Attenuation of low-frequency electromagnetic waves in the Earth's lower ionosphere under the influence of strong local disturbances in the atmosphere.

The value and duration of attenuation of low frequency waves (1...10 kHz) in the presence of a strong local disturbance of the atmosphere have been estimated. Sources of significant local disturbances of the atmosphere are, for example, precipitation of energetic particles of radiation belts; electromagnetic pulses of lightning discharges; radiation of powerful low-frequency ground-based transmitters; invasion of large meteors. Strong local disturbances lead to an increase of ionization (concentration of free electrons) of the environment by several orders of magnitude in the region of space whose characteristic dimensions are comparable to the length of the wave (tens and hundreds of kilometers). As such a disturbance, we use the previously developed macroscopic model of an instantaneous, point release of a relatively large amount of energy in the atmosphere below the ionosphere. This model makes it possible to estimate the features of the propagation of low-frequency waves through the disturbed layer of the lower ionosphere by changing only two initial parameters: the disturbance energy and its initial height. It is shown that the attenuation value is almost independent of frequency and geo- and heliophysical conditions. For initial heights up to 50 km, the fading duration does not exceed ~ 2 min. With an increase of the initial altitude, the attenuation in the lower ionosphere becomes extremely large. However, for heights of 50 ... 70 km (depending on the value of energy), the horizontal size of the disturbance decreases significantly, which leads to a decrease in the fading time to tens of seconds for initial heights of more than 80 km.

On the size dependence of cumulus cloud spacing

<p>The spatial variability of Trade wind cumulus cloud fields has been found to be of great importance for understanding their role in Earth's climate system. In this study the focus is on the spacing between individual cumulus clouds. The main objective is to establish how inter-cloud spacing depends on cloud size, information that is crucial for understanding cloud-radiation interaction and spatial organization, and for informing grey zone parametrizations. To this end, a large-domain high resolution ICON LES simulation of marine shallow cumulus cloud fields is used. The domain is located at the subtropical Atlantic and the simulations are performed for the time of the recent NARVAL South campaign (December 2013). The simulations are compared with MODIS satellite imagery and research flight measurements, showing a good agreement between observations and the simulation, on both cloud size statistics and the vertical structure of the boundary layer.  </p><p>To determine the size and locations of the clouds, a cluster analysis was applied to the data. The inter-cloud, or nearest neighbor spacing (NNS), is addressed using four different expressions, classic definitions but also novel ones. For all definitions the spacing increases with cloud size, but the dependence is strongly influenced by the used definition. The classic definition, the spacing between clouds of any size, shows a well-defined logarithmic dependence on cloud size. The logarithmic relation can be explained by the abundance of closely-spaced small clouds. The small distances between these clouds form an upper bound for the NNS for the larger cloud sizes. In contrast, the spacing between clouds of a similar size increases exponentially with size. We argue that the exponential size-dependence reflects the mesoscale dynamics that drive the horizontal size of large convective cells, of which the cumulus clouds are the visible parts.  </p>

Hydrogen Tank Rupture in Fire in the Open Atmosphere: Hazard Distance Defined by Fireball

The engineering correlations for assessment of hazard distance defined by a size of fireball after either liquid hydrogen spill combustion or high-pressure hydrogen tank rupture in a fire in the open atmosphere (both for stand-alone and under-vehicle tanks) are presented. The term “fireball size” is used for the maximum horizontal size of a fireball that is different from the term “fireball diameter” applied to spherical or semi-spherical shape fireballs. There are different reasons for a fireball to deviate from a spherical shape, e.g., in case of tank rupture under a vehicle, the non-instantaneous opening of tank walls, etc. Two conservative correlations are built using theoretical analysis, numerical simulations, and experimental data available in the literature. The theoretical model for hydrogen fireball size assumes complete isobaric combustion of hydrogen in air and presumes its hemispherical shape as observed in the experiments and the simulations for tank rupturing at the ground level. The dependence of the fireball size on hydrogen mass and fireball’s diameter-to-height ratio is discussed. The correlation for liquid hydrogen release fireball is based on the experiments by Zabetakis (1964). The correlations can be applied as engineering tools to access hazard distances for scenarios of liquid or gaseous hydrogen storage tank rupture in a fire in the open atmosphere.

DETECTION OF ROAD CAVITIES IN URBAN CITIES BY 3D GROUND PENETRATING RADAR

Cavities under urban roads have increasingly become a great threat to the traffic safety in many cities. As a quick, effective, and high-resolution geophysical method, ground penetrating radar (GPR) has been widely used to detect and image near-surface objects. However, the interpretation of field GPR data is still challenging. For example, it is hard to distinguish reflections caused by road cavities or other urban utilities by a conventional 2D GPR survey. The superiority of 3D GPR in data interpretation is demonstrated by a laboratory experiment. Two pipes and a glass-made cavity buried in a sandpit show similar hyperbolic reflections in the 2D GPR profiles, and are hard to be discriminated. In contrast, their geometric shapes and dimensions are readily identified in the 3D image reconstructed from the synthetic 3D GPR dataset. Thus, a car-mounted 3D GPR system with two antenna arrays oriented in different polarization directions is developed, and has detected over 100 cavities in three Chinese cities over the past one year. The field data of two of the cavities are presented. As a result, the cavity depth, horizontal size and height can be accurately estimated from the 3D GPR dataset. Both laboratory and field experimental results indicate that 3D GPR possesses a great potential in detection and recognition of road cavities and utilities in the complicated urban environment.

Smoke with Induced Rotation and Lofting (SWIRL) in the Stratosphere

The Australian bushfires of 2019/20 produced an unusually large number of pyrocumulonimbus (pyroCb) that injected huge amounts of smoke into the lower stratosphere. The pyroCbs from 29 December 2019 to 4 January 2020 were particularly intense, producing hemispheric-wide aerosol that persisted for months. One plume from this so-called Australian New Year (ANY) event evolved into a stratospheric aerosol mass ~1000 km across and several kilometers thick. This plume initially moved eastward toward South America in January, then reversed course and moved westward passing south of Australia in February and eventually reached South Africa in early March. The peculiar motion was related to the steady rise in plume potential temperature of ~8 K day−1 in January and ~6 K day−1 in February, due to local heating by smoke absorption of solar radiation. This heating resulted in a vertical temperature anomaly dipole, a positive potential vorticity (PV) anomaly, and anticyclonic circulation. We call this dynamical component of the smoke plume “smoke with induced rotation and lofting” (SWIRL). This study uses Navy Global Environmental Model (NAVGEM) analyses to detail the SWIRL structure over 2 months. The main diagnostic tool is an anticyclone edge calculation based on the scalar Q diagnostic. This provides the framework for calculating the time evolution of various SWIRL properties: PV anomaly, streamfunction, horizontal size, vertical thickness, flow speed, and tilt. In addition, we examine the temperature anomaly dipole, the SWIRL interaction with the large-scale wind shear, and the ozone anomaly associated with lofting of air from the lower to the middle stratosphere.

Features of formation of karst falls on the earth’s surface

Karst manifestations are widespread in many regions and pose a significant danger to residence and economic activity. Failing funnels arise during the collapse of rocks over underground voids (caves, workings, etc.), formed during karst formation or in the process of anthropogenic doing in the rock massive. However, not every karst or technogenic cavity gives rise to a failure of the earth’s surface, and as a rule, its occurrence is unexpected. In this work, we consider the dynamics of the formation of dips of the earth’s surface in the form of a collapse pipe. To do this, the geomechanical problem is solved by the meshless code Smoothed Particle Hydrodynamics (SPH). The method allows to obtain a solution to the problem taking into account large deformations and possible discontinuities in the process of changing the stress-strain state. The Drucker-Prager fracture criterion is used, the parameters of which change over time in accordance with the accumulation of damage, which determines the temporary development of the fracture process, its beginning and speed. Various options for the formation of a vertical dip are considered depending on the geometrical parameters of the initial cavity, its depth and materials composing the rock mass, as well as the features of the destruction of various materials composing the mass during the formation of the dip. Relations are obtained that relate the depth of the cavity, the horizontal size of the hole, the strength properties of the rocks (adhesion, angle of internal friction), the coefficient of lateral pressure in the array. The features of wave processes generated by the formation of a dip are considered, for which a velocity field is obtained near the fracture zone at various time points in the fracture process.

A Microwave Tomography Strategy for Underwater Imaging via Ground Penetrating Radar

Detection and monitoring of underwater structures is one of the most challenging applicative scenarios for remote sensing diagnostic techniques, among which ground penetrating radar (GPR). With this aim, an imaging strategy belonging to the family of microwave tomographic approaches is proposed herein. This strategy allows the imaging of objects located into a wet sand medium below a freshwater layer and it can find application in investigation of lakes, rivers, and hydraulic structures. The proposed strategy accounts for the layered structure of the scenario under test by exploiting a spatially variable equivalent permittivity in the inverse scattering model. This allows a reliable reconstruction of depth and horizontal size of underwater hidden objects. The imaging capabilities of the strategy are verified by processing experimental data referred to a laboratory environment reproducing a submerged archeological site at scale 1:1. The results are compared with those obtained by modelling the reference scenario as a homogeneous medium, in order to verify the effective improvement in terms of reconstruction accuracy.

Structure Analysis of Tropical Cyclone Hudhud Over Bay of Bengal Using WRF Model

Tropical cyclones (TCs) over Bay of Bengal (BoB) have significant socio-economic impacts on the countries bordering the BoB. In this study, we have examined the structure and thermodynamic features of the TC Hudhud (7th -14th October, 2014) using WRF model. Simulated outputs are in good agreement with the available observations of India Meteorological Department and Joint Typhoon warning Center. At maximum intensity stage, the system’s horizontal size is found around 690 km. Wind and vorticity distributions capture the circulation of the system very well. Most strong winds of 60 ms−1 are extended vertically from 850 hPa to about 700 hPa. Simulation has shown intensification of the system above 200 hPa with wind speed of about 30ms−1. Relative humidity of the order of 90 % is found up to 400 hPa. Dhaka Univ. J. Sci. 66(1): 79-86, 2018 (January)

The increase of signal-to-noise ratio in the spatial separation of signals by the curvature of the wavefront

The article presents the results of the research of the dependence of the signal-to-noise ratio in the focal spot of the reflecting passive phased antenna array (PPAA), made of planar diffraction elements, on the orientation of the PPAA relative to the structure of the interference field, its dimensions and distance from PPAA plane to the focal spot. The results of the research show that the signal-to-noise ratio mainly depends on the horizontal size of the PPAA. The increase of the vertical size of the array leads to the fact that the focal distance corresponding to the maximum of the signal-to-noise ratio increases with the practically unchanged value of this maximum.

Сomputed tomography anatomy of the orbital apex

The apex of the bony orbit and its soft tissues are most difficult to investigate. Meanwhile just pathological processes in this area cause several serious conditions which could lead to blindness and in many cases to disability. Purpose: to study linear and volume indices of the bony orbital apex and its soft content in normal conditions. Material and methods: 210 patients (266 orbits) are examined. Both orbits were investigated in 56 patients (112 orbits) with no orbital pathology. In patients with unilateral orbital involvement, the normal orbit was investigated (154 orbits). Among examined patients, 86 were men and 124 women. Mean age was 41.2 ± 10.4 years. The CT scan according to the standard technique obtaining axial and frontal sections was carried out in all patients (section thickness was 1.0 mm; interval - 1.0 mm). Results and discussions: The average horizontal size of the external part of an orbit in men was 22.2 ± 0.41 mm (range 17-28 mm). The same size in women was 21.4 ± 0.23 mm (17-26 mm). The vertical size of the external part of the orbit in men is equal to 23.12 ± 0.38 mm, in average and at women - 23.4 ± 0.31 mm. Orbital apex length is 16-24 mm (average 20.1 ± 0.47 mm) in men, in women it is 15-23 mm (average 19,2 ± 0,35 mm). In the article, normal volume of the orbital apex, of the optic nerve, extraocular muscles and orbital fat are presented. Ratios of volume characteristics of studied structures of the orbital apex are displayed. Conclusions: Volume characteristics of the orbital apex and its soft content could be useful to understand the pathogenesis of pathological processes in this area. They could be also used to carry out the differential diagnosis between true and false proptosis, and for surgery planning.