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Hydroxyl airglow temperatures above Davis Station, Antarctica

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French, W. John R.(William John Reginald) (2001) Hydroxyl airglow temperatures above Davis Station, Antarctica. PhD thesis, University of Tasmania.

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Abstract

The hydroxyl airglow (6-2) band has been monitored above Davis station, Antarctica
(68.6°S, 78.0°E) by means of a Czerny-Turner spectrometer since 1990. This thesis
is an investigation of the long-term trends and variability in OH(6-2) rotational-temperatures
over an 11 year time span.
Tropospheric warming, due to increased greenhouse gases concentrations over the
last 150 years, is expected to be associated with enhanced cooling in the stratosphere
and mesosphere. Modelling studies indicate a maximum cooling response in the high
latitude mesosphere. Some reported observations suggest that pronounced cooling,
(up to 7 K/decade) in excess of model predictions, has already taken place.
Hydroxyl airglow emissions originate near 87 km. The layer is ideally located to
monitor mesopause region temperatures by ground based spectroscopic means. The
Davis OH(6-2) database contains 8 years of observations that span 1990 to 2000;
over 126,000 spectra, which yield 1310 nightly averages. Observations are limited by
day length at Davis. Night-time observations are possible between day-of-year 45
(14-Feb) and 300 (27-Oct).
Analysis techniques are developed to optimise the rotational temperature
determinations for the P 1 (2), P 1(4) and P 1(5) ratio's in this band for climate change
studies. Precise calibration of the instrument spectral response is important for long-term
trend assessment. Inter-year spectral response variation of less than 0.3% has
been achieved. Temperature errors associated with the response calibration amount
to 1.5 K for earlier years and less than 0.3 K since 1996.
A detailed investigation of auroral and OH satellite line emissions, solar Fraunhofer
and water vapour absorption across the OH(6-2) region (λ837.5-851.5 nm) is
undertaken. These features can modify the apparent intensity of the P-branch lines.
P1(3) is rejected from further analysis due to un-thermalised contributions from the
OH(5-1)P1(12) lines. A correction is applied to account for the Q1(5) contribution
under P1(2). N2 1PG and Nᶧ2 Meinel auroral emissions and solar Fraunhofer
absorption are accounted for by appropriate selection of the backgrounds for each
line. Water vapour absorption is not found to be significant. Correction factors are
also applied to account for the difference in A-doubling between the P-branch lines.
These are derived from frequency-stabilised laser determinations of the instrument
bandwidth (which is 0.15nm). Errors associated with each correction are assessed.
Serendipitously, auroral emissions due to atmospheric Argon are identified for the
first time in this investigation. Two argon lines (at λ840.82 nm and λ842.46 nm,
between P1 (2) and P1 (3)) are apparent during intense auroral activity but do not
influence the rotational temperatures at the resolution of the Davis instrument.
As a scanning instrument is used, a 'sampling' error is also associated with the time
taken to acquire each spectrum due to possible intensity variations. A mean trend of
—1.1 K due to an average intensity decrease across the night, with a 7 K standard
deviation is found from coincident photometer observations. As a result of the
investigation of background features, acquisition times were reduced from the order
of 1 hour (in 1990) to 7.3 minutes (from the end of 1996) by scanning only selected
P-branch lines and background regions, which reduces the standard deviation.
Furthermore, an analysis technique for time interpolation of sampled branch lines
and backgrounds between spectra is developed, which removes the trend component.
Three sets of published transitions probabilities yield a 12 K range in the absolute
temperature derived. An experimental investigation of OH(6-2) Qi/Pi and R1/P1
emission intensity ratios, for rotational states up to j'=4.5, is undertaken to determine
which set is most suitable for application to the OH(6-2) band.
Selection criteria are established to reject spectra that do not yield consistent
temperatures for each of the three possible ratios, suffer low signal-to-noise, or are
contaminated by strong aurora, scattered moonlight or changing cloud conditions.
Nightly averages are determined from spectra that pass all criteria. Annual variations
are characterised by an extended warm (206 ± 4 K) winter period, with a gradual
decline (-0.04 K/day) over the interval DOY 106-258 and including episodic 10-20
day (planetary scale) variations of amplitude up to 30 K. Equinoctial transitions from
cold summer temperatures show a sharp rise in autumn (1.2 K/day; DOY 49-80) and
a more gradual spring decline (-0.65 K/day; DOY 275-296). The autumn transition
occurs earlier, and the spring transition later than either CIRA86 or MSISE-90 model
predictions. The midwinter local minimum in MSISE-90 is also not supported.
Mean winter temperatures calculated from the daily averages for each year are
consistent with a positive solar cycle association of 0.066 K/solar-flux-unit,
considerably lower than most values reported in the literature. Multivariate analysis
supports a long term cooling trend of the order of 0.5 K/year in the winter average
temperatures over Davis.

Item Type: Thesis (PhD)
Keywords: Middle atmosphere, Infrared spectroscopy, Atmospheric radiation, Atmosphere, Airglow
Copyright Holders: The Author
Copyright Information:

Copyright 2001 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Since 1990 infra-red emissions from the Hydroxyl airglow layer have been monitored above Davis station, Antarctica using a Czerny-Turner spectrometer, with the aim of measuring and recording middle atmosphere temperatures. This thesis is an investigation of the long-term trends and variability in OH(6-2) rotational-temperatures over an 11 year time span with its possible implications in tropospheric warming and corresponding cooling in the stratosphere and mesosphere. "Enrolled in this degree ... with the Institute of Antarctic and Southern Ocean Studies (IASOS)"- -Contents page iii. Thesis (Ph.D.)--University of Tasmania, 2002. Includes bibliographical references. Since 1990 infra-red emissions from the Hydroxyl airglow layer have been monitored above Davis station, Antarctica using a Czerny-Turner spectrometer, with the aim of measuring and recording middle atmosphere temperatures. This thesis is an investigation of the long-term trends and variability in OH(6-2) rotational-temperatures over an 11 year time span with its possible implications in tropospheric warming and corresponding cooling in the stratosphere and mesosphere

Date Deposited: 09 Dec 2014 00:07
Last Modified: 11 Mar 2016 05:54
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