Influence of Tropical Waves on the Lifecycle of Mesoscale Convective Systems over West Africa

<p>Rainfall variability over West Africa remains a major challenge for numerical weather prediction (NWP). Due to the largely stochastic and sub-grid nature of tropical convection, current NWP models still fail to provide reliable precipitation forecasts – even for a 1-day leadtime – and are barely more skillful than climatology-based forecasts. Thus, several recent studies have investigated the presumably more predictable influence of tropical waves on environmental conditions for convection and found distinct and coherent (thermo-)dynamical patterns depending on the type and phase of the wave. Of particular interest in this context is the interaction of the wave with the lifecycle of usually westward propagating mesoscale convective systems (MCSs), which are the major providers of rain in the region and can occasionally even lead to flooding. The exact mechanisms and strength of this interaction are still not entirely known.</p><p>This study combines two recent datasets in a novel way in order to systematically investigate the influence of tropical waves on MCS characteristics and lifecycle. First, MCSs are tracked within northern tropical Africa (20°W-30°E / 2°-15°N) over an 11-year period during the West African rainy season (April-October) using infrared brightness temperature fields provided by the Spinning enhanced visible and infrared imager (SEVIRI). Second, tropical waves are isolated by applying a filtering method in the wave-frequency domain to precipitation data of the Tropical Rainfall Measuring Mission (TRMM) within the 5°-15°N latitude band for the same target period. By combining the two datasets in space and time, the magnitude and phase of each wave is known at every timestep of the MCS tracks, which enables a systematic investigation of MCS characteristics as a function of wave properties.</p><p>Preliminary results suggest that long-lived MCSs (lifetime ≥ 12h) frequently couple with the “wet” phase of high-frequency tropical waves, in particular Kelvin, eastward inertia-gravity (EIG), and African easterly waves (AEW). Showing an enhanced occurrence frequency of MCS initiation, the wet phase of AEWs appears to have strong modulation capabilities during the genesis stage and further accompanies these long-lived MCSs during their entire lifetime. In the case of Kelvin waves and EIGs, the wet phase overlaps only with the intensification and maturity stage of these MCSs as a consequence of opposite directions of movement. Similar coupling patterns also exist for mixed Rossby gravity waves (MRGs), although to a weaker extent. Furthermore, no consistent coupling tendencies with long-lived MCSs are evident for low-frequency waves (Madden-Julian Oscillation (MJO), equatorial Rossby wave (ER)), arguably since they act on larger spatio-temporal scales. For short-lived MCSs (lifetime < 6h), the coupling with high-frequency waves is substantially weaker.</p><p>In the future we will also address potential influences of wave-wave interactions on MCSs as well as potential differences in coupling mechanisms between the Guinea Coast region and the Sahel farther north. With increasing efforts in the prediction of tropical waves, this study has the potential to aid the short-term forecasting of MCS development and its lifecycle. This can be of particular importance for the anticipation of extreme rainfall events and subsequent risk assessment in West Africa.</p>

Extreme convection in the Lofoten Basin of the Norwegian Sea

<p>One of the factors affecting the variability of the global climate is strong oceanic convection. Current research declares the results of the investigation on the extreme convection in the Lofoten Basin (LB) using the Argo profilers data. The most common parameter reflecting the convection intensity is Mixed Layer Depth (MLD). In the frames of the understudied period, MLD exceeds 1000 m in March-April and December 2010 in the Lofoten Basin Eddy (LBE), whereas the average MLD is about 200 m and rarely exceeds 400 m in the basin. Water volume formed at mid-depth of the central LB, between 1000 m depth and the isosteric surface s07 is connected with the extreme convection events. We analytically assess the final mixing depth that corresponds well to measured values of the MLD. Such a correspondence indicates the variations in the buoyancy flux and stratification as the main reasons for MLD variability in the LB. We easily explain this variability due to heat release in the basin. Atmospheric patterns during the extreme convection are described. It occurs that northerly winds are as common as dominating south-westerly winds during the months with extreme convection. 32 cases of extreme convective events with MLD exceeding 350 m were analyzed and we reveal that correspondent composite maps of Sea Level Pressure (SLP) and surface heat flux match well NAO-/EAP- atmospheric pattern in the Northern Atlantic, while negative NAO pattern prevails in climate during winter-spring. We define the heat release as the major trigger of strong convection. Heat release associated with extreme convection events in the LB is twice stronger than usual.</p>

Ground-level atmospheric electricity of mid-latitude Nimbostratus and Stratus cloud at Swider station, Poland

<p>We present main results of our analysis of the ground-level atmospheric electricity under  Nimbostratus and Stratus clouds at mid-latitude Geophysical Observatory in Swider. Atmospheric electricity data from the Geophysical Observatory in Swider was analysed according to the calculation scheme allowing to obtain the main components of the current density in such conditions, i.e. conduction current density and precipitation or convection current, based on the basic measured parameters: electric field, Maxwell current density and total air conductivity. The atmospheric electric field and conduction current is more likely downward under Stratus cloud as is the precipitation or convection current. The electric field under Nimbostratus during snow at the ground is downward and during rain is upward and sometimes also upward precipitation current occurs during heavier rain. Mean values of electric field, conductivity, conduction and precipitation current have been obtained and an average mean current budget was calculated. Another analysis concerns the dependence of precipitation current density on the electric field at the Earth's surface in the conditions of Nimbostratus with continuous, stable precipitation, in historical cases reported as linear. The dependence of the linear regression coefficients on the value of electrical conductivity of the air was particularly investigated from the angle of the theoretical results of the work of Ette and Oladiran (1980).</p>

Numerical Investigation of Natural Convection and Entropy Generation of Water near Density Inversion in a Cavity Having Circular and Elliptical Body

In this chapter, a water-filled square cavity with left hot wall and right cold wall was numerically investigated. The hot and cold wall temperatures are 10°C and 0°C respectively to examine the density inversion of natural convection water, i.e. water at 4°C. In the middle of the square, there are circular and elliptical bodies to study fluid–structure interaction in terms of the thermohydraulic behavior and entropy generation. 2D numerical simulation was performed using finite volume method in Ansys fluent software with the assumption of laminar flow. The simulation results are compared with benchmark data to determine reliability. The results indicate that the body insertions increase the convection heat transfer coefficients at the best heat transfer positions due to impingement heat transfer. An increase in heat transfer rate of 1.06 times is observed in the case of circular body compared to none. There are three primary eddies in the cavity with bodies, whereas the cavity without body has two primary eddies. Maximum entropy generation was found in the upper right corner of cavity mainly due to high horizontal temperature gradient. Bodies of circle and vertical ellipse have almost the same thermohydraulic and entropy generation characteristics due to the same horizontal dimension which mainly effects on the downward natural convection current. The entropy generation of cavity with circular body is 1.23 times higher than that of the cavity without body. At positions y/L = 1 on the hot wall and y/L = 0.74 on the cold wall, the convection heat transfer coefficient is close to zero due to stagnant fluid.

Preparation, characterization, and study of the nonlinear optical properties of a new prepared nanoparticles copolymer

The nonlinear optical properties of a nanoparticle polyaniline/polyacrylonitrile (PAn/PANr) copolymer newly prepared via experimental and theoretical findings are reported. Diffraction ring patterns result in the far field when a CW, visible, 473 nm, low power laser beam traverses the solution of PAn/PANr in the solvent ethanol. The diffraction ring patterns are usually used to estimate the nonlinear refractive index while the Z-scan is used to measure both nonlinear refractive index and its sign and nonlinear absorption coefficient. The experimentally observed ring patterns are numerically reproduced via the use of the Fresnel–Kirchhoff procedure. Good agreement between experimental findings and the numerical ones are obtained. The obtained diffraction ring patterns suffer modification in the vertical direction due to convection current as a result of local heating of the sample and the presence of gravity.

Numerical simulation and experiments to improve throwing power for practical PCB through-holes plating

Purpose The purpose of this study is to investigate the synergism of convection, current density distribution and additives by numerical simulation and electrochemical experiments for good throwing power (TP) of copper electro-deposition in printed circuit board (PCB) manufacture. Design/methodology/approach The flow field of THs and current density distribution on various AR of THs are calculated and analyzed. Meanwhile, corresponding simulation is used to study the performance of plating electrolytes on TP. Two electrochemical parameters, overpotential (η) and potential difference (△η), are chosen to evaluate the electrochemical properties of different plating solutions by galvanostatic measurement and potentiodynamic cathode polarization at different rotating speeds. Findings By combining both the results of simulation and practical plating, these two electrochemical properties of electrolytes exhibit significant impact to the system at varied conditions. Especially, the electrolyte with higher polarizing η and △η values lead to the elevated TP for AR of more than 2:1. Originality/value The harring cell model is built as a bridge between the theoretical and experimental study for control of uniformity of plating THs in PCB manufacturing. This dual-parameter evaluation is validated to be a promising decisive method to guide the THs plating with particular AR in industry.

Fully developed mixed convection flow in a vertical microtube with time periodic heating boundary condition

Purpose The purpose of this paper is to investigate fully developed mixed convection flow in the steady-periodic regime for a Newtonian fluid in a vertical microtube in the presence of velocity slip and temperature jump, which has not been accounted for in the literature. Design/methodology/approach To achieve this objective, the governing equations for the problem are separated into steady and oscillatory components using separation of variable method; this gives a pair of independent boundary value problems. This is then solved along with its boundary conditions and constraint equations using the method of undetermined coefficient. The exact solutions of momentum and energy equations are obtained under the velocity slip and temperature jump conditions. Findings The significant result from the study is that increase in rarefaction parameter as well as fluid–wall interaction parameter decreases the oscillation amplitude of the dimensionless velocity. Furthermore, it was found that the product of dimensionless frequency and Prandtl number initiate a strong convection current inside the microtube. Practical implications Such type of study may be used on the determination of the thermal and tangential momentum accommodation coefficients and be applicable to the designs and fabrications of microheat exchanger. Moreover, it provides the possibility to get a bench mark for numerical solvers with reference to basic flow configuration. Originality/value These solutions generally deserve great attention, since the application of a magnetic field has been found to be effective tool in controlling the convection current. The current work is aimed as an extension of the previous analytical studies to prove some insight into a number of industrial applications, which use similar configurations.

Study of Aerosol Charging in Electro-Aerosol Generator

The urgency of the study is conditioned by the necessity of perfection of the methods of electro-aerosol disinfection and disinsection of the air and the surfaces of premises, medical and preventive treatment of animals, and plant protection. The objective of the paper is aimed at revealing the regularities of electro-physical processes when charging an aerosol in electro-aerosol generators, improvement of the methods of electro-aerosol generation. The leading approach to the research of this problem is mathematical modelling of electrotechnical, dynamic and kinetic processes in electro-aerosol generators, allowing revealing the basic regularities of electro-aerosol generation. Experimental studies are aimed at assessment of the main characteristic of the electro-aerosol generator – convection current. As a result of the studies, a mathematical model of electro-aerosol generation with the mechanical method of atomization was obtained. The regularities of charging a fluid thread, arising at the edge of the generator, when applying electric field. The configuration of the electric field of cylindrical electrodes, taking into account the influence of the bulk charge of the electro-aerosol cloud, was considered. Electrostatic pressure of the electric field of the generator on fluid streams was taken into consideration. An equation for determining the average radius of drops of the electro-aerosol during mechanical atomization by the perforated cage was obtained. The mathematical model of electro-aerosol generation is obtained in the form of the equation of convection current, being one of the basic characteristics of the process of electro-aerosol generation, allowing defining the efficiency of electrization of aerosol particles and the degree of their fragmentation. Theoretical calculations are verified experimentally. Materials of the paper can be useful for the researchers, post-graduate students, engineers, engaged in designing, manufacturing and exploiting electro-aerosol equipment.