J. Stegman

Review of mesospheric temperature trends

In recent times it has become increasingly clear that releases of trace gases from human activity have a potential for causing change in the upper atmosphere. However, our knowledge of systematic changes and trends in the temperature of the mesosphere and lower thermosphere is relatively limited compared to the Earths lower atmosphere, and not much effort has been made to synthesize these results so far. In this article, a comprehensive review of long-term trends in the temperature of the region from 50 to 100 km is made on the basis of the available up-to-date understanding of measurements and model calculations. An objective evaluation of the available data sets is attempted, and important uncertainly factors are discussed. Some natural variability factors, which are likely to play a role in modulating temperature trends, are also briefly touched upon. There are a growing number of experimental results centered on, or consistent with, zero temperature trend in the mesopause region (80–100 km). The most reliable data sets show no significant trend but an uncertainty of at least 2 K/decade. On the other hand, a majority of studies indicate negative trends in the lower and middle mesosphere with an amplitude of a few degrees (2–3 K) per decade. In tropical latitudes the cooling trend increases in the upper mesosphere. The most recent general circulation models indicate increased cooling closer to both poles in the middle mesosphere and a decrease in cooling toward the summer pole in the upper mesosphere. Quantitatively, the simulated cooling trend in the middle mesosphere produced only by CO 2 increase is usually below the observed level. However, including other greenhouse gases and taking into account a “thermal shrinking” of the upper atmosphere result in a cooling of a few degrees per decade. This is close to the lower limit of the observed nonzero trends. In the mesopause region, recent model simulations produce trends, usually below 1 K/decade, that appear to be consistent with most observations in this region

ETON 2: Quenching parameters for the proposed precursors of O2(b1Σg+) and O(1S) in the terrestrial nightglow

Abstract Volume emission profiles of the O 2 ( b 1 Σ g + − X 3 Σ g t - )( O - O ) Atmospheric Band and the O( 1 S- 1 D) green line are used together with coordinated measurements of the atomic oxygen concentrations to test the hypothesis that both emissions are excited by Barth type mechanisms. By considering O 2 ( b 1 Σ g + ) to be produced from an excited O 2 precursor, with O 2 as transfer agent, and O( 1 S) to be formed from a similar precursor with atomic oxygen as the transfer agent, precursor quenching rates are obtained as a function of altitude. These quenching profiles can be well resolved into components corresponding to collisional deactivation by O( 3 P) and O 2 (or N 2 ), and support the suggestion that Barth type mechanisms are involved. Minimum efficiencies for the production of the two precursors in oxygen atom recombination and ratios for the quenching of each by O( 3 P) and O 2 (or N 2 ) are deduced. Differences in the quenching coefficients for the two precursors are discussed.

A measurement of the O2(b1Σg+−X3Σg−) atmospheric band and the OI(1S) green line in the nightglow

Abstract Simultaneous measurements of the nightglow profiles of the O2(b1Σg+−X3Σg−) A-band, the atomic oxygen green line and the OH (8−3) Meinel band are presented. The altitude profiles are used to determine both the excitation mechanisms for the oxygen emissions and the atomic oxygen altitude distribution. It is shown that the measurements are consistent with a green line excitation through the Barth mechanism and that the molecular oxygen emission is excited through oxygen recombination and the reaction between OH∗ and atomic oxygen. The derived atomic oxygen concentrations,6.2 × 1011cm−3at 98km, are consistent with the Jacchia (1971) model.

ETON 1: A data base pertinent to the study of energy transfer in the oxygen nightglow

Abstract A comprehensive group of experiments, assembled and flown on a series of Petrel rockets in March 1982 for the primary purpose of investigating the extent to which energy transfer is important in the excitation of the oxygen nightglow, is briefly described. Aspects of the data reduction methods are summarised and the bulk of the processed data is presented in profile and tabular form as a base for modelling and analyses in papers to follow and in evidence of the quality and success of this memorable international collaborative campaign.

Interannual variations of the quasi‐16‐day oscillation in the polar summer mesospheric temperature

Nighttime measurements of the hydroxyl Meinel (4,2) rotational band have been used to infer the mesospheric temperature over Scandinavia from June to August during the years 1992–1995. While the nightly averaged temperatures show a statistically significant, quasi-16-day oscillation in the 1992 and 1994 summer data, none is observed during 1993 and 1995. When present, the period, amplitude, and temporal behavior of this oscillation agree with both model predictions and previous wind measurements of the (1,3) Rossby normal mode in the summer mesosphere. Thus this temperature oscillation appears to correspond to the thermal signature of the 16-day Rossby mode. Its appearance in the summer mesosphere is shown to occur when the oscillating zonal flows in the upper stratosphere near the equator are in an eastward phase, while it appears to be blocked during the westward phases. This correspondence of the 16-day wave in the summer mesosphere with the eastward equatorial wind would favor the explanation that it is generated in the winter hemisphere and propagates vertically and toward the summer pole following the westerly mean winds.

An assessment of proposed O(1S) and O2(b1Σg+) nightglow excitation parameters

Abstract A database consisting of a number of simultaneously measured O( 1 D- 1 S) green line and O 2 ( b 1 Σ g + ) − X 3 Σ g )(0,0) atmospheric band nightglow emission profiles is examined to assess the general validity of the airglow excitation parameters recently proposed by McDade et al . (1986, Planet. Space Sci ., 34 , 789). The measured profiles were obtained under quite diverse atmospheric conditions and should, therefore, allow a critical assessment of the proposed parameters. Model green line emission profiles, calculated from the measured O 2 atmospheric band emission profiles using the proposed parameters, are compared with the green line profiles actually measured on each occasion. The measured and modelled green line profiles are found to be in good agreement under most conditions. The cases for which discrepancies exist are discussed in terms of inadequacies in either the background atmosphere adopted for the analyses or in the parameters themselves.

Trends and variability of mesospheric temperature at high-latitudes

Abstract Using ground-based measurements of the hydroxyl (OH) Meinel (3,1) band nightglow near 1500 nm, nightly means of mesospheric temperature and OH radiance from 1991 to 1998 have been derived over Stockholm (59.5°N, 18.2°E). Time-series analysis techniques applied both to the eight-year data set as well as to an annual superposed epoch revealed several statistically significant periodic components. A trend analysis that included these periodic components revealed a small positive trend over the eight-year temperature time series. However, examining the trends on a month-to-month basis revealed positive trends during winter, small negative trends during equinox, and no significant trend during summer. This seasonal variability indicates that dynamic feedbacks, rather than radiative forcing of the mesosphere by infrared active gases, may dominate the response of the mesosphere to greenhouse gas emissions. In support of this an examination of the variability in the superposed epoch of OH temperature and radiance showed strong impulses near equinox. A simple gravity-wave transmission and dissipation model indicates that these are due in part to seasonal increases in the gravity-wave transmission of the lower atmosphere, and enhanced wave heating and mixing in the mesosphere.

Polar vortex evolution during the 2002 Antarctic major warming as observed by the Odin satellite

In September 2002 the Antarctic polar vortex split in two under the influence of a sudden warming. During this event, the Odin satellite was able to measure both ozone (O3) and chlorine monoxide (ClO), a key constituent responsible for the so-called “ozone hole”, together with nitrous oxide (N2O), a dynamical tracer, and nitric acid (HNO3) and nitrogen dioxide (NO2), tracers of denitrification. The submillimeter radiometer (SMR) microwave instrument and the Optical Spectrograph and Infrared Imager System (OSIRIS) UV-visible light spectrometer (VIS) and IR instrument on board Odin have sounded the polar vortex during three different periods: before (19–20 September), during (24–25 September), and after (1–2 and 4–5 October) the vortex split. Odin observations coupled with the Reactive Processes Ruling the Ozone Budget in the Stratosphere (REPROBUS) chemical transport model at and above 500 K isentropic surfaces (heights above 18 km) reveal that on 19–20 September the Antarctic vortex was dynamically stable and chemically nominal: denitrified, with a nearly complete chlorine activation, and a 70% O3 loss at 500 K. On 25–26 September the unusual morphology of the vortex is monitored by the N2O observations. The measured ClO decay is consistent with other observations performed in 2002 and in the past. The vortex split episode is followed by a nearly complete deactivation of the ClO radicals on 1–2 October, leading to the end of the chemical O3 loss, while HNO3 and NO2 fields start increasing. This acceleration of the chlorine deactivation results from the warming of the Antarctic vortex in 2002, putting an early end to the polar stratospheric cloud season. The model simulation suggests that the vortex elongation toward regions of strong solar irradiance also favored the rapid reformation of ClONO2. The observed dynamical and chemical evolution of the 2002 polar vortex is qualitatively well reproduced by REPROBUS. Quantitative differences are mainly attributable to the too weak amounts of HNO3 in the model, which do not produce enough NO2 in presence of sunlight to deactivate chlorine as fast as observed by Odin.

Stratospheric profiles of nitrogen dioxide observed by Optical Spectrograph and Infrared Imager System on the Odin satellite

[1] Vertical profiles of nitrogen dioxide in the 19–40 km altitude range are successfully retrieved over the globe from Optical Spectrograph and Infrared Imager System (OSIRIS) limb scatter observations in late 2001 and early 2002. The inclusion of multiple scattering in the radiative transfer model used in the inversion algorithm allows for the retrieval of NO2 down to 19 km. The slant column densities, which represent the observations in the inversion, are obtained by fitting the fine structure in normalized radiance spectra over the 435–449 nm range, where NO2 electronic absorption is readily observable because of long light paths through stratospheric layers rich in this constituent. Details of the spectral fitting and inversion algorithm are discussed, including the discovery of a pseudo-absorber associated with pixelated detectors and a new method to verify altitude registration. Comparisons are made with spatially and temporally coincident profile measurements of this photochemically active trace gas. Better than 20% agreement is obtained with all correlative measurements over the common retrieval altitude range, confirming the validity of OSIRIS NO2 profiles. Systematic biases in the number densities are not observed at any altitude. A ‘‘snapshot’’ meridional cross section between 40� N and 70� S is shown from observations during a fraction of an orbit. INDEX TERMS: 0340 Atmospheric Composition and Structure: Middle atmosphere—composition and chemistry; 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 0394 Atmospheric Composition and Structure: Instruments and techniques; 3334 Meteorology and Atmospheric Dynamics: Middle atmosphere dynamics (0341, 0342); KEYWORDS: optical, Sun-synchronous, polar-orbiting, Fraunhofer, Ring effect, iterative onion peel

Mean state densities, temperatures and winds during the MAC/SINE and MAC/EPSILON campaigns

Abstract During 1987 two major field campaigns were conducted, mainly in northern Norway (in summer and late autumn), in which a total of 41 (26+15) in-situ temperature profiles were obtained by different techniques such as passive falling spheres, ionizalion gauges and mass spectrometers. Simultaneously, ground-based measurements of OH-temperatures and sodium lidar temperatures were performed for approximately 85 h and 104 h, respectively. In addition, a total of 67 (37 + 30) wind profiles were measured by in-situ techniques. Several radar systems measured winds almost continuously before, during and after the campaigns. The mean temperature profile for the summer campaign showed major deviations from a recently published reference atmosphere (CIRA 1986), whereas the differences between observations and model are smaller in autumn. In general, both the summer and autumn mean wind profiles agreed with CIRA 1986. Minor differences were attributed to tidal biases of the observations and ageostrophic components.