Neil Arnold

Global observations of gravity waves from High Resolution Dynamics Limb Sounder temperature measurements: A yearlong record of temperature amplitude and vertical wavelength

[1] Global observations of gravity waves have been performed using the High Resolution Dynamics Limb Sounder (HIRDLS) temperature data. A background field that was derived by dynamically calculating 31 day (±15 day) means to block the stationary component and the slowly varying planetary-scale waves, was first subtracted from the HIRDLS temperature measurements. An additional step was then taken to remove rapidly moving planetary-scale waves by developing an along-track temperature filter, which was created by averaging the profiles within a 1000 km along-track window. Finally, each individual temperature perturbation vertical profile was analyzed using a fast Fourier transform to estimate gravity wave temperature amplitudes and vertical wavelengths. The investigation of the monthly mean gravity wave temperature amplitudes for the year 2006 found that gravity wave activity in the stratosphere is highly variable with season and can be very orographically dependent, especially in the winter extratropics. The monthly zonal means show that the peak vertical wavelengths correspond closely to the peak amplitudes. The increasing amplitudes and vertical wavelengths are faster and generated at lower altitudes in the winter extratropical and high-latitude stratosphere than those in the summer tropical stratosphere. This is consistent with the lower source altitudes of orographic gravity waves in the extratropics and high latitudes and the higher source altitudes of convectively generated gravity waves in the tropics. Three cases were studied for the observed gravity waves over large mountain ranges using the European Centre for Medium-Range Weather Forecasts wind data. Investigations of episodes of enhanced gravity wave activity over the southern Andes, the Cascade Range, and the Rockies in winter months of 2006 indicate that orographic gravity waves refract downwind from the mountains and propagate along the direction of the intense winds. By way of contrast, observations of gravity waves around the Himalayas show a strong relationship with the cyclones in that region.

Solar magnetic flux influences on the dynamics of the winter middle atmosphere

A number of observational studies have reported a connection between various lower atmospheric parameters and a range of geomagnetic and solar coronal indices. The bulk of the Suns magnetised plasma energy is absorbed in the high-latitude upper atmosphere, far removed from the troposphere and the stratosphere, so a physical mechanism which accounts for these correlations has proved elusive. In this paper, a mechanistic three-dimensional model of the atmosphere between 10 and 130 km has been developed to demonstrate that high energy particles from the solar wind can perturb the winter stratosphere significantly. Planetary waves provide an effective means of coupling solar-induced changes in the thermosphere down to the stratosphere. A qualitatively similar response to forcing by increasing solar ultraviolet radiation was obtained even though there was no in situ forcing in the stratosphere.

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)

The solar factor

Rasmus Benestad raises some legitimate concerns about the likelihood that our climate is affected by the activity of the Sun (July p 19). He takes issue with the hypothesis, first proposed by Henrik Svensmark and others, that recent increases in the Suns magnetic field have shielded the Earth from galactic cosmic rays, which have in turn reduced the number of clouds at low altitudes. Fewer clouds would let more solar energy in and cause less energy to be reflected back into space. Benestad argues that before we can make any inferences about the effect of changes in the Suns magnetic field on our climate, we must first determine the contribution made by Earths magnetic field.

SUN'S MAGNETIC SURPRISE

Over two thousand years ago Aristotle would have been surprised to learn that the Sun is not a perfect heavenly sphere, and that its brightness varies with time. Yet had Aristotle heard of the work of Meton, another ancient Greek scientist, he would have known that blemishes on the Sun had been seen at sunrise over a 20-year period. Meton also speculated about the possible impact of these sunspots on the weather, since they were seen predominantly during periods when the rainfall was above average.

Radio limb sounding: a technique for measuring global profiles of free electrons in the ionosphere

Abstract A technique is proposed to measure the electron density profile of the ionosphere between 100 and 800 km using a microwave frequency receiver on board a conventional Earth Observations Satellite and a series of transmitters on micro satellites in appropriate orbits. A pair of pulsed frequencies will enable the relative signal delay to be determined and hence the total electron content between the two devices. The vertical profiles of N along the satellite track can be retrieved using a modified form of tomography. Regular global coverage from a single experiment over several years would provide a unique data set for comparison with models of the ionosphere, and also its interaction with the neutral atmosohere and its response to changes in solar activity.