Francis Dalaudier

Direct Evidence of “Sheets” in the Atmospheric Temperature Field

Abstract This paper presents experimental evidence showing the ubiquitous presence in the lower atmosphere (at least up to 25 km) of very strong (positive) temperature gradients within very thin layers. The presence of such “sheets” in the temperature field of the free atmosphere was frequently hypothesized in order to account for the aspect sensitivity of VHF radar measurements. Owing to their high vertical resolution (20 cm) and to the fast-response thermometers used, the in situ balloon measurements discussed in this paper constitute the first direct evidence of their true existence. Statistical study of the properties of the sheets results in the following typical values: thickness 3–20 m, temperature increase 0.2–0.8 K, gradient 30–100 K/km. The sheets are frequently observed in groups, associated with and taking part in regions of high static stability. Local measurements using two pairs of sensors one meter apart indicate that the sheets are not flat and horizontal. Sometimes, clear evidence of ong...

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.

Retrieving parameters of the anisotropic refractive index fluctuations spectrum in the stratosphere from balloon-borne observations of stellar scintillation

Scintillation effects are not negligible in the stratosphere. We present a model based on a 3D model of anisotropic and isotropic refractive index fluctuations spectra that predicts scintillation rates within the so-called small perturbation approximation. Atmospheric observations of stellar scintillation made from the AMON-RA (AMON, Absorption par les Minoritaires Ozone et NO(x); RA, rapid) balloon-borne spectrometer allows us to remotely probe wave-turbulence characteristics in the stratosphere. Data reduction from these observations brings out values of the inner scale of the anisotropic spectrum. We find metric values of the inner scale that are compatible with space-based measurements. We find a major contribution of the anisotropic spectrum relative to the isotropic contribution. When the sight line plunges into the atmosphere, strong scintillation occurs as well as coupled chromatic refraction effects.

Strong Mixing Events Observed near the Tropopause with the MU Radar and High-Resolution Balloon Techniques

Abstract In the present paper, strong isotropic radar echo enhancements are shown that are related to the existence of nearly statically neutralized layers (40–120 m thick) observed with high-resolution (10 cm) temperature measurements performed during the Middle and Upper Atmosphere Radar, Temperature Sheets and Interferometry (MUTSI) campaign (8–26 May 2000, Japan). These events have been observed in the immediate vicinity of the tropopause, just above a jet stream maximum. They most likely result from strong turbulent mixing generated by shear instabilities and could be one of the mechanisms responsible for the generation of strong temperature gradients (temperature gradient sheets) at the mixed layer edges. Further investigations will be necessary to evaluate the occurrence of this neutralization mechanism and its contribution to the generation of strong temperature gradient sheets within the free atmosphere.

Nighttime ozone profiles in the stratosphere and mesosphere by the Global Ozone Monitoring by Occultation of Stars on Envisat

[1] The Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument on board the European Space Agency’s Envisat satellite measures ozone and a few other trace gases using the stellar occultation method. Global coverage, good vertical resolution and the self-calibrating measurement method make GOMOS observations a promising data set for building various climatologies. In this paper we present the nighttime stratospheric ozone distribution measured by GOMOS in 2003. We show monthly latitudinal distributions of the ozone number density and mixing ratio profiles, as well as the seasonal variations of profiles at several latitudes. The stratospheric profiles are compared with the Fortuin-Kelder daytime ozone climatology. Large differences are found in polar areas and they can be shown to be correlated with large increases of NO2. In the upper stratosphere, ozone values from GOMOS are systematically larger than in the Fortuin-Kelder climatology, which can be explained by the diurnal variation. In the middle and lower stratosphere, GOMOS finds a few percent less ozone than Fortuin-Kelder. In the equatorial area, at heights of around 15–22 km, GOMOS finds much less ozone than Fortuin-Kelder. For the mesosphere and lower thermosphere, there has previously been no comprehensive nighttime ozone climatology. GOMOS is one of the first new instruments able to contribute to such a climatology. We concentrate on the characterization of the ozone distribution in this region. The monthly latitudinal and seasonal distributions of ozone profiles in this altitude region are shown. The altitude of the mesospheric ozone peak and the semiannual oscillation of the number density are determined. GOMOS is also able to determine the magnitude of the ozone minimum around 80 km. The lowest seasonal mean mixing ratio values are around 0.13 ppm. The faint tertiary ozone peak at 72 km in polar regions during wintertime is observed.

First simultaneous global measurements of nighttime stratospheric NO2 and NO3 observed by Global Ozone Monitoring by Occultation of Stars (GOMOS)/Envisat in 2003

The Global Ozone Monitoring by Occultation of Stars (GOMOS) stellar occultation instrument on board the Envisat European satellite provides global coverage of ozone and other stratospheric species with good vertical resolution and a self-calibrating method. In this paper we present the first simultaneous global distribution of stratospheric NO 2 and NO 3 from 1 year of nighttime GOMOS data in 2003. Most previous NO 2 satellite observations have been made using the solar occultation technique. They are difficult to interpret due to the fast photochemical evolution of NO 2 at sunrise and sunset. There are no published observations of NO 3 from space because this constituent is rapidly photodissociated during daytime and is not observable by solar occultation. It is shown that the NO 2 mixing ratio reaches a maximum around 40 km with values between 14 and 16 ppbv at low and middle latitudes. The global distribution of NO 2 observed by GOMOS is very similar to the NO + NO 2 Halogen Occultation Experiment climatology deduced from sunset measurements from 1999 to 2004. At high latitude a high mixing ratio is observed in the north vortex in November 2003 after a strong solar proton event and in the south vortex in July 2003. The NO 3 mixing ratio peaks at 40–45 km. NO 3 follows a semiannual variation at low latitudes with maxima at equinoxes and an annual variation at middle and high latitudes with a maximum in summer. In the upper stratosphere the mixing ratio of NO 3 is strongly correlated with temperature due to the thermal dependence of its formation rate. Citation: Hauchecorne, A., et al. (2005), First simultaneous global measurements of nighttime stratospheric NO 2 and NO 3 observed by Global Ozone Monitoring by Occultation of Stars (GOMOS)/Envisat in 2003

Validation of Winds Measured by MU Radar with GPS Radiosondes during the MUTSI Campaign

Abstract For many years, mesosphere–stratosphere–troposphere (MST) radar techniques have been used for studying the structure and dynamics of the lower and middle atmosphere. In particular, these instruments are unique tools for continuously monitoring vertical and horizontal components of the atmospheric wind at high spatial and temporal resolutions. From the very beginning, many studies have been carried out analyzing the reliability of the MST radar wind measurements and their accuracy. However, until now, very few studies have been presented confirming the high performances of the VHF Middle and upper Atmospheric (MU) radar of Japan (35°N, 136°E) for measuring the wind field. The present paper thus gives original comparisons between horizontal velocities measured by MU radar and by instrumented balloons using global positioning system (GPS) radiosondes. Twelve radiosondes were successfully used during the French–Japanese MU Radar Temperature Sheets and Interferometry (MUTSI) campaign (10–26 May 2000, ...

Chromatic refraction with global ozone monitoring by occultation of stars. I. Description and scintillation correction

We describe refractive and chromatic effects, both regular and random, that occur during star occultations by the Earths atmosphere. The scintillation that results from random density fluctuations, as well as the consequences of regular chromatic refraction, is qualitatively described. The resultant chromatic scintillation will produce random features on the Global Ozone Monitoring by Occultation of Stars (GOMOS) spectrometer, with an amplitude comparable with that of some of the real absorbing features that result from atmospheric constituents. A correction method that is based on the use of fast photometer signals is described, and its efficiency is discussed. We give a qualitative (although accurate) description of the phenomena, including numerical values when needed. Geometrical optics and the phase-screen approximation are used to keep the description simple.

Turbulence in the stratified atmosphere: Recent theoretical developments and experimental results

Abstract Above the boundary layer and outside convective systems, atmospheric turbulence generally occurs within a stably stratified environment. Experimental data clearly reveal that classical ideas about isotropy and inertiality of turbulence may then be questioned. Recent theoretical developments show a wide range of possible turbulent regimes, depending upon different physical hypotheses relating the dominant terms in the momentum and energy balances. The scaling of stratified turbulence proposed by /1/ will be extensively reviewed, such as present views of the tubulent buoyancy subrange. Key measurements appear to be those of temperature, vertical and horizontal velocities along vertical paths and, most of all, estimations of vertical heat flux (or mass flux) spectra. Recent atmospheric measurements acquired by a balloon borne instrumentation in the upper troposphere and lower stratosphere will be presented as they give some insight in these debates. Owing to its great practical importance, the problem of the relationships between temperature and velocities fluctuations will be specifically discussed.

A global climatology of the mesospheric sodium layer from GOMOS data during the 2002–2008 period

This paper presents a climatology of the mesospheric sodium layer built from the processing of 7 years of GOMOS data. With respect to preliminary results already published for the year 2003, a more careful analysis was applied to the averaging of occultations inside the climatological bins (10 in latitude-1 month). Also, the slant path absorption lines of the Na doublet around 589 nm shows evidence of partial saturation that was responsible for an underestimation of the Na concentration in our previous results. The sodium climatology has been validated with respect to the Fort Collins lidar measurements and, to a lesser extent, to the OSIRIS 2003–2004 data. Despite the important natural sodium variability, we have shown that the Na vertical column has a marked semi-annual oscillation at low latitudes that merges into an annual oscillation in the polar regions,a spatial distribution pattern that was unreported so far. The sodium layer seems to be clearly influenced by the mesospheric global circulation and the altitude of the layer shows clear signs of subsidence during polar winter. The climatology has been parameterized by time-latitude robust fits to alCorrespondence to: D. Fussen (didier.fussen@oma.be) low for easy use. Taking into account the non-linearity of the transmittance due to partial saturation, an experimental approach is proposed to derive mesospheric temperatures from limb remote sounding measurements.