Development Of Convective Cells Research Articles

Influences of Urbanization on an Afternoon Heavy Rainfall Event over the Yangtze River Delta Region

Abstract An afternoon heavy rainfall event occurred over coastal Nantong, an area of 70–100 km downstream from the Shanghai–Suzhou–Wuxi–Changzhou city belt over the Yangtze River Delta, under the influences of weak southwesterly monsoonal flows and lake/sea breezes on 26 July 2018. An observational analysis shows the emergence of pronounced urban heat island (UHI) effects along the city belt during the late morning hours. A series of nested-grid cloud-permitting model simulations with the finest grid spacing of 1 km are performed to examine the impacts of urbanization on convection initiation (CI) and the subsequent heavy rainfall event. Results reveal the generation of lake breezes and warm anomalies in the planetary boundary layer along the city belt and low-level convergence, thereby inducing upward motion for CI. The southwesterly flows of the monsoonal warm–moist air, enhanced by the UHI effects along the city belt, allow the development of convective cells along the city belt, some of which merge with convective clusters during their downstream propagation, and the ultimate generation of several distinct heavy rainfall centers by local convective clusters over coastal Nantong where atmospheric columns are more moist and potentially unstable under the influences of sea breezes. Sensitivity simulations show small contribution of Nantong’s UHI effects to the heavy rainfall event. The above findings help elucidate how the UHI effects could assist the CI in a weak-gradient environment, and explain why urbanization can contribute to increased downwind mean and extreme precipitation under the influences of favorable regional forcing conditions. Significance Statement The urban heat island (UHI) effects tend to produce more rainfall on its downwind side than that on the other sides, but alone could hardly account for the generation of heavy rainfall. This study examines the influences of the UHI effects associated with a city belt over the Yangtze River Delta on generating an afternoon heavy rainfall event over coastal Nantong that is 70–100 km downwind from the city belt. Results show (i) the downwind advection of the UHI effects; (ii) the initiation of convective storms along the city belt, and their subsequent downstream propagation, leading to the generation of heavy rainfall over Nantong; and (iii) an important role of sea breezes in generating the heavy rainfall event.

Isolating Large‐Scale Smoke Impacts on Cloud and Precipitation Processes Over the Amazon With Convection Permitting Resolution

AbstractAbsorbing aerosol from biomass burning impacts the hydrological cycle and radiation fluxes both directly and indirectly via modifications to convective processes and cloud development. Using the ICOsahedral Non‐hydrostatic modelling framework in a regional configuration with 1,500 m convection‐permitting resolution, we isolate the response of the Amazonian atmosphere to biomass burning smoke via enhanced cloud droplet number concentrations Nd (aerosol‐cloud interactions; ACI) and changes to radiative fluxes (aerosol‐radiation interactions; ARI) over a period of 8 days. We decompose ARI into contributions from reduced shortwave radiation and localized heating of the smoke. We show ARI influences the formation and development of convective cells: surface cooling below the smoke drives suppression of convection that increases with smoke optical depth, while the elevated heating promotes initial suppression and subsequent intensification of convection overnight; a corresponding diurnal response (repeating temporal response day‐after‐day) from high precipitation rates is shown. Enhanced Nd (ACI) perturbs the bulk cloud properties and suppresses low‐to‐moderate precipitation rates. Both ACI and ARI result in enhanced high‐altitude ice clouds that have a strong positive longwave radiative effect. Changes to low‐cloud coverage (ARI) and albedo (ACI) drive an overall negative shortwave radiative effect, that slowly increases in magnitude due to a moistening of the boundary layer. The overall net radiative effect is dominated by the enhanced high‐altitude clouds, and is sensitive to the plume longevity. The considerable diurnal responses that we simulate cannot be observed by polar orbiting satellites widely used in previous work, highlighting the potential of geostationary satellites to observe large‐scale impacts of aerosols on clouds.

Entropy generation analysis of Rayleigh convection in gas–liquid mass transfer process

An exploration of the transient state of Rayleigh convection is implemented via the entropy generation analysis, which is always used to identify the irreversibility with the available energy losses for the non-equilibrium thermodynamic processes of CO2 absorbed into some solvents. The calculation of local entropy generation rate for the convective mass transfer is worked out with the quantitative Schlieren technology, and its profiles perpendicular to the gas–liquid interface are applied to analyze the evolution of Rayleigh convection. The results show that entropy generation reaches approximately 2.50WK−1m−3 for the CO2–ethanol system until interfacial convection occurs to reduce local irreversibility with the synergistic effects of velocity vector and concentration gradient, as a result of providing a quantified thermodynamic criterion for predicting the onset of Rayleigh convection. Moreover, the critical value caused by mass transfer is determined by the density differences and boundary conditions for different solvents. The variation of global entropy generation shows that Rayleigh convection changes the irreversible route from a linear to non-linear thermodynamic branch with the emergence of the ordered plume-like structures. The detailed entropy generation analysis for the onset and development of convective cells is discussed to investigate Rayleigh convection and deepen the comprehension for the internal mechanism of convection enhancing mass transfer from the viewpoint of irreversible thermodynamics.

Stability analysis of stratified Rayleigh–Bénard–Poiseuille convection. Part II: Influence of thermocapillary forces

The development of convective cells in horizontal stratified systems can be induced by buoyancy or thermocapillary forces (Marangoni effect). When both mechanisms of instability are present it is expected a non-trivial influence of one over the other, with the main characteristics based primarily on the ratio between the buoyancy forces acting in each layer. In this study, linear stability analysis is used to investigate the influence of the thermocapillary effect in double layer stratified systems confined between two solid walls and heated from below, including cases where a pressure gradient induces fluid flow parallel to the walls. To simplify the analysis, it is assumed that the fluids in both layers have similar properties and the interface deformation is neglected. The presence of thermocapillary forces acting on the interface generates a complex behavior, particularly when the upper layer is deeper than the lower layer, which causes the convective cells to emerge initially in the upper layer. In this case, oscillatory states can appear as a result of the competition between the mechanisms of instability. This behavior is not observed if the thermocapillary forces are neglected; therefore, in order to obtain a complete picture of the system stability, it is important to include the Marangoni effect. The increases in the Reynolds and Prandtl numbers showed a similar effect. For low Re or Pr values, the thermocapillary forces initially stabilize the system, however, after a certain threshold the increase in the Marangoni number tends to destabilize the system. The presence of parallel flow also hinders the formation of oscillatory states. When the convective cells emerge initially on the lower layer, the increase in the Marangoni number facilitates the development of the cells, since the thermocapillary forces always act in the same direction as the buoyancy forces at the interface.

The Metal Content of Silicate Melts and Aqueous Fluids in Subeconomically Mo Mineralized Granites: Implications for Porphyry Mo Genesis

Abstract To better understand the factors leading to porphyry Mo mineralization, we studied melt and fluid inclusions in three subeconomically Mo mineralized granites in well-known Mo provinces: the Treasure Mountain dome in the Colorado mineral belt (USA), and the Drammen and Glitrevann granites in the Oslo rift (Norway). Melt and fluid inclusions were investigated in samples ranging from coarsely crystallized whole rocks to euhedral quartz crystals within miarolitic cavities. The major and trace element chemistry of individual inclusions was determined by laser ablation-inductively coupled plasma-mass spectrometry. Melt inclusions are rhyolitic in composition and record a clear trend of increasing Mo concentrations with increasing degree of melt differentiation as monitored by Cs, extending from ~5 to 10 ppm Mo at 5 ppm Cs to ~17 to 40 ppm Mo at 100 ppm Cs. Coexisting magmatic fluids were single phase, had a salinity of 4 to 6 wt % NaCl equiv and a density of 0.6 to 0.7 g/cm3, and contained ~0.5 wt % S and up to 6 mol % CO2. Molybdenum concentrations in these fluids ranged from ~20 to ~200 ppm Mo, except for some highly evolved fluids that had lower Mo contents. Comparison of our data with published fluid and melt inclusion data from porphyry Mo deposits, porphyry Cu (Mo, Au) deposits, and barren intrusions reveals that most subduction-related magmas have lower Mo/Cs ratios than within-plate magmas, but that within these two groups there are no systematic differences between barren and productive intrusions. This suggests that the mineralization potential was not primarily controlled by the metal content of the melts and fluids, but rather by other factors such as size of the magma chamber and the efficiency of residual melt and fluid extraction from the magma chamber and their focusing into a small apophysis at its top. Based on our data, it can be calculated that at least several tens of km3 of magma were necessary to form intermediate-sized Mo deposits, and at least several hundred km3 to form giant (≥1 Mt Mo) deposits. All three granites investigated in this study would have been large enough to produce at least an intermediate-sized Mo deposit, but they nevertheless are only subeconomically mineralized. Their low productivity thus appears to be the result of poor fluid focusing. Factors promoting a high degree of fluid focusing include (1) accumulation of major volumes of fractionated, crystal-poor melts at the top of the magma chamber, (2) formation of an apophysis, and (3) development of convection cells, leading to an efficient circulation of these fractionated melts through the apophysis.

The Diagnoses of Quasi‐Balanced Flows in Asymmetric Intense Hurricane

AbstractBased on the discussion of the balanced, quasi‐balanced and non‐balanced dynamic characteristics in hurricanes life cycle, the potential vorticity (PV)‐w inversion system is applied to analyze the hurricane Bonnie (1998), which exhibited the features of asymmetry and steady intensity. The results show that the basic vortex can be well described by the balanced flows derived through the nonlinear balanced model, whereas by adding the quasi‐balanced ! equation, the quasi‐balanced flows can reflect the structure of convergent inflow in planetary boundary layer (PBL), divergent outflow in upper troposphere, strong slantwise updraft in eyewall and subsidence motion in hurricane eye. Because the quasi‐balanced flows can describe the organized severe convective systems with long life span and the correlative convergence motion have the same magnitude with vortex motion, the quasi‐balanced flows have the co‐existence feature of divergence and vorticity motions. It is found, however, that the inflow area in PBL and outflow area in upper‐level contain significant unbalanced supergradient flows. Under the quasi‐balanced downshear environments, the moving direction left of mesoscale severe convective bands has the favorable condition for the genesis and the development of convective cells, and the opposite of the right side. So the severe convective systems in eyewall and spiral rain bands weaken and die out as soon as propagating to moving direction right with the basic cyclonic rotational flows, which causes the formation and maintenance of asymmetric structure in eyewall and spiral rain bands.

2005년 12월에 발생한 호남대설의 발달 환경에 관한 연구

Heavy snowfall was occurred over the south-western part of the Korean Peninsula called as Honam Districts, on two days from 21 December 2005. The development mechanism of snowfall and its characteristics were analysed using observation and numerical data provided by Korea Meteorological Administration. In comparison with other years Arctic air mass developed and maintained during all December 2005 due to active planetary waves with three branches. And jet streams at lower and higher levels make easy development of snow convection cells. Especially thermal low induced by mesoscale heat and dynamic sources, also help the developments of convection cells in strong ascension. The understanding the relation between synoptic and mesoscale circumstance, therefore, is also important to predict the heavy snowfall and to prevent the disaster.

Deep Convection and “First Echoes” within Anvil Precipitation

The development of convective cells within anvil precipitation, in a region of moderate convective activity that might be called a small mesoscale convective system, is described and discussed. The presence of precipitation-sized hydrometeors in the air as the convection develops makes early stages visible to radar that might not otherwise be seen. Two kinds of convective initiation are illustrated. In one, a vigorous cell is initiated over an outflow boundary, but within light precipitation. In the other, the initiation is evidently by an instability created by the melting layer, perhaps by a mechanism first discussed by Findeisen. In this latter type, the new convective elements are not severe but they generate supercooled cloud within the anvil, extend entirely through the anvil to altitudes above 12 km MSL, and produce graupel showers with rain at the ground exceeding 50 dBZ. The instability itself may be generated in large part by moistening and cooling the sounding by the falling precipitation.

Effects of land-surface heterogeneity upon surface fluxes and turbulent conditions

In this paper, we examine the effects of land-surface heterogeneity on the calculation of surface-energy and momentum fluxes in a meso-scale atmospheric model. A series of numerical experiments has been carried out with a combination of different resolutions for the atmosphere and the land surface, which allows an examination of the aggregation and dynamic effects associated with land-surface heterogeneity. The numerical results show that for a given atmospheric model resolution, increased land-surface resolution leads to better estimates of surface-energy and momentum fluxes, and for a given land-surface resolution, increased atmospheric model resolution also improves the estimates of these fluxes. This latter result contradicts the prevailing view that subgrid variation in atmospheric data plays only a minor role in estimating the fluxes. It is also shown that subgrid land-surface heterogeneity leads to increased turbulent fluctuations. The responsible mechanisms of this effect are both the subgrid variation of surface-energy fluxes and their impact upon the development of convective cells. It is suggested that subgrid atmospheric motions induced by surface heterogeneity may be an important factor which needs to be considered in subgrid closure schemes for atmospheric models.

MIXED CONVECTION IN A PLANE CHANNEL WITH A BUILT-IN TRIANGULAR PRISM

The superposition of Von Karman street and convective cells in a horizontal plane channel containing a triangular prism and heated from below constitute the principal subject of this numerical investigation. The numerical scheme is based on the control volume finite element method (CVFEM) adapted to the standard staggered grid with the SIM PLER algorithm for pressure-velocity coupling and an Alternating Direction Implicit (ADI) scheme for the time integration. Many standard test flows are simulated successfully. Results are obtained for a Grashof number ranging from 0 to 1.5 10 4 at Pr 0.71 and Re 100 with constant physical properties. At the outlet of the computational domain a convective boundary condition (CBC) is used. Results are presented to show the effect of development of convective cells on the flow pattern and on the Strouhal number. Regarding the heat transfer rate, we focus on the effect of the presence of the triangular prism on the heat flux transferred from the hot wall to the flow.

Density-dependent solute transport in discretely-fractured geologic media: is prediction possible?

The development of a dense solute plume in a fractured geologic medium can be highly irregular due to both the complexity of the fracture network as well as the presence of convection cells that may arise as a result of the density contrast between the invading solute and the ambient groundwater. A two-dimensional numerical model has been developed here to investigate density-dependent groundwater flow and solute transport in geologic materials that contain discrete fractures in order to examine some of the complex forms into which plumes can evolve, particularly with regard to fracture–matrix interactions. Results from simulations which involve parallel vertical fractures show that the evolution of the solute plume is affected by the development of convection cells in the porous matrix blocks between the vertical fractures. In a geologic medium containing a network of regularly spaced horizontal and vertical fractures, complex migration pathways can develop that are unexpected even though the geometry and interconnectivity of the fractures are known a priori. Downward solute migration can occur in some vertical fractures, while upward migration of less dense fluid can occur in others with transient circulation patterns developing in the intervening porous matrix. Because of the inherent uncertainty associated with fracture delineation, and because of the irregular nature of unstable dense plumes, deterministic prediction of dense-plume migration pathways and travel times in fractured geologic media will be subject to considerable uncertainty.

Thermohaline convection in flowing groundwater

Geothermal activity creates destabilising temperature gradients which are significant in some aquifers. Usually, in such aquifers stabilising salinity gradients also exist. The combination of temperature and salinity distribution in the aquifer may induce various types of hydrodynamic instabilities which were identified in a previous article. The present article concerns the effect of anisotropic characteristics of the hydrodynamic dispersion on the growth of instabilities in the aquifer. Three different mechanisms may lead to instability of the flow field: (a) buoyancy forces may induce convection currents; if the difference between the convection velocity of salt, due to the hydraulic gradients, and that of heat is negligible, then this mechanism is generally most effective in planes parallel to the hydraulic velocity of the fluid (velocity due to the hydraulic gradient); (b) the difference between heat and salt effective diffusivities may lead to overstability; this mechanism is most effective in planes perpendicular to the hydraulic velocity; (c) the difference between the convection velocity of salt and that of heat may induce oscillations which are most effective in planes parallel to the hydraulic velocity. The growth of instabilities in an aquifer of unlimited length is different from their growth in an aquifer of limited length. In the latter thermohaline convection develops in planes perpendicular to the hydraulic velocity, whereas in the former it develops in planes forming an angle θ with the hydraulic gradient. The development of convection cells in the flow field is identified by numerical experiments. These experiments identify the convection cell length and the angle formed between the thermohaline convection plane and the hydraulic gradient.