Vol 59, No 6 (2023)

Various methods of cluster analysis are used for identification of large-scale atmospheric circulation regimes or weather regimes (WRs). In this paper we compare four most commonly used clustering methods – k-means (KM), Ward’s hierarchical clustering (HW), Gaussian mixture model (GM) and self-organizing maps (SOM) to analyze WRs in Euro-Atlantic region. The data used for WRs identification are 500 hPa geopotential height fields (z500) from the ERA5 reanalysis for the 1940–2022 period. Four classical wintertime weather regimes are identified by the KM method – two regimes associated with positive and negative phases of the North Atlantic Oscillation (NAO+ and NAO–), a regime associated with the Scandinavian blocking (SB) and a regime characterized by elevated pressure over the Northern Atlantics. For summer months KM method gets WRs that are similar by their spatial structure to the classical winter ones. The SOM method yields results that are almost identical to the results of KM method. Unlike KM and SOM methods, HW and GM do not catch the spatial structure of all four classical winter Euro-Atlantic weather regimes and their summer analogues. Compared to WRs of the KM and SOM methods, WRs obtained by HW and GM methods explain less z500 variance, they have different occurrences, persistence and transition features. Summer and winter WRs obtained by HW and GM methods are less similar to each other compared to WRs provided by KM method. Average spatial correlation coefficients between mean z500 fields of WRs obtained by KM and HW methods are 0.76 in winter and 0.83 in summer, 0.70 in winter and 0.72 in summer for KM and GM methods and 0.41 in winter and 0.44 in summer for the regimes between HW and GM methods, respectively. There are statistically significant trends of seasonal occurrence of WRs found by some of the studied clustering methods – a positive trend for the occurrence of the NAO+ regime and a negative trend for the occurrence of the NAO– regime.

Based on the data of the ERA5 reanalysis, the dates of spring and autumn rearrangements of the stratospheric circulation on isobaric surfaces of 30, 20 and 10 gPa in the latitude zone of 30–90° C. in the period 1979–2020 were obtained. Of the 42 cases of spring restructuring, 10 belong to the early, 15 to the middle and 17 to the late. The spread in the dates of spring rearrangements on the surface of 10 hPa is 69 days. Most often, the spring restructuring of the circulation occurs from top to bottom, in some years, the delay of spring restructuring on the surface of 30 gPa relative to the surface of 10 gPa reaches 22–25 days. Autumn perestroika takes place from the bottom up and their terms at the 3 levels under consideration are close to each other. The relationship between the timing of the spring restructuring of the stratospheric circulation with solar activity and large sudden winter stratospheric warming is shown. Analysis of the fields of anomalies of daily temperature values and zonal wind velocity in the 1000-1 hPa layer in the period January-May showed their significant spatio-temporal difference in the case of early and late spring perestroika. Thus, foci of positive anomalies of temperature and wind speed are formed initially in the upper stratosphere, and then shifted from top to bottom. The interrelations between the layers of the atmosphere in different seasons are considered.

According to the monitoring data of the optical and microphysical characteristics of smoke aerosol at AERONET stations during forest fires in the summer of 2019 in Alaska, anomalous selective absorption of smoke aerosol was detected in the visible and near infrared spectral range from 440 to 1020 nm. With anomalous selective absorption, the imaginary part of the refractive index of smoke aerosol reached 0.315 at a wavelength of 1020 nm. A power-law approximation of the spectral dependence of the imaginary part of the refractive index with an exponent from 0.26 to 2.35 is proposed. It is shown that for anomalous selective absorption, power-law approximations of the spectral dependences of the aerosol optical extinction and absorption depths are applicable with an Angstrom exponent from 0.96 to 1.65 for the aerosol optical extinction depth and from 0.97 to –0.89 for the aerosol optical absorption depth, which reached 0.72. Single scattering albedo varied from 0.62 to 0.96. In the size distribution of smoke aerosol particles with anomalous selective absorption, the fine fraction of particles of condensation origin dominated. The similarity of the fraction of particles distinguished by anomalous selective absorption with the fraction of tar balls detected by electron microscopy in smoke aerosol, which, apparently, arise during the condensation of terpenes and their oxygen-containing derivatives, is noted.

The results of back-trajectory analysis of nine-year (2013–2021) measurements of organic (OC) and elemental (EC) aerosol carbon concentrations made at the atmospheric monitoring station near St. Petersburg (Peterhof, 59.88° N, 29.83° E) are presented. The spatial location of sources was estimated by the concentration weighted trajectory method (CWT) in the geographic area 16°–44° E × 48°–68° N. The obtained data allow us to identify the territories with the strongest organic and elemental carbon emissions and to estimate the seasonal variability of these emissions. In particular, the obtained estimates show that the most intense sources of organic and elemental aerosol carbon in the studied region are located in the Volga-Oka interfluve and on the adjacent territories. It is demonstrated that linear regression coefficients between CWT function values for organic and elemental carbon differ for different regions and seasons and may indicate the prevailing type of sources of carbon-containing aerosol particles.

The contributions of various wave components (second, third and difference harmonics) to the formation of probability distributions of extreme wave heights, as well as amplitudes of crests and troughs, are evaluated based on the direct numerical simulation of irregular nonlinear deep water waves within three-dimensional potential equations of hydrodynamics. The simulation results taking into account 4- and 5-wave nonlinear interactions are analyzed. Different non-linear harmonics participate in the formation of the probability distributions in nontrivial way, essentially not satisfying the principles of linear superposition and ordering of the contribution by the parameter of weak nonlinearity.