Comparison between S. T. radar and in situ balloon measurements

A campaign for simultaneous in situ and remote observation of both troposphere and stratosphere took place near Aire-sur-l'Adour (in southeastern France) on May 4, 1984. The aim of this campaign was a better understanding of the physics of radar echoes. The backscattered signal obtained with a stratosphere-troposphere radar both at the vertical and 15 deg. off vertical is compared with the velocity and temperature measurements made in the same region (about 10 km north of the radar site) by balloon-borne ionic anenometers and temperature sensors. In situ measurements clearly indicate that the temperature fluctuations are not always consistent with the standard turbulent theory. Nevertheless, the assumptions generally made (isotropy and turbulent field in k) and the classical formulation so derived for radar reflectivity are able to reproduce the shape of the radar return power profiles in oblique directions. Another significant result is the confirmation of the role played by the atmospheric stratification in the vertical echo power. It is important to develop these simultaneous in situ and remote experiments for a better description of the dynamical and thermal structure of the atmosphere and for a better understanding of the mechanisms governing clear-air radar reflectivity

Can one detect small-scale turbulence from standard meteorological radiosondes?

International audienceIt has been recently proposed by Clayson and Kantha (2008) to evaluate the climatology of atmospheric turbulence through the detection of overturns in the free at mosphere by applying a Thorpe analysis on relatively lowy vertical resolution (LR) profiles from standard radiosoundings. Since then, several studies based on this idea have been published. However, the impact of instrumental noise on the detection of turbulent layers was completely ig- nored in these works. The present study aims to evaluate the feasibility of overturns detection from radiosoundings. For this purpose, we analyzed data of two field campaigns dur- ing which high-resolution (HR) soundings (10-20 cm) were performed simultaneously with standard LR soundings. We used the raw data of standard meteorological radiosondes, the vertical resolution ranging from 5 to 9 m. A Thorpe analysis was applied to both LR and HR po- tential temperature profiles. A denoising procedure was first applied in order to reduce the probability of occurrence of artificial overturns, i.e. potential temperature inversions due to instrumental noise only. We then compared the empirical probability density functions (pdf) of the sizes of the selected overturns from LR and HR profiles. From HR profiles measured in the troposphere, the sizes of the detected overturns range from 4 to ∼1000 m. The shape of the size pdf of overturns is found to sharply decrease with increasing scales. From LR profiles, the smallest size of de- tected overturns is ∼32 m, a similar decrease in the shape of the pdf of sizes being observed. These results suggest that overturns, resulting either from small-scale turbulence or from instabilities, can indeed be detected from meteoro- logical radiosonde measurements in the troposphere and in the stratosphere as well. However they are rather rare as they belong to the tail of the size distribution of overturns: they only represent the 7 % largest events in the troposphere, and 4 % in the stratosphere

Using the International Monitoring System infrasound network to study gravity waves

International audienceThe infrasound network of the International Monitoring System (IMS) has been designed for the detection of atmospheric pressure fluctuations produced in the [0.02 Hz-4 Hz] frequency range. However, the majority of the measuring chains used in this network also record pressure fluctuations at lower frequencies. The objective of this paper is to demonstrate the accuracy of IMS pressure measurements in the gravity wave band, whose period usually ranges from a few minutes to 24 hours. Application examples such as the monitoring of worldwide gravity wave time-spectra and the characterization of surface pressure fluctuations produced by atmospheric tides are presented. This study opens the way to the analysis of gravity waves using IMS data, which constitute a unique and accurate set of pressure measurements

Going forth and back in time: a fast and parsimonious algorithm for mixed initial/final-value problems

We present an efficient and parsimonious algorithm to solve mixed initial/final-value problems. The algorithm optimally limits the memory storage and the computational time requirements: with respect to a simple forward integration, the cost factor is only logarithmic in the number of time-steps. As an example, we discuss the solution of the final-value problem for a Fokker-Planck equation whose drift velocity solves a different initial-value problem -- a relevant issue in the context of turbulent scalar transport.Comment: 12 pages, 4 figure

Mesure de la réfractivité atmosphérique par radar météorologique : comparaison avec un réseau de capteurs au sol.

International audienceLes radars météorologiques peuvent mesurer les changements de l'indice de réfraction de l'air dans les basses couches de l'atmosphère (Fabry et al., 1997, Fabry 2004). En utilisant les changements de phase provenant de cibles fixes aux alentours du radar, cette mesure permet d'obtenir une mesure de la réfractivité atmosphérique. Celle-ci dépend de la pression, la température et l'humidité. Les échos exploitables proviennent en général des cibles fixes telles que des châteaux d'eau, des tours ou des pylônes électriques. Pendant la campagne HyMeX (Hydrological cycle in Mediterranean expriment), cette mesure a été implémentée avec succès sur les radars bande S du réseau opérationnel de Météo-France. Afin de mieux comprendre les sources d'erreur autour de cette mesure, en particulier lorsque l'on monte en fréquence, Besson et al. 2012 a mené une simulation à partir des données de stations météorologiques automatiques. Cela a permis de mettre en avant une plus forte variabilité du signal l'été et en fin d'après-midi, quand la réfractivité est très sensible aux changements d'humidité. Cette simulation a ensuite été confirmée par des mesures radar. Est-il alors possible d'obtenir une information sur la turbulence à partir de cette mesure ? Pour échantillonner la variabilité spatiale et temporelle de la réfractivité, une analyse a été menée sur un an de données provenant du radar en bande C de Trappes, conjointement à une comparaison avec les stations automatiques alentours. L'étude présentée ici a permis de montrer qu'un lien qualitatif et quantitatif peut être établi entre les variabilités de la réfractivité mesurée par radar ou par les stations automatiques, qui sont liées à la turbulence atmosphérique de basses couches

First climatology of polar mesospheric clouds from GOMOS/ENVISAT stellar occultation instrument

GOMOS (Global Ozone Monitoring by Occultation of Stars), on board the European platform ENVISAT launched in 2002, is a stellar occultation instrument combining four spectrometers and two fast photometers which measure light at 1 kHz sampling rate in the two visible channels 470–520 nm and 650–700 nm. On the day side, GOMOS does not measure only the light from the star, but also the solar light scattered by the atmospheric molecules. In the summer polar days, Polar Mesospheric Clouds (PMC) are clearly detected using the photometers signals, as the solar light scattered by the cloud particles in the instrument field of view. The sun-synchronous orbit of ENVISAT allows observing PMC in both hemispheres and the stellar occultation technique ensures a very good geometrical registration. Four years of data, from 2002 to 2006, are analyzed up to now. GOMOS data set consists of approximately 10 000 cloud observations all over the eight PMC seasons studied. The first climatology obtained by the analysis of this data set is presented, focusing on the seasonal and latitudinal coverage, represented by global maps. GOMOS photometers allow a very sensitive PMC detection, showing a frequency of occurrence of 100% in polar regions during the middle of the PMC season. According to this work mesospheric clouds seem to be more frequent in the Northern Hemisphere than in the Southern Hemisphere. The PMC altitude distribution was also calculated. The obtained median values are 82.7 km in the North and 83.2 km in the South

Hydrogen wall and heliosheath Lyman-alpha absorption toward nearby stars: possible constraints on the heliospheric interface plasma flow

In this paper, we study heliospheric Ly-$\alpha$ absorption toward nearby stars in different lines of sight. We use the Baranov-Malama model of the solar wind interaction with a two-component (charged component and H atoms) interstellar medium. Interstellar atoms are described kinetically in the model. The code allows us to separate the heliospheric absorption into two components, produced by H atoms originating in the hydrogen wall and heliosheath regions, respectively. We study the sensitivity of the heliospheric absorption to the assumed interstellar proton and H atom number densities. These theoretical results are compared with interstellar absorption toward six nearby stars observed by the Hubble Space Telescope.Comment: 10 pages, accepted for publication in JGR-Space Physic

GOMOS: Gobal Ozone Monitoring by Occultation of Stars

In this paper we report on the progress and status of the Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument, and imaging spectrometer under development for flight on the European Space Agency's Polar Orbiting Earth Mission (POEM-1) mission in 1998. Employing occultation of stars as a light probe of the Earth's atmosphere from a sun-sychronous polar orbit, the instrument will monitor ozone and other atmospheric trace gases over the entire globe. Atmospheric transmission resolution of approximately 1.7 km. When data are combined regionally, it will be possible to detect ozone concentration trends as small as 0.05 percent/year, depending on the degree of combination

Turbulence energetics in stably stratified geophysical flows: strong and weak mixing regimes

Traditionally, turbulence energetics is characterized by turbulent kinetic energy (TKE) and modelled using solely the TKE budget equation. In stable stratification, TKE is generated by the velocity shear and expended through viscous dissipation and work against buoyancy forces. The effect of stratification is characterized by the ratio of the buoyancy gradient to squared shear, called Richardson number, Ri. It is widely believed that at Ri exceeding a critical value, Ric, local shear cannot maintain turbulence, and the flow becomes laminar. We revise this concept by extending the energy analysis to turbulent potential and total energies (TPE and TTE = TKE + TPE), consider their budget equations, and conclude that TTE is a conservative parameter maintained by shear in any stratification. Hence there is no "energetics Ric", in contrast to the hydrodynamic-instability threshold, Ric-instability, whose typical values vary from 0.25 to 1. We demonstrate that this interval, 0.25<Ri<1, separates two different turbulent regimes: strong mixing and weak mixing rather than the turbulent and the laminar regimes, as the classical concept states. This explains persistent occurrence of turbulence in the free atmosphere and deep ocean at Ri>>1, clarify principal difference between turbulent boundary layers and free flows, and provide basis for improving operational turbulence closure models.Comment: 23 pages, 4 figures, Quarterly Journal of Royal Meteorological Society, in pres