Alain Hauchecorne

Climatology and trends of the middle atmospheric temperature (33-87 KM) as seen by Rayleigh lidar over the south of France

The technique of the Rayleigh lidar provides temperature profiles with a good temporal and vertical resolution in the middle atmosphere. Data obtained by 2 Rayleigh lidars set up at the Observatory of Haute-Provence (44°N, 6°E) and at Biscarrosse (44°N, 1°W) from 1978 to 1989 led to a unique set of data of 1200 night-mean temperature profiles from 37 to 87 km. A climatology of the temperature over the south of France has been established from this data base and new results concerning both the short-term and long-term variability of the middle atmosphere are presented in this paper. The observed temperature around 44°N, 0°W is in general colder than the mean temperature given in the COSPAR International Reference Atmosphere (1986) near 75 km and warmer above 80 km. A clear semiannual variation is observed near 65 km with maxima occurring just after the equinoxes. The study of the short-term variability indicates clearly two frequency domains: long periods below 65 km with a maximum in December–January which relate to planetary waves and shorter periods above 65 km induced by the breaking of gravity waves. A correlation with the 11-year solar cycle, negative in the stratosphere and positive in the mesosphere, is found to be well above the 95% confidence level with an amplitude larger in winter than in summer. However, with only 11 years of data it is obviously difficult to conclude that this atmospheric perturbation is induced by the solar forcing. A cooling of the mesosphere of about −4 K/decade at 60–70 km is found but is at the limit of the 95% confidence level, whereas we do not observe any significant trend in the stratosphere.

Gravity waves in the middle atmosphere observed by Rayleigh lidar: 2. Climatology

A large data set obtained by Rayleigh lidars during 4 years has been analyzed in order to describe the gravity wave climatology in the 30- to 75-km altitude range at mid-latitude. The lidar data were collected in two sites different with respect to orography, both located in the south of France (44°N). The seasonal variability of the wave activity, the vertical growth of potential energy density per unit mass, and the power spectral density versus vertical wave number are shown. The seasonal variability of the wave activity is found to be mainly annual, the maximum of activity occurring during winter. A semiannual component, with a secondary maximum in summer, is superposed to the annual cycle above 60-km altitude. The power spectral density increases from the stratosphere to the mesosphere in the entire spectral range. A significant positive correlation is found between the wave activity and the wind intensity in the stratosphere. Finally, some simple hypotheses, in terms of wave sources and wave transmission, are advanced in order to get an insight into the causes of the observed seasonal and geographical variability of the wave activity.

GOMOS on Envisat: an overview

Abstract GOMOS (Global Ozone Monitoring by Occultation of Stars) on board Envisat measures O 3 , NO 2 , NO 3 , neutral density, aerosols, H 2 O, and O 2 , in the stratosphere and mesosphere by detecting absorption of starlight in ultraviolet, visible and near-infrared wavelengths. During bright limb conditions GOMOS will also observe scattered solar radiation. GOMOS will deliver ozone concentration profiles at altitudes 15–100 km with a vertical sampling better than 1.7 km and with a global coverage. As a self-calibrating method stellar occultation measurements provide a basis for a long-term global monitoring of ozone profiles. We will present here the status of the GOMOS instrument and show samples of first results obtained in 2002.

A warm layer in Venus' cryosphere and high-altitude measurements of HF, HCl, H2O and HDO

Venus has thick clouds of H2SO4 aerosol particles extending from altitudes of 40 to 60 km. The 60–100 km region (the mesosphere) is a transition region between the 4 day retrograde superrotation at the top of the thick clouds and the solar–antisolar circulation in the thermosphere (above 100 km), which has upwelling over the subsolar point and transport to the nightside. The mesosphere has a light haze of variable optical thickness, with CO, SO2, HCl, HF, H2O and HDO as the most important minor gaseous constituents, but the vertical distribution of the haze and molecules is poorly known because previous descent probes began their measurements at or below 60 km. Here we report the detection of an extensive layer of warm air at altitudes 90–120 km on the night side that we interpret as the result of adiabatic heating during air subsidence. Such a strong temperature inversion was not expected, because the night side of Venus was otherwise so cold that it was named the ‘cryosphere’ above 100 km. We also measured the mesospheric distributions of HF, HCl, H2O and HDO. HCl is less abundant than reported 40 years ago. HDO/H2O is enhanced by a factor of ∼2.5 with respect to the lower atmosphere, and there is a general depletion of H2O around 80–90 km for which we have no explanation.

Midlatitude long-term variability of the middle atmosphere : trends and cyclic and episodic changes

A study of variability and long-term trend detection in the middle atmosphere is performed using the temperature database obtained since 1979 by French Rayleigh lidars at 44°N. A multivariable analysis is used to consider natural variability, including solar activity, quasi-biennial oscillation (QBO), and volcanic eruption effects. As the solar proxy has an important role in this analysis, its choice is first discussed. Changes induced by solar activity reported here, mainly large negative correlation observed in the winter upper stratosphere, are not reproduced by numerical models without an unrealistic solar UV change. A significant warming of 6 K in the mesosphere is observed in summer 1992 and 1993. This large change could be due to aerosols injected in the stratosphere by the Mount Pinatubo eruption. Observed volcanic aerosol effects are compared with some numerical simulations and are shown to present some agreements. A long-term cooling of 0.4 K/year is observed in the mesosphere that is statistically significant but is larger than anthropogenic effects simulated in numerical models. In the upper stratosphere the small trend observed is not yet significant but in better agreement with model predictions.

Infrasound monitoring for atmospheric studies

The infrasound field, the science of low-frequency acoustic waves, has developed into a broad interdisciplinary field encompassing academic disciplines of physics and recent technical and scientific developments. The infrasound network of the International Monitoring Network (IMS) of the CTBT-Organization has demonstrated its capability for detecting and locating infrasonic sources such as meteorites, volcanic eruptions, earthquakes, auroras, mountain associated waves... Nearly 70% of the global network is now operational and regional cluster arrays are deployed around the globe. Systematic investigations into low-frequency acoustic signals have evidenced an unprecedented potential of the monitoring of infrasonic waves permanently generated by natural and man-made events. Furthermore, recent studies point out new insights on quantitative relationships between observables and atmospheric specifications, and therefore opening new fields into the mathematics of geophysical inverse problems for atmospheric remote sensing. This volume reviews the most important areas of infrasound, with emphasis on the latest researches and applications, e.g. instrumentation, engineering, signal processing, source monitoring, propagation modeling, atmospheric dynamics, global changes, remote sensing methods. Researchers and students will benefit from a comprehensive content of infrasound related topics, where both fundamental and applied topics are discussed by authors from international institutions, all experts in their fields.

A Critical Review of the Database Acquired for the Long-Term Surveillance of the Middle Atmosphere by the French Rayleigh Lidars

Abstract The database obtained by Rayleigh lidars over the south of France is now used for monitoring the middle-atmosphere structure and to validate satellite data. For these reasons it is crucial to ensure the quality of the data. The purpose of this paper is to review all possible sources of errors that could induce random or systematic bias in the temperature measurements. The characteristics of the lidars, the procedures used, as well as the data reduction software are then reviewed. Comparisons made between the lidar and other available techniques and between lidars of different characteristics lead to the conclusion that an accuracy of 1 K can be attained between 30 and about 70 km depending on the lidar power. The method itself is not affected by drift with time and provides absolute temperature data without any need of calibration and therefore is one of the best instruments for long-term monitoring.

Rotational Raman lidar to measure the atmospheric temperature from the ground to 30 km

A lidar method using the anti-Stokes rotational lines of N/sub 2/ and O/sub 2/ Raman spectra to determine the temperature of the atmosphere up to 30 km is described. The method uses the variation with the temperature of the envelope of the intensities of the backscattered rotational Raman spectrum. For each temperature of the gas, the ratio of the fluxes through two narrow and close-by filters takes a definite value directly related to the temperature. The difficulty of eliminating the near-by contribution of the Mie backscattering was solved by doubling the filters to produce a rejection factor of 10/sup +8/ at the central wavelength. The validity of the method is illustrated by comparing a number of temperature profiles obtained simultaneously with radiosonde and by this new Raman lidar. The theoretical calculation of the method led to an analytic calibration function which, once adjusted with a radiosonde, can provide the temperature on successive days of measurement in the height range of 50 to 25 km. >

Gravity waves in the middle atmosphere observed by Rayleigh lidar: 1. Case studies

Density and temperature mesoscale fluctuations as observed in the stratosphere and mesosphere by means of two Rayleigh lidars with high resolution in time (15 min) and space (300 m), have been analyzed in some particular cases corresponding to different seasonal conditions. These case studies are characteristic of recurrently observed patterns and thus provide a description of the mesoscale fluctuation field in the middle atmosphere. The spatial, temporal, and spectral characteristics of the fluctuations are described and discussed in the framework of the gravity wave interpretation. Dominant wave modes with large period and large vertical wavelength (inertia-gravity waves) are frequently observed in the stratosphere and lower mesosphere. These low-frequency modes are not generally observed above 50- to 55-km altitude, suggesting a strong damping of such waves in the mesosphere. The vertical growth of potential energy density indicates that the wave motions are generally not conservative in the middle atmosphere. The gravity waves amplitude appears too small to produce convective instabilities in the stratosphere. On the contrary, the amplitude of the fluctuations is close to the convective saturation limit deduced from the linear theory for wavelengths up to 3–5 km in the lower mesosphere, and up to 6–8 km above 60 km altitude. Furthermore, convectively instable layers, which can persist for periods longer than 1 hour, have been frequently observed in the mesosphere.

Recent observations of mesospheric temperature inversions

The recent results of observed mesospheric temperature inversions are presented. Using two Rayleigh lidars (located in the south of France) and ISAMS (Improved Stratospheric And Mesospheric Sounder) and HALOE (Halogen Occultation Experiment) temperature measurements from UARS, a new detailed climatology of the inversions (occurring between 65 and 85 km of altitude) has been developed. A strong annual cycle is observed at midlatitudes, with a maximum during the winter months (monthly mean amplitude of 15 K observed by lidar at 44°N, 5°E). This annual cycle is also observed by ISAMS and HALOE for all longitudes, so that a midlatitude belt of strong inversions forms during the southern and northern winters. In addition, a strong semiannual cycle is observed by ISAMS and HALOE at lower latitudes, with its maxima 1 month after the equinoxes. Both solstice and equinoctial events are the most significant observed signatures of the inversions. The frequency of occurrence of the inversions follows the same annual and semiannual cycles. The winter inversion events are located about 70 km of altitude, and the equinoctial events are located 5-10 km above, suggesting that different processes could be involved. The winter events observations are consistent with some of the proposed mechanisms, involving the breaking gravity waves, while no evidence of similar mechanisms was found for the equinoctial events.