E. Arnone

Observation of intrinsically bright terrestrial gamma ray flashes from the Mediterranean basin

Abstract We present three terrestrial gamma ray flashes (TGFs) observed over the Mediterranean basin by the Reuven Ramaty High Energy Solar Spectroscope Imager (RHESSI) satellite. Since the occurrence of these events in the Mediterranean region is quite rare, the characterization of the events was optimized by combining different approaches in order to better define the cloud of origin. The TGFs on 7 November 2004 and 16 October 2006 came from clouds with cloud top higher than 10–12 km where often a strong penetration into the stratosphere is found. This kind of cloud is usually associated with heavy precipitation and intense lightning activity. Nevertheless, the analysis of the cloud type based on satellite retrievals shows that the TGF on 27 May 2004 was produced by an unusual shallow convection. This result appears to be supported by the model simulation of the particle distribution and phase in the upper troposphere. The TGF on 7 November 2004 is among the brightest ever measured by RHESSI. The analysis of the energy spectrum of this event is consistent with a production altitude ≤12 km, which is in the upper part of the cloud, as found by the meteorological analysis of the TGF‐producing thunderstorm. This event must be unusually bright at the source in order to produce such a strong signal in RHESSI. We estimate that this TGF must contain ∼3 × 1018 initial photons with energy >1 MeV. This is 1 order of magnitude brighter than earlier estimations of an average RHESSI TGF.

Seeking sprite-induced signatures in remotely sensed middle atmosphere NO2: latitude and time variations

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 ◦ Nt o 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. (Some figures in this article are in colour only in the electronic version)

Two-dimensional tomographic retrieval of MIPAS/ENVISAT measurements of ozone and related species

Observations from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) were analysed with the two-dimensional GMTR retrieval system in order to obtain fields of ozone and several molecular species related to ozone chemistry: HNO3, N2O, NO2, N2O5, ClONO2, COF2, CFC-11 and CFC-12. MIPAS measures mid-infrared emission of the atmosphere both during the day and at night time with global coverage. Observing the atmosphere with limb viewing geometries, the instrument is able to resolve finer vertical structures than with nadir instruments, thus enabling the investigation of ozone height-dependent processes. With the currently planned mission extended up to 2014, MIPAS can provide both short-term resolution and long-term trends needed for studying ozone. The adopted GMTR algorithm permits us to resolve the horizontal inhomogeneities of the atmosphere that are modelled using a two-dimensional discretization of the atmosphere. It is therefore especially suitable for analysing portions of the atmosphere where strong gradients such as at the ozone hole may be poorly reproduced by common horizontal homogeneous one-dimensional retrievals. The adopted strategy is well suited for a refined analysis and a correct monitoring of the ozone recovery, as required by the Montreal Protocol and successive amendments.

WACCM climate chemistry sensitivity to sprite perturbations

Transient luminous events affect Earths atmosphere between thunderstorm tops and the lower ionosphere through ion-neutral chemistry reactions. Particular emphasis has been given to sprites, with models and observations suggesting a capability of perturbing atmospheric nitrogen oxides at a local level, as it is known to occur for tropospheric lightning and laboratory air discharges. However, it is as yet unknown whether sprites can be a relevant source of nitrogen oxides for the upper atmosphere. In this paper, we study the sensitivity of the Whole Atmosphere Community Climate Model (WACCM) to sprite-like nitrogen oxide perturbations. We take a top-down approach to estimate what magnitude sprite perturbations should have to become significant as compared to other relevant atmospheric processes and study the sensitivity of the model response within the given uncertainties. We show that, based on current predictions by sprite streamer chemistry models, sprites can perturb Tropical NOx at 70 km altitude between 0.015 ppbv (buried in the background variability) and 0.15 ppbv (about 20%), adopting a local NOx production per sprite of 1.5·1023 and 1.5·1024 molecules respectively at this altitude. Below the lowest of the adopted values, sprites are irrelevant at global scales. Sprite NOx may build up to significantly larger amounts locally above active thunderstorms, further aided by other transient luminous events and possibly terrestrial gamma ray flashes. We also use model results to interpret the available observational studies and give recommendations for future campaigns.

Retrieving cloud geometrical extents from MIPAS/ENVISAT measurements with a 2-D tomographic approach

Clouds represent a critical factor in regulating the Earths atmosphere and its energy balance. Satellite instruments can measure the energy balance and global atmospheric properties only through an accurate knowledge of the vertical profile of cloudiness, which is as yet one of the key shortages in atmospheric science. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on-board the ENVISAT satellite, designed to infer the amount of atmospheric trace-gases, demonstrated also sensitivity to the radiation emitted from clouds. In order to model the effect of the geometrical extent of a cloud on MIPAS measurements, we developed a retrieval model capable to simulate cloud effects on broad spectral intervals accounting for the two-dimensional (2-D) variability of the atmosphere in the satellite orbit plane. The 2-D analysis revealed a sensitivity of MIPAS spectra to both the vertical and horizontal extents and the position of clouds along the instrument line of sight. One-dimensional models were found to underestimate Cloud Top Height (CTH) by approximating clouds as an infinite horizontal layer with a finite vertical extents. With the 2-D approach, we showed it is possible, for optically thin Polar Stratospheric Clouds (PSCs), to retrieve both CTH and horizontal dimension by analyzing simultaneously all the limb observations that come across the cloud with their field of view. For a selected case study we found a very good agreement for both PSC CTH and horizontal extents retrieved from MIPAS measurements and those retrieved from coincident CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarisation) measurements.

CHIMTEA—Chemical Impact of Thunderstorms on Earth’s Atmosphere

Since their accidental discovery in the 1990s, lightning-related sprites, other transient luminous events (TLEs), and terrestrial gamma-ray flashes have shown us how the impact of thunderstorms extends from the troposphere up to the upper atmosphere and ionosphere. Thunderstorms are a key player for the climate system, in particular through lightning-produced NOx and troposphere–stratosphere exchange. The CHemical Impact of Thunderstorms on Earth’s Atmosphere (CHIMTEA) project focused on TLE-producing thunderstorms and their possible impact on stratospheric NOx and ozone. The distribution and seasonal cycle of thunderstorm activity were studied through global lightning data and TLE observations over Europe. Michelson Interferometer for Passive Atmosphere Sounding (MIPAS)/Environmental Satellite (ENVISAT) measurements of NOx, ozone, and other related constituents from the upper troposphere to the mesosphere were analyzed with a 2D tomographic approach to quantify thunderstorm-induced changes and explore how to improve their detectability. The study included observations from Global Ozone Monitoring by Occultation of Stars (GOMOS)/ENVISAT, other satellites, and in situ measurements. The sensitivity of the measurements to sprite-NOx was investigated through ad hoc radiative transfer simulations quantifying reference thresholds. Global and regional observations showed sprite-NOx to be at the edge of current detectability, with no detectable impact on ozone. Model simulations were performed including for the first time a sprite-NOx parameterization in the Whole Atmosphere Community Climate Model (WACCM): it was shown that sprites may contribute significantly to tropical NOx in the middle mesosphere and reach detectable levels above particularly active thunderstorms. Extension of the adopted strategy to study lightning-NOx was recommended, whereas the modeling and multi-satellite approach was shown to be suitable in support to the upcoming space missions.

The VLF fingerprint of elves: Step-like and long-recovery early VLF perturbations caused by powerful ±CG lightning EM pulses

Summary form only given. Subionospheric VLF recordings are investigated in relation with intense cloud-to-ground (CG) lightning data. Lightning impacts the lower ionosphere via heating and ionization changes which produce VLF signal perturbations known as early VLF events. Typically, early events recover in about 100 s, but a small subclass does not recover for many minutes, known as long-recovery early events (LORE). In this study, we identify LORE as a distinct category of early VLF events, whose signature may occur either on its own or alongside the short-lived typical early VLF event. Since LORE onsets coincide with powerful lightning strokes of either polarity (±), we infer that they are due to long-lasting ionization changes in the uppermost D region ionosphere caused by electromagnetic pulses emitted by strong ±CG lightning peak currents of typically>250 kA, which are also known to generate elves. The LORE perturbations are detected when the discharge is located within ~250 km from the great circle path (GCP) of a VLF transmitter-receiver link. The probability of occurrence increases with stroke intensity and approaches unity for discharges with peak currents greater than ~300 kA. LOREs are nighttime phenomena that occur preferentially, at least in the present regional data set, during winter when strong ±CG discharges are more frequent and intense. The evidence suggests LORE as a distinct signature representing the VLF fingerprint of elves, a fact which, although was predicted by theory, it escaped identification in the long-going VLF research of lightning effects in the lower ionosphere.