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Stratospheric Processes And their Role in Climate
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| Home | Initiatives | Organisation | Publications | Meetings | Acronyms and Abbreviations | Useful Links |
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Stratospheric Processes And their Role in Climate
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| Home | Initiatives | Organisation | Publications | Meetings | Acronyms and Abbreviations | Useful Links |
The GRIPS initiative (GCM-Reality Intercomparison Project for SPARC) aims at evaluation of the performance of GCMs that can resolve the stratospheric circulation. The initiative is open to all, but a model must include a realistic representation of the hydrological cycle and a full radiation scheme, so that it can be used as a fully interactive climate model. The workshop in Victoria provided the first opportunity to formally discuss the project structure/tasks. Of the 14 participating groups, 12 were represented. Expenses were covered by the Canadian AES, the WCRP and participating institutions.
The objectives were:
- For each group to make a presentation on their model, to bring standard documentation about their models, detailing the numerical, physical, and (where appropriate) chemical aspects of their code. A list of standard diagnostics were also requested in a form that should facilitate the intercomparison of the simulations.
- To discuss the GRIPS structure/objectives (a plan for 1-3 years and ideas for the longer term).
- To consider how GRIPS could liaise with other intercomparisons, particularly AMIP (Peter Gleckler represented AMIP at the workshop).
S. Pawson spoke on the objectives of GRIPS and how it may fit with other model intercomparison projects. G. Boer outlined his experience of model intercomparisons during the last two decades pointing out possible pitfalls as well as the successes.
Of the participating models, some have been reported in the literature for several years (e.g., NCAR CCM [B. Boville], MRI/LRF [M. Chiba], UCLA [J. Farrara], GFDL SKYHI [J. Wilson]); for a few models the first results are just becoming available (e.g., the Canadian Middle Atmosphere Model (MAM) [S. Beagley], the UKMO model [R. Swinbank]), while others are of intermediate age. The location of the upper boundary varies between the models: both the UCLA and the MRI/CL [K. Kodera] models extend to 1 hPa, the highest tops being at 0.001 hPa in the Canadian MAM and the UGAMP model [J. Thuburn]. The treatment of physical processes differs considerably. The tropospheric physics varies in complexity, generally being more comprehensive in those models developed recently from tropospheric climate models, such as the FUB [U. Langematz] and MPI [E. Manzini] models. The differing histories of the models are also shown by their applications: two have been actively used for the problem of stratospheric data assimilation, at NASA-GSFC [L. Coy] and the UKMO, while others applied extensively for climate change research (notably the NASA-GISS model [N. Balachandran]). In most models the trace gas distributions are specified, but four of them -MAM, UCLA, NCAR [B. Boville], and CNRM [P. Simon] models- include the option to transport ozone and other reactive trace gases and include chemistry modules of differing complexity. Chemistry-transport packages are being actively developed by several other groups. P. Gleckler summing up the AMIP status highlighted technical problems associated with such projects, and the need to tightly define any experiments and to ensure good quality control for all data was emphasised. Stratospheric aspects of the AMIP simulations (J. Farrara representing R. Mechoso) were presented (few of the AMIP models extend far into the middle atmosphere). L. Gates, the leader of the PCMDI where AMIP is coordinated, offered any help to GRIPS (storing data at PCMDI and the use of their comprehensive analysis and graphics packages). P. Merilees (AES) spoke about the Canadian middle atmosphere data assimilation initiative (modelling and observation). Some observational data available for model validation were discussed by S. Pawson (radiosonde, satellite), R. Swinbank (UKMO assimilations) and L. Coy (GSFC assimilations). K. Kodera studied recently the statistical links between stratosphere and troposphere in three different GCMs; even with three models the results showed considerable variations (this type of study is an excellent example of how intercomparisons of the model output can be made and has direct relevance to the central themes of SPARC).
Much discussion was devoted to the first stage of the project, when existing runs should be analysed. The idea is to provide some overview of the current state of modelling to report to the SPARC SSG (which would then be used as a basis for future intercomparisons including dedicated integrations).
Task1.'Documentation of the models' is to ensure that all models are documented to the same standard (that produced by Phillips, 1995, for the AMIP models, is a level to aim at): numerical and physical aspects of the models, boundary conditions used in the integrations, and the storage of data. It should include references to published work and also allow for easy updates when the model is changed (while retaining the ability to cross-check the older integrations).
Task 2.'Basic Climatology', is a documentation of the basic performance of the models. The quantities to be compared are (for 12 months, 10 years, at standard levels): monthly means of u, T, Z, eddy fluxes of heat and momentum, meridional/vertical circulation, forcing for u and potential temperature, tracers (O3, H2O) and clouds, zonal wind at 60deg.N/S and TNP/SP. These ought to be provided to FUB as ASCII files or SUN/UNIX-compatible binary files, along with a data description and routines to read them. An alternative form is input files for the GRADS plotting, along with the 'ctl' files. These data would only be used for an intercomparison of the models; any publications which may use these data would include all contributors as co-authors. Groups are requested to provide data for each month of 10 years of their integration, or for the longest period possible. The levels are: 1000, 850, 700, 500, 300, 200, 100, 70, 50, 30, 20, 10, 5, 3, 2, 1, 0.5, 0.3, 0.2, 0.1, 0.05, 0.03, 0.02, 0.01 hPa (some flexibility allowed).
'Optional' tasks are: Task 3 (K. Kodera) 'Statistical connections between the stratosphere and troposphere' can be performed using the zonal-mean fields and the monthly-mean temperature and height fields.
Task 4 'Stratospheric warmings' (synoptic evolution) is investigated individually for all models, with discussion at the next workshop.
Tasks 5-8 are pilot studies with a small number of models. [5-'Travelling waves and tides' (J. Thuburn), 6-'Tropical oscillations and waves' (J. Wilson), 7-'Strat-trop exchange' (P. Simon), 8-'Spatial wavenumber spectra' (J. Koshyk)].
These involve some specific experiments within 1-2 years (some off-line tests of the radiative transfer schemes, diagnostics of GW drag, some GCM integrations).
5.1. Radiation schemes. Many (but not all) schemes were validated in the WCRP/ICRCCM (Intercomparison of Radiation Codes for Climate Models); further, the tests were for five profiles and concentrated on the troposphere. It is thus meaningful to perform some additional tests for the middle atmosphere:
- Zonal-mean (latitude-height) sections of radiative heating would be calculated using each radiation scheme, with identical temperature and trace gas distributions, surface albedo and emissivity, and incoming solar radiation. The calculations would first be performed for January, then for other months if motivated (U. Langematz coordinator).
- The possibility of obtaining reference line-by-line calculations for the same input data should be investigated. This would serve as a reference for the current study but also a unique opportunity to define a set of reference heating rates for the middle atmosphere.
5.2 GW drag.GRIPS should coordinate activities with the SPARC GWPP Group. Also, two possible aspects of GRIPS concern the distribution of the drag in the atmosphere:
- From the assimilated data it may be possible to determine the required sub-gridscale drag by examining the 'correction' to the model fields introduced at the assimilation timesteps. L. Coy would investigate this using the DAO assimilated data.
- From the zonal net heating rates determined in the radiation calculations it is possible (often after some correction of the global thermal balance) to determine the diabatic circulation and thereby infer the total eddy forcing of the mean flow (see Shine, 1989, and others). If the large-scale wave drag can be determined from the eddy fields, some estimate of the drag can be made for the different radiation schemes. This approach could place some bounds on the estimates of GW drag in the upper stratosphere and mesosphere.
5.3. GCM integrations. A set of dedicated experiments for GRIPS should be performed to give a directly comparable series of integrations with all the models, but also so that all participants obtain some useful results for their own work. The model integration should be for one winter season in the northern hemisphere. All model runs should use the same: incoming solar radiation, prescribed CO2 concentration, zonal-mean O3, prescribed stratospheric H2O, and SST and sea-ice extents: then the boundary conditions and prescribed trace gases should not affect the results (also desirably the same initial conditions). All groups should then perform two integrations:
- The first should include no parameterisation of sub-gridscale drag on the zonal mean state but some artificial dissipation of waves at the upper model levels is permitted (to prevent spurious reflections from the upper boundary).
- The second is identical to the first, but an additional forcing is added at the top levels as a crude representation of sub-gridscale drag. All other forcing terms would be calculated interactively, using the usual model formulation. A prescribed body force should be applied imposed above 1 hPa in the models and tuned so that all models impose the same net force per unit mass, although the vertical distribution would be flexible to accommodate the different upper boundaries and positioning of the model levels.
These model integrations would be compared quite thoroughly. Some groups may perform a third integration, using their usual formulation of sub-gridscale drag.
One of possible directions for GRIPS would be to perform long integrations for intercomparison; a close liaison with the AMIP subproject directed towards the stratosphere may be possible.
One central issue may be the radiative forcing due to stratospheric trace gases, and the impact of ozone change (with potential coupling with dynamics) on climate. Important here is the ability of the GCMs to adequately transport the trace gases and the success of the chemical components of the models (e.g., the NCAR CCM recent results concerning transport show apparently severe numerical deficiencies in the formulation of the vertical advection). Chemical modules for the GCMs are in various stages of development. It is anticipated that GRIPS would eventually coordinate some aspects of modelling the stratospheric chemistry-climate feedbacks. Close liaisons with the SPARC initiatives for 'middle atmospheric water vapour' and 'UT/LS chemistry' are anticipated, with GRIPS concentrating on modelling aspects, possibly including long-term prediction and making attempts to put error bars on the certainty of the models.
GRIPS should attempt to initiate a comprehensive home page system in Berlin, where all information would be available (documentation, reports). An attempt would be made to introduce a bulletin board for up to date discussion.
The second GRIPS workshop (a small, closed meeting) will be held in Berlin in the spring of 1997.
CMAM, Canada (S. Beagley); CGAM, UK. (W. Lahoz); CNRM, France (P. Simon); FUB, Germany (U. Langematz); GFDL, USA (K. Hamilton); GISS, USA (D. Rind); GSFC, USA (L. Coy); JMA, Japan (T. Iwasaki); LaRC, USA (W. Grose); MPI, Germany (E. Manzini); MRI/LRF, Japan (M. Chiba); MRI/CL, Japan (A. Kitoh); NCAR, USA (B. Boville); UCLA, USA (R. Mechoso); UKMO, UK. (R. Swinbank).
Steven Pawson
The Stratospheric Temperature Trends Assessment (STTA) group met at Berlin (Free Univ.) on December 18-19, 1995 to discuss further developments regarding this SPARC project. The meeting focused on two principal subjects: the data and time series, and trend determination strategies and analyses. Data has now been made available by 11 different groups. Not all the datasets are strictly independent. The datasets include those derived from instrumental measurements (e.g., satellite, radiosonde, lidar), and those obtained using, in addition, an analysis technique involving a model. In general, the time series from the different datasets show a reasonable coherence of the temperature anomalies, especially in the lower stratosphere. A deadline of July 1996 was established for receipt of additional new datasets that the STTA will consider in the present phase of the project. (Since the Berlin meeting, some data from the Russian radiosonde stations has been received).
Three different time periods were selected on a preliminary basis to examine the trends: 1979-1994, 1965-1994 and 1965-1979. The trends will be analysed in 10deg. latitude belts and at five height levels ranging from the lower to the upper stratosphere. Since different trend methods have been used in the literature in the past, it was decided that tests be conducted to identify differences between the various techniques. It was expected that an optimal technique will eventually be chosen for the project's objectives. Some preliminary determination of the trends and their significance using one of the techniques has been made. The possible influences of the QBO, volcanic aerosols, solar variations, and dynamical variability on the trends, and the need to consider these factors explicitly in the understanding of the trends, was reiterated.
Fixed Dynamical Heating (FDH) and General Circulation Model (GCM) simulations will be utilised in analysing the possible causes of the trends. Simulations include the stratospheric temperature changes expected due to increases in the well-mixed greenhouse gases, and due to the global decreases in ozone as reported by various satellite measurements. In the case of ozone, estimates will be made of the uncertainties in the calculated effects due to uncertainties in the vertical profile of the ozone loss. In this regard, further calculations of temperature change in the stratosphere using the various ozone loss datasets were reported, and the relative roles of changes in different trace gases on the long-term trends were discussed. Models will also be employed to assess the extent of dynamical variability, which could prove to be a decisive factor in interpreting high latitude temperature trends. Discussions regarding the structure and contents of the report to be prepared by the group were also initiated.
Vincent Ramaswamy
The upper troposphere and the lower stratosphere are regions of the atmosphere where it is difficult to separate, in time scale, the chemistry and atmospheric motions. This is also the coldest region of the lower atmosphere, the pressures are still large, and condensable species are significant. Heterogeneous chemistry plays a critical role in this region. In addition, this is the region where the chemistry of ozone is least well understood but has the most impact in terms of climate. The current concerns are losses of ozone in the lower stratosphere and increases in the upper troposphere, both of which have important climate effects. Understanding the UT/LS region is also of immediate practical concern for issues such as the effects of subsonic and supersonic aircraft on the atmosphere.
Therefore, the SPARC Group on UT/LS will focus its efforts on better understanding of the chemistry in this region and facilitating inclusion of the chemistry in numerical models. The group will also bring the unique conditions of this region to the attention of laboratory chemists who need to make measurements under temperature and pressure conditions that are not normally used in the laboratory. Close interactions with the active field measurements community will be fostered. To this end, a working relation ship with IGAC will be developed.
Composition of the SPARC group on UT/LS:
Co-Chairs: I. Isaksen (Norway) ivar.isaksen@geofysikk.uio.no, A. R. Ravishankara (USA) ravi@al.noaa.gov
Current Members: J. McConnell (Canada), M. Chipperfield (UK), S. Solomon (USA), A. Thompson (USA), A. Wahner (Germany).
During the last years there were many significant findings by individual PIs about the LS/UT (it clearly shows that our field is very active and good work is being done). Yet, it is important to bring to focus these activities and also foster free exchange of information between scientists who are from highly varied disciplines. These are the two main interests of the group.
Near-Term Plans
It has been identified that one of the primary areas of uncertainty in dealing with UT/LS is the treatment of heterogeneous processes in numerical models. Laboratory data on heterogeneous processes have to be parameterised in a physically sound basis into forms that are easy to use in the models. Further, the growth in laboratory information is fast but its transfer to modelers is slow. Similarly, information from modelers is slow in reaching the laboratory scientists. To facilitate the transfer and explore the approaches that will be most fruitful in including heterogeneous and homogeneous processes in the models, the group is planning two workshops: (1) How to include heterogeneous processes in numerical models and (2) What are the kinetics and photochemical parameters that most affect UT/LS and how to obtain that information.
The first workshop will be held in Strasbourg, France during October 21-23, 1996 (close to the European Symposium on effects of aircraft, which will be held in Paris the week before) The local host has been identified (Prof. P. Mirabel) (see the announcement of the workshop in this newsletter). The workshop will include presentation of the latest results from laboratory scientists, field measurements scientists, and numerical modelers. A format for parameterisation, where heterogeneous reaction rates can be expressed as first order rate coefficients for removal from the gas phase, will be explored. The findings of the meeting will be documented as a report. The second workshop is still in its formative stages. It will most likely be held in Boulder CO during the early part of 1997. In addition to the workshops, the group will expand the involvement of the very large group of atmospheric scientists who are making field measurements in UT/LS.
A. R. Ravishankara