STUDYING THE VARIATIONS IN AEROSOL LOADING AND THERMAL REGIME OF THE STRATOSPHERE OVER TOMSK ON THE BASIS OF LIDAR MONITORING

One of the important applications of lidar techniques is the study of aerosol content and thermal regime of the stratosphere. Such investigations in monitoring mode were initiated at the Institute of Atmospheric Optics since 1986 (for aerosol) and 1994 (for temperature), and are continued to the present. The main attention is paid to studying the annual variations in aerosol content in the stratosphere and sudden disturbances caused by winter stratospheric warming. In this paper we present the results of studying the aerosol content and its vertical stratification and vertical distribution of temperature in the stratosphere over Tomsk for the last three years.


INTRODUCTION
The period of 2016-18, which presents the results was characterized by the absence of volcanic eruptions which could affect the disturbance of the aerosol component of the stratosphere of the northern hemisphere including western Siberian region. So it was a convenient possibility to observe the peculiarities of the temporal variability of the background aerosol loading of the stratosphere over Western Siberia during quite long time interval. In this period, as in previous years, winter stratospheric warming's were observed. This gave the opportunity to continue to study the peculiarities of their manifestations. The represented data array of 222 total signals collected in some nights was used as initial data for analysis in 2016-18.The height interval was from 15 till 50-60 km, the spatial resolution was 192 m. Lidar signals were received in the photopulses counting mode with accumulation of ɩɨ 12×10 4 laser pulses (accumulation time was about 2 hours during a night).

METHODOLOGY
Lidar methods of the Raman and elastic aerosol and molecular scattering of light (Raman, Mie and Rayleigh scattering) were used to determine the vertical profile of aerosol characteristics and temperature in the stratosphere. The atmospheric sounding was carried out by laser radiation at a wavelength of 532nm, the reception of the lidar signals at wavelengths 532 and 607 nm. The optical characteristics of the scattering ratio R(H) and the integral aerosol backscattering coefficient B(H) were used to estimate the aerosol filling of the stratosphere. where ȕK-aerosol backscattering coefficient, ‫ܪ‬ ଵ is the height of the tropopause, and ‫ܪ‬ ଶ is chosen about 30km.
The vertical profiles of temperature T(H) were retrieved from the Rayleigh and Raman signals using the formula and Ɋ2(ɇ) are the transparencies of the atmosphere from the level of arrangement of the lidar up to the height H at the wavelength of 532 nm (Rayleigh signals) and 532 and 607 nm (Raman signals), respectively; R* is the universal gas constant, g(h) is the gravity acceleration, Hm is the maximum height, from which quite reliable for processing signals are detected (so-called calibration height, at which the temperature values Ɍ(Hm) are set).

Aerosol
The summary results of lidar observations of the stratospheric aerosol layer for the period 2016-18 are presented in Fig.1. In this picture measurements of the monthly mean dynamics of the aerosol vertical stratification are shown. The observations for this period of time indicate that the common feature for the region of the Western Siberia is the maximal aerosol loading of the stratosphere in winter months, almost no aerosol loading for the entire stratospheric layer in summer months, and no loading for the upper stratospheric layer from 30 to 50 km, except in winter months. Also, there are substantial differences in aerosol stratification. For instance, the aerosol loading was stronger in December 2017 as compared to 2016 and 2018, it was stronger in January 2016 and 2018, and stronger in February 2016.

Temperature.
Observations for January 2016 are presented in Fig. 1. In this figure, like in figures below, the vertical temperature profiles (VTP), obtained from lidar measurements, are compared with those, measured on Aura satellite [3] and at aerological sensing station in Novosibirsk [4], as well as those from CIRA-86 model [5].
From Fig. 3 it can be seen that the stratospheric warming (SW) began in the second decade of January, and continued on the next days of observations in a classical form of half-waves with a positive deviation from monthly average temperature in the upper half of the stratosphere and a negative deviation in its lower half.
The maximal positive deviation reached 40 K (January 12, altitude of 45 km), and maximal negative deviation was 35 K (January 5 and 7, altitude of about 30 km). From January 19 to 26, the vertical temperature distribution stabilized somewhat (approached the model distribution) in the upper stratosphere. However, in the entire layer of the lower atmosphere, the temperature profiles obtained from lidar, balloon, and satellite data remained shifted toward negative values relative to the model profile. The last overshoot of warming was observed on February 1 of 2016 in the height range of 40-60 km (see Fig.4). The analogous warming, but in the narrower height range of 45-60 km, was also observed from the data of satellite measurements. According to the data of the following observations, the final stage of stabilization of the vertical temperature distribution occurred. From Fig. 4, we can see that the temperature profiles obtained in the entire height range of 10-60 km from lidar, satellite, and balloon measurements become quantitatively and qualitatively closer to the model profile. The observed SW in 2016 was of the minor type, as indicated by the data taken from the website of the European center for medium-term weather forecasts [9]. Temperature profiles for the quiescent period of 2016 (from April to September) are shown in Fig. 5. In this long period of time, the stratosphere evolved into a stable thermal regime, in which the vertical temperature distribution well agrees with the monthly average model distribution. The next stratospheric warming began in December (Fig. 6). For instance, on December 11 at the altitude of about 50 km there had been a positive deviation of temperature from monthly average by about 30 Ʉ. Then, the weather conditions were unfavorable, so that observations had been resumed as late as the end of January 2017. From Fig. 6 it can be seen that, as indicated by both lidar and satellite observations, the stratospheric warming continued for the third decade of January. It is noteworthy that the lidar and satellite measurements of temperature well agree. Based on these independent measurements, the warming pattern shows two peaks at altitudes of 30-32 and 47-50 km.
Then, in the next period of observations from February to December, the thermal regime of the stratosphere evolved into the stable state, indicated by a good correspondence between lidar/satellite and modelbased temperature profiles (not shown).
Occurrence of stratospheric warming in winter 2017/2018 was recorded both by lidar and satellite measurements in the last decade of January 2018 (Fig. 7). The warming peaked in the stratopause region (January 21, ɇ=50 km). On next days of observations, the stratopause altitude decreased to 40 km, and the maximal positive deviation reached 50 Ʉ. The vertical temperature distribution during the SW episode in December 2018 -January 2019 is shown in Fig. 8. A pronounced warming was localized at altitudes from 20 to 50 km, and was accompanied by a cooling at altitudes from 10 to 20 km. We note that the stratospheric warming was as long as the whole month and had a large (up to 60 K) amplitude of the positive  The SW in 2018-2019 was major in type, i.e., westerly air mass transport in the stratosphere reversed to easterly transport (see Fig. 9). The lidar studies of the stratosphere over Tomsk in the period of 2016-2018, as well as earlier years, revealed the following: 1. The loading of the stratosphere with background aerosol occurs in the cold period of a year: start in October, maximum in January, and end in April. In the warm period, there is practically no aerosol in the stratosphere. 2. Every year the winter warming takes place in the stratosphere. It begins in December, is most pronounced in January, and is sometimes extended to February. Weak and, more rarely, strong warmings are observed. The amplitude of positive temperature deviations from the monthly average value can achieve 60 K, and the height of the stratopause can go down to 30 km. In the period of March till November, the vertical temperature distribution is inasatisfactory agreement with the CIRA-86 model distribution.