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Update

The SPARC Polar Stratospheric Cloud Assessment

The SPARC Polar Stratospheric Cloud Assessment (SPA) has kicked off with a chapter scoping meeting at the Coolfont (Spa) resort in West Virginia, USA from 12-13 May. The meeting was kindly hosted by our local organiser Mike Fromm of the US Naval Research Laboratory (NRL).

The aim of SPA is to assess our understanding of polar stratospheric clouds (PSCs). The motivation for the assessment is that there remain genuine gaps in our understanding of PSC distribution, formation and long term change that are important to stratospheric chemistry. Our understanding is patchy and specific rather than global and integrated and there has been a tendency to undertake detailed process studies rather than integrative studies. There is also no consensus on how to describe PSCs and denitrification in global models, which means we are unable to reliably predict changes that might occur in a future stratosphere. An important factor that limits our understanding is the lack of a large-scale consistent and evaluated dataset for model testing, the creation of which is one aim of SPA.

The purpose of the meeting was to agree on the organisation of the chapters and distribute writing tasks. We also managed to create an additional chapter on meteorological processes and agreed to produce a “Twenty Questions and Answers About PSCs” document for the stratospheric community, along the lines of David Fahey’s excellent WMO Ozone Assessment pamphlet. If written well, we believe that such a document will allow the stratospheric community to better comprehend the rather acronym burdened world of PSCs.

The chapters are as follows:

• 1.               PSC processes (Niels Larsen)
  This chapter will set the scene and also serve to pull the whole assessment together.
  
• 2.               Temperatures and meteorological diagnostics for PSC studies (Gloria Manney and Steve Eckermann) This chapter was seen as a very important addition to SPA and was created during the meeting. Much of the analysis of PSC occurrence and the interpretation of datasets in terms of microphysical quantities requires accurate meteorological analyses. We also know that mesoscale processes play an important role in forming solid PSCs. The aim of this chapter will be to assess how accurate the meteorological analyses need to be and what effect uncertainties have on our interpretation of PSC measurements.


• 3.               PSC detection and discrimination (Beiping Luo and Mike Fromm) The overall aim of this chapter is to define the range of PSC properties that can be detected by different instruments. Statements have previously been made about the frequency of occurrence of various cloud types that may have more to do with instrument detection thresholds than with actual PSC existence, particularly when it comes to the existence of a few large solid particles. This chapter will provide a consistent way of converting PSC signatures into ranges of microphysical properties, where possible.


• 4.               PSC observations and their interpretation (Terry Deshler and Lamont Poole) This is the chapter that most closely resembles a PSC climatology. Climatologies have been produced in the past. The aim here is to refine them to provide information about different types of PSC.


• 5.               Denitrification and dehydration observations (Michelle Santee and Gerald Nedoluha) The occurrence of denitrification and dehydration is closely related to that of PSCs. This chapter will mirror chapter 4 by examining the satellite and in situ record of denitrification and dehydration.


• 6.               Using models to assess our understanding of PSCs (Katja Drdla and Ken Carslaw) How good are the different PSC models that are used? Models range from those that include, as far as possible, all the microphysical processes to those that parametrise PSC processes for use in large scale models. This chapter will assess these models, including denitrification and dehydration, for a few well defined cases based on the data from chapters 4 and 5. An important aspect will be the extent to which laboratory measurements of PSC formation, models and field observations are consistent.


• 7.               PSCs in a changing stratosphere (Markus Rex and Richard Bevilacqua) The aim of this chapter is to understand what controls long term changes and interannual variability in PSCs. It will also explore simple ways for global models such as CCMs to test whether they have the right physics to correctly predict long term changes that are relevant to ozone depletion studies.

SPA has ambitious aims that go beyond a straightforward review of understanding. A 10 month period was felt to be essential for all participants to make substantial progress, particularly in cases where large datasets need to be assembled and analysed. Our next meeting is planned for early spring 2006 in Europe.

Co-chairs: Ken Carslaw, University of Leeds, UK (carslaw@env.leeds.ac.uk)
Katja Drdla, Ames Research Centre, USA (Katja.Drdla-1@nasa.gov)

Meeting participants: Chapter lead authors (Niels Larsen, Gloria Manney, Terry Deshler, Lamont Poole, Beiping Luo, Mike Fromm, Michelle Santee, Gerald Nedoluha, Richard Bevilacqua) and coauthors (Harald Flentje, John Remedios, Christiane Voigt, Hideaki Nakajima, Jerome Alfred, Tony Strawa)

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