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

Horizontal maps of echo power in the lower stratosphere using the MU radar

International audienceIn recent works, zenithal and azimuthal angle variations of echo power measured by VHF Stratosphere-Troposphere (ST) radars have been analyzed in detail using different radar multi-beam configurations. It was found that the azimuthal angle corresponding to maximum echo power is closely related to the direction of the horizontal wind shear. These properties indicate that local wind shear affects the tilt of the scatterers. Moreover, horizontal maps of echo power collected using a large set of beams steered pulse-to-pulse up to 40 degrees off zenith revealed that the power distribution pattern in the troposphere is often skewed. In this work, a three-dimensional description of echo power variations up to 24 degrees off zenith is shown for measurements in the lower stratosphere (i.e. up to approximately 20km) using a "sequential multi-beam" (SMB) configuration. Such a description was not possible above the tropopause with classical multi-beam configurations because of the loss of radar sensitivity due to the limited integration time by the use of a large number of beams. This work attempts to complete previous descriptions of the phenomenon by some observations in the lower stratosphere discussed in association with complementary balloon measurements. Key words. Meteorology and atmospheric dynamics (turbulence) – Radio Science (remote sensing

High-resolution vertical imaging of the troposphere and lower stratosphere using the new MU radar system

International audienceIn the present paper, a new application of the range imaging technique called Frequency Interferometry Imaging (FII) or Range Imaging (RIM), performed in April 2005, is shown using the new 46.5-MHz Middle and Upper (MU) atmosphere radar system (Shigaraki, Japan). Height-time images of brightness distribution have been computed at the highest resolution ever obtained for imaging with VHF radars in the troposphere and, for the very first time, in the lower stratosphere, up to about 22 km. The images were produced by processing signals obtained with an initial range-resolution of ?r=150 m and five equally-spaced frequencies within ?f=1.0 MHz, with the adaptive Capon method. These values represent an improvement of a factor 2 over all the previous published experiments at VHF, which were performed with ?r=300 m and ?f=0.5 MHz. The Capon images present realistic and self-consistent features, and reveal many more organized structures than the height-time SNR plots at the initial range-resolution. For example, the Capon images show persistent enhanced brightness layers significantly thinner than 150 m in the stratosphere, which are impossible to track with the standard single-frequency mode owing to a lack of range resolution. These observations thus support the idea of strong stratification even at vertical scales much smaller than 100 m, as suggested by recent high-resolution temperature observations by balloons (Dalaudier et al., 1994). We also present comparisons of Capon images with patterns obtained from the dual-FDI technique and two parametric methods (the MUSIC algorithm and the newly-introduced Maximum Entropy Method based on an auto-regressive (AR) model). The comparisons confirm the insufficiencies of the dual-FDI technique and indicate that parametric methods such as MEM and the MUSIC algorithm can help to validate the Capon images when the parametric methods provide similar patterns

Atmospheric Kelvin-Helmholtz b illows captured by the MU radar, lidars and a fish-eye camera

On June 11, 2015, a train of large-amplitude Kelvin–Helmholtz (KH) billows was monitored by the Middle and Upper Atmosphere (MU) radar (Shigaraki MU Observatory, Japan) at the altitude of ~ 6.5 km. Four to five KH billows in formation and decay stages were observed for about 20 min at the height of a strong speed shear (> ~ 30 m s⁻¹km⁻¹), just a few hundred meters above a mid-level cloud base. The turbulent billows had a spacing of about 3.5–4.0 km (3.71 km in average) and an aspect ratio (depth/spacing) of ~ 0.3. The turbulence kinetic energy dissipation rate estimated was of the order of 10–50 mWkg⁻¹, corresponding to moderate turbulence according to ICAO (2010) classification. By chance, an upward-looking fish-eye camera producing pictures once every minute detected smooth protuberances at the cloud base caused by the KH billows so that comparisons of their characteristics could be made for the first time between the radar observations and the pictures. The main characteristics of the KH wave (horizontal wavelength, phase front direction and phase speed) obtained from the analysis of the pictures were fully consistent with those found from radar data. The pictures indicated that the billows were advected by the wind observed at the height of the critical level. They also revealed a very small transverse extent (about twice the horizontal spacing) suggesting that the large-amplitude KH billows were generated by a very localized source. Micro-pulse lidar and Raman–Rayleigh–Mie lidar data also collected during the event permitted us to confirm some of the characteristics of the billows

Turbulence Kinetic Energy Dissipation Rates Estimated from Concurrent UAV and MU Radar Measurement s

We tested models commonly used for estimating turbulence kinetic energy dissipation rates ε from very high frequency stratosphere–troposphere radar data. These models relate the root-mean-square value σ of radial velocity fluctuations assessed from radar Doppler spectra to ε. For this purpose, we used data collected from the middle and upper atmosphere (MU) radar during the Shigaraki unmanned aerial vehicle (UAV)—radar experiment campaigns carried out at the Shigaraki MU Observatory, Japan, in June 2016 and 2017. On these occasions, UAVs equipped with fast-response and low-noise Pitot tube sensors for turbulence measurements were operated in the immediate vicinity of the MU radar. Radar-derived dissipation rates ε estimated from the various models at a range resolution of 150 m from the altitude of 1.345 km up to the altitude of ~ 4.0 km, a (half width half power) beam aperture of 1.32° and a time resolution of 24.6 s, were compared to dissipation rates (εU) directly obtained from relative wind speed spectra inferred from UAV measurements. Firstly, statistical analysis results revealed a very close relationship between enhancements of σ and εU for εU≳10⁻⁵m²s⁻³, , indicating that both instruments detected the same turbulent events with εU above this threshold. Secondly, εU was found to be statistically proportional to σ³, whereas a σ² than the longitudinal and transverse dimensions of the radar sampling volume. The σ³ dependence was found even after excluding convectively generated turbulence in the planetary boundary layer and below clouds. The best agreement between εU and radar-derived ε was obtained with the simple formulation based on dimensional analysis ε=σ³ /Lc where LC ≈ 50–70 m. This empirical expression constitutes a simple way to estimate dissipation rates in the lower troposphere from MU radar data whatever the sources of turbulence be, in clear air or cloudy conditions, consistent with UAV estimates

Estimation de profils de distance à haute résolution avec un radar ST

La technique de compression d'impulsion par saut de fréquence couplée à des méthodes adaptatives et Haute Résolution, généralement utilisées en traitement d'antennes, permet d'accroître considérablement la résolution des radars atmosphériques Doppler à impulsion fonctionnant dans le bas de la gamme VHF (50 MHz). Ainsi il est possible de construire des profils de distance à Haute Résolution dans le but de détecter des couches d'extension verticale beaucoup plus petite que la résolution initiale classique fixée par la durée des impulsions émises. Nous présentons les différentes méthodes utilisées ainsi que leur application sur les données du mini-radar VHF du LSEET installé à Toulon, France (43.7° N, 5.58° E)

Estimation of Turbulence Parameters in the Lower Troposphere from ShUREX (2016-2017) UAV Data

Turbulence parameters in the lower troposphere (up to ~4.5 km) are estimated from measurements of high-resolution and fast-response cold-wire temperature and Pitot tube velocity from sensors onboard DataHawk Unmanned Aerial Vehicles (UAVs) operated at the Shigaraki Middle and Upper atmosphere (MU) Observatory during two ShUREX (Shigaraki UAV Radar Experiment) campaigns in 2016 and 2017. The practical processing methods used for estimating turbulence kinetic energy dissipation rate &epsilon; and temperature structure function parameter C T 2 from one-dimensional wind and temperature frequency spectra are first described in detail. Both are based on the identification of inertial (&minus;5/3) subranges in respective spectra. Using a formulation relating &epsilon; and C T 2 valid for Kolmogorov turbulence in steady state, the flux Richardson number R f and the mixing efficiency &chi; m are then estimated. The statistical analysis confirms the variability of R f and &chi; m around ~ 0.13 &minus; 0.14 and ~ 0.16 &minus; 0.17 , respectively, values close to the canonical values found from some earlier experimental and theoretical studies of both the atmosphere and the oceans. The relevance of the interpretation of the inertial subranges in terms of Kolmogorov turbulence is confirmed by assessing the consistency of additional parameters, the Ozmidov length scale L O , the buoyancy Reynolds number R e b , and the gradient Richardson number Ri. Finally, a case study is presented showing altitude differences between the peaks of N 2 , C T 2 and &epsilon; , suggesting turbulent stirring at the margin of a stable temperature gradient sheet. The possible contribution of this sheet and layer structure on clear air radar backscattering mechanisms is examined.</p

Twenty-three unsolved problems in hydrology (UPH) – a community perspective

This paper is the outcome of a community initiative to identify major unsolved scientific problems in hydrology motivated by a need for stronger harmonisation of research efforts. The procedure involved a public consultation through on-line media, followed by two workshops through which a large number of potential science questions were collated, prioritised, and synthesised. In spite of the diversity of the participants (230 scientists in total), the process revealed much about community priorities and the state of our science: a preference for continuity in research questions rather than radical departures or redirections from past and current work. Questions remain focussed on process-based understanding of hydrological variability and causality at all space and time scales. Increased attention to environmental change drives a new emphasis on understanding how change propagates across interfaces within the hydrological system and across disciplinary boundaries. In particular, the expansion of the human footprint raises a new set of questions related to human interactions with nature and water cycle feedbacks in the context of complex water management problems. We hope that this reflection and synthesis of the 23 unsolved problems in hydrology will help guide research efforts for some years to come

Latest progresses in ShUREX (2015-2017) data analyses

[第379回生存圏シンポジウム ; 第12回MUレーダー・赤道大気レーダーシンポジウム (MU-EAR)] 開催日時: 2018年9月5日(水)-8日(木), 開催場所: 京都大学宇治キャンパス, 主催者: 京都大学生存圏研究