ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-101-2016Sensitivity of polar stratospheric cloud formation to changes in water vapour and temperatureKhosrawiF.farahnaz.khosrawi@kit.eduUrbanJ.https://orcid.org/0000-0001-7026-793XLossowS.https://orcid.org/0000-0003-2833-0522StillerG.https://orcid.org/0000-0003-2883-6873WeigelK.https://orcid.org/0000-0001-6133-7801BraesickeP.PittsM. C.https://orcid.org/0000-0001-8240-7223RozanovA.BurrowsJ. P.https://orcid.org/0000-0003-1547-8130MurtaghD.https://orcid.org/0000-0003-1539-3559Department of Meteorology, Stockholm University, Stockholm, SwedenDepartment of Earth and Space Science, Chalmers University of Technology, Gothenburg, SwedenInstitute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, GermanyInstitute of Environmental Physics, University of Bremen, Bremen, GermanyNASA Langley Research Center, Hampton, USAnow at: Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germanydeceased, 14 August 2014F. Khosrawi (farahnaz.khosrawi@kit.edu)15January20161611011219March20151July201510December201515December2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/16/101/2016/acp-16-101-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/101/2016/acp-16-101-2016.pdf
More than a decade ago it was suggested that a cooling of stratospheric
temperatures by 1 K or an increase of 1 ppmv of stratospheric
water vapour could promote denitrification, the permanent removal of nitrogen
species from the stratosphere by solid polar stratospheric cloud (PSC)
particles. In fact, during the two Arctic winters 2009/10 and 2010/11 the
strongest denitrification in the recent decade was observed. Sensitivity
studies along air parcel trajectories are performed to test how a future
stratospheric water vapour (H2O) increase of 1 ppmv or
a temperature decrease of 1 K would affect PSC formation. We perform
our study based on measurements made during the Arctic winter 2010/11. Air
parcel trajectories were calculated 6 days backward in time based on
PSCs detected by CALIPSO (Cloud Aerosol Lidar and Infrared Pathfinder
satellite observations). The sensitivity study was performed on single
trajectories as well as on a trajectory ensemble. The sensitivity study shows
a clear prolongation of the potential for PSC formation and PSC existence
when the temperature in the stratosphere is decreased by 1 K and
water vapour is increased by 1 ppmv. Based on 15 years of
satellite measurements (2000–2014) from UARS/HALOE, Envisat/MIPAS, Odin/SMR,
Aura/MLS, Envisat/SCIAMACHY and SCISAT/ACE-FTS it is further investigated if
there is a decrease in temperature and/or increase of water vapour
(H2O) observed in the polar regions similar to that observed at
midlatitudes and in the tropics. Performing linear regression analyses we
derive from the Envisat/MIPAS (2002–2012) and Aura/MLS (2004–2014)
observations predominantly positive changes in the potential temperature
range 350 to 1000 K. The linear changes in water vapour derived from
Envisat/MIPAS observations are largely insignificant, while those from
Aura/MLS are mostly significant. For the temperature neither of the two
instruments indicate any significant changes. Given the strong inter-annual
variation observed in water vapour and particular temperature the severe
denitrification observed in 2010/11 cannot be directly related to any changes
in water vapour and temperature since the millennium. However, the
observations indicate a clear correlation between cold winters and enhanced
water vapour mixing ratios. This indicates a connection between dynamical and
radiative processes that govern water vapour and temperature in the Arctic
lower stratosphere.
Introduction
Polar stratospheric clouds (PSCs) form in the polar winter stratosphere at
altitudes between 15 to 30 km. PSCs consist of liquid and solid
particles and have been classified into three different types based on their
composition and physical state: (1) supercooled ternary solutions (STS),
(2) Nitric Acid Trihydrate (NAT) and (3) ice. The formation of PSCs is
strongly temperature dependent. Liquid PSC cloud particles (STS) form by the
condensation of water vapour (H2O) and nitric acid (HNO3) on
the liquid stratospheric background sulfate aerosol particles at temperatures
2–3 K below the NAT existence temperature TNAT (∼195K at 20 km) while for the formation of solid cloud
particles (ice) much lower temperatures are required, usually 3–4 K
below the ice frost point Tice (∼185K at
20 km) e.g.. The formation of the
liquid STS particles is quite well understood, however, the exact formation
mechanism of NAT and ice PSC particles still leaves some unresolved questions
and is still an active area of research.
Progress in understanding PSC particle formation processes has been made
recently in the frame of the European project RECONCILE (Reconciliation of
essential process parameters for an enhanced predictability of Arctic
stratospheric ozone loss and its climate interactions) .
For example, CALIPSO measurements for the Arctic winter 2009/10 presented by
showed that widespread NAT formation occurred, albeit in
low number densities, before ice clouds had been formed at temperatures well
above Tice. Further, lidar measurements performed during recent
years have also indicated that there must be a formation mechanism for NAT
PSCs above the ice frost point Tice without ice particles
necessarily serving as a nucleation kernel for NAT particles. Until the
RECONCILE project, the only known pathway to form NAT was through
heterogeneous nucleation on ice particles, forming NAT clouds downstream of
mountain wave ice clouds .
Heterogeneous nucleation on particles such as meteoric smoke has been
suggested to be a potential pathway for NAT formation
. performed box model
simulations along air parcel trajectories based on the observations by
CALIPSO made during the Arctic winter 2009/10 applying a new parameterisation
for heterogeneous NAT nucleation, assuming NAT formation on particles as e.g.
meteoric smoke. The CALIPSO observations were well reproduced by the model
simulations applying this new parameterisation thus indicating that NAT
nucleation on other particles than ice is possible. Further, both the
modelling study by and the one by using
the Zurich Optical and Microphysical box Model (ZOMM) showed that small-scale
temperature fluctuations usually not represented in meteorological data
needed to be considered to reproduce the CALIPSO observations.
Denitrification, the permanent removal of HNO3 by sedimenting polar
stratospheric cloud particles, limits the deactivation process of the ozone
destroying substances in springtime and thus leads to a prolongation of the
ozone destroying cycles. Stratospheric cooling caused by increasing
greenhouse gas concentrations will have significant implications on
denitrification and ozone loss. Model simulations predict that very large
ozone losses will occur more frequently in the future in the Arctic and that
the recovery of the ozone layer will be delayed by more than a decade due to
increased greenhouse gas concentrations
e.g..
Water vapour is one of the most important greenhouse gases and plays a key
role in the chemistry and radiative balance of the upper troposphere and
lower stratosphere (UT/LS). Several studies have been performed in the past
investigating stratospheric water vapour trends using in situ and
remote-sensing measurements
e.g..
combined 10 data sets covering the time period
1954–2000 and found a 1 % yr-1 (0.054 ppmv yr-1) increase in
lower stratospheric water vapour in the mid-latitudes. Long-term
balloon-borne measurements at Boulder, Colorado
(40∘ N/105∘ W) indicate an increase of lower
stratospheric water vapour abundances, on average by 1 ppmv, during
the last 30 years (1980–2010) .
Recently, analysed a merged satellite time series
spanning from the late 1980s to 2010, which did not confirm the findings from
the Boulder data set, arguing the representativeness of these data on a
larger spatial scale. In the lower stratosphere negative changes were
dominating, while positive changes were found only in the upper part of the
stratosphere. The decrease in the lower stratosphere was attributed to a
strengthened lower stratospheric circulation. A decisive role here played a
pronounced drop in water vapour in 2000 (also known as the millennium drop)
, that first started to
recover in 2004 to 2005. This drop was caused by a reduced transport of water
vapour from the troposphere into the stratosphere in response to a colder
tropical tropopause. The temperature decrease has been due to variations of
the QBO (quasi-biennial oscillation), ENSO (El Niño Southern Oscillation)
and the Brewer-Dobson circulation that collectively acted in the same
direction lowering the tropopause temperatures. In 2011 such a drop happened
again, however more short-lived .
analysed satellite data together with a trajectory model.
They did not see any firm evidence of trends (neither positive nor negative)
in the data since the mid 1980s. However, they cannot rule out that a trend
exists that is just too small to be identified given the large inter-annual
and inter-decadal variability. presented model
simulations of 18 coupled Chemistry Climate Models (CCMs) in the tropical
tropopause layer (TTL). The models simulate decreases in the tropopause
pressure in the 21st century, along with ∼1K increases per
century in cold point temperature and 0.5–1 ppmv per century
increases in water vapour above the tropical tropopause.
Any changes in atmospheric water vapour bring important implications for the
global climate. Increases in stratospheric water vapour cool the stratosphere
but warm the troposphere. Both the cooling of the stratosphere and the
increase in water vapour enhance the potential for the formation of polar
stratospheric clouds. More than a decade ago it was already suggested that
a cooling of stratospheric temperatures by 1 K or an increase of
1 ppmv of stratospheric water vapour could promote denitrification
. During the two Arctic winters 2009/10 and
2010/11, the strongest denitrification in the recent decade was observed
. In the latter winter, denitrification led
also to severe ozone depletion with a magnitude comparable to the Antarctic
“ozone hole”
.
In this study, the correlation between observed water vapour variability and
the recent temperature evolution in the Arctic together with PSC observations
are considered to investigate a possible connection between the increase in
stratospheric water vapour and polar stratospheric cloud
formation/denitrification. This study aims at (1) performing a sensitivity
study on how an increase in water vapour and decrease in temperature will
affect PSC formation and existence and (2) on assessing the H2O
variability during the 15-year period 2000–2014.
A sensitivity study is performed to investigate what effect changes in
water vapour and temperature (due to a trend or variability) would have on
PSC formation and occurrence. Therefore, air parcel back trajectories are
calculated according to PSC observations by CALIPSO during the Arctic winter
2010/11. On the basis of this trajectory ensemble the increase in time the
air parcels would be exposed to temperatures below TNAT or
Tice along the trajectories are considered for a water vapour
increase of up to 1 ppmv and a temperature decrease of up to
1 K in the stratosphere.
Measurements from several different satellites together with
temperatures from ECMWF are used to investigate water vapour trends and
variability in the polar stratosphere. So far trend studies in stratospheric
water vapour have focused on the tropics and mid-latitudes. Here, for the
first time such an analysis has been performed for the polar stratosphere. We
use satellite measurements that were derived for the 15-year period
2000–2014.
Satellite data
To investigate a possible water vapour trend as well as water vapour
variability in the polar lower stratosphere, satellite observations of water
vapour from the Odin Sub-Millimetre Radiometer (Odin/SMR), the Aura Microwave
Limb Sounder (Aura/MLS), the Envisat Michelson Interferometer for Passive
Soundings (Envisat/MIPAS), the Scanning Imaging Absorption spectrometer for
Atmospheric Chartography (Envisat/SCIAMACHY), the SCISAT Atmospheric
Chemistry Experiment Fourier Transform Spectrometer (SCISAT/ACE-FTS) and the
UARS Halogen Occultation Experiment (UARS/HALOE) are used. A short
description of these satellite instruments will follow below. A detailed
intercomparison of water vapour derived from these instruments can be found
in . For performing case studies along air parcel
trajectories that are based on PSC measurements we apply measurements from
the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on board of
CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations).
Odin/SMR
Odin/SMR was launched on 20 February 2001 and it observes the thermal
emission of trace gases from the Earth's limb. Odin carries two instruments,
the Optical Spectrograph and Infrared Imaging System (OSIRIS)
and the Sub-Millimetre Radiometer (SMR)
. Observations by Odin/SMR were performed in a time-sharing
mode with astronomical observations until 2007 and solely in aeronomy mode
thereafter. In aeronomy mode, various target bands are dedicated to profile
measurements of trace constituents relevant to stratospheric and mesospheric
chemistry and dynamics such as O3, ClO, N2O, HNO3,
H2O, CO, HO2 and NO, as well as minor isotopologues of
H2O and O3e.g.. Stratospheric mode
measurements were performed every third day until April 2007 and every other
day thereafter. A typical stratospheric mode scan covers the altitude range
from 7 to 70 km with a resolution of ∼1.5km in terms
of tangent altitude below 50 km and of ∼5.5km above.
Usually, the latitude range between 82.5∘ S and 82.5∘ N is
observed . Water vapour measurements are derived
by Odin/SMR in several different bands in the sub-millimetre range. Here,
level-2 data from the 544.6 GHz band of version 2.0 for the lower
stratosphere are used .
Aura/MLS
MLS on board Aura is part of the NASA/ESA “A-train” satellite
constellation. MLS was launched in July 2004 and is an advanced successor of
the MLS instrument on the Upper Atmosphere Research Satellite (UARS) that was
launched in 1991 and provided measurements until 1999. MLS is a limb sounding
instrument that measures the thermal emission at millimetre and
sub-millimetre wavelengths using seven radiometers to cover five broad
spectral regions . Measurements are performed from the
surface to 90 km with a global latitude coverage from 82∘ S
to 82∘ N. Global water vapour measurements are derived from a line
close to the 183 GHz band . Version 3.3 data is
used in this study .
Envisat/MIPAS
MIPAS is a middle infrared Fourier transform spectrometer and was launched in
March 2002 on board Envisat. MIPAS was operational until the sudden loss of
contact with Envisat on 8 April 2012. MIPAS measured the atmospheric emission
spectrum in the limb sounding geometry. MIPAS operated in its nominal
observation mode from June 2002 to March 2004, thus approximately 2 years.
Measurements during this time period were performed in its full spectral
resolution measurement mode with a designated spectral resolution of
0.035 cm-1. Measurements were performed covering the altitude
range from the mesosphere to the troposphere with a high vertical resolution
(about 3 km in the stratosphere). After a failure of the
interferometer slide at the end of March 2004, MIPAS resumed measurements in
January 2005 with a reduced spectral resolution of 0.0625 cm-1,
but with improved spatial resolution. Data products of MIPAS are up to 30
trace species, e.g. H2O, O3, HNO3, CH4,
N2O, NO2 as well as temperature
. Here, the MIPAS data version
V5H_H2O_20 and V5R_H2O_220/221 derived with the IMK/IAA retrieval
processor covering the periods 2002–2003 and 2005–April 2011/May
2011–2012, respectively, have been used (updated version of the retrieval as
described in ).
Envisat/SCIAMACHY
SCIAMACHY was launched on board Envisat in March 2002 and was in operation
from August 2002 until the sudden loss of contact with Envisat on
8 April 2012. SCIAMACHY observed electromagnetic radiation upwelling from the
Earth's atmosphere in three measurement modes: occultation, nadir, and limb
geometry. The instrument and mission objectives are provided by
and . In this study, measurements
of the scattered solar light in limb viewing geometry are used. In this
geometry, the instrument scanned the horizon in 3.3 km steps from
-3 to 92 km (0 to 92 km since October 2010). This vertical
sampling and the instantaneous field of view (∼2.6km in
vertical direction at the tangent point) resulted in a vertical resolution of
typically 3–4 km. Envisat was in a Sun-synchronous orbit with an
inclination of 98∘. This resulted in global coverage for
SCIAMACHY limb measurements being achieved within 6 days at the
Equator and less elsewhere . SCIAMACHY target species are
O3, BrO, OClO, ClO, SO2, H2CO, NO2, CO,
CO2, CH4, H2O, N2O, aerosol and clouds. In
the limb viewing geometry water vapour is retrieved at about 10 to
25 km altitude from the near-infrared spectral range
(1353–1410 nm). Here, we use SCIAMACHY water vapour from data
version 3.01. The IUP Bremen water vapour retrieval algorithm V3.01 follows
the retrieval concept presented in for V3 and the new
data set is described in .
SCISAT/ACE-FTS
The ACE mission was launched on 12 August 2003 on-board the SCISAT satellite.
SCISAT is a Canadian-led satellite mission and carries two instruments, the
ACE Fourier Transform Spectrometer (ACE-FTS) and the Measurement of Aerosol
Extinction in the Stratosphere and Troposphere Retrieved by Occultation
(ACE-MAESTRO). ACE-FTS is a solar occultation instrument and has been
providing measurements since 2004 . Measurements are
performed during sunrise and sunset (resulting in 15 sunrise and 15 sunset
measurements per day). A seasonally varying coverage of the globe is
provided, with an emphasis on mid-latitudes and the polar regions. The
ACE-FTS measurements provide vertical profiles of more than 30 atmospheric
species as well as temperature and pressure. The 14 baseline atmospheric
species measured by ACE-FTS are O3, H2O, HCl, CCl3F,
CCl2F2, CH4, HF, N2O, CO, NO, NO2,
HNO3, ClONO2, and N2O5. ACE-FTS
version 3.5 data have been used in this study.
UARS/HALOE
The Halogen Occultation Experiment (HALOE) was launched aboard the Upper
Atmosphere Research Satellite (UARS). HALOE is as ACE-FTS a solar occultation
instrument . The geometry of the UARS orbit (57∘
inclination, circular at 585 km with orbit period of 96 min)
results in 15 sunrise and 15 sunset measurements daily. Measurements between
80∘ N and 80∘ S in about 45 days are performed.
HALOE was launched in September 1991 and provided measurements until 2005,
thus over a time period of 14 years. Therefore,
HALOE provides the longest satellite data set though it is not used to its
full extent in this study since we focus on measurements obtained since the
millennium. HALOE Version 19 data are used in this study.
CALIPSO/CALIOP
CALIPSO is part of the NASA/ESA “A-train” satellite constellation and has
been in operation since June 2006. Measurements of PSCs are provided by
CALIOP . CALIOP is a two-wavelength, polarisation sensitive
lidar. High vertical resolution profiles of the backscatter coefficient at
532 and 1064 nm as well as two orthogonal (parallel and
perpendicular) polarisation components at 532 nm are provided
. The lidar pulse rate is 20.25 Hz,
corresponding to one profile every 333 m horizontally. The vertical
resolution of CALIOP varies with altitude from 30 m in the lower
troposphere to 180 m in the stratosphere. For the PSC analyses, the
CALIPSO profile data are averaged to a resolution of 180 m vertically
and 5 km horizontally. The determination of the composition of PSCs
is based on the measured aerosol depolarisation ratio (ratio of parallel and
perpendicular components of 532 nm backscatter) and the inverse
scattering ratio (1/R532), where R532 is the ratio of the total
to molecular backscatter at 532 nm. Using
these two quantities, PSCs are classified into STS, water ice, and three
classes of liquid/NAT mixtures (Mix-1, Mix-2 and Mix-2 enhanced). Mix-1
denotes mixtures with very low NAT number densities (from about 3×10-4 to 10-3cm-3), Mix-2 denotes mixtures with
intermediate NAT number densities of (10-3cm-3), and Mix-2
enhanced denotes mixtures with sufficiently high NAT number densities
(>0.1cm-3) and volumes (>0.5µm3cm-3) that their presence is not masked by the more
numerous STS droplets at temperatures well below TNAT. In
addition, intense mountain-wave induced ice PSCs are identified as a subset
of CALIPSO ice PSCs through their distinct optical signature in R532.
Arctic winter 2010/11
The Arctic winter 2010/11 was one of the coldest in the last 2 decades
. The 2010/11 winter was characterised by an
anomalously strong vortex with an atypically long cold period that was
persistent from mid-December to mid-March . The polar
vortex formed at the end of November 2010 and remained stable until the end
of April. The long cold period, lasting over 4 months, was interrupted by
short warmer periods in the beginning of January, February and March due to
minor warmings. In February and March, temperatures were colder than in
previous years of the last decade. The final warming during the 2010/11
Arctic winter occurred in mid-April, thus later than usual
.
The PSC season during the 2010/11 winter can be divided into four PSC phases
according to the four cold phases that occurred over the 4-month period
from December 2010 to March 2011. The time periods of these four phases and
the PSC types that occurred during each phase were derived from CALIPSO
observations and are as follows : (1) 23 December 2010 to
8 January 2011: STS, Mix 1/2 and ice clouds. (2) 20–28 January 2011:
mainly Mix-1 and Mix-2 with some STS and ice. (3) 5–27 February: STS, Mix-1
and Mix-2 as well as ice clouds. (4) 5–19 March: STS clouds (Note: no
CALIPSO data are available from 8 to 13 March).
A graphic presentation of the temporal evolution of time, VPSC,
T-TNAT and several trace gases during this winter can be found in
and . performed
a similar analysis on PSC occurrence as we did, but used MIPAS observations
for PSC detection. They also derive four PSC phases from MIPAS, however with
somewhat different time periods for each phase as the ones we derive from
CALIPSO. Differences in PSC detection between both instruments are caused by
the different measurement principle (active lidar in the visible vs. passive
spectroscopy in the infrared spectral region). CALIPSO generally detects PSCs
in a greater fraction than MIPAS. This can be explained by the patchier
nature of PSCs in the Arctic and the different spatial resolutions of the two
instruments, which makes the clouds more likely to be detected by the CALIPSO
lidar . The Arctic winter 2010/11 has been well analysed,
especially with respect to ozone loss
while
the dynamical perspective, thus the exceptional dynamical conditions of this
winter so far were only discussed in detail by .
Sensitivity studies
The sensitivity study is performed based on measurements of PSCs by CALIPSO
during the Arctic winter 2010/11. The basic approach is demonstrated on
single trajectories, but the final results rely on a statistical assessment
of a trajectory ensemble. Based on the PSCs observed by CALIPSO during the
Arctic winter 2010/11 air parcel trajectories were calculated 6 days
backwards with the NOAA HYSPLIT (Hybrid Single Particle Lagrangian Integrated
Trajectory Model) model based on GDAS (Global Data Assimilation System)
analyses
http://ready.arl.noaa.gov/HYSPLIT.php
. GDAS
analyses are provided by the National Center for Environmental Predictions
(NCEP) four times a day (00:00, 06:00, 12:00 and 18:00 UTC) with
a horizontal resolution of 1∘×1∘ on 23
pressure levels (1000 to 20 hPa). An isentropic method was used for
the calculation of vertical motion. The trajectories were started for each
PSC detection at three different altitudes, corresponding to the bottom,
middle and top of the cloud.
The Arctic winter 2010/11 has been chosen for the sensitivity study because
it was one of the coldest Arctic winters leading to a high number of PSC
occurrences. This makes the statistics more reliable than if we would have
chosen a warmer winter with less PSC occurrences. During the Arctic winter
2010/11 PSCs were detected by CALIPSO on 47 days on 259 orbit tracks.
In total, 738 trajectories were calculated based on the CALIPSO observations
and considered for the sensitivity study on the trajectory ensemble. For the
sensitivity study on single trajectories we selected two trajectories, one
trajectory where temperatures below TNAT were reached, but not
below Tice and one where temperatures below both,
TNAT and Tice, were reached along the trajectory.
Figure shows a map with locations where the trajectories
were started according to the PSCs detected by CALIPSO, colour coded by the
four cold phases during the Arctic winter 2010/11. PSCs were observed around
Greenland during Phase 1, during Phase 2 PSCs were observed over Russia and
during Phase 3 over the entire Arctic. During Phase 4 only a few PSCs were
detected which were located around Greenland
Note: no CALIPSO
observations are available from 8 to 13 March.
.
Start points where the back trajectories were started according to
the PSCs observed by CALIPSO during the four cold phases during the Arctic
winter 2010/11 (Phase 1: 23 December–8 January
(magenta), Phase 2: 20–28 January (green), Phase 3: 5–27 February (blue)
and Phase 4: 5–18 March (cyan)).
Sensitivity studies on single back trajectoriesCase 1: sensitivity to H2O enhancements
Figure shows the CALIPSO measurement on
26 February 2011 around 00:04 UTC. A PSC was measured at altitudes
between 16 and 24 km (between 76∘ N, 61∘ E to
70∘ N, 49∘ E). The PSC was located east of Novaya Zemlya
and was composed of all kinds of PSC particles but mainly of STS with a thick
ice layer in between (Fig. ). Based on the PSC
observed on 26 February 2011 air parcel trajectories were calculated
6 days backwards with the HYSPLIT model. The trajectories were
started at 00:00 UTC at 20, 22 and 24 km (started at
71∘ N, 61∘ E) and ended at 20 February at
00:00 UTC. During the course of the 6 days the trajectories
followed the circular flow within the polar vortex and thus the air masses
were transported twice around in the polar regions (see Fig. S3 in the
Supplement).
CALIPSO PSC composition for the PSC measured along the orbit track
starting at 25 February 2011, 23:56 UTC. Shown is the composition for
the measurement on 26 February 2011 at 00:04 UTC and the GEOS-5
temperatures and geopotential height fields (in gpkm) at 30 hPa at
12:00 UTC (bottom). The white line marks the CALIPSO orbit track.
Temperature history of the back trajectory calculated with HYSPLIT
based on the PSC measured by CALIPSO on 26 February 2011 (back trajectory
started at 20 km at 00:00 UTC). Top: for a typical
H2O mixing ratio of 5 ppmv in the polar lower stratosphere,
middle: for an H2O enhancement of 0.5 ppmv
(5.5 ppmv), bottom: for an H2O enhancement of 1 ppmv
(6 ppmv). The NAT existence temperature TNAT and ice
formation temperature Tice are given as solid and dashed lines,
respectively. Temperatures drop below the NAT formation temperature at time
periods t=-140 to -100 h and t=-20 to 0 h. The
temperature ranges during these time periods are denoted by T1 and T2,
respectively (grey solid lines).
Figure shows the temperature along the trajectory started at
20 km (black line) together with the threshold temperatures for
TNAT and Tice (red solid and dashed line,
respectively). The threshold temperatures were calculated according to the
parameterisations of and , respectively.
TNAT and Tice were calculated for 5 ppmv
(typical water mixing ratio during polar winter) and for increased water
vapour mixing ratios of 5.5 and 6 ppmv, respectively. Along the
trajectory temperatures drop twice below TNAT (at t=-140 to
-100 h, temperature range T1, and at t=-20 to 0 h,
temperature range T2) but temperatures did not reach Tice. The
temperature range T2 corresponds to the time period when a PSC was
measured by CALIPSO on that day. The temperature drops sufficiently low below
TNAT to allow STS formation, which is in agreement with the
CALIPSO observation at 20 km (Fig. ). Although
ice was measured on that day, the ice layer was located in the middle of the
PSC, between 21 and 23 km. The trajectory considered here was started
at 20 km, thus at the bottom of the PSC and therefore below the ice
layer (Fig. ). With increasing H2O
mixing ratio the TNAT threshold temperature is higher and
temperatures can more easily drop below TNAT. However, a slight
prolongation of temperatures below TNAT is found only for the
temperature range T1 (Table ). Although the effect on
TNAT seems to be insignificant in this example, the effect on
Tice seems to be more significant. Temperatures did not drop
below Tice but came very close to the Tice threshold
when water vapour mixing ratios were increased. Therefore, another trajectory
has been chosen, one where Tice was reached along the trajectory
using a water vapour mixing ratio of 5 ppmv (typical water vapour
mixing ratio during polar winter). This trajectory is discussed in the
following section.
Time periods when T1 and T2 were below the NAT and ice
threshold temperature along the back trajectory. TNAT and
Tice were derived for H2O mixing ratios of 5, 5.5. and
6 ppmv for the trajectory started on 26 February 2011 at
00:00 UTC (Case 1). Only increases in water vapour were considered.
CALIPSO PSC composition for the PSC observation along the orbit
track starting at 23 January 2011, 19:52 UTC. Shown is the
composition for the measurement on 23 January 2011 at 20:03 UTC (top)
and the GEOS-5 temperatures and geopotential height fields (in gpkm) at
30 hPa at 12:00 UT (bottom). The white line marks the CALIPSO
orbit track.
Temperature history of the back trajectory calculated with HYSPLIT
based on the PSC measured with CALIPSO on 23 January 2011 (back trajectory
started at 18 km at 20:00 UTC). Top: for a typical
H2O mixing ratio of 5 ppmv in the polar lower stratosphere,
middle: for an H2O enhancement of 0.5 ppmv
(5.5 ppmv), bottom: for an H2O enhancement of 1 ppmv
(6 ppmv). The NAT existence temperature TNAT and ice
formation temperature Tice are given as solid and dashed lines,
respectively. Temperatures drop below the NAT formation temperature at time
periods t=-135 to -105 h and t=-45 to 0 h. The
temperature ranges during these time periods are denoted by T1 and T2,
respectively (grey solid lines).
Case 2: sensitivity to H2O enhancements and additional cooling
Figure shows the CALIPSO measurement on
23 January 2011. A PSC was measured over Russia at altitudes between 16 and
23 km (80∘ N, 139∘ E to 66∘ N,
105∘ E). Based on the PSC measured on 23 January 2011, back
trajectories were calculated with HYSPLIT 6 days backwards starting
at 20:00 UTC at three different altitudes within the PSC, namely at
18, 20 and 22 km (started at 72∘ N, 113∘ E). During
the course of the 6 days the trajectories followed the circular flow
within the polar vortex and thus the air masses were transported twice around
in the polar regions (see Fig. S4 in the Supplement). As in the previous
example, the PSC was composed of all kinds of PSC particles, but STS, Mix 2
enhanced (liquid/NAT mixture with intermediate NAT number densities of
10-3cm-3) and some ice in between was dominating
(Fig. ).
Same as Fig. but with an additional temperature
decrease along the back trajectory of 1 K.
Figure shows the temperature along the trajectory (black)
that was started at 18 km as well as the threshold temperatures for
TNAT and Tice (red solid and dashed line,
respectively). As in the case discussed in Sect. , the threshold
temperatures were calculated for 5 ppmv (typical water mixing ratio
during polar winter (, and references therein,
) and for increased water vapour mixing ratios of 5.5
and 6 ppmv (middle and bottom panel). In this case, the temperatures
drop twice below TNAT along the trajectory, at t=-135 to
-105 h (temperature range T1) and t=-45 to 0 h
(temperature range T2). Temperatures during the second time period with
T2<TNAT were colder and even reached Tice. The
time periods where temperatures were lower than TNAT are
prolonged when the atmospheric water vapour mixing ratio is increased
(Table ). For example, while the T1 temperatures did not
reach below TNAT under normal stratospheric conditions,
temperatures reach 15 and 30 h below TNAT with
an increase in H2O mixing ratio of 0.5 and 1 ppmv,
respectively, thus allowing STS and NAT PSC formation and existence during
a longer time period.
Time periods when T1 and T2 are below the NAT and ice
threshold temperature along the trajectory. TNAT and
Tice were derived for H2O mixing ratios of 5, 5.5. and
6 ppmv for the back trajectory started on 23 January 2011 at
20:00 UTC (Case 2). Water vapour increases (Case 2a) as well as an
additional temperature cooling by 1 K (Case 2b) are considered.
Case 2aCase 2bH2OT1<TNATT2<TNATT2<TiceT1<TNATT2<TNATT2<Tice(ppmv)(h)(h)(h)(h)(h)(h)51038–4535205.52541154542226304420>454725
The effect becomes even stronger when additionally the temperature is
decreased (Fig. ). Time periods where T1 or T2 are
below TNAT and Tice become much longer, as can be
seen from Table . Further, the effect becomes more
pronounced for Tice as can be expected, but there seems to be
also an increase in T2 below TNAT due to strong temperature
cooling along the trajectory.
Sensitivity studies on back trajectory ensemble
The back trajectory ensemble was calculated starting at dates and times when
PSCs were measured by CALIPSO during the Arctic winter 2010/11. For each PSC
measurement, trajectories were calculated 6 days backward in time at
three different altitudes, corresponding to the top, middle and bottom of the
cloud. In total 738 trajectories were calculated. During the course of the
6 days the trajectories in general followed the circular flow within
the polar vortex and thus the air masses were transported once or several
times around in the polar regions. The temperature thresholds for PSC
formation encountered along the trajectories derived with HYSPLIT are in good
agreement with the corresponding PSC types measured by CALIPSO.
Using the entire trajectory ensemble the total time (sum over all 738
trajectories) where the temperature was below TNAT and
Tice, respectively, was estimated applying an H2O mixing
ratio of 5 ppmv (same as in Sect. and ,
typical water vapour mixing ratio for the Arctic polar lower stratosphere
(, and references therein, ) and
observed by the satellite instruments considered in this study). This
calculation was repeated applying a H2O increase of
0.25–1 ppmv (ΔH2O = 0.25 ppmv, as in
Sect. and , according to the estimated trends from
and ) as well as a decrease in
temperature by 0.5 and 1 K. Additionally, the calculation was
repeated for a water vapour decrease of 0.25 ppmv to also investigate
what the effect of an opposite change would be, which could result from the
natural H2O variability. To quantify the effect a change in
H2O mixing ratio and a decrease in temperature would have, we
calculated the enhancement in time that would result when the temperatures
would be exposed accordingly longer to temperatures below TNAT
and Tice.
Histograms of the total time where the temperature along the back
trajectory is below the NAT existence threshold temperature (sum over all 738
back trajectories) for stratospheric H2O mixing ratios of 4.75,
5.0, 5.25, 5.5 and 6.0 ppmv.
The calculation was performed assuming an HNO3 mixing ratio of 3 (top), 5 (middle) and 7 ppbv
(bottom).
The results of the sensitivity study with the trajectory ensemble are
summarised in Figs. and for
TNAT and in Figs. and
for Tice. In Figs. and
the total time the temperature is below
TNAT and Tice, respectively, is given while in
Figs. and the additional time is
given when the temperature would be below TNAT and Tice,
respectively, if the H2O mixing ratio would increase and temperature
decrease (see also tables in Supplement). The calculation of extra exposure
time to TNAT and Tice was done assuming HNO3
mixing ratios of 7, 5 and 3 ppmv which corresponds to the conditions
in the polar lower stratosphere at the beginning of the winter and later in
the winter when HNO3 has been taken up by the PSCs and HNO3
has been permanently removed by sedimenting PSC particles (denitrification).
Histogram of increase in time where the temperature along the back
trajectory is below the threshold temperature (sum over all 738 back
trajectories) for a stratospheric H2O increase of 0.25, 0.5, 0.75 and
1.0 ppmv, respectively, and for a stratospheric H2O decrease
of 0.25 ppmv. The calculation was performed assuming an HNO3
mixing ratio of 3 (top), 5 (middle) and 7 ppbv (bottom).
For the reference conditions (normal stratospheric winter conditions,
beginning of the winter, thus prevailing gas phase abundances of
5 ppmvH2O and 7 ppmvHNO3) temperatures were
in total below TNAT for 43 512 h
(Fig. ). Note: total trajectory time is
107 010 h (738 trajectories × 145 h). If
HNO3 decreases during the course of the winter to 5 ppbv the
air will be ∼3600h less exposed to temperatures below
TNAT. If HNO3 will further decrease to 3 ppbv the
total time will be ∼5000h less than during reference
conditions. On the other hand, if in the future H2O increases and the
temperature decreases in the stratosphere, the total time where temperature
falls below TNAT will increase independent of the HNO3
abundance (3, 5 or 7 ppbv) in the stratosphere
(Fig. ). Any H2O increase of 0.25 ppmv
will result in 1500 h more where the temperature along the
trajectories will be below TNAT. The effect is much stronger when
the temperature is decreased by 0.5 K. For each 0.5 K cooling
the time will be increased by ∼4000h
(Fig. and tables in the Supplement).
Histogram of total time where the temperature along the back
trajectory is below the ice formation threshold temperature (sum over all 738
back trajectories) for a stratospheric H2O increase of 4.75, 5.0,
5.25, 5.5, 5.75 and 6.0 ppmv.
Temperatures below Tice are rarely reached in the Arctic.
However, the Arctic winter 2010/11 was exceptionally cold and temperatures
below Tice were reached along several trajectories. Under typical
stratospheric conditions (H2O= 5 ppmv) temperatures were
below Tice for 571 h (Fig. ). If
the H2O abundance in the stratosphere is 0.25 ppmv less (thus
4.75 ppmv) than the total time where the temperatures are below
Tice decreases to 340 h, 231 h less than under the
reference stratospheric conditions (Fig. and the
Supplement). If the H2O mixing ratio increases by 0.25 ppmv,
from 5 to 5.25 ppmv, temperatures below Tice would
persist 299 h longer than for reference conditions. If water vapour
increases further from 5 to 5.5 or 6 ppmv, the time where
temperatures are below Tice will increase by 669 and
1728 h, respectively. Thus, the higher the water vapour gets in the
stratosphere the stronger the impact of a further increase will be. The same
behaviour is found when the temperature in the stratosphere is cooled by 0.5
to 1 K. In the extreme case when H2O mixing ratios would
increase by 1 ppmv and the temperature would decrease by 1 K
the total time where temperatures are below Tice would increase
from 571 to 6789 h, thus by ∼6000h which corresponds
to an enhancement by a factor of 12.
Water vapour and temperature variability (2000–2014)
In the previous section we demonstrated that a water vapour increase and
temperature decrease would increase the potential for PSC formation. More
than a decade ago it was already suggested that a cooling of stratospheric
temperatures by 1 K or an increase of 1 ppmv of stratospheric
water vapour could promote denitrification .
During the two Arctic winters 2009/10 and 2010/11, the strongest
denitrification in the recent decade was observed
.
Histogram of increase in time where the temperature along the back
trajectory is below the ice formation threshold temperature (sum over all 738
back trajectories) for a stratospheric H2O increase of 0.25, 0.5,
0.75 and 1.0 ppmv, respectively, and for a stratospheric H2O
decrease of 0.25 ppmv.
Here, we investigate the variability of Arctic water vapour and temperature
since the new millennium to see if there is a connection to the severe
denitrification observed in the past years. For that we used observations
from UARS/HALOE, Odin/SMR, Envisat/MIPAS, SCISAT/ACE-FTS, Envisat/SCIAMACHY
and Aura/MLS as well as ERA interim reanalysis data. In a first step these
data sets were interpolated onto a regular potential temperature grid with a
resolution of 25 K. Temperature and pressure data needed for the
conversion from the native vertical coordinate to potential temperature were
taken from the individual data sets themselves. Then for every profile the
equivalent latitude as function of altitude was derived from ERA interim
potential vorticity data. Data within high equivalent latitudes, i.e. between
70 to 90∘ N, were subsequently binned into monthly and
zonally averaged time series. Finally the resulting time series were
de-seasonalised to make variability on inter-annual to decadal scales more
visible. For a given month a multi-year average was calculated from the
individual monthly averages which was then subtracted from the latter. The
multi-year averages were based on the entire time period covered by the
individual data sets.
Anomaly of the monthly mean temperature (top) and water vapour for
the polar regions at equivalent latitudes (70 to 90∘ N). The
data were averaged within the potential temperature layers 475–525 K
(18–22 km). Top: temperature from ERA-interim (blue), Aura/MLS
(red), Envisat/MIPAS (green) and SCISAT/ACE-FTS (orange). Bottom: water
vapour derived from Odin/SMR at 544 GHz band (orange), Aura/MLS
(red), Envisat/MIPAS (green), Envisat/SCIAMACHY (purple), SCISAT/ACE-FTS
(yellow) and UARS/HALOE (brown).
Figure shows the de-seasonalised time series for temperature
(upper panel) and water vapour (lower panel) covering the time period
2000–2014 averaged over the potential temperature range 475–525 K
(∼ 18–22 km). Temperature information is provided by
ERA-Interim, Aura/MLS, Envisat/MIPAS and SCISAT/ACE-FTS; for water vapour
there is in addition also data from UARS/HALOE and Envisat/SCIAMACHY. In
terms of temperature there is a very good agreement among the different data
sets. The overall variability is dominated by the winter season. Strong
inter-annual variability can be found for this season with pronounced cold
and warm events that show absolute anomalies of more than 10 K. For
water vapour there is more scatter and less consistency between the
individual data sets. Deviations exceed occasionally 0.5 ppmv, which
is a typical level of uncertainty of these observations .
Unlike temperature the water vapour variability is not as apparently
dominated by the winter season. The pronounced events seen in temperature can
be still observed in water vapour in an anti-correlated sense, but not as
obvious. There are indications of a substantial decrease throughout 2003,
however not in all data sets. A similar feature can be observed in 2011.
Likewise there are indications of an increase in 2006 and 2007. Overall this
demonstrates significant changes over short time periods.
Same as Fig. 2, but for 525–825 K.
Figure is of the same kind as Fig. however it
considers the potential temperature range between 525 and 825 K
(∼ 22–28 km). Please note that there are no water vapour data
from Envisat/SCIAMACHY available here. For temperature very similar
characteristics can be observed as in Fig. . This is also true
for water vapour. The scatter is somewhat reduced providing a clearer picture
on inter-annual to decadal variability. In particular the variations in the
aftermath of sudden stratospheric warmings in early 2009 and 2013 are very
pronounced here .
Visually, both Figs. and do not indicate any
obvious linear changes in temperature or water vapour overall. To investigate
this aspect more rigorously we performed separately a regression analysis of
the Envisat/MIPAS and the Aura/MLS time series. The selection of these two
data sets was based on their extensive and regular observational coverage.
The regression model considered an offset, a linear term, periodic variations
of 3, 4, 6 and 12 months as well as the QBO in the form of Singapore winds at 50
and
30 hPa
.
In addition the model considered autocorrelation effects and a possible
offset between the MIPAS full and reduced resolution data
. Figure shows
the linear change estimates for water vapour (left panel) and temperature
(right panel). Even though the two data sets cover slightly different time
periods they indicate largely positive changes in water vapour in the
altitude range between 350 and 1000 K potential temperature. The
change in absolute terms is typically larger for Aura/MLS than for
Envisat/MIPAS, just at the lowest altitudes the behaviour is opposite. For
Envisat/MIPAS the linear changes are mostly not significant at the 2σ
uncertainty level. The only exception is the altitude range between 375 and
450 K. For Aura/MLS significance is visible at more altitudes, yet
there is a number of altitude levels where the changes are close to
insignificance. At 1000 K the regression did not converge. The
Envisat/MIPAS temperature time series shows largely negative changes that
roughly increase with altitude. However none of these changes are significant
at the 2σ level. The same is true for the Aura/MLS temperatures, where
the trend estimates are more close to zero. Similar results are derived when
instead of all seasons only the winter months DJF (the predominant time for
PSC formation) is considered (see Supplement).
Linear change in water vapour (left) and temperature (right) vs.
potential temperature derived from Envisat/MIPAS (2002–2012) and Aura/MLS
(2004–2014). For the linear change in water vapour derived from
Envisat/MIPAS an offset of 0.1 ppmv between the two measurement
periods has been considered. As error bars the 2σ uncertainty is
given.
As noted earlier water vapour and temperature show a clear anti-correlation,
e.g. enhanced water vapour mixing ratios occur in cold winters and vice
versa. This connection is shown in Fig. , that
considers as Fig. the altitude range between 475–525 K
potential temperature. The individual data points shown here are averages
over January, February and March. Both for Envisat/MIPAS and Aura/MLS the
correlation is substantial, with correlation coefficients of -0.65 and
-0.87, respectively. This clearly indicates a connection between the
processes that govern water vapour and temperature in the polar lower
stratosphere, as the subsidence inside the polar vortex, sudden stratospheric
warmings and radiative cooling. A corresponding figure for the altitude range
between 525–825 K is provided in the Supplement. There the
correlation is less strong.
Correlation of temperature (blue) and water vapour (red) anomaly
derived from Envisat/MIPAS (top) and Aura/MLS (bottom) for potential
temperature range 475–525 K (3 month average consisting of
the months January, February and March; MIPAS: 2002–2012, MLS: 2005–2014).
Altitude-time evolution of water vapour in the polar regions
(70–90∘ N) derived from Envisat/MIPAS observation for the time
period 2002–2012.
Figure shows the altitude time evolution of water vapour
in the polar regions derived from Envisat/MIPAS observations for the time
period 2002–2012. Differences in the downward transport of water vapour from
year to year are clearly visible during the time period 2002–2012. E.g. the
transport of high H2O mixing ratios (e.g. 6 ppmv) reaches
much further down into the lower stratosphere (to 20 km and even
below) during the Arctic winters as 2006/07, 2007/08 and 2010/11 than in
other Arctic winters.
Discussion
In the present work we were focusing on the polar regions to understand how
water vapour and temperature changes in the lower polar stratosphere affect
PSC formation which eventually can also have an impact on denitrification.
The influence of water vapour and temperature on PSCs is two-fold. There is a
background component and on top there is inter-annual variability. The
long-term changes of low-latitude water vapour described in the introduction
eventually also influence the high latitudes. On the other hand vortex
dynamics play an essential role for PSC variations from year to year. Over
the time period from 2002 to 2012 Envisat/MIPAS observations indicate a
significant change in water vapour only between 375 and 450 K in form
of an increase. Aura/MLS observations considering the time period from 2004
to 2014 show positive changes over a wider altitude range, many of them even
significant. Figure indicates that the later start and end of
the Aura/MLS measurements relative to the MIPAS observations could play a
decisive role for this difference. In principle an increase is expected.
After the long-standing millennium drop a new increase has been observed in
the tropics, starting around 2004 to 2005 . This
increase was only interrupted by the 2011 drop that was however much more
short-lived than the millennium drop. By early 2014 the volume mixing ratios
had more or less recovered . For the time period since 2002
we cannot find any significant changes in temperature in the altitude range
between 350 K and 1000 K potential temperature.
The inter-annual variability component is driven by the vortex-related
dynamics and other short-term variations, like QBO and drops in water vapour.
During wintertime we found in the satellite data a significant correlation
between cold/warm winters and enhanced/reduced water vapour mixing ratios.
This correlation indicates a connection between dynamical processes that
influence the polar winter dynamics. On the one hand there is the subsidence
within the polar vortex and its variations. On the other hand there are
sudden stratospheric warmings that break up the polar vortex and completely
revert the dynamical conditions. During polar winter vigorous descent occurs
within the polar vortex, transporting air masses from the upper stratosphere
and mesosphere down to the lower stratosphere . As
water vapour typically exhibits a maximum around the stratopause this descent
also transports moister air towards the lower stratosphere.
analysed SCIAMACHY data from 2002–2009 and found that
the QBO west phase is associated with larger PSC occurrences and stronger
chemical ozone destruction than the QBO east phase. Their findings are in
agreement with the Holton-Tan mechanism which relates the
QBO west phase to a colder and more stable vortex. During cold Arctic
winters, as 2010/2011, the subsidence within the polar vortex is strongly
enhanced as shown e.g. by , causing positive water vapour
anomalies. This explains already qualitatively the correlation we observed.
As an amplifying effect act sudden stratospheric warmings, like in early 2009
or 2013, that led to high positive temperature anomalies and low water
vapour. The latter are mainly explained by the poleward transport of dry air
once the horizontal mixing barrier in the form of the polar vortex edge is
removed.
The signatures of the water vapour drops in 2000 and 2011 are not easily
distinguishable in the Arctic. In the altitude range between 475 to
525 K the decrease throughout 2003 may be attributed to the
millennium drop. Arctic observations of POAM III indicated the drop already
in early 2001 . This seems to be consistent with studies by
that showed a delay of up to 12 months between the drop
occurrence in the tropics and at 50 ∘ latitude at these low
altitudes. The UARS/HALOE observations employed here do not show a clear sign
of a decrease in 2001, however admittedly the measurement coverage of this
instrument has not been optimal for these high latitudes. The decrease in the
Arctic in 2011 may correspond to the drop observed in the tropics. Yet, the
length of the decrease is shorter than observed at the low latitudes. Higher
up, between 525 K and 825 K potential temperature, a longer
delay to the drop occurrence in the tropics can be expected
. Thus, the decrease observed here in 2002 and
2003 is more likely attributed to the millennium drop. The decrease in 2011
on the other hand is unlikely to be connected to the tropical event.
Overall, based on the observations since the new millennium we can conclude
that such strong denitrification events as in 2010/11 are for the time being
driven by inter-annual variability than any changes in water vapour and/or
temperature since the millennium.
By performing sensitivity studies we, on one hand, investigate what
implications water vapour and temperature changes and/or variability would have on
PSC formation and existence. On the other hand this sensitivity study also
shows what implications uncertainties in water vapour from measurements and
temperature measurements and/or reanalyses would have on Arctic studies. Gravity
waves can affect PSC occurrence and composition in the Arctic and Antarctic
. Temperature perturbations
that are caused by gravity waves are usually not represented in
meteorological analyses. Further, meteorological analyses tend to have cold
or warm biases as was shown by e.g. . PSC formation and
existence is quite sensitive to temperature and water vapour changes as well
as uncertainties of these. An increase of e.g. 1 ppmv in water vapour
and a decrease of 1 K in temperature would significantly alter the
estimates of PSC volume and area (VPSC and APSC,
respectively) and thus would affect the estimates on e.g. ozone loss and
chlorine activation . However, irrespective of if there is
a cold or warm bias in the trajectory temperature or in the water vapour
mixing ratio in the stratosphere, an increase in water vapour mixing ratios
or a cooling of temperature will definitely result in a prolongation of the
potential for PSC existence as shown in our sensitivity study.
Our sensitivity study was performed on the basis of the 2010/11 winter, which
was the coldest Arctic winter in the recent decade. The anomalously strong
polar vortex and the atypically long cold period that persisted from
mid-December to mid-March led to extensive PSC formation. PSCs were observed
by CALIPSO over 47 days during the Arctic winter 2010/11. During the
Arctic winter 2009/10, a cold period extended over 4 weeks, from
mid-December to mid-January, and PSCs were only observed on 26 days.
Thus, if this study would have been performed on the basis of another
(warmer) winter less PSC observations would have been served as a basis for the
trajectory calculations and thus as a basis for the statistic. As a consequence
the total times where the temperature was below TNAT or
Tice, respectively, would have been shorter as for the Arctic
winter 2010/11. However, the resulting increase in time due to a decrease in
temperature and an increase in water vapour can be expected to be similar,
thus as dramatic as for the 2010/11 winter.
As shown in this study, increases in stratospheric water vapour as well as
decreases in stratospheric temperature can prolong PSC formation and
existence. An increase of water vapour of 1 ppmv and a decrease of
temperature of 1 K increased the times where the temperature is below
TNAT by 38 %. A much stronger increase in time was found
for ice. The increase in time where temperatures were below Tice
were enhanced by a factor of 12 for an increase of H2O by
1 ppmv and a temperature decrease of 1 K. Generally,
temperatures sufficiently low for ice formation are rarely reached in the
Arctic. Therefore, this strong increase in time where temperatures would be
below Tice would mainly be of importance for very cold, extreme
Arctic winters as e.g. the Arctic winter 2010/11. However, if cold Arctic
winters will become colder in the future as suggested by
, ice formation will also become more common in the
Arctic. Thus, changes in stratospheric H2O mixing ratio and
temperature can significantly alter PSC formation and existence and thus the
chemistry of the polar stratosphere, as e.g. increasing denitrification and
thus ozone loss.
Conclusions
The Arctic winter 2010/11 was one of the coldest in the last 2 decades. The
2010/11 winter was characterised by an anomalously strong vortex with an
atypically long cold period that was persistent from mid-December to
mid-March. During the Arctic winter 2010/11 the strongest denitrification
during the recent decade was observed. More than a decade ago it was already
suggested that a cooling of stratospheric temperatures by 1 K or an
increase of 1 ppmv of stratospheric water vapour could promote
denitrification, the removal of HNO3 by sedimenting PSC particles.
Based on the 2010/11 winter a sensitivity study was performed to investigate
how a change of up to 1 ppmv in water vapour and temperature decrease
of up to 1 K (due to a trend or variability) would affect PSC
formation and occurrence. Air parcel trajectories were calculated
6 days backward according to PSC observations by CALIPSO. In total, 738
trajectories were calculated. On the basis of this trajectory ensemble the
increase in time the air parcels would be exposed to temperatures below
TNAT or Tice along the trajectories was calculated.
Measurements from several different satellites derived for the
15-year period 2000–2014 were used together with temperatures from
ECMWF to investigate water vapour trends and variability in the polar
stratosphere. So far trend studies on stratospheric water vapour have focused
on the tropics and mid-latitudes. Here, for the first time such an analysis
has been performed for the polar stratosphere.
Our sensitivity studies based on air parcel trajectories confirm that PSC
formation is quite sensitive to water vapour and temperature changes.
Increased H2O (and further cooling of the stratosphere) would
increase the potential for PSC formation and prolong PSC existence and thus
the chemistry of the polar stratosphere, as e.g. increasing denitrification
and thus ozone loss. On the other hand an increase in temperature and
a decrease in water vapour will reduce PSC formation and existence. From the
Envisat/MIPAS (2002–2012) and Aura/MLS (2004–2014) observations we derive
predominantly positive changes in the altitude range between 350 to
1000 K potential temperature. Those from Envisat/MIPAS observations
are largely insignificant, while those from Aura/MLS are mostly significant.
For the temperature neither of the two instruments indicate any significant
changes. Given the strong inter-annual variation we observed in water vapour
and particular temperature the severe denitrification observed in 2010/11
cannot be directly related to any changes in water vapour and/or temperature
since the millenium. However, we found from the satellite observations that
cold winters coincide with high water vapour mixing ratios, as e.g. for the
winters 2006/07, 2007/08 and 2010/11. This correlation is quite significant
in the lower stratosphere at 475–525 K and indicates a connection
between dynamical and radiative processes that govern water vapour and
temperature in the Arctic lower stratosphere.
Dedication to Jo Urban
This work is dedicated to our highly valued colleague Jo Urban who
passed away much too early. Without his devoted work on the Odin/SMR data
processing over many years this work would not have been possible. In
particular the retrieval of water vapour from the SMR observations and the
combination of these data with other data sets to understand the long-term
development of this trace constituent comprised a large part his life's work.
With his death, we lost not only a treasured colleague and friend, but also
a leading expert in the microwave and sub-millimetre observation community.
The Supplement related to this article is available online at doi:10.5194/acp-16-101-2016-supplement.
Acknowledgements
We are grateful to the European Space Agency (ESA) for providing Odin/SMR
data. Odin is a Swedish-led satellite project jointly funded by the Swedish
National Space Board (SNSB), the Canadian Space Agency (SCA), the National
Technology Agency of Finland (Tekes) and the Centre National d'Etudes
Spatiales (CNES) in France. SCISAT/ACE is a Canadian led mission mainly
supported by the CSA and National Sciences and Engineering Research Council
of Canada (NSERC). Provision of MIPAS level-1b data by ESA is gratefully
acknowledged. We also would like to thank the MLS team for providing their
data. MLS data were obtained from the NASA Goddard Earth Sciences Data and
Information Center. The SCIAMACHY limb water vapour data V3.01 are a result
of the DFG Research Unit “Stratospheric Change and its role for Climate
Prediction (SHARP)” and the ESA Project SPIN (ESA SPARC Initiative) and were
partly calculated using resources of the German HLRN (High-Performance
Computer Center North). We also would like to thank M. Hervig for providing
a program to calculate the NAT existence temperature. We gratefully
acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the
HYSPLIT READY website (http://ready.arl.noaa.gov/HYSPLIT.php). We
further acknowledge the helpful comments from the two anonymous referees.
This study was performed in the frame of the FP7 project RECONCILE (Grant
number: RECONCILE-226365-FP7-ENV-2008-1). We are also grateful to Swedish
National Space Board (SNSB) for funding F. Khosrawi (2012–2013) and the
German Research Foundation (DFG) for funding S Lossow within the project
SHARP under contract STI 210/9-2. We acknowledge support by Deutsche
Forschungsgemeinschaft and Open Access Publishing Fund of Karlsruhe Institute
of Technology.The article processing charges
for this open-access publication were covered by a Research
Centre of the Helmholtz
Association.Edited by: C. von Savigny
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