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At the altitudes of the mesosphere – lower thermosphere, OH and HO2 play a significant role in many physicochemical processes. Thus, monitoring of their spatiotemporal evolution together with other chemically active trace gases is one of the most important problems for this atmosphere region, in which direct measurements are difficult. The paper studies the nighttime OH and HO2 chemical equilibria using the 3D chemical transport modeling within the general approach including the extraction of the main sources and sinks in the equilibrium space-time areas and derivation of analytical criteria for equilibrium validity. The presented analysis shows that there are extended areas, where nighttime HO2 and OH are close to their local equilibrium concentrations determined mainly by the reaction between HOx – Ox components themselves and with H2O2, N, NO, NO2, and CO. In the upper mesosphere – lower thermosphere, the equilibrium expressions can be shortened, including the HOx – Ox chemistry only. These conditions describes the HO2 and OH equilibrium from the top to the some lower borders, the altitude position of which vary in the interval between 72–73 and 85 km and depends essentially on season and latitude. The developed analytical criteria almost everywhere well reproduce the main features of these borders. The obtained results allow to extend the abilities of previously proposed methods for the retrieval of poorly measured components from measurement data and to develop new approaches. [ABSTRACT FROM AUTHOR]

The assumption of nighttime ozone chemical equilibrium (NOCE) is widely used for retrieval of the Ox-HOx components in the mesopause from rocket and satellite measurements. In this work, recently developed analytical criterion of determining the NOCE boundary is applied (1) to study of connection of this boundary with O and H spatiotemporal variability basing on the 3D chemical transport modeling, and (2) to retrieve and analyze the spatiotemporal evolution of the NOCE boundary in 2002-2021 from SABER/TIMED data set. It was revealed, first, that the NOCE boundary well reproduces the transition zone dividing deep and weak diurnal oscillations of O and H at the low and middle latitudes. Second, the NOCE boundary is sensitive to sporadic abrupt changes in the middle atmosphere dynamics, in particular, due to powerful sudden stratospheric warmings leading to events of elevated (up to ~80 km) stratopause, which took place in January-February 2004, 2006, 2009, 2010, 2012, 2013, 2018 and 2019. Third, the space-time evolution of this characteristics expressed via pressure-height contains a clear signal of 11-year solar cycle in the range of 55°S-55°N. In particular, average annual the NOCE boundary averaged in this range of latitudes anticorrelates well with F10.7 index with the coefficient of -0.96. Moreover, it shows a weak linear trend of 49.2±36.2 m/decade. [ABSTRACT FROM AUTHOR]

An increase in the frequency of atmospheric hazards in a changing climate has attracted interest in the study of regional features of mesoscale convective systems and trends of lightning activity. Severe convective storms are the most destructive weather events causing substantial damage and fatalities. In this paper, we analyze general trends in the lightning activity in the Upper Volga region and identify the particular features of a severe thunderstorm on 13–14 July 2020. The analysis of the annual variability of the lightning activity in the Upper Volga region, carried out according to the World-Wide Lightning Location Network (WWLLN) data, has shown that the total number of discharges during the convective seasons of 2016–2021 increased. For the convective season of 2020, the daily number of discharges in the neighborhood of Nizhny Novgorod (the center of the region) is calculated according to the WWLLN data and the electric field mill (EFM) measurements. It is revealed that the most powerful (per convective season) thunderstorms have a number of similarities both according to observations and numerical simulation. The thunderstorm on 13–14 July was numerically simulated using the Weather Research and Forecasting (WRF) model in combination with calculations of the electric parameters (such as electric potential and intracloud electric field). The simulations were carried out using two parametrizations of microphysical processes to reveal the features associated with allowance for aerosol particles. The data from the meteorological radar and WWLLN were used to validate the radar reflectivity simulation results of the WRF model. Some features of the thunderstorm evolution characteristic for each of the microphysical parametrizations were identified. In general, the non-aerosol parametrization gives a more correct description of the development of severe thunderstorms in the Upper Volga region (with the exception of the final stage of the convective system development). For a convective event to have the required duration (more than 6 h), aerosol particles should be taken into account. [ABSTRACT FROM AUTHOR]