[Show abstract][Hide abstract] ABSTRACT: Recent research on sprites shows these and other transient luminous events can exert a local impact on atmospheric chemistry, although with minor effects at global scales. In particular, both modelling and remote sensing work suggest perturbations to the background NOx up to a few tens of per cent can occur above active sprite-producing thunderstorms. In this study we present a detailed investigation of MIPAS/ENVISAT satellite measurements of middle atmospheric NO2 in regions of high likelihood of sprite occurrence during the period August to December 2003. As a proxy of sprite activity we used ground based WWLLN detections of large tropospheric thunderstorms. By investigating the sensitivity of the analysis to the characteristics of the adopted strategy, we confirm the indication of sprite-induced NO2 enhancements of about 10% at 52 km height and tens of per cent at 60 km height immediately after thunderstorm activity, as previously reported by Arnone et al (2008b Geophys. Res. Lett. 35 5807). A further analysis showed the enhancement to be dominated by the contribution from regions north of the Equator (5°N to 20°N) during the first 30 to 40 days of the sample (i.e. the tail of Northern Hemisphere summer) and in coincidence with low background winds.
[Show abstract][Hide abstract] ABSTRACT: We present an order of magnitude estimate of the impact of sprites and other transient luminous events (TLEs) on the atmospheric temperature via ozone changes. To address the effects of expected TLE-ozone changes of at most a few percent, we first study the linearity of the radiatively driven response of a stratosphere-mesosphere model and of a general circulation model (GCM) to a range of uniform climatological ozone perturbations. The study is limited to Northern Hemisphere winter conditions, when planetary wave activity is high and the non linear stratosphere-troposphere coupling can be strong. Throughout most of the middle atmosphere of both models, the radiatively driven temperature response to uniform 5% to 20% ozone perturbations shows a close-to linear relationship with the magnitude of the perturbation. A mid-latitude stratopause ozone perturbation is then imposed as an idealised experiment that mimics local temperature gradients introduced by the latitudinal dependence of TLEs. An unrealistically high 20% magnitude is adopted for the regional ozone perturbation to obtain statistical significance in the model response. The local linearity of the radiatively driven response is used to infer a first order estimate of TLE-induced temperature changes of the order of 0.015 K under typical conditions, and less than a peak temperature change of 0.3 K at 60–70 km height in coincidence of extraordinarily active TLE-producing thunderstorms before horizontal mixing quickly occurs. In the latter case, dedicated mesoscale modelling is needed to study the relevance of regional non linear processes which are expected to impact these radiatively driven responses.
Full-text · Article · Sep 2008 · Advances in Geosciences
[Show abstract][Hide abstract] ABSTRACT: The paper reviews recent advances in studies of electric discharges in the stratosphere and mesosphere above thunderstorms,
and their effects on the atmosphere. The primary focus is on the sprite discharge occurring in the mesosphere, which is the
most commonly observed high altitude discharge by imaging cameras from the ground, but effects on the upper atmosphere by
electromagnetic radiation from lightning are also considered. During the past few years, co-ordinated observations over Southern
Europe have been made of a wide range of parameters related to sprites and their causative thunderstorms. Observations have
been complemented by the modelling of processes ranging from the electric discharge to perturbations of trace gas concentrations
in the upper atmosphere. Observations point to significant energy deposition by sprites in the neutral atmosphere as observed
by infrasound waves detected at up to 1000km distance, whereas elves and lightning have been shown significantly to affect
ionization and heating of the lower ionosphere/mesosphere. Studies of the thunderstorm systems powering high altitude discharges
show the important role of intracloud (IC) lightning in sprite generation as seen by the first simultaneous observations of
IC activity, sprite activity and broadband, electromagnetic radiation in the VLF range. Simulations of sprite ignition suggest
that, under certain conditions, energetic electrons in the runaway regime are generated in streamer discharges. Such electrons
may be the source of X- and Gamma-rays observed in lightning, thunderstorms and the so-called Terrestrial Gamma-ray Flashes
(TGFs) observed from space over thunderstorm regions. Model estimates of sprite perturbations to the global atmospheric electric
circuit, trace gas concentrations and atmospheric dynamics suggest significant local perturbations, and possibly significant
meso-scale effects, but negligible global effects.
Full-text · Article · Sep 2008 · Surveys in Geophysics
[Show abstract][Hide abstract] ABSTRACT: The local chemical impact of sprites is investigated for the first time. This study is motivated by the current understanding of the streamer nature of sprites: streamers are known to produce NOx, thus sprites are expected to lead to a local enhancement of the background abundances, and possibly impact ozone. We adopted the following strategy: middle atmosphere MIPAS/GMTR NO2 satellite measurements were correlated with ground-based WWLLN detections of large tropospheric thunderstorms as a proxy for sprite activity. We found no evidence of any significant impact at a global scale, but an indication of a possible sprite induced NO2 enhancement of about 10% at 52 km height in correspondence with active thunderstorms. This local enhancement appears to increase with height from a few percent at 47 km to tens of percent at 60 km.
Full-text · Article · Mar 2008 · Geophysical Research Letters
[Show abstract][Hide abstract] ABSTRACT: To assess a possible solar modulation of the planetary waves, the 300 hPa geopotential height of the two major reanalysis data sets, NCEP/NCAR and ERA40, is studied for the difference between years of solar maximum and minimum activity for three solar cycles in the period 1963-1999. We find a significant positive difference over the North Pacific ocean for December through February which resembles that found in sea level pressures in earlier studies. Furthermore, there are statistically significant amplitude differences in the climatological state of the first two planetary waves extracted from the geopotential heights. We also perform a model study of the middle atmospheric response to the tropospheric solar signal. The 300 hPa geopotential height is used as the lower boundary in a middle atmosphere model which is run with a constant solar cycle minimum radiative forcing, so that only the lower boundary is forcing the middle atmosphere. The zonal wind and temperature at 300-0.01 hPa from the model experiment are studied in boreal winter for the same years that were studied in the geopotential height analysis. Differences of the means of the two data sets reproduce the structure and amplitude seen in observations and in reanalysis data. Whether this is a direct effect of the solar forcing of the troposphere or a feedback coupling of the solar forcing of the stratosphere is not presently clear, but the modulation of the tropospheric planetary waves seems to be important for the observed solar modulation of the stratosphere.
Full-text · Article · Oct 2007 · Journal of Geophysical Research Atmospheres
[Show abstract][Hide abstract] ABSTRACT: The Atmosphere-Space Interactions Monitor (ASIM) is an instrument suite to be mounted on one of the external platforms on the International Space Station (ISS). It will study the coupling of thunderstorms processes to the upper atmosphere, ionosphere and radiation belts, and energetic space particle precipitation effects in the mesosphere and thermosphere. The scientific objectives include (1) investigations into sprites, jets, elves and relativistic electron beams injected into the magnetosphere above thunderstorms, (2) studies of gravity waves in the thermosphere above severe thunderstorms, (3) auroral energetics, and (4) ozone and NOx concentrations in the upper atmosphere. The instruments are 6 TV frame-rate, narrow-band optical cameras, 6 100kHz-photometers, and one X-ray sensor. The mission includes instrument teams from Denmark, Spain, Norway and the US, and science teams from around the world. The mission is developed within the European Space Agency (ESA). ASIM is entering Phase B with expected launch in 2011.
[Show abstract][Hide abstract] ABSTRACT: Sprites and other transient luminous events have been suggested to impact atmospheric chemistry, in particular through production of NO x . Recent ion-chemistry modelling (1) estimated sprites to locally perturb atmospheric NO x by a few tens of percentage. Even though consideration of chemical and dynamical timescales, and of the occurrence rate of sprites leads to a negligible global impact, sprite-induced chemical changes appear to be locally significant. Satellite observations of NO 2 from the MIPAS/ENVISAT spectrometer were analysed in coincidence with regions of high likelihood of sprite occurrence in order to find evidence of sprite perturbations. Given the paucity of available sprite observations, intense tropospheric thunderstorms were used as a proxy of sprite activity and a one-to-one correlation between these active regions and night time NO 2 was performed during the period August to December 2003 (2). MIPAS NO 2 data were retrieved with the 2-dimensional GMTR algorithm (3) thus enhancing the sensitivity to small changes. Intense thunderstorms were localised through the WWLLN lightning detection network (4) that had higher efficiency towards the Maritime Continent during the coincidence period. Figure 1 compares the distribution of anomalies of MIPAS background night time NO 2 at 52 km height (black) with that of NO 2 anomalies in correspondence of intense thunderstorm activity (and thus high likelihood of sprite occurrence - red) over 5° degrees latitude bands. The analysis showed a statistically significant enhancement of night time NO 2 of about 10% at 52 km height in correspondence of intense thunderstorm activity (WWLLN-NO 2 ) over the 5° to 10°N and 15° to 20°N latitude bands. The region 5°S to 5°N showed no significant change, while other regions had too few coincidences to be meaningful. The statistical significance was tested applying a numerical bootstrap analysis to the difference of the background NO 2 distribution to the WWLLN-NO 2 distribution.