The ground-based microwave radiometer MIAWARA-C recorded the upper
stratospheric and lower mesospheric water vapour distribution continuously
from June 2011 to March 2013 above the Arctic station of Sodankylä, Finland
(67.4
The Arctic atmosphere is highly variable. Over the year, it is affected by the extremes of solar radiation ranging from long daylight periods in summer to the complete lack of Sun light in winter. The absence of radiative heating in the stratosphere leads to strong eastward winds, the polar vortex, and descent of air over the Arctic region in winter. As a consequence of the polar winter condition, the temperature of the stratosphere decreases allowing polar stratospheric clouds to form. These clouds play a major role in the heterogeneous catalytic destruction of ozone in spring. In addition to influencing the temperatures the polar vortex acts as a mixing barrier for trace gases. This mixing barrier can give rise to sharp gradients in trace gases such as nitrous oxide, ozone or water vapour.
Thanks to its relatively long chemical lifetime in the order of months in the
stratosphere and weeks in the mesosphere
There are two major sources of middle atmospheric water vapour. The first one is vertical transport through the tropical transition layer. The lower stratosphere is extremely dry because of the cold tropopause temperatures in the Tropics resulting in freeze-drying. The second source of middle atmospheric water vapour is the oxidation of methane leading to a positive vertical gradient in volume mixing ratio (VMR) throughout the stratosphere. The increasing photo-dissociation with altitude results in a negative gradient in the mesosphere.
The latitudinal distribution of water vapour in the middle atmosphere is mainly determined by the large-scale residual circulation. Above the winter polar region dry mesospheric air descends inducing horizontal gradients in the water vapour VMR, which are sustained by the vortex edge. These horizontal gradients make water vapour a valuable tracer for short-term transport in the winter hemisphere, e.g. in the course of SSW. Above the summer polar region, upwelling of relatively humid stratospheric air results in high water vapour VMR in the mesosphere.
Sudden stratospheric warmings are events occurring in the winter hemisphere
and are characterised by a fast and strong increase of stratospheric
temperature and simultaneous cooling of the mesosphere in the polar region.
In the course of SSWs the polar circulation is strongly distorted from normal
winter conditions with stratospheric zonal winds reversing to westward and
the temperature increases from 60
Uneven in situ heating of the atmosphere and the asymmetric distribution of
land can excite planetary waves. Numerous studies identified the most
prominent periods of these waves to be approximately 2 days, 5 days,
10 days and 16 days. The most prominent planetary wave component in
the mesosphere is the quasi-2-day wave (Q2DW) with amplitudes larger than
10 K in temperature and wind amplitudes of several tens of m s
Recent studies discussed a strong Q2DW activity around winter solstice at
high latitudes
Measurements of the state of the atmosphere are essential for increasing the
understanding of the dynamically variable Arctic winter atmosphere and help
to improve the quality of models. Examples of recent studies using SSW events
for comparing models and measurements are given in
The ground-based Middle Atmospheric WAter vapour RAdiometer for Campaigns
(MIAWARA-C) measured in Sodankylä for 20 months from June 2011 to March
2013. The high temporal resolution of the order of 1 h above one
location can only be obtained by ground-based microwave radiometry. In
addition, microwave radiometry is the only remote sensing technique capable
of monitoring water vapour in the middle atmosphere from the ground
After describing the instrument, the measured water vapour time series is presented. The measured water vapour data are spectrally analysed and signatures of the summertime and wintertime Q2DW are discussed. Additionally, the two SSWs of 2012 and 2013 are discussed complementing the measured water vapour profiles with temperature data from Aura MLS and ECMWF model data.
MIAWARA-C is a compact microwave radiometer designed for campaigns to measure
middle-atmospheric water vapour profiles. It is controlled remotely and
operated continuously under all weather conditions except rain. The pressure
broadened emission line of water vapour at 22.235 GHz is measured with
a heterodyne receiver and spectrally analysed with a fast Fourier transform
spectrometer and a spectral resolution of 30.5 kHz and a usable spectral
bandwidth of 400 MHz. A detailed description of the instrument is presented
in
For MIAWARA-C the reliable altitude range of the retrieval v1.1 is defined as
the region where the area of the averaging kernel (AoA) is larger than 0.8.
This definition results in a sensitive altitude range from 4 hPa (37 km) to
0.017 hPa (75 km). Beyond these limits the instrument is still sensitive,
however the contribution from the a priori profile increases and the quality
of the assignment to altitude levels decreases. The full width at half
maximum of the averaging kernels is a measure for the vertical resolution of
the water vapour profiles and is approximately 12 km in the reliable
altitude range. The measured and calibrated spectra are integrated prior to
the retrieval in order to increase the signal-to-noise ratio. For v1.1 the
spectra are integrated until they reach a fixed noise level which results in
a constant altitude range and in a varying integration time. The number of
retrieved profiles per day is mainly determined by the tropospheric
conditions and is presented in
MIAWARA-C monitored middle-atmospheric water vapour above Sodankylä over 20 months without major data gaps from 13 June 2011 until 7 March 2013. There are only three measurement gaps of more than 24 h over the whole measurement period. The interruptions are mainly caused by rain. With a constant noise level in brightness temperature of 0.0141 K, a total of 8823 profiles could be retrieved.
An overview of the time series is presented in Fig.
The data set obtained by MIAWARA-C is ideally suited to investigate the
temporal variability in water vapour caused by periodic phenomena on short
timescales or effects of events such as SSWs. Investigation of the quasi-16-day wave in mesospheric water vapour during the boreal winter 2011/12
based on data from MIAWARA-C has already been presented in
In MIAWARA-C's time series the effects of two SSWs on water vapour are
clearly visible, the first one taking place in January 2012 and the second
one in January 2013. The central dates of the SSWs defined as the occurrence
of the maximum zonal mean temperature at 1 hPa and 60
In addition to the two reversals to winter conditions and the two SSWs, MIAWARA-C observed the change from winter to summer circulation in 2012.
Ground-based microwave instruments for water vapour such as MIAWARA-C can
achieve a high temporal resolution in the order of hours and offer the
possibility to investigate short-term variations in the amount of the trace
gas. The median of the integration time over the whole measurement period
from June 2011 to March 2013 is below 1 h allowing the investigation of
periodic structures in water vapour in an ideal way. Spectral decomposition
of MIAWARA-C's time series showed dominant variations with periods of
approximately 16, 10, 5 and 2 days. A detailed analysis of the evolution and
regional differences of the quasi-16-day wave for winter 2011/12 has already
been presented in
Overview of the water vapour time series measured by MIAWARA-C. The two black dashed lines mark the central dates of the two SSWs. The reliable altitude range is indicated by white lines (AoA > 0.8).
For the spectral decomposition a wavelet-like approach is chosen as described
in
The 2-day amplitude analysis of our data set is presented in
Fig.
A periodogram for the two periods with enhanced Q2DW activity is presented in
Fig.
An illustration of the Q2DW for the two periods showing both filtered time
series and measurement is given in Figs.
MIAWARA-C has monitored the water vapour evolution above Sodankylä during
three SSWs in 2010, 2012 and 2013. The 2010 event is discussed thoroughly in
Amplitude of the 2-day activity in MIAWARA-Cs water vapour time series. The two black dashed lines mark the central dates of the two SSWs.
Mean amplitude of different periods obtained by filtering the water vapour time series on two altitude levels (top: 0.05 hPa, bottom: 1 hPa) for 1 to 30 November 2011 (blue) and 27 June to 24 July 2012 (red).
Measured water vapour data (black dots and grey lines) and filtered Q2DW signal (blue lines) for 0.05 and 1 hPa for November 2011. The mean value over the time period has been added to the filtered signal and contributions with periods of 5, 10 and 16 days have been subtracted from the measurement. In winter, the Q2DW is strong at 1 hPa and accounts for most observed variations with periods shorter than 5 days.
Measured water vapour data (black dots and grey lines) and filtered Q2DW signal (blue lines) for 0.05 and 1 hPa for July 2012. The mean value over the time period has been added to the filtered signal and contributions with periods of 5, 10 and 16 days have been subtracted from the measurement. In summer, the Q2DW is strong at 0.05 hPa but cannot explain the observed variations with periods shorter than 5 days on 1 hPa.
SSW 2012, the vertical green line marks the
central date of the SSW (16 January 2012). First panel: water vapour measured
by MIAWARA-C. Second panel: distance to the vortex edge (negative distance:
polar vortex above Sodankylä, positive distance: non-vortex air above
Sodankylä). Third panel: zonal mean temperature at 80
SSW 2013, the vertical green line marks the
central date of the SSW (9 January 2013). First panel: water vapour measured
by MIAWARA-C. Second panel: distance to the vortex edge (negative distance:
polar vortex above Sodankylä, positive distance: non-vortex air above
Sodankylä). Third panel: zonal mean temperature at 80
The 2012 event shows two maxima in temperature at 1 hPa with the second one
coinciding with the wind reversal. Trajectory analysis of the origin of the
air above Sodankylä shows that both maxima coincide with transport from
mid-latitudinal air (not shown here). The effects on water vapour can be seen
for both temperature maxima as an increase between 1 and 0.03 hPa caused by
the transport of humid mid-latitudinal air into the polar region. The
evolution of 80
The zonal mean water vapour distribution is determined by the general
circulation. The descent above polar regions results in horizontal gradients
of water vapour at the vortex edge. Inside the vortex, stratospheric air
below 40–45 km is more humid than outside whereas the mesospheric vortex
air is characterised as being drier than non-vortex air
The water vapour measurements for the 2012 and the 2013 SSWs as well as the
distance to the vortex edge above Sodankylä are shown in
Figs.
The measured local water vapour time series shown in Figs.
MIAWARA-C has observed three warmings at Sodankylä. After the warmings
normal polar winter conditions are reforming and the humid air which has
entered in the course of the event is descending with the mean residual
circulation. Following
Observational data of middle atmospheric
water vapour obtained at the Arctic research station in Sodankylä have been
presented. The focus has been put on discussing the water vapour time series
measured by the ground-based microwave radiometer MIAWARA-C. This instrument
has monitored the water vapour evolution above Sodankylä from June 2011 to
March 2013 without major measurement gaps. The temporal resolution of the
retrieved profiles is approximately 1 h for average conditions. The high
temporal resolution of the data set allows the investigation of short-term
variations in this key atmospheric constituent. The data set of MIAWARA-C is
used to investigate variations with periods close to 2 days related to the
Q2DW. A wavelet-like analysis showed a varying activity in the mesosphere
with generally higher amplitudes in the Arctic summer. Additionally, there is
an enhanced activity in 2-day oscillation for both 2012 and 2013 winters
below 0.1 hPa reaching a maximum value of 0.8 ppmv in November 2011. In
addition to the strong Q2DW activity, there was a SSW in both winters
affecting the measured water vapour profiles. Around the zonal mean wind
reversal there is a sharp increase in water vapour caused by transport of
humid mid- to low-latitudinal air into polar regions from 1 to 0.06 hPa.
After the SSW the circulation returns to normal winter conditions and the air
descends over the Arctic region. The descent rates after the SSWs are
364 m d
This work has been supported by the Swiss National Science Foundation grant
number 200020-146388. Participation at the LAPBIAT campaign was funded
through the EU Sixth Framework Programme, Lapland Atmosphere–Biosphere
Facility (LAPBIAT2). We thank the team of the Finnish Weather Service for
their hospitality and support during the campaigns. Assistance provided by
the ARTS/Qpack team is greatly appreciated. Additionally, we would like to
thank