D. Cariolle

The ARPEGE/IFS atmosphere model: a contribution to the French community climate modelling

A new atmospheric model has been developed jointly by Météo-France, and the European Centre for Medium-range Weather Forecasts (ECMWF) under the acronyms ARPEGE (action de recherche petite echelle grande echelle, which means research project on small and large scales) and IFS (integrated forecast system). This model includes, inter alia, an atmospheric general circulation model (GCM) which is intended by the French climate modelling community to be used for studying the anthropogenic climate impact. A preliminary version of this model has been available since 1992. This paper describes its main characteristics. Three 10-year integrations of this model having spectral horizontal resolutions of T21, T42, and T79 have been performed using prescribed monthly mean sea surface temperatures (SST) observed from 1979 until 1988. The results of these integrations are presented and compared with the observed climatology. The comparison is made for the winter (DJF) and summer (JJA) periods. It is shown that the model is capable of reproducing the observed climatology in a generally successful manner.

A three-dimensional modeling study of trace species in the Arctic lower stratosphere during winter 1989–1990

A three-dimensional (3D) radiative-dynamical-chemical model has been developed and used to study the evolution of trace gases in the Arctic lower stratosphere during winter 1989–1990. A series of 10-day model integrations were performed throughout this period. The model includes a comprehensive scheme of gas phase chemical reactions as well as a parameterization of heterogeneous reactions occurring on polar stratospheric cloud (PSC) surfaces. An important element of a 3D chemical model is the transport scheme. In this study the transport of chemical species is achieved by a non diffusive method well suited to the preservation of sharp gradients. During the winter studied temperatures were cold enough for the formation of both type I and type II polar stratospheric clouds from early December to early February. Model simulations in late December show that inside the polar vortex air is rapidly processed by polar stratospheric clouds converting HCl and ClONO2 to active chlorine. The possibility of ozone destruction depends strongly on the amount of sunlight. In early February an average ozone loss of 15 ppbv (parts per billion by volume) /day is predicted in PSC-processed air at 50 hPa, giving a column loss of just under 1 DU/day. This loss increases to 25 ppbv/day if PSCs persist until March with a column loss of around 1.5 DU/day. The relatively small magnitude of the ozone loss predicted in the model, compared to the variability of ozone induced by dynamics, highlights the problems in identifying the signature of chemical ozone loss in the Arctic. In future years significant ozone depletion could occur if PSCs persist until late March. The efficiency of the catalytic cycles responsible for the ozone loss has been analyzed as a function of latitude, altitude and time. In general, the cycle involving ClO + ClO is the dominant loss mechanism in the polar lower stratosphere. Cycles involving BrO can make a relatively large contribution early in the season and when the levels of ClO are low. The cycle initiated by ClO + O destroys ozone at altitudes above 30 hPa but the loss is compensated, to some extent, by in situ ozone production. The results for trace species are validated, where possible, by comparison with the available measurements, although the sparse nature of the observations does not effectively constrain the model.

Response of the Météo-France climate model to changes in CO2 and sea surface temperature

The climate response to an increase in carbon dioxide and sea surface temperatures is examined using the Météo-France climate model. This model has a high vertical resolution in the stratosphere and predicts the evolution of the ozone mixing ratio. This quantity is fully interactive with radiation and photochemical production and loss rates are accounted for. Results from a 5-year control run indicate a reasonable agreement with observed climatologies. A 5-year simulation is performed with a doubled CO2 concentration using, as lower boundary conditions, mean surface temperatures anomalies and sea ice limits predicted for the years 56–65 of a 100-year transient simulation performed at Hamburg with a global coupled atmosphere-ocean model. The perturbed simulation produces a global mean surface air warming of 1.4 K and an increase in global mean precipitation rate of 4%. Outside the high latitudes in the Northern Hemisphere, the model simulates a strong cooling in the stratosphere reaching 10 K near the stratopause. Temperature increases are noticed in the lower polar stratosphere of the Northern Hemisphere caused by an intensification in the frequency of sudden warmings in the perturbed simulation. The low and mid-latitude stratospheric cooling leads to an ozone column enhancement of about 5%. Other features present in similar studies are exhibited in the troposphere such as the stronger surface warming over polar regions of the Northern Hemisphere, the summer time soil moisture drying in mid-latitudes and the increase in high convective cloudiness in tropical regions.

Filamentation and layering of an idealized tracer by observed winds in the lower stratosphere

The isentropic transport of a passive tracer on synoptic time scales in the winter lower stratosphere is modelled with the use of a high-resolution transport model, which is forced by winds derived from global meteorological analyses. The study has focused on a meteorological situation which occured in late January 1992. Repeated poleward intrusions of mid-latitude air are shown to lead to the filamentation of a tracer distribution, which was initially compact and located inside the polar vortex. The effect of wind resolution on the filamentation process is examined. By performing isentropic advection on many closely spaced independent levels, the vertical structure of these filaments can be studied.

Coupled Atmosphere‐Wildland Fire Modelling

A tight interaction exists between the development of a wildfire and the local meteorology near the front. The convective effects induced by the fire heat release can modify the local wind circulation and consequently affect the fire propagation. In this study we use a meso-scale numerical model in a Large Eddy Simulation (LES) configuration coupled to a simplified physical front tracking wildfire model to investigate the differences induced by the atmospheric feedback in propagation speed and behaviour. Simulations of typical experimental configurations show a good response of the coupled fire-atmospheric model. Numerical results matches qualitatively observed values for fire induced winds and convection. Both numerical models already have operational usage and might ultimately be run to support decisions in wildfire management.

Simulation des changements climatiques au cours du XXIe siècle incluant l'ozone stratosphérique

Abstract Two climate simulations of 150 years, performed with a coupled ocean/sea-ice/atmosphere model including stratospheric ozone, respectively with and without heterogeneous chemistry, simulate the tropospheric warming associated with an increase of the greenhouse effect of carbon dioxide and other trace gases since 1950 and their impact on sea–ice extent, as well as the stratospheric cooling and its impact on ozone concentration. The scenario with heterogeneous chemistry reproduces the formation of the ozone hole over the South Pole from the 1970s and its deepening until the present time, and shows that the ozone hole should progressively fill during the coming decades. To cite this article: J.-F. Royer et al., C. R. Geoscience 334 (2002) 147–154.

A 3D transport model study of chlorine activation during EASOE

A 3D chemical transport model (CTM) has been used to study the behaviour of stratospheric trace species during the EASOE campaign. The model contains a comprehensive gas phase chemistry scheme as well as a treatment of heterogeneous reactions on PSCs and sulphate aerosols. The CTM is forced using the ECMWF analyses providing realistic meteorological conditions throughout the model simulations. Experiments have been performed to examine the evolution of chlorine species throughout the winter and to estimate the magnitude of chemical ozone loss. Heterogeneous reactions on PSCs lead to large (over 1.5ppbv) abundances of active chlorine in the model polar lower stratosphere in early January. The level of active chlorine then decreases from mid January as PSCs become less frequent. In the model PSCs are more efficient at activating chlorine than the sulphate aerosols although the latter cause extensive denoxification which maintains the high ClOx levels after processing. Despite the high abundances of ClO and BrO the model results show that the opportunity for chemical ozone destruction during this time was limited by the lack of sunlight.

Ridge formation in the lower stratosphere and its influence on ozone transport: A general circulation model study during late January 1992

Satellite total ozone measurements showed the development of an ozone mini-hole over northern Europe in late January 1992, during the observational phase of the European Arctic Stratospheric Ozone Experiment (EASOE). During the same period, ozone profiles, recorded with ozonesondes, showed that layers below 20 km were strongly depleted in ozone at some locations over northern and central Europe. The perturbed chemistry involving the reactive species, which play a role in the ozone-destroying catalytic cycles, was the focus of intensive observational investigations during the EASOE campaign. These processes are likely to take place between 15 and 30 km, and the question of mixing between vortex air and midlatitude air in the lower stratosphere, especially at the base of the polar vortex, is a central one. High-resolution (T106) 7-day forecast simulations have been performed with a GCM, in which the ozone field was realistically initialized, in order to study the formation and evolution of the ozone mini-hole, and the nature of large-scale mixing in the lower stratosphere. In particular, we tried to examine whether the model could reproduce with some degree of realism ozone tongues seen in the satellite data. The study has revealed that the formation of the mini-hole was linked to the poleward extension in the lower stratosphere of an anticyclonic ridge. The effect of tropospheric forcing was evident up to at least 40 hPa. Ozone-poor air from subtropical latitudes was advected toward Scandinavia at the same time as a tongue of polar air extended northeasterly toward central Europe. During the course of the 1-week simulation, the modeled ozone mixing ratio and potential vorticity (PV) revealed strong large-scale isentropic mixing between high, middle, and low latitudes in the lower stratosphere. This mixing may occur through either the formation of narrow ozone/PV tongues, or of more vortex-like blobs of ozone/PV also seen to be peeled off from the vortex. There is good correspondence with the structures seen in the satellite-derived total ozone field at medium and synoptic scales.

Sensitivity to prescribed changes in sea surface temperature and sea ice in doubled carbon dioxide experiments

Time sclice experiments are performed with the atmospheric GCM ARPEGE, developed at Météo-France, to study the impact to increases in the atmospheric carbon dioxide. This spectral model runs at T42 horizontal resolution with 30 vertical layers including a comprehensive tropospheric and stratospheric resolution and a prognostic parameterization of the ozone mixing ratio. The model is forced in a 5-year control run by climatological SSTs and sea-ice extents in order to obtain an accurate simulation of the present-day climate. Two perturbed runs are performed using SSTs and sea-ice extents for doubled CO2 concentration, obtained from transient runs performed by two coupled atmospheric-oceanic models run at the Max Planck Institute (MPI) in Hamburg and the Hadley Centre (HC). A global surface temperature warming of 1.6 K is obtained with the MPI SST anomalies and 1.9 K with the HC SST anomalies. The precipitation rate increases by 4.2% (and 4.7%). The features obtained in the stratosphere (a cooling increasing with the altitude and an increase in the ozone mixing ratio) are not sensitive to the oceanic forcing. On the contrary, the anomalies in the troposphere such as a warming increasing with altitude, an acceleration of westerly jets and a raised cloud height, depend on the oceanic forcing imposed in the two perturbed runs. Special attention is given to continental areas where the impact of the oceanic forcing is studied over eight regions around the globe. Regions sensitive to oceanic forcing such as Europe are identified in contrast with areas where the patterns are driven by land-surface physical processes, such as over continental Asia. Finally, the Köppen classification is applied to the climate simulated in the three experiments. Both doubled CO2 runs show the same predominance of global warming over precipitation changes in the Kbppen analyses.

Total ozone from the TIROS operational vertical sounder during the formation of the 1987 “ozone hole”

Total ozone maps obtained from the infrared radiances measured by the TOVS/HIRS2 instrument on board the NOAA 10 satellite are used to study the formation of the 1987 Antarctic “ozone hole.”We use in this study an improved version of the retrieval algorithm described by Muller and Cayla (1983), now calibrated for middle and high latitudes and taking into account the unusually depleted ozone profiles of the Antarctic spring. Error analysis indicates that our method has an accuracy of the order of 5–7% in clear sky conditions. Values determined from the TIROS operational vertical sounder (TOVS) are in good agreement with Dobson measurements in the mid-latitudes and with the ozonesondes launched from the Antarctic stations during the Airborne Antarctic Ozone Experiment (AAOE). The agreement with the total ozone mapping spectrometer (TOMS) data at mid-latitudes is also good, but significant differences are found in early September in the high latitudes. In particular, the large-scale zonally symmetric minimum representative of the ozone hole appears later in the TOVS maps than in the TOMS data. The ozone hole was already apparent in the TOMS map on the first days of September, while TOVS detected only localized ozone deep minima associated with optically thick polar stratospheric clouds (PSCs) and did not observe any circular depletion structure until September 17. This discrepancy seems to be the consequence of high solar zenith angles and climatological errors in the TOMS algorithm, which tends to underestimate the ozone content in late winter. It is only in mid-September that TOVS data show a rapid ozone decrease affecting the whole vortex. The low ozone amounts are first recorded in the vicinity of the PSCs detected in the ozone field and then spread into the vortex. TOVS observations suggest that a rapid ozone decrease might take place during or just after the formation of major water ice PSCs. Since the vortex is exposed to UV sunlight in mid-September, this could be the direct consequence of both a sudden increase of free chlorine and an efficient denitrification occurring during type 2 PSC events. It is concluded that since the algorithm presented in this paper allows reliable ozone determinations in middle and high latitudes and accurate type 2 PSC detection, measurements from TOVS could play an important role in the ozone layer monitoring, especially in the wintertime polar regions where UV techniques are ineffective or affected by the lack of intense sunlight.