Jean Vernin

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...

Outer scale of turbulence appropriate to modeling refractive-index structure profiles.

The outer scale of turbulence L (0) has been calculated from values of the refractive-index structure coefficient C(2)(N) obtained from spatio-angular correlation measurements of stellar scintillation. It is found that L(0) </= 5 m for a large range of observations in France, U.S.A., and Chile and that its dependence on altitude Z follows the same general form at all these sites. The prediction of C(2)(N)(Z) profiles is shown to be feasible utilizing standard meteorological radiosonde data and this L(0)(Z) curve. A simple model based on dimensional analysis and a more complicated stochastic model are compared, but the latter appears to have no advantage.

Whole atmospheric-turbulence profiling with generalized scidar

Statistical analysis of stellar scintillation on the pupil of a telescope, known as the scidar (scintillation, detection, and ranging) technique, is sensitive only to atmospheric turbulence at altitudes higher than a few kilometers. With the generalized scidar technique, recently proposed and tested under laboratory conditions, one can overcome this limitation by analyzing the scintillation on a plane away from the pupil. We report the first experimental implementation of this technique, to our knowledge, under real atmospheric conditions as a vertical profiler of the refractive-index structure constant C (N)(2) (h). The instrument was adapted to the Nordic Optical Telescope and the William Hershel Telescope at La Palma, Canary Islands. We measure the spatial autocorrelation function of double-star scintillation for different positions of the analysis plane, finding good agreement with theoretical expectations.

An analysis of temperatures and wind speeds above Dome C, Antarctica

Department of Geography, University of Idaho, Moscow, Idaho, USA-Abstract. A good astronomical site must fulflll several criteria including low atmospheric turbulence and lowwind speeds. It is therefore important to have a detailed knowledge of the temperature and wind conditions ofa location considered for future astronomical research. Antarctica has unique atmospheric conditions that havealready been exploited at the South Pole station. Dome C, a site located on a local maximum of the Antarcticplateau, is likely to have even better conditions. In this paper we present the analysis of two decades of windspeed measurements taken at Dome C by an automated weather station (AWS). We also present temperature andwind speed proflles taken over four Antarctic summers using balloon-borne weather sondes. We will show that aswell as having one of the lowest average wind speed ever recorded at an existing or potential observatory, DomeC also has an extremely stable upper atmosphere and a very low inversion layer.Key words. Site Testing { Atmospheric afiects { Balloons

First Whole Atmosphere Nighttime Seeing Measurements at Dome C, Antarctica

ABSTRACT We report site‐testing results obtained in the nighttime during the polar autumn and winter at Dome C. These results were collected during the first Concordia winterover by A. Agabi. They are based on seeing and isoplanatic angle monitoring, as well as in situ balloon measurements of the refractive index structure constant profiles \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Atmospheric turbulence and wind profiles monitoring with generalized scidar

C^{2}_{n}( h)

Optical turbulence profiles at Mauna Kea measured by MASS and SCIDAR

\end{document} . Atmosphere is divided into two regions: (1) a 36 m high surface layer responsible for 87% of the turbulence, a...

Turbulence Profiles with Generalized Scidar at San Pedro Mártir Observatory and Isoplanatism Studies

We present a method for simultaneous measurement of the vertical distribution of the optical turbulence strength C N(h) and wind velocity V (h) in the Earth’s atmosphere, based on an analysis of spatio-temporal correlations of stellar scintillation images obtained with generalized scidar. A statistical comparison of V (h) obtained with this method and instrumented balloons supports the use of this method. The algorithm used allows for the identification of dome seeing, which can be subtracted from C N(h), to obtain a turbulence profile free of dome contribution. Examples of simultaneous C N(h) and V (h) monitorings are presented.

Wind and CN2 profiling by single-star scintillation analysis

The vertical distribution of turbulence over Mauna Kea has been measured on four nights in 2002 October, simultaneously using two different instruments based on stellar scintillation—the generalized SCIDAR (scintillation detection and ranging) and MASS (multiaperture scintillation sensor). The turbulence integrals match within 20%, and the low-resolution profiles delivered by MASS correctly reveal the localization of the strongest high-altitude turbulent layers. As deduced from DIMM (differential image motion monitor), MASS, and SCIDAR measurements, optical turbulence in the first 0.7 km above the summit contributed typically half of the total integral, the latter corresponding to a seeing of 0 .5. The ground layer and free atmosphere are not correlated.

Optical Turbulence Profiling with Balloons Relevant to Astronomy and Atmospheric Physics

The results obtained from 3398 vertical profiles of atmospheric turbulence measured during 11 nights at the Observatorio Astronomico Nacional in San Pedro Martir (Baja California, Mexico) are presented. The observations were carried out with the generalized scidar (GS) installed at the 1.5 m and the 2.1 m telescopes of that site, in 1997 March and April. The open-air seeing was measured with a differential image motion monitor (DIMM). The GS can detect turbulence profiles along the whole optical path, unlike the classical scidar, which is insensitive to low-altitude turbulence. For the first time, to our knowledge, profiles including turbulence near the ground are monitored and statistically analyzed. Isoplanatic angles for speckle interferometry and adaptive optics (AO) in either full or partial compensation are deduced, as well as the focus anisoplanatism parameter for sodium laser guide stars. The advantage of minimizing the distance between the turbulent layers and the conjugated plane of the deformable mirror of an AO system is studied. The comparison of GS profiles obtained at both telescopes, together with DIMM measurements, show that the turbulence near the ground is more strongly dominant at the 1.5 m telescope than at the 2.1 m telescope, where the median values of the seeing near the ground, in the free atmosphere and in the whole optical path are , and , respectively. These values are comparable to or better than those of the major astronomical observatories, although a larger data sample is needed for a definitive comparison.