E. Blanc

Sprites over Europe

Results are presented from the first European campaign for observation of sprites, conducted during the summer of 2000 from the French astronomical observatory, Observatoire Midi-Pyrenees. The primary objective was to establish if sprites are generated over Europe and to identify the characteristics of the associated thunderstorms. During the one-month campaign local weather conditions allowed observations approximately half of the nights. Sprites were observed two nights over the Alps and one night over southeastern France in connection with cold fronts moving in from the Atlantic. In all, 40 sprites were recorded, including dancing sprites, multiple carrot sprites and c-sprites. The weather conditions were almost identical during the 3 nights, with the active area forming on the front-side of the cold fronts. The storms are not of the same magnitude as active systems often observed over the North American plains. Even so, sprites seem to be a common occurrence also over Europe.

Response of the lower atmosphere and the ionosphere to the eclipse of August 11, 1999

Abstract At the time of the last total eclipse occurring in the North of France on August 11th 1999, the Laboratoire de Detection et de Geophysique set up multi-component microbarograph stations to measure the variation of atmospheric pressure on the ground as well as an ionospheric sounding array. The stations were located at distances from 0 to 283 km from the path of totality. Measurements show that gravity waves were generated by the eclipse and that their sources are situated at two different altitudes, one in the lower atmosphere and the other at higher levels in the thermosphere. The period was 9– 12 min on the ground level and ∼60 min in the ionosphere.

Comparison of co-located independent ground-based middle atmospheric wind and temperature measurements with numerical weather prediction models

High-resolution, ground-based and independent observations including co-located wind radiometer, lidar stations, and infrasound instruments are used to evaluate the accuracy of general circulation models and data constrained assimilation systems in the middle atmosphere at northern hemisphere mid-latitudes. Systematic comparisons between observations, the Medium-Range Weather Forecasts (ECMWF) operational analyses including the recent Integrated Forecast System (IFS) cycles 38r1 and 38r2, the NASAs Modern Era Retrospective analysis for Research and Applications (MERRA) re-analyses and the free running climate Max Planck Institute Earth System Model (MPI-ESM-LR) are carried out in both temporal and spectral domains. We find that ECMWF and MERRA are broadly consistent with lidar and wind radiometer measurements up to ~40 km. For both temperature and horizontal wind components, deviations increase with altitude as the assimilated observations become sparser. Between 40 and 60 km altitude, the standard deviation of the mean difference exceeds 5 K for the temperature and 20 m/s for the zonal wind. The largest deviations are observed in winter when the variability from large-scale planetary waves dominates. Between lidar data and MPI-ESM-LR, there is an overall agreement in spectral amplitude down to 15-20 days. At shorter time-scales, the variability is lacking in the model by ~10 dB. Infrasound observations indicate a general good agreement with ECWMF wind and temperature products. As such, this study demonstrates the potential of the infrastructure of the Atmospheric Dynamics Research Infrastructure in Europe project (ARISE) that integrates various measurements and provides a quantitative understanding of stratosphere-troposphere dynamical coupling for numerical weather prediction applications.

Misty Picture: A Unique Experiment for the Interpretation of the Infrasound Propagation from Large Explosive Sources

In the framework of the Comprehensive Nuclear-Test-Ban Treaty, the International Monitoring System develops a 60 micro-barometric stations network. These stations, which records infrasound, detect various powerful natural and artificial sources like long range explosions, oceanic swell, and volcano eruptions. For data analysis, the CEA, in collaboration with the LMFA, develops specific methods based on measurements, data processing and numerical simulation. The Misty Picture experiment is a high explosive event (4685 Tons of ANFO) realized in 1987 in New Mexico (US). Infrasounds were recorded by 22 sensors until a distance of 1000 km in a quiet background noise condition. Multi-reflected tropospheric, stratospheric and thermospheric phases are detected. Signals recorded near the source (1 km away) and observed in the geometrical shadow zone (between 150 km and 250 km from the point source) are of particular interest. This reference experiment very well documented is used to improve our understanding of the atmospheric propagation of infrasound as well as to evaluate our models. Using various methods such as ray tracing, parabolic equation and finite dierences, we investigate eects

Evaluation of wind and temperature profiles from ECMWF analysis on two hemispheres using volcanic infrasound

In this paper, we evaluate vertical wind and temperature profiles that are produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric analysis. The evaluation is carried out on both hemispheres: we make use of stratospheric infrasound arrivals from Mount Etna (37°N) and Mount Yasur (22°S). The near-continuous, high activity of both volcanoes permits the study of stratospheric propagation along well-defined paths with a time resolution ranging from hours to multiple years. Infrasound observables are compared to theoretical estimates obtained from acoustic propagation modeling using the ECMWF analysis. While a first-order agreement is found for both hemispheres, we report on significant discrepancies around some of the equinox periods and other intervals during which the atmosphere is in a state of transition and dynamical oscillations of the atmosphere dominate over the general circulation. We present an inversion study in which we make use of measured trace velocity estimates to estimate first-order effective sound speed model updates in a Bayesian framework. Deviations from the a priori models around the stratopause up to 10% (≈ 30 m s−1) are estimated. Such updates are in line with the results from comparisons between ECMWF analysis and observations from lidar and microwave Doppler spectroradiometer facilities that were colocated during the course of the 2012–2013 Atmospheric dynamics Research and InfraStructure in Europe (ARISE) measurement campaign.

Global Scale Monitoring of Acoustic and Gravity Waves for the Study of the Atmospheric Dynamics

The development of the Infrasound International Monitoring System, used for the verification of the Comprehensive nuclear Test Ban Treaty, represents a powerful tool to measure permanently, at a global scale and over large periods of time, the characteristics of the waves and dynamics of the atmosphere in relation with the climate. The first way is to monitor quasi-continuous infrasound sources such as ocean swells or volcanic eruptions to determine the fluctuations of the stratosphere and mesosphere in relation to the activity of planetary waves and large scale polar disturbances such as Vortex Intensification or Sudden Stratospheric Warming. The second way is to monitor gravity waves which are observed in the lower frequency range of the infrasound data. Large scale waves, mainly produced in tropical regions, influence the mean circulation of the middle atmosphere by transporting moment and energy from tropical to polar regions with a possible role on tropospheric climate. This paper demonstrates through different examples the potential of the network to observe these waves as well as changes in the atmospheric wave guide in relation to atmospheric parameters. As the network will provide long duration observations, it is suggested to use them to study the atmosphere in relation with the climate evolution.

HF signatures of powerful lightning recorded on DEMETER

[1] Emissions in the HF range (1.7 – 3 MHz) are observed at the times of powerful lightning on the low-altitude satellite DEMETER. At � 700 km, wave activity observed on the E field spectrograms recorded on DEMETER during nighttime is mainly dominated by upgoing 0+ whistlers and higher-order dispersed whistlers. Over a period of 30 months, 130 events with HF emissions at frequency � 2 MHz have been observed at the time of intense 0+ whistlers on VLF spectrograms. A global map of the distribution of events indicates that they do not occur above regions of most thunderstorm activity such as the upper part of South America or the middle of Africa. This lack of occurrence above these two near-equatorial regions is consistent with the high value of the critical frequency of the F layer which prevents the transionospheric propagation of the HF component of the lightning pulses up to 700 km. The time, location, intensity, and polarity of the lightning discharges related to a subset of these HF events are determined above the North American region with the National Lightning Detection Network. As most of the events are recorded in Survey Mode when full resolution of the data is not available, the neural network on board DEMETER is used to determine the times of the 0+ whistlers recorded by the satellite with a time accuracy of � 0.1 s. The impulsive HF events correspond to intense lightning discharges occurring in regions immediately below the satellite. It is assumed that the propagation of these HF pulses up to the altitude of the satellite is favored by local ionospheric heating due to high thunderstorm activities.

Ten year observations of gravity waves from thunderstorms in western Africa

A new study of gravity waves produced by thunderstorms was performed using continuous recordings at the IS17 (Ivory Coast) infrasound station of the International Monitoring System developed for the verification of the Comprehensive Nuclear Test-Ban Treaty. A typical case study is presented for a large thunderstorm on 10–11 April 2006 lasting near 14 h. Comparison with cloud temperature measured by the Meteosat 6 satellite shows that wave activity is large when the cloud temperature is low inside convection cells located over the station. Statistics based on 10 year data show that the wave activity is intense throughout the year with peak periods in May and October and less intense activity in January, in good agreement with the local keraunic level. The seasonal variations of the wave azimuth highlight clear trends from northward direction from February to August to southward direction from August to December. Lightning flashes, observed from space, show a similar motion confirming that thunderstorms are the main sources of the gravity wave activity. The gravity wave azimuth follows the seasonal motion of the tropical rain belt partly related to the Intertropical Convergence Zone of the winds. The contribution of other possible sources, such as wind over relief, is weak because surface winds are weak in this region and only oceans are present south of the station. We conclude that the large observed wave activity is mainly produced by convection associated to thunderstorms.

Main results of LSO (Lightning and sprite observations) on board of the international space station

The experiment LSO (Lightning and Sprite Observations), on board of the International Space Station, was the first experiment dedicated to nadir observations of sprites from space. Such observations are innovative as sprites are generally observed at the horizon. At the nadir, sprites are superimposed with lightning flashes and the observation concept is based on a spectral differentiation of sprites and lightning by using an adapted filter. The experiment is composed of two micro-cameras, fixed on a station window. One camera is equipped with a filter and measures the sprites in the N2 1P most intense sprite emission line, which coincides also to the atmospheric absorption band of the molecular oxygen. The second microcamera provides observations of lightning flashes in the visible. Measurements were performed during four ESA missions: Andromede, Odissea, Cervantes and Delta. During 19h of effective observations, 180 flashes were analyzed and several possible sprites were identified, demonstrating the interest of this differentiation method. In addition during sunset and sunrise conditions when the lower atmosphere is in the dark, LSO observed the airglow of the secondary ozone maximum at about 90 km modulated by gravity wave activity.

Toward an Improved Representation of Middle Atmospheric Dynamics Thanks to the ARISE Project

This paper reviews recent progress toward understanding the dynamics of the middle atmosphere in the framework of the Atmospheric Dynamics Research InfraStructure in Europe (ARISE) initiative. The middle atmosphere, integrating the stratosphere and mesosphere, is a crucial region which influences tropospheric weather and climate. Enhancing the understanding of middle atmosphere dynamics requires improved measurement of the propagation and breaking of planetary and gravity waves originating in the lowest levels of the atmosphere. Inter-comparison studies have shown large discrepancies between observations and models, especially during unresolved disturbances such as sudden stratospheric warmings for which model accuracy is poorer due to a lack of observational constraints. Correctly predicting the variability of the middle atmosphere can lead to improvements in tropospheric weather forecasts on timescales of weeks to season. The ARISE project integrates different station networks providing observations from ground to the lower thermosphere, including the infrasound system developed for the Comprehensive Nuclear-Test-Ban Treaty verification, the Lidar Network for the Detection of Atmospheric Composition Change, complementary meteor radars, wind radiometers, ionospheric sounders and satellites. This paper presents several examples which show how multi-instrument observations can provide a better description of the vertical dynamics structure of the middle atmosphere, especially during large disturbances such as gravity waves activity and stratospheric warming events. The paper then demonstrates the interest of ARISE data in data assimilation for weather forecasting and re-analyzes the determination of dynamics evolution with climate change and the monitoring of atmospheric extreme events which have an atmospheric signature, such as thunderstorms or volcanic eruptions.