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Stratospheric Processes And their Role in Climate
A project of the World Climate Research Programme

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SPARC Brochure, June 2001 - The SPARC Initiatives

Stratospheric Processes and their Relation to Climate

The dynamical, chemical and radiative processes which occur in the stratosphere all influence climate. An assessment of the current knowledge of these processes has shown that research is needed to better understand the following important topics:

Upper Tropospheric and lower Statospheric Processes

Co-Chairs: A.R. Ravishankara (USA) and T. Shepherd (Canada)

What is happening in the UT/LS region ?

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Figure 1: The stratosphere is characterized by high ozone and low water vapour, and the troposphere by low ozone and high water vapour. The transition between the two regions occurs over a layer of finite thickness corresponding to a minimum in temperature, which may be called the tropopause layer. Climate chemistry is highly dependent on the OH radical, whose production is proportional to the product of ozone and water vapour and so is large in the UT/LS region. [Courtesy of A.R. Ravishankara.]

Objectives

It is in the upper troposphere and lower stratosphere (UT/LS) that the role of chemistry in climate comes into greatest prominence, and SPARC expects to play a key role in developing the science in this area through carefully targeted workshops and review papers.

Scientific motivation

The UT/LS is a critical region for climate sensitivity. Chemical and radiative timescales are relatively long, which means that dynamical forcings (chemical transport and adiabatic warming or cooling) play a particularly strong role in controlling the structure of the region. But equally, this means that chemical concentrations and temperature are highly sensitive to changes in rates of chemical and radiative processes. Transport of ozone through the UT/LS region plays a key role in determining chemical abundances in the troposphere as a whole. Low temperatures also imply the importance of condensed matter (liquid and solid clouds and aerosols) in this region, and, therefore, of heterogeneous and multiphase chemical reactions. Finally, the tropical tropopause controls the amount of stratospheric water vapour through the freeze drying mechanism.

Activity within SPARC

Given the need for an interdisciplinary understanding of climate science in the UT/LS region, SPARC has brought together scientists with different expertise (chemistry, microphysics, radiation, dynamics, transport) and methodology (theory, modelling, measurement, laboratory studies). Special workshops organized in collaboration with IGAC have focused on particular gaps in understanding and led to key papers reducing the uncertainty of photochemistry of ozone and rates of peroxy radical reactions that affect ozone in the UT/LS region. A recent workshop in Bad Tölz, Germany led to a new understanding of the UT/LS region as a transition region between the troposphere and stratosphere. A review paper is expected to result. A recent workshop on nitrogen oxides in the atmosphere and their partitioning in the UT/LS region was held in Heidelberg, Germany. The next workshop on the current state of gas phase reactions in the UT/LS region is scheduled for July 22-27, 2001 in Breckenridge, Colorado.

Future Plans

The goal is to continue to bring together the tropospheric and stratospheric climate modelling communities, as well as the radiative-dynamical and chemical communities.
What is SPARC doing about UT/LS ?

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Figure 2: The UT/LS region involves a complex interplay between dynamics, transport, radiation, chemistry, and microphysics. Dynamics and radiation lead to the low temperatures that form condensed matter through microphysical processes; microphysics in turn affects chemistry, as do temperatures, solar radiation, and transport of chemical species; chemistry in turn feeds back onto climate through radiation. [Courtesy of A.R. Ravishankara.]

Gravity Wave Processes and Parameterisation

Co-Chairs: K. Hamilton (USA) and R. Vincent (Australia)

Radiosonde station network

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Figure 1: The locations of all the stations from which there have been some high-resolution wind or temperature data contributed to the SPARC GWPP initiative.

The Gravity Wave Processes and Parameterisation (GWPP) initiative of SPARC is coordinating international projects aimed at understanding the internal gravity wave field in the stratosphere and its interaction with the large-scale circulation and climate. Thus far, most of the effort has gone into projects designed to improve the empirical database for characterizing and understanding the gravity wave field.

Activities

Routine balloon soundings of wind and temperature contain valuable information about the gravity wave field, although much of this information is lost when these data are archived only at the usual mandatory and significant levels, as is the standard practice in most countries. Modern radiosonde systems actually record data at quite high vertical resolution (~100 m). The SPARC GWPP initiative has coordinated the accumulation and analysis of these "raw" high-resolution data. This has been done primarily by involving scientists from a number of countries who have interacted with their own national meteorological services to obtain, save and analyse the high resolution data. Routine data have now been obtained from the meteorological services of 12 countries (Australia, Canada, Finland, France, Germany, Iceland, Japan, Korea, New Zealand, Switzerland, UK and USA). The data provided by the US include some from Caribbean locations as well as from US territories in the western Pacific. Limited amounts of temperature data from the special SHADOZ campaign of ozonesonde profiles have also been obtained for several stations, including some in tropical Africa and South America. The data are now being analysed to characterize aspects of the wave climatology in the lower stratosphere. Another project being coordinated by the SPARC GWPP initiative (in collaboration with SCOSTEP) is an international field experiment to be held in late 2001 in the Australian-Indonesian region. This Darwin Area Wave Experiment (DAWEX) will study the waves in the stratosphere and higher altitudes in relation to strong diurnal convection observed just north of Darwin, Australia.

Future Plans

The SPARC GWPP initiative in the future will:

  • Coordinate the continuing analysis of the global high-resolution data set. It is hoped that most of the high-resolution soundings used in this project will be archived at the SPARC Data Center,
  • Build on the DAWEX experience to organize an even more ambitious field experiment in 2003 or later. This Effects of Tropical Convection Experiment (ETCE) will examine the role of deep convection on the dynamical, chemical and microphysical aspects of the tropical upper troposphere and stratosphere.
  • Coordinate modelling work designed to formulate practical parameterisations of gravity wave effects in global climate models.
Potential energy (J kg-1)

Figure 2: Meridional and seasonal dependence of the potential energy associated with gravity waves in the lower stratosphere determined from the high-resolution data acquired and analysed by the scientists in the SPARC GWPP initiative. The contour labels are in J/kg

Specific Topics

Sensitivity to changes of the height and temperaure of the tropopause

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Sensitivity of the height and temperature of the tropopause to idealized changes in a general circulation model. The dark curves show the control case. The red curves show the impact of increased stratospheric planetary wave drag, which acts to raise and cool the tropopause in the tropics and to lower and warm it in the extratropics. The blue curves show the impact of a surface cooling, which acts to lower and cool the tropopause everywhere. The different character of the response to dynamical and radiative changes is a useful fingerprint for attribution of observed changes. [Courtesy of J. Thuburn, Reading Univ.]
Spectral cooling rates for the mid-latitude summer atmosphere
Including water vapour, carbon dioxide, and ozone

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Thermal infrared cooling rates for H2O, CO2 (355 ppm), and O3 as a function of wavenumber and pressure for midlatitude summer conditions. Colour scale x 10-3 is in units of K d-1 (cm-1)-1. The figure demonstrates the special nature of the UT/LS region in the atmosphere's radiative balance. The band centred at 667 cm-1 is dominated by CO2, and exhibits strong stratospheric cooling (explaining why a CO2 increase leads to stratospheric cooling) but a local warming at the tropopause. The band around 1043 cm-1 is dominated by O3 and exhibits warming throughout the lower stratosphere. Water vapour makes a strong contribution to upper tropospheric cooling near 300 cm-1, although its effects are moderated by CO2. In the UT/LS region the thermal infrared warming and cooling effects tend to largely cancel, implying a strong radiative sensitivity to greenhouse gas changes. [Courtesy of M. Iacono, AER.]
Ultrathin tropical tropopause cloud (UTTC) at 17 km altitude

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Ultrathin tropical tropopause cloud at 10°S and 17 km altitude, located about 300 m below the tropical tropopause, detected through backscatter ratio R at 1064 nm measured by OLEX on the German research aircraft Falcon during the APE-THESEO experiment in February 1999 (top panel). In situ measurements of backscatter ratio at 532 nm measured by the aerosol sonde MAS on board the Russian high-altitude research aircraft Geophysica, whose flight track is indicated by the white line in the top panel, are shown in the bottom panel. The cloud is only about 200-300 m thick and has an optical density of about 10-4, 300 times lower than that required for visibility from the ground. The low temperatures of the tropical tropopause region lead to the formation of cirrus clouds, which might be important for chemistry via heterogeneous reactions and possibly also for scavenging (e.g. of HNO3). They also provide useful constraints on dehydration mechanisms. These results suggest that subvisible cirrus clouds may be far more prevalent than has previously been imagined. [Courtesy of DLR/Oberpfaffenhofen, CNR/Rome, and the APE-THESEO community.]

SPARC/IGAC Activities

The chemistry of peroxy radicals

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Reference:
G. S. Tyndall, et al., The atmospheric chemistry of small organic peroxy radicals,
J. Geophys. Res., in press, 2001.
Ozone is a key trace constituent in the troposphere because it is a greenhouse gas, initiates chemistry that controls greenhouse gases, and is toxic to the biological system. The chemistry by which ozone is photochemically produced in the troposphere is inherently different from the chemistry that pervades ozone generation in the stratosphere. Peroxy radicals, produced by the oxidation of hydrocarbons in the presence of oxygen, play a central role in the photochemical production of ozone in the troposphere. Quantification of the ozone production rate and the abundance of ozone in the troposphere requires accurate information about the mechanism of chemical reactions of the peroxy radical and the rates of their processes in the atmosphere. [Courtesy of A.R. Ravishankara.]
Photochemistry of O3

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Reference:
Y. Matsumi, et al., Quantum yields for production of O(1D) in the ultraviolet photolysis of ozone : Recommendation based on evaluation of laboratory data,
J. Geophys. Res., submitted, 2001.
The photochemical decomposition of ozone to yield O(1D), the first electronically excited state of the oxygen atom, is a key process in the troposphere and the stratosphere. The reaction of O(1D) with H2O is the major sources of OH radicals, which are the essential species for the initiation of chemistry in the atmosphere. The reaction of O(1D) with N2O is the major source of nitrogen oxides in the stratosphere. Therefore, knowing how much O(1D) is produced in the photodissociation of ozone is essential. A panel of experts was collected as a part of the SPARC/IGAC joint activity to evaluate the quantum yield for O(1D), i.e., the number of O(1D) atoms produced for a photon absorbed by ozone, which led to a revision of the previous picture of ozone photolysis. The final obtained result shows that the quantum yield varies as a function of wavelength and temperature. [Courtesy of A.R. Ravishankara.]

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Last update: July 04, 2001