Showing papers by "Kaley A. Walker published in 2018"
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TL;DR: In this article, the authors reported that the strong dynamical variability in the Arctic means that large ozone depletion events like those of 2010-2011 and 2015-2016 may still occur until the concentrations of ozone-depleting substances return to their 1960 values.
22 citations
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Karlsruhe Institute of Technology1, Earth System Research Laboratory2, Chalmers University of Technology3, University of Bremen4, Langley Research Center5, Spanish National Research Council6, National Center for Atmospheric Research7, University of Colorado Boulder8, National Institute of Information and Communications Technology9, United States Naval Research Laboratory10, International Federation of Accountants11, Jet Propulsion Laboratory12, University of Toronto13
TL;DR: In this paper, a comparison of time series of stratospheric and lower mesospheric water vapour using 33 data sets from 15 different satellite instruments were compared in the framework of the second SPARC (Stratosphere-troposphere Processes And Their Role in Climate) water vapours assessment (WAVAS-II).
Abstract: . Time series of stratospheric and lower mesospheric water vapour
using 33 data sets from 15 different satellite instruments were compared in
the framework of the second SPARC (Stratosphere-troposphere Processes And
their Role in Climate) water vapour assessment (WAVAS-II). This comparison
aimed to provide a comprehensive overview of the typical uncertainties in the
observational database that can be considered in the future in observational
and modelling studies, e.g addressing stratospheric water vapour trends. The
time series comparisons are presented for the three latitude bands, the
Antarctic (80 ∘ –70 ∘ S), the tropics
(15 ∘ S–15 ∘ N) and the Northern Hemisphere mid-latitudes
(50 ∘ –60 ∘ N) at four different altitudes (0.1, 3, 10 and
80 hPa ) covering the stratosphere and lower mesosphere. The combined
temporal coverage of observations from the 15 satellite instruments allowed
the consideration of the time period 1986–2014. In addition to the
qualitative comparison of the time series, the agreement of the data sets is
assessed quantitatively in the form of the spread (i.e. the difference
between the maximum and minimum volume mixing ratios among the data sets),
the (Pearson) correlation coefficient and the drift (i.e. linear changes of
the difference between time series over time). Generally, good agreement
between the time series was found in the middle stratosphere while larger
differences were found in the lower mesosphere and near the tropopause.
Concerning the latitude bands, the largest differences were found in the
Antarctic while the best agreement was found for the tropics. From our
assessment we find that most data sets can be considered in future
observational and modelling studies, e.g. addressing stratospheric and lower
mesospheric water vapour variability and trends, if data set specific
characteristics (e.g. drift) and restrictions (e.g. temporal and spatial
coverage) are taken into account.
18 citations
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TL;DR: In this paper, an evaluation has been performed of six different ozone loss estimation methods based on the same single observational dataset to determine the Arctic ozone loss (mixing ratio loss profiles and the partially-column ozone losses between 380 and 550 K).
Abstract: . Stratospheric ozone loss
inside the Arctic polar vortex for the winters between 2004–2005 and
2012–2013 has been quantified using measurements from the space-borne
Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS).
For the first time, an evaluation has been performed of six different ozone
loss estimation methods based on the same single observational dataset to
determine the Arctic ozone loss (mixing ratio loss profiles and the
partial-column ozone losses between 380 and 550 K). The methods used are the
tracer-tracer correlation, the artificial tracer correlation, the average
vortex profile descent, and the passive subtraction with model output from
both Lagrangian and Eulerian chemical transport models (CTMs). For the
tracer-tracer, the artificial tracer, and the average vortex profile descent
approaches, various tracers have been used that are also measured by ACE-FTS.
From these seven tracers investigated ( CH4 , N2O , HF ,
OCS , CFC-11, CFC-12, and CFC-113), we found that CH4 ,
N2O , HF , and CFC-12 are the most suitable tracers for
investigating polar stratospheric ozone depletion with ACE-FTS v3.5. The
ozone loss estimates (in terms of the mixing ratio as well as total column
ozone) are generally in good agreement between the different methods and
among the different tracers. However, using the average vortex profile
descent technique typically leads to smaller maximum losses (by approximately
15–30 DU) compared to all other methods. The passive subtraction method
using output from CTMs generally results in slightly larger losses compared
to the techniques that use ACE-FTS measurements only. The ozone loss
computed, using both measurements and models, shows the greatest loss during
the 2010–2011 Arctic winter. For that year, our results show that maximum
ozone loss (2.1–2.7 ppmv) occurred at 460 K. The estimated partial-column
ozone loss inside the polar vortex (between 380 and 550 K) using the
different methods is 66–103, 61–95, 59–96, 41–89, and 85–122 DU for
March 2005, 2007, 2008, 2010, and 2011, respectively. Ozone loss is difficult
to diagnose for the Arctic winters during 2005–2006, 2008–2009, 2011–2012,
and 2012–2013, because strong polar vortex disturbance or major sudden
stratospheric warming events significantly perturbed the polar vortex,
thereby limiting the number of measurements available for the analysis of
ozone loss.
17 citations
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TL;DR: In this article, the authors describe the stratospheric and mesospheric ozone (version V5r_O3_m22) distributions retrieved from MIPAS observations in the three middle atmosphere modes (MA, NLC, and UA) taken with an unapodized spectral resolution of 0.0625 cm−1 from 2005 until April 12, 2012.
Abstract: . In this paper we describe the stratospheric and mesospheric ozone (version V5r_O3_m22) distributions retrieved from MIPAS observations in the three middle atmosphere modes (MA, NLC, and UA) taken with an unapodized spectral resolution of 0.0625 cm−1 from 2005 until April 2012. O3 is retrieved from microwindows in the 14.8 and 10 µm spectral regions and requires non-local thermodynamic equilibrium (non-LTE) modelling of the O3 v1 and v3 vibrational levels. Ozone is reliably retrieved from 20 km in the MA mode (40 km for UA and NLC) up to ∼ 105 km during dark conditions and up to ∼ 95 km during illuminated conditions. Daytime MIPAS O3 has an average vertical resolution of 3–4 km below 70 km, 6–8 km at 70–80 km, 8–10 km at 80–90, and 5–7 km at the secondary maximum (90–100 km). For nighttime conditions, the vertical resolution is similar below 70 km and better in the upper mesosphere and lower thermosphere: 4–6 km at 70–100 km, 4–5 km at the secondary maximum, and 6–8 km at 100–105 km. The noise error for daytime conditions is typically smaller than 2 % below 50 km, 2–10 % between 50 and 70 km, 10–20 % at 70–90 km, and ∼ 30 % above 95 km. For nighttime, the noise errors are very similar below around 70 km but significantly smaller above, being 10–20 % at 75–95 km, 20–30 % at 95–100 km, and larger than 30 % above 100 km. The additional major O3 errors are the spectroscopic data uncertainties below 50 km (10–12 %) and the non-LTE and temperature errors above 70 km. The validation performed suggests that the spectroscopic errors below 50 km, mainly caused by the O3 air-broadened half-widths of the v2 band, are overestimated. The non-LTE error (including the uncertainty of atomic oxygen in nighttime) is relevant only above ∼ 85 km with values of 15–20 %. The temperature error varies from ∼ 3 % up to 80 km to 15–20 % near 100 km. Between 50 and 70 km, the pointing and spectroscopic errors are the dominant uncertainties. The validation performed in comparisons with SABER, GOMOS, MLS, SMILES, and ACE-FTS shows that MIPAS O3 has an accuracy better than 5 % at and below 50 km, with a positive bias of a few percent. In the 50–75 km region, MIPAS O3 has a positive bias of ≈ 10 %, which is possibly caused in part by O3 spectroscopic errors in the 10 µm region. Between 75 and 90 km, MIPAS nighttime O3 is in agreement with other instruments by 10 %, but for daytime the agreement is slightly larger, ∼ 10–20 %. Above 90 km, MIPAS daytime O3 is in agreement with other instruments by 10 %. At night, however, it shows a positive bias increasing from 10 % at 90 km to 20 % at 95–100 km, the latter of which is attributed to the large atomic oxygen abundance used. We also present MIPAS O3 distributions as function of altitude, latitude, and time, showing the major O3 features in the middle and upper mesosphere. In addition to the rapid diurnal variation due to photochemistry, the data also show apparent signatures of the diurnal migrating tide during both day- and nighttime, as well as the effects of the semi-annual oscillation above ∼ 70 km in the tropics and mid-latitudes. The tropical daytime O3 at 90 km shows a solar signature in phase with the solar cycle.
13 citations
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TL;DR: In this paper, a regression model is used to fit a 2-D surface to all available ACE-FTS OCS measurements as a function of day-of-year and latitude.
Abstract: . When computing climatological averages of atmospheric trace gas mixing ratios obtained from satellite-based measurements, sampling biases arise if data coverage is not uniform in space and time. Complete homogeneous spatio-temporal coverage is essentially impossible to achieve. Solar occultation measurements, by virtue of satellite orbits and the requirement of direct observation of the sun through the atmosphere, result in particularly sparse spatial coverage. In this study, a method is presented to adjust for such sampling biases when calculating climatological means. The method is demonstrated using carbonyl sulfide (OCS) measurements at 16 km altitude from the ACE-FTS (Atmospheric Chemistry Experiment Fourier Transform 15 Spectrometer). At this altitude, OCS mixing ratios show a steep gradient between the poles and equator. ACE-FTS measurements, which are provided as vertically resolved profiles, and integrated stratospheric OCS columns are used in this study. The bias adjustment procedure requires no additional observations other than the satellite data product itself and is expected to be generally applicable when constructing climatologies of long-lived tracers from sparsely and heterogeneously sampled satellite data. In a first step of the adjustment procedure, a regression model is used to fit a 2-D surface to all available ACE-FTS OCS measurements as a function of day-of-year and latitude. The regression model fit is used to calculate an adjustment factor, 20 which is then used to adjust each measurement individually. The mean of the adjusted measurement points of a chosen spatio-temporal frame is then used as the bias-free climatological value. When applying the adjustment factor to seasonal averages in 30° zones, the maximum spatio-temporal sampling bias adjustment was 11 % for OCS mixing ratios at 16 km and 5 % for the stratospheric OCS column. The adjustments were validated against the much denser and more homogeneous OCS data product from the limb-sounding MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) instrument, and both the direction and sign of the adjustments were in agreement with the adjustment of the ACE-FTS data.
2 citations
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05 Nov 2018TL;DR: In this paper, the authors describe the current validation results for the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) with a focus on long-term validation efforts and overall mission status for this instrument.
Abstract: This paper will describe the current validation results for the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) with a focus on long-term validation efforts and overall mission status for this instrument.
2 citations
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05 Nov 2018
TL;DR: In this paper, the authors describe Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) N2O measurements in the middle atmosphere along with its production mechanisms and long-term variation.
Abstract: This paper will describe Atmospheric Chemistry – Experiment Fourier Transform Spectrometer (ACE-FTS) N2O measurements in the middle atmosphere along with its production mechanisms and long-term variation. Comparisons with model N2O simulations will also be discussed.