Horizontal wavenumber spectra across the middle atmosphere are investigated based on density measurements with the Airborne Lidar for Middle Atmosphere research (ALIMA) in the vicinity of the ...Southern Andes, the Drake passage and the Antarctic peninsula in September 2019. The probed horizontal scales range from 2000 to 25 km. Spectral slopes are close to k−5/3 in the stratosphere and get shallower for horizontal wavelengths <200 km in the mesosphere. The spectral slopes are shown to be statistically robust with the presented number of flight legs despite the unknown orientation of true wave vectors relative to the flight track using synthetic data and a Monte Carlo approach. The largest spectral amplitudes are found over the ocean rather than over topography. The 2019 sudden stratospheric warming caused a critical level for MWs and a reduction of spectral amplitudes at horizontal wavelengths of about 200 km in the mesosphere.
Plain Language Summary
The spectral analysis of observations along extended flight tracks helps to determine the contribution of different length scales to atmospheric processes. In this study we calculate horizontal wavenumber spectra in the altitude range between 20 and 80 km, the middle atmosphere, based on observations from the Airborne Lidar for Middle Atmosphere research onboard the HALO aircraft. The observations were performed in the vicinity of the Southern Andes, the Drake passage and the Antarctic peninsula during September 2019. The observed horizontal scales range from 2000 km to about 25 km and cover almost the entire mesoscale range of atmospheric dynamics in the middle atmosphere. This study finds that vertical oscillations in the atmosphere, called gravity waves, cause the slopes and power of the spectra at the observed horizontal scales in the middle atmosphere. The slopes and power of the horizontal spectra vary with varying gravity wave activity during the period of observations.
Key Points
Horizontal wavenumber spectra across the middle atmosphere are computed using airborne lidar observations during the 2019 sudden stratospheric warming (SSW)
Horizontal wavenumber spectra are close to k−5/3 in the stratosphere, and become shallower in the mesosphere during the SSW
Observational evidence is provided that the mesoscale spectral slope in the middle atmosphere is caused by the occurrence of gravity waves
Abstract
Monitoring and predicting space weather activity is increasingly important given society’s growing reliance on space-based infrastructure but is hampered by a lack of observational data. ...Airglow at 1083 nm from metastable helium He(2
3
S) in the thermosphere has long been a target for remote-sensing instruments seeking to fill that gap; however, passive measurements of He(2
3
S) fluorescence are limited by low brightness, and interpretation of these observations is complicated by the > 500 km depth of the He(2
3
S) layer. Here, we demonstrate a lidar instrument that is able to stimulate and detect He(2
3
S) fluorescence, and we present measured profiles of He(2
3
S) density. These measurements provide crucial validation to space weather models, support predictions of peak number density ( ~ 1 cm
−3
) and the dependence of density on altitude, solar zenith angle, and season, and extend by a factor of 4 the maximum probed altitude range by an atmospheric profiling lidar. These measurements open the door for the development of more sophisticated lidars: by applying well-established spectroscopic lidar techniques, one can measure the Doppler shift and broadening of the He(2
3
S) line, thereby retrieving profiles of neutral wind speed and temperature, opening a window for studying space weather phenomena.
Annual cycles of horizontal winds and gravity wave (GW) momentum fluxes in the mesosphere/lower thermosphere (MLT) are presented for the medium frequency Doppler radar at Saura (SMF radar, located at ...69°N, 16°E) for the first time. Four year mean wind and momentum flux fields for 2008 through 2011 clearly show the coupling and interactions between GWs and the mean flow especially in the summer months. GW breaking at mesopause heights results in momentum flux divergence and affects the wind field by forcing a reversal of the wind profile in summer. Height‐time cross sections for the individual years (2008 to 2011) illustrate the year‐to‐year variation of horizontal winds and the vertical fluxes of zonal and meridional momentum. They show similar annual patterns from year to year which are more consistent in the summer months than during winter and have maximum absolute values in 2009. Furthermore, the precise SMF radar measurements give an excellent possibility to evaluate momentum flux estimates from the colocated meteor radar at Andenes. Both radars have different capabilities, and different techniques are applied to derive momentum fluxes. They show comparable results for the 4 year mean annual cycles of horizontal winds and momentum fluxes especially in summer. This holds for both structure and magnitudes in the overlapping heights, where the SMF radar data provide a wider vertical coverage. The best agreement is found for the zonal components of both radars, whereas there are some larger discrepancies in the meridional components, especially in the vertical flux of meridional momentum.
Key Points
First annual cycles of GW momentum fluxes with the comprehensive Saura MF radar
The year‐to‐year variation of horizontal winds and GW momentum fluxes is studied
Results from colocated MF and meteor radars agree very well in zonal direction
Temperature profiles based on radio occultation (RO) measurements with the operational European METOP satellites are used to derive monthly mean global distributions of stratospheric (20–40 km) ...gravity wave (GW) potential energy densities (EP) for the period July 2014–December 2016. In order to test whether the sampling and data quality of this data set is sufficient for scientific analysis, we investigate to what degree the METOP observations agree quantitatively with ECMWF operational analysis (IFS data) and reanalysis (ERA-Interim) data. A systematic comparison between corresponding monthly mean temperature fields determined for a latitude–longitude–altitude grid of 5° by 10° by 1 km is carried out. This yields very low systematic differences between RO and model data below 30 km (i.e., median temperature differences is between −0.2 and +0.3 K), which increases with height to yield median differences of +1.0 K at 34 km and +2.2 K at 40 km. Comparing EP values for three selected locations at which also ground-based lidar measurements are available yields excellent agreement between RO and IFS data below 35 km. ERA-Interim underestimates EP under conditions of strong local mountain wave forcing over northern Scandinavia which is apparently not resolved by the model. Above 35 km, RO values are consistently much larger than model values, which is likely caused by the model sponge layer, which damps small-scale fluctuations above ∼ 32 km altitude. Another reason is the well-known significant increase of noise in RO measurements above 35 km. The comparison between RO and lidar data reveals very good qualitative agreement in terms of the seasonal variation of EP, but RO values are consistently smaller than lidar values by about a factor of 2. This discrepancy is likely caused by the very different sampling characteristics of RO and lidar observations. Direct comparison of the global data set of RO and model EP fields shows large correlation coefficients (0.4–1.0) with a general degradation with increasing altitude. Concerning absolute differences between observed and modeled EP values, the median difference is relatively small at all altitudes (but increasing with altitude) with an exception between 20 and 25 km, where the median difference between RO and model data is increased and the corresponding variability is also found to be very large. The reason for this is identified as an artifact of the EP algorithm: this erroneously interprets the pronounced climatological feature of the tropical tropopause inversion layer (TTIL) as GW activity, hence yielding very large EP values in this area and also large differences between model and observations. This is because the RO data show a more pronounced TTIL than IFS and ERA-Interim. We suggest a correction for this effect based on an estimate of this artificial EP using monthly mean zonal mean temperature profiles. This correction may be recommended for application to data sets that can only be analyzed using a vertical background determination method such as the METOP data with relatively scarce sampling statistics. However, if the sampling statistics allows, our analysis also shows that in general a horizontal background determination is advantageous in that it better avoids contributions to EP that are not caused by gravity waves.
Considerable progress has been made over the past years concerning the experimental capabilities to observe mesospheric ice particles from space, from the ground, and in situ. Despite this progress ...regarding the observational data base, a quantitative description of related physical and chemical processes is still a challenging task due to uncertainties of several microphysical aspects concerning ice evolution in the harsh environment of the polar summer mesopause region. In the current paper, we review our current knowledge of the microphysics of mesospheric ice particles including issues like ice particle nucleation, the water vapor saturation pressure at mesopause temperatures, particle sedimentation, the equilibrium temperature of mesospheric ice particles, and particle coagulation. In addition, we consider the effect of variations of the atmospheric forcing variables like temperature, humidity, and turbulent transport. The sensitivity of ice particle properties towards these microphysical uncertainties and external forcings is assessed using the community aerosol and radiation model for atmospheres (CARMA). Simulated ice particle size distributions are analyzed applying Mie scattering calculations. Defining a hierarchy of uncertainties, our simulations suggest that the nucleation rate and number density of ice nuclei are most important, followed by the water vapor saturation pressure, and the accommodation coefficient affecting the particle temperature and sedimentation speed, and coagulation processes. Our study of the cloud sensitivity to changes of the forcing variables further reveals that close to the prevailing conditions in the polar summer mesopause region the cloud properties most strongly depend on a variation of water vapor, followed by temperature and eddy diffusion. Interestingly, our calculations suggest that the cloud brightness under the observing conditions of the SBUV/SBUV-2 suite of instruments is much more strongly controlled by variations of water vapor than temperature. Finally, we find that modeled ice particle size distributions are closely described by a Gaussian distribution. In contrast, the use of a lognormal distribution leads to a severe overestimate of the abundance of large particles.
An exceptionally deep upper-air sounding launched from Kiruna
airport (67.82∘ N, 20.33∘ E) on 30 January 2016 stimulated
the current investigation of internal gravity waves excited during a minor
...sudden stratospheric warming (SSW) in the Arctic winter 2015/16. The analysis
of the radiosonde profile revealed large kinetic and potential energies in
the upper stratosphere without any simultaneous enhancement of upper
tropospheric and lower stratospheric values. Upward-propagating
inertia-gravity waves in the upper stratosphere and downward-propagating
modes in the lower stratosphere indicated a region of gravity wave generation
in the stratosphere. Two-dimensional wavelet analysis was applied to vertical
time series of temperature fluctuations in order to determine the vertical
propagation direction of the stratospheric gravity waves in 1-hourly
high-resolution meteorological analyses and short-term forecasts. The
separation of upward- and downward-propagating waves provided further evidence
for a stratospheric source of gravity waves. The scale-dependent
decomposition of the flow into a balanced component and inertia-gravity waves
showed that coherent wave packets preferentially occurred at the inner edge
of the Arctic polar vortex where a sub-vortex formed during the minor SSW.
This study focuses on meteor smoke particle (MSP) induced effects on the D region ion chemistry. Hereby, MSPs, represented with an 11 bin size distribution, have been included as an active component ...into the Sodankyä Ion and Neutral Chemistry model. By doing that, we model the diurnal variation of the negatively and positively charged MSPs as well as ions and the electron density under quiet ionospheric conditions. Two distinct points in time are studied in more detail, i.e., one for sunlit conditions (Solar zenith angle is 72°) and one for dark conditions (Solar zenith angle is 103°). We find nightly decrease of free electrons and negative ions, the positive ion density is enhanced at altitudes above 80 km and reduced below. During sunlit conditions the electron density is enhanced between 60 and 70 km altitude, while there is a reduction in negative and positive ions densities. In general, the MSP influence on the ion chemistry is caused by changes in the electron density. On the one hand, these changes occur due to nightly electron scavenging by MSPs resulting in a reduced electron‐ion recombination. As a consequence positive ion density increase, especially water cluster ions are highly affected. On the other hand, the electron density is slightly increased during daytime by a MSP‐related production due to solar radiation. Thus, more electrons attach to neutrals and short‐lived negative ions increase in number density. The direct attachment of ions to MSPs is a minor process, but important for long living ions.
Key Points
Ionospheric modeling of the D region ion chemistry including meteor smoke particles
Diurnal variations of charged MSP number density and plasma composition differences
Case studies during sunlit and dark conditions for the analysis of individual ion behavior
During winter the wind field in the mesosphere/lower thermosphere (MLT) at middle and polar latitudes is characterized by a strong variability due to enhanced planetary wave activity and related ...stratospheric sudden warming (SSW) events. Such events are considered as distinct vertical coupling processes influencing the atmosphere below and above the stratosphere. In the last 12 years, an enhanced number of SSW, compared to the period from 1989 to 1998, has been observed in the northern hemisphere. Every SSW is connected with different effects in the MLT (strength and temporal development of wind reversals, temperature changes, wave activity, longitudinal dependence). To characterize the average behavior of the mesospheric response to strong SSWs, we combine high-resolution wind measurements from MF- and meteor radar at Andenes (69°N, 16°E) with global temperature observations from MLS aboard the Aura satellite for SSW events with a return to the middle atmosphere normal winter condition afterwards. Our aim is to identify characteristic wave patterns which are common to the majority of these events and to define the average characteristics of the SSW-related wave activity in the MLT. These will be compared to the relatively quiet winter 2011 with only a short minor warming without a wind reversal and to the wave activity in 2009 and 2010. The results show clear signatures of enhanced mesospheric planetary wave activity before and during the SSW and an earlier onset of the short term wind reversal in the mesosphere compared to wind and temperature changes in the stratosphere. The strong eastward winds at altitudes below 80km after SSW are connected with an enhanced gravity wave activity caused by changed filter conditions. This provides evidence for a strong modulation of semidiurnal tidal amplitudes before and during SSW by planetary waves. However, no clear relation has been found in the temporal development of tides relative to the onset of the selected SSW events.
► Mean waves as composite during SSW from mesospheric winds and MLS temperatures. ► PW is dominated by a 10-day wave synchronously with SSW. ► Weaker 16-day wave before the SSW. ► Evidence for PW modulation of semidiurnal tides.
The charging by secondary electron emission (SEE) from particles is known as a significant charging process in astrophysical plasmas. This work aims at evaluating the significance of SEE for charging ...of meteoric smoke particles (MSPs) in the Earth's polar atmosphere. Here, the atmosphere is subject to a bombardment of energetic electrons from the magnetosphere (and partly the sun). We employ the SEE formalism to MSPs in the upper mesosphere using electron precipitation fluxes for three different precipitation strengths. In addition, we address the possible effect of tertiary electron emission (TEE) from MSPs induced by atmospheric secondary electrons for one precipitation case. The SEE and TEE rates from MSPs of different sizes are compared to plasma attachment and photodetachment and photoionization rates of MSPs. The needed concentration of electrons and ions have been modeled with the Sodankylä Ion and Neutral Chemistry (SIC) model with included electron precipitation spectra as an additional ionization source. We find that secondary electron emission from MSPs is not a relevant charging mechanism for MSPs. The electron attachment to MSPs and photodetachment of negatively charged MSPs are the most important processes also during energetic electron precipitation.
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