Direct measurements of middle-atmospheric wind oscillations with periods between 5 and 50 days in the altitude range between mid-stratosphere (5 hPa) and upper mesosphere (0.02 hPa) have been made using a novel ground-based Doppler wind radiometer. The oscillations were not inferred from tracer measurements, as the radiometer offers the unique capability of near-continuous horizontal wind profile measurements. Observations from four campaigns at high, middle and low latitudes with an average duration of 10 months have been analyzed. The dominant oscillation has mostly been found to lie in the extra-long period range (20–50 days), while the well-known atmospheric normal modes around 5, 10 and 16 days have also been observed. Comparisons of our results with ECMWF operational analysis data revealed remarkably good agreement below 0.3 hPa but discrepancies above.
The dynamics of the middle atmosphere is characterized by waves and
oscillations with distinct periods. An accurate representation of the
middle-atmospheric dynamics can improve the forecast skills of numerical
weather prediction models, especially on timescales beyond 1 week
Measurements of zonal and meridional wind are the most direct way to observe atmospheric dynamics. For studying long period oscillations long time series of continuous measurements are required. However, wind observations in the upper stratosphere and lower mesosphere are practically non-existent and the few measurements available are not present on a continuous basis (see Supplement Text S1).
Rocket soundings
Oscillations of horizontal wind in the (upper) mesosphere/lower thermosphere
(MLT) have been extensively studied using radar observations
Here we present an analysis of oscillations in upper stratospheric and
mesospheric horizontal wind profiles with periods between 5 and
50
WIRA is a novel ground-based
passive microwave heterodyne receiver designed for the observation of horizontal wind
profiles from the mid-stratosphere (5
The zonal and meridional wind time series measured by WIRA during four different measurement campaigns analyzed in the present study. The gray areas correspond to data points judged untrustworthy according to the conditions indicated in the text. Please note the different color scale for zonal and meridional wind.
A strength of microwave radiometers is their ability to take measurements
during day and night and under overcast conditions. This strength, combined
with low operation costs, allows for the recording of long continuous time
series. The present study is based on measurements taken by WIRA at four
different locations at high, middle and low latitudes:
Sodankylä (67
ECMWF is a major service
provider of weather and climate data products. The operational
analysis used in this study combines
meteorological data from a variety of different observing platforms with a
continually updated general circulation model. The observations assimilated
in a 4-D-Var assimilations window of 12
It is known from earlier research that atmospheric waves with periods ranging
from 5 to 50
Temporally averaged periodograms of zonal and meridional wind
profiles measured by WIRA. The black, gray and white contour lines mark
The spectral method used in the present study will be described in some more
details in the following: for each altitude level a wind time series
The normalized periodogram
In our case
For comparison, the pseudo-wavelet approach used by
As Fig.
Spectral analyses have been performed on daily average wind profiles by WIRA
and ECMWF operational analysis. In
order to allow direct comparisons between measurements and model, the ECMWF
data were convolved with WIRA's averaging kernels to account for the limited
vertical resolution of the radiometer and data gaps were added at the times
The altitude-dependent temporally averaged periodograms of the horizontal
wind measurements by WIRA are shown in Fig.
Figure
As Fig.
Normal modes in the atmosphere are known to have oscillation periods around
2, 5, 10 and 16
Temporal evolution of the periodogram at stratopause level
(0.9
As Fig.
High interannual variability has to be expected (e.g., compare the results
from the Bern and the Provence campaign which were sampled at very close
geographical locations). Despite this variability, one might conclude from
the WIRA data that zonal wind oscillations tend to be strongest at midlatitudes and that meridional wind oscillations are weaker in the tropics
than at midlatitudes. This hypothesis is supported by Figs. S5 to S8 showing
ECMWF data for more extended time intervals at the campaign sites. It also
confirms previous studies based on observations or assimilated model data
The only major exception to the quiet mesosphere in
Fig.
The analysis for the scenario ECMWF at WIRA shown in
Fig.
Above 0.3
A previous study
Atmospheric waves and oscillations can be intermittent in nature (i.e.,
wave packets) and/or seasonally constrained. Accordingly, the temporal
evolution of the oscillations was examined.
Figures
From the analyses it becomes obvious that the dominant oscillation in the extra-long period range is always highly significant. The oscillation peaks for ECMWF data are slightly more significant, which is consistent with the absence of measurement noise.
A clear seasonality is apparent for all observations and model data with
oscillation activities being much stronger in the winter half-year for all
oscillation periods covered by the present study. The seasonality is also
visible for other years at the campaign sites as shown by ECMWF data
(Figs. S9 to S12 of the Supplement) and is in accordance with other
observational studies of stratopause level oscillations
The same pattern of seasonality as for the high- and midlatitude stations is
observed at La Réunion although it is located in the southern tropics
(21
During WIRA's measurement campaigns in the Northern Hemisphere two major
sudden stratospheric warmings occurred in mid-January 2012 and at the
beginning of January 2013. Previous studies
The period of the extra-long period oscillations is not constant between the
different campaigns. It can even vary within a single occurrence of the
oscillation as seen in the example of Bern where the period decreases from 35
to 25
As Fig.
In general the agreement between WIRA and ECMWF at stratopause level is very
good in terms of timing, amplitude and frequency. The extra-long oscillations
in zonal wind at the two midlatitude stations of Bern and Provence are
slightly stronger in the WIRA time series and the amplitude of the
quasi-16-day wave in the zonal wind is slightly enhanced for the
measurements. However, the most notable difference between WIRA and ECMWF
appears at shorter periods. Although mostly not statistically significant,
ECMWF seems to underestimate variabilities with periods shorter than
10
Long and extra-long period oscillations in the horizontal wind have been
observed by the novel ground-based Doppler wind radiometer
WIRA in the altitude range
between mid-stratosphere (5
The dominant oscillations were found to lie in the extra-long period band
(20–50
WIRA observations and ECMWF model data agree remarkably well below
0.3
We acknowledge ECMWF for the operational analysis data
(
This work has been supported by the Swiss National Science Foundation grants 200020–146388 and 200020–160048. We especially thank the staff of the Observatoire du Maïdo, the Observatoire de Haute-Provence and the Finnish Meteorological Institute in Sodankylä for the hospitality and support during the measurement campaigns. Edited by: N. Harris