACP field campaign collaborators from agencies such as NOAA and NASA have access to aircraft and ozonesonde data that can be used to investigate stratosphere-tropospheric exchange. For example, differential absorption lidars aboard aircraft have been used to measure ozone profiles. Collaborative experiments involving appropriate agencies and investigators should be sought for future ACP field campaigns.

Modeling

Because of the deficiencies of existing observational data, models have been employed to obtain an improved picture of how substances are exchanged between the stratosphere and troposphere. The projects currently supported by ACP bring together a wide spectrum of modeling techniques to address stratosphere-troposphere exchange. For climate change applications, 3-D and 2-D models will continue to be used. A global stratospheric model is being improved to incorporate prognostic tropospheric chemistry processes so that the long-range transport can be treated more realistically. For air-quality applications, higher horizontal and vertical resolution is clearly needed to adequately represent stratosphere-troposphere exchange. Regional models will be applied to produce the high-resolution meteorological fields required to simulate atmospheric processes associated with stratosphere-troposphere exchange ozone for individual episodes.

Scientists working on stratosphere-troposphere interactions need to collaborate with regional and global modelers and help to link the regional and large-scale domains. For example, a number of global modeling projects addressing long-range transport of pollutants have focused on the North Atlantic Regional Experiment (NARE) and the Pacific Exploratory Mission-WEST (PEM-West). In these cases, the long-range transport of interest can be affected for long periods of time by stratosphere-troposphere interactions acting to modify trace-gas species in the middle to lower troposphere. For regional modeling, lateral and upper boundary conditions for trace-gas species concentrations must be specified, which can be addressed with large-scale models. In addition, a better understanding of stratospheric influences on the lower troposphere would improve the capability of regional models to interpret field data.