For automatic searches for sudden stratospheric warming (SSW) events, their beginning/ending dates are defined as extrema of the first-order derivative simultaneous with zero values of the second derivative of temperature and zonal wind in time. A search for SSW dates at altitudes 30 and 40 km was performed, using the JRA-55 database for 59 years. The dates of the fasters change in temperature and zonal wind differ not more than two days for SSW searching the JRA- 55 database. The SSW dates correspond, within the limits of the uncertainties, to those obtained with other standard methods. Various types of SSW were analyzed. An alternative SSW classification was proposed. Frequently, before SSW developments, increases in the heat fluxes directed to the North Pole occur, which can heat the polar stratosphere.
Increased attention is currently paid to studying the so-called "secondary" acoustic-gravity waves (AGWs), which appear due to instabilities and nonlinear interactions of "primary" wave modes generated by atmospheric sources. This report is devoted to the study of horizontal spatial spectra of primary and secondary AGWs at fixed altitude levels in the middle and upper atmosphere using a high-resolution three-dimensional nonlinear model AtmoSym. It is found that in a short time after turning on the source of plane waves at the lower boundary of the model, the spectrum contains mainly a peak related to the primary AGW. Later, spectral peaks corresponding to secondary AGWs appear at horizontal wave numbers that are multiples of the wave numbers of the primary wave. This study allows estimating relative contributions of secondary AGWs at different heights, different times, and for different atmospheric conditions.
3-dimensional numerical nonlinear model of general circulation of the middle and upper atmosphere (MUAM) is used to investigate reaction of the atmospheric circulation in the middle and upper atmosphere to changes in phases of equatorial stratospheric quasi-biennial oscillation (QBO). To estimate changes in transport of atmospheric gas species, residual meridional circulation (RMC) is calculated based on the modelled atmospheric hydrodynamic fields for easterly, westerly and transitional (so-called “easterly-shear” and “westerly-shear”) QBO phases. For this purpose, four 10-members ensembles of MUAM simulations have been obtained corresponding to the aforementioned QBO phases. To determine QBO phases, empirical orthogonal functions (EOF) are applied for the equatorial zonal wind profiles. Statistically significant results are obtained illustrating how changes in direction of equatorial stratospheric winds influence extratropical circulation. It is shown in particularly, that the strongest changes in thermal and dynamical conditions of the middle- and high-latitude stratosphere-mesosphere occur during easterly-shear QBO phase.
For automated searches for sudden stratospheric warming (SSW) events, their beginning/ending dates are defined as extremes of the first-order time derivative simultaneous with zero values of the second-order time derivative of temperature and zonal wind. The dates of the fastest changes in temperature and zonal wind differ not more than two days for SSW searching the JRA-55 database. The SSW dates correspond, within the limits of the uncertainties, to those obtained with other standard methods. Frequently, before SSW developments, increases in the heat fluxes directed to the North Pole occur, which can heat the polar stratosphere.
Spectra of the critical frequency of the F2 ionospheric layer in the range of periods 0.5 – 40 days are analyzed using the data of measurements with the ionosonde DPS-4 at the Peterhof scientific station of Saint Petersburg State University (60°N, 30°E). Spectral analysis is executed using the Lomb –Skargle method for 60-day running intervals. Spectra show maxima at periods of 1 day and 0.5 day corresponding to diurnal and semidiurnal changes, and maxima in the range of periods 2 – 40 days. These waves have frequently maximum amplitudes in spring and summer, which is opposite to the PW amplitudes observed in the lower and middle atmosphere. This may be caused by different mechanisms including possible PW propagation from the middle atmosphere of the winter hemisphere to the summer thermosphere along the waveguides crossing the equator at altitudes above 60 km.
A high-resolution three-dimensional numerical model is used for studying nonlinear acoustic-gravity waves (AGWs), propagating from the Earth's surface into the upper atmosphere. Wave sources contain the superposition of two AGW harmonics with different periods, wavelengths and phase speeds. Large-scale AGWs change background conditions for the propagation of smaller-scale wave modes and can modulate their amplitudes. Simulations showed that nonlinear interactions might create small-scale structures in the upper atmosphere. Largest amplitudes of temperature disturbances occur at altitudes 100 – 200 km, producing convective instabilities at altitudes 100 – 120 km. Largest wave-induced increases in the mean temperature exist at altitudes 100 – 150 km. Above 200 km, changes in the mean temperature are mainly negative for the smaller-scale wave mode and are positive for the larger-scale mode and for their superposition. Interactions of two waves propagating in opposite directions produce the mean flows directed opposite and along the x-axis at different altitudes. Simulated wave-induced changes in the mean temperature and horizontal velocities produced by wave sources composed of two wave modes in the nonlinear model are different from the sums of respective changes created by the individual modes. These differences show that nonlinear interactions may significantly influence dynamical and thermal effects produced by sets of AGW spectral modes propagating in the atmosphere.
The intensity of mesoscale wind speed variations with periods 1.7 – 5.6 hr was determined by digital filtering of the measurements at mesosphere and lower thermosphere region, which had been performed at the Collm Observatory of Leipzig University (51.3° N, 13.0° E) using the ionospheric drift method in the years 1983 – 2007 and with a meteor radar in 2004 – 2018. This made it possible to obtain the continuous series of measurements for the interval of 1983 - 2018. Interannual changes at 88 km altitude are studied. The annual mean zonal wind was directed to the east in 1983 – 2018 and grew from 3 to 10 m/s. The meridional southward wind reached a minimum of 2 m/s around 2002 and a maximum of 10 m/s in 2012. The intensity of mesoscale waves varied within the limits of up to 10%.
Simple digital filtering is used for studying mesoscale variations of the rotational temperature of excited hydroxyl (OH*) at heights 85 – 90 km according to the data of spectral measurements at observatories Zvenigorod (56°N, 37°E.) in years 2004 – 2016, Tory (52°N, 103°E) in 2012 – 2017 and Maymaga (63°N, 130°E) in 2000 - 2015. Monthly-mean values and standard deviations of OH* temperature disturbances with periods 0.7 – 8 hr are determined, which may reflect the intensity of internal gravity waves in the mesopause region. The filtering of mesoscale variations was performed by calculating the differences between the measured values of OH* temperature separated with time intervals of 0.5 - 2 hr. Seasonal and interannual changes in the mesoscale variances of the temperature at the observational sites are studied.
Multiyear global climatology of mesoscale variations of atmospheric parameters at altitudes 2 - 35 km from radio occultation experiments with low-orbit satellites receiving GNSS signals is considered. Numerical filtering of vertical radio refraction profiles gives information about standard deviations of variations with vertical scales less than 8 km. Global distributions of the standard deviations are subjects for significant inter-annual changes in the tropo-stratosphere. Seasonal changes of radio refraction and temperature standard deviations obtained with Microlab 1, CHAMP and COSMIC satellites are analyzed and compared. At low latitudes, quasi-biennial oscillations exist. The causes of mesoscale variability may be convection, acoustic-gravity waves and turbulence in the atmosphere.
A parameterization of the dynamical and thermal effects of orographic gravity waves (OGWs) and assimilation quasibiennial oscillations (QBOs) of the zonal wind in the equatorial lower atmosphere are implemented into the numerical model of the general circulation of the middle and upper atmosphere MUAM. The sensitivity of vertical ozone fluxes to the effects of stationary OGWs at different QBO phases at altitudes up to 100 km for January is investigated. The simulated changes in vertical velocities produce respective changes in vertical ozone fluxes caused by the effects of the OGW parameterization and the transition from the easterly to the westerly QBO phase. These changes can reach 40 - 60% in the Northern Hemisphere at altitudes of the middle atmosphere.
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