The potential climatic importance of the stratosphere has been recognised by the WMO by the initiation of the SPARC (Stratospheric Processes and their Role in Climate) programme (WMO 1993). The aims of SPARC are to develop a more complete understanding of stratospheric processes, with particular emphasis on their importance for the circulation and energy balance of the troposphere. One SPARC initiative,GRIPS (GCM-Reality Intercomparison for SPARC), will assess our capability of modelling the atmosphere from the surface to the stratopause using troposphere-stratosphere-mesosphere GCMs. GRIPS is co-ordinated by Dr. Pawson (FU Berlin, Germany) and Dr. Kodera (MRI, Tsukuba, Japan). In the EU-funded EuroGRIPS programme, four European members of the GRIPS community (FU Berlin, UKMO, CGAM and CNRM) have established a formal link to investigate several aspects of this problem.
GRIPS is split into two (or more) phases. In the first phase a set of intercomparisons will be undertaken. The results obtained and links established between the GRIPS participants during the first phase will allow the identification of a realistic set of numerical experiments (such as those in AMIP) which can be performed during the second phase of the project. The aims of AMIP are: (i) identification of systematic errors in atmospheric GCMs (AGCMs); (ii) improvements in model formulations. The first phase of GRIPS is well-advanced and, given the overlap between AMIP and GRIPS, it is natural to carry out numerical experiments which address the aims of both AMIP and GRIPS.
In this diagnostic subproject, we focus on the climatology of the stratosphere. Studies of the variability in the stratosphere (e.g. sudden warmings) will be carried out as well.
The question to be addressed by this subproject is: What elements are needed in AGCMs to provide an accurate representation of the stratosphere? The work in this subproject will build toward addressing the following questions: (1) How important is an accurate representation of the stratosphere in climate models? (2) What is the impact of changes in stratospheric composition on the climate?
Current climate models vary widely in the resolution and sophistication with which they represent the stratosphere. The impact of a realistically represented stratosphere (whether it be a higher upper boundary in climate models and/or higher resolution in the stratosphere) on medium range forecasts and climate simulations needs to be investigated, particularly in those models used for climate change. Boville (1984) showed how such influences can lead to significant changes in the modelled climate. Kodera (1993) showed how perturbations to the polar night jet can propagate downwards from the upper stratosphere to the troposphere throughout a winter. Further studies have isolated connections between the non-zonal circulation in the middle atmosphere with that in the troposphere. Recent statistical work (Baldwin et al. 1994, Perlwitz and Graf 1995) has provided evidence for a connection between the North Atlantic Oscillation in the troposphere and the strength of the stratospheric polar vortex; however, these statistical studies do not show the causal link between the stratosphere and the troposphere.
There are at least three possible advantages of including the stratosphere in climate models. First, by removing the upper boundary in the models away from the lower and middle stratosphere, possible adverse effects should be reduced. Secondly, resolving the middle atmosphere allows an improved representation of stratospheric processes; this is important for possible studies of the impact of stratospheric changes on tropospheric climate. Since stratospheric observations may provide early signals of climate change, an accurate representation of the stratosphere in climate models would be fundamental in understanding these signals. Thirdly, having an adequate representation of the stratospheric circulation in climate models allows a more physically-based treatment of important trace gases such as H2O, CH4 and N2O.
Future climate change will be influenced by trends in the concentration of stratospheric ozone and other radiatively active trace gases. Understanding the response of GCMs to modifications in the atmospheric composition is crucial to our ability to predict climate change due to anthropogenically induced variations in trace gas concentrations and aerosols. The importance of stratospheric processes is highlighted by the study of Santer et al. (1996), who found that the effects of CO2 and sulphate aerosols alone cannot explain the observed spatial distribution of the temperature trends; inclusion of a reduction of stratospheric ozone led to a better agreement between the modelled and observed temperature trends.
Before we can address the question of whether, in AGCMs, the stratosphere has an impact on the tropospheric climate, we must ascertain that these models have a realistic representation of the stratosphere. This subproject addresses this last issue.
Assess the simulation of the stratospheric circulation by AGCMs with a troposphere-stratosphere configuration.
The AGCMs developed at institutions participating in both AMIP-II and GRIPS and which have model levels extending to pressures less than 1 hPa (UKMO, NCAR, CNRM and perhaps others) will form the basis of the studies in this subproject. The models at UKMO (Cullen 1993) and NCAR (Boville 1995) have been developed for use in climate and middle atmosphere studies. These models provide an ideal test-bed for controlled simulations of the stratospheric circulation. (Note that both the UKMO and NCAR models will have AMIP runs with two different configurations: one focusing on the troposphere and a second configuration with additional levels extending through the stratosphere. These will be available for assessing the impact on the simulated tropospheric climate of using a higher upper boundary and improving the vertical resolution.)
The AMIP runs of the stratospheric configurations of the models will be compared with climatologies of the stratosphere produced from UKMO analyses, NCEP analyses and from the ERA dataset to evaluate the AGCMs' performance in reproducing the mean climate and variability of the stratosphere. The data requirements from Tables 1 and 6, and the extra stratospheric data requested, indicate the geophysical parameters which will be used in this subproject.
The stratospheric circulation in the models will be assessed in the following manner:
(2) Interannual variations of the monthly-mean data will be calculated from the model integrations, and compared with the equivalent fields from analysed data. The nature of the interannual variations will be compared, as well as whether the variations in the simulated data are correlated with the equivalent observed variations.
(3) The simulated planetary-wave structure (amplitudes and phases) will be assessed from the monthly-mean fields.
(4) Time series of monthly-mean fields will be constructed, in order to assess the simulation of variations on seasonal and longer time-scales. For example, a time series of the variations of winds over the equator will be used to compare the model simulations of the Semi-Annual and Quasi-Biennial Oscillations.
(5) Time series will be constructed from the high frequency model data to compare the model simulations of higher frequency phenomena. For example, a time series of 50 hPa temperature at the poles will be used to compare the model simulations of stratospheric warmings.
(6) The model simulations of low temperatures at 50 hPa will be compared, since these are important for the simulation of ozone depletion, because of the formation of polar stratospheric clouds (PSCs) at low temperatures. Time series will be produced of the minimum temperatures at middle to high latitudes. Time series will also be produced of the areas below particular threshold temperatures relevant to PSC formation.
(7) The high frequency fields will also be used to assess the model
simulations of such phenomena as stratospheric sudden warmings and vortex
mergers, by plotting synoptic maps for periods of particular dynamical
Standard AMIP output
Upper-air monthly mean (Table 1):
Supplementary output (high frequency 6-hourly) (Table 6):
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Boville, B. A., 1994: The influence of the polar night jet on the tropospheric circulation in a GCM. J. Atmos. Sci., 41, 1132-1142.
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