Maximum Vertical Velocity Research Articles

The Influences of Effective Inflow Layer Streamwise Vorticity and Storm-Relative Flow on Supercell Updraft Properties

Abstract The relationship between storm-relative helicity (SRH) and streamwise vorticity ωs is frequently invoked to explain the often robust connections between effective inflow layer (EIL) SRH and various supercell updraft properties. However, the definition of SRH also contains storm-relative (SR) flow, and the separate influences of SR flow and ωs on updraft dynamics are therefore convolved when SRH is used as a diagnostic tool. To clarify this issue, proximity soundings and numerical experiments are used to disentangle the separate influences of EIL SR flow and ωs on supercell updraft characteristics. Our results suggest that the magnitude of EIL ωs has little influence on whether supercellular storm mode occurs. Rather, the transition from nonsupercellular to supercellular storm mode is largely modulated by the magnitude of EIL SR flow. Furthermore, many updraft attributes such as updraft width, maximum vertical velocity, vertical mass flux at all levels, and maximum vertical vorticity at all levels are largely determined by EIL SR flow. For a constant EIL SR flow, storms with large EIL ωs have stronger low-level net rotation and vertical velocities, which affirms previously established connections between ωs and tornadogenesis. EIL ωs also influences storms’ precipitation and cold-pool patterns. Vertical nonlinear dynamic pressure acceleration (NLDPA) is larger at low levels when EIL ωs is large, but differences in NLDPA aloft become uncorrelated with EIL ωs because storms’ midlevel dynamic pressure perturbations are substantially influenced by the tilting of midlevel vorticity. Our results emphasize the importance of considering EIL SR flow in addition to EIL SRH in the research and forecasting of supercell properties.

Thermal Chains and Entrainment in Cumulus Updrafts. Part II: Analysis of Idealized Simulations

AbstractResearch has suggested that the structure of deep convection often consists of a series of rising thermals, or “thermal chain,” which contrasts with existing conceptual models that are used to construct cumulus parameterizations. Simplified theoretical expressions for updraft properties obtained in Part I of this study are used to develop a hypothesis explaining why this structure occurs. In this hypothesis, cumulus updraft structure is strongly influenced by organized entrainment below the updraft’s vertical velocity maximum. In a dry environment, this enhanced entrainment can locally reduce condensation rates and increase evaporation, thus eroding buoyancy. For moderate-to-large initial cloud radius R, this breaks up the updraft into a succession of discrete pulses of rising motion (i.e., a thermal chain). For small R, this leads to the structure of a single, isolated rising thermal. In contrast, moist environments are hypothesized to favor plume-like updrafts for moderate-to-large R. In a series of axisymmetric numerical cloud simulations, R and environmental relative humidity (RH) are systematically varied to test this hypothesis. Vertical profiles of fractional entrainment rate, passive tracer concentration, buoyancy, and vertical velocity from these runs agree well with vertical profiles calculated from the theoretical expressions in Part I. Analysis of the simulations supports the hypothesized dependency of updraft structure on R and RH, that is, whether it consists of an isolated thermal, a thermal chain, or a plume, and the role of organized entrainment in driving this dependency. Additional three-dimensional (3D) turbulent cloud simulations are analyzed, and the behavior of these 3D runs is qualitatively consistent with the theoretical expressions and axisymmetric simulations.

Thermal Chains and Entrainment in Cumulus Updrafts. Part I: Theoretical Description

AbstractRecent studies have shown that cumulus updrafts often consist of a succession of discrete rising thermals with spherical vortex-like circulations. In this paper, a theory is developed for why this “thermal chain” structure occurs. Theoretical expressions are obtained for a passive tracer, buoyancy, and vertical velocity in axisymmetric moist updrafts. Analysis of these expressions suggests that the thermal chain structure arises from enhanced lateral mixing associated with intrusions of dry environmental air below an updraft’s vertical velocity maximum. This dry-air entrainment reduces buoyancy locally. Consequently, the updraft flow above levels of locally reduced buoyancy separates from below, leading to a breakdown of the updraft into successive discrete thermals. The range of conditions in which thermal chains exist is also analyzed from the theoretical expressions. A transition in updraft structure from isolated rising thermal, to thermal chain, to starting plume occurs with increases in updraft width, environmental relative humidity, and/or convective available potential energy. Corresponding expressions for the bulk fractional entrainment rate ε are also obtained. These expressions indicate rather complicated entrainment behavior of ascending updrafts, with local enhancement of ε up to a factor of ~2 associated with the aforementioned environmental-air intrusions, consistent with recent large-eddy simulation (LES) studies. These locally large entrainment rates contribute significantly to overall updraft dilution in thermal chain-like updrafts, while other regions within the updraft can remain relatively undilute. Part II of this study compares results from the theoretical expressions to idealized numerical simulations and LES.

Estimating the Maximum Vertical Velocity at the Leading Edge of a Density Current

AbstractThe maximum upward vertical velocity at the leading edge of a density current is commonly <10 m s−1. Studies of the vertical velocity, however, are relatively few, in part owing to the dearth of high-spatiotemporal-resolution observations. During the Plains Elevated Convection At Night (PECAN) field project, a mobile Doppler lidar measured a maximum vertical velocity of 13 m s−1 at the leading edge of a density current created by a mesoscale convective system during the night of 15 July 2015. Two other vertically pointing instruments recorded 8 m s−1 vertical velocities at the leading edge of the density current on the same night. This study describes the structure of the density current and attempts to estimate the maximum vertical velocity at their leading edges using the following properties: the density current depth, the slope of its head, and its perturbation potential temperature. The method is then be applied to estimate the maximum vertical velocity at the leading edge of density currents using idealized numerical simulations conducted in neutral and stable atmospheres with resting base states and in neutral and stable atmospheres with vertical wind shear. After testing this method on idealized simulations, this method is then used to estimate the vertical velocity at the leading edge of density currents documented in several previous studies. It was found that the maximum vertical velocity can be estimated to within 10%–15% of the observed or simulated maximum vertical velocity and indirectly accounts for parameters including environmental wind shear and static stability.

Using a two-phase method for numerical natural convection simulation in a cavity containing multiwalled carbon nanotube/water

In the present study, using two-phase mixing method, the natural heat transfer (HT) in a nanofluid cavity is investigated by numerical method. The first phase is the water, and the second phase is the multiwalled carbon nanotubes. To investigate the various factors, numerical study on Rayleigh numbers (Ra) between 103 and 106 and nanofluid volume fraction (VF) range of 0–3% is performed. The effect of porous media is also investigated at Darcy numbers between 0 and 60. Results were presented through temperature contour, velocity contour, and mean Nusselt number (Nu) diagram. The results show that increasing the nanofluid VF will result in an improvement in HT and an increase in average Nu. As the Ra number increases and the convective term dominates, the nanofluid flow becomes turbulent, resulting in an increase in the average Nu number. At low Ra, due to the weak buoyancy force, the temperature lines will be almost the same shape as the cavity, but at Ra of 106, an increase in buoyancy forces will result in disrupting the uniform arrangement of the temperature lines. Using Taguchi method and signal-to-noise ratio, the order of importance of the parameters is investigated. Among the three different input parameters (Ra, VF, and porosity), the porosity or non-porosity of the medium is the most effective parameter on the average Nu number. Based on results, increasing Ra number will increase the average Nu number by 2300% for non-porous media and about 34–140% for porous mediums. Also, the maximum vertical velocity increases by about 249 times because of the increase in the Ra number.

On the Vorticity Balance over Steep Slopes: Kuroshio Intrusions Northeast of Taiwan

AbstractThe circulation of the Kuroshio northeast of Taiwan is characterized by a large anticyclonic loop of surface intrusion and strong upwelling at the shelfbreak. To study the mechanisms of Kuroshio intrusions, the vorticity balance is examined using a high-resolution nested numerical model. In the 2D depth-averaged vorticity equation, the advection of geostrophic potential vorticity (APV) term and the joint effect of baroclinicity and relief (JEBAR) term are dominant. On the other hand, in the 2D depth-integrated vorticity equation, the main balance is between nonlinear advection and bottom pressure torque. It is shown that JEBAR and APV tend to compensate, and their difference is comparable to bottom pressure torque. Perhaps most significantly, a general framework is provided for examination of vorticity balance over steep slopes through a full 3D depth-dependent vorticity equation. The 3D analysis reveals a well-defined bottom boundary layer over the shelfbreak, about 40 m deep and capped by the vertical velocity maximum. In the upper frictionless layer from the surface to about 100 m, the primary balance is between nonlinear advection and horizontal divergence. In the lower frictional layer, viscous stress is balanced by nonlinear advection and horizontal divergence. The bottom pressure torque, which corresponds to the depth-integrated viscous effect, is a proxy for viscous stress divergence at the bottom. The importance of nonlinear advection is further demonstrated in a sensitivity experiment by removing advective terms from momentum equations. Without nonlinear advection, the bottom pressure torque becomes trivial, the boundary layer vanishes, and the on-shelf intrusion is considerably weakened.

Joint Modeling of Severe Dust Storm Events in Arid and Hyper Arid Regions Based on Copula Theory: A Case Study in the Yazd Province, Iran

Natural disasters such as dust storms are random phenomena created by complicated mechanisms involving many parameters. In this study, we used copula theory for bivariate modeling of dust storms. Copula theory is a suitable method for multivariate modeling of natural disasters. We identified 40 severe dust storms, as defined by the World Meteorological Organization, during 1982–2017 in Yazd province, central Iran. We used parameters at two spatial vertical levels (near-surface and upper atmosphere) that included surface maximum wind speed, and geopotential height and vertical velocity at 500, 850, and 1000 hPa. We compared two bivariate models based on the pairs of maximum wind speed–geopotential height and maximum wind speed–vertical velocity. We determined the bivariate return period using Student t and Gaussian copulas, which were considered as the most suitable functions for these variables. The results obtained for maximum wind speed–geopotential height indicated that the maximum return period was consistent with the observed frequency of severe dust storms. The bivariate modeling of dust storms based on maximum wind speed and geopotential height better described the conditions of severe dust storms than modeling based on maximum wind speed and vertical velocity. The finding of this study can be useful to improve risk management and mitigate the impacts of severe dust storms.

An improvement of convective precipitation nowcasting through lightning data dynamic nudging in a cloud-resolving scale forecasting system

A lightning data dynamic nudging (LDN) method was designed to adjust the dynamic field in convective clouds based on the physical relationship between lightning and vertical velocity. Under this framework, vertical velocity data retrieved from the Guangdong–Hongkong–Macau lightning location system were assimilated by a nudging approach using the Global/Regional Assimilation and Prediction System in mesoscale model (GRAPES-Meso). The effect of the assimilation on short-term (<6 h) precipitation forecast was evaluated by simulating the squall line event on 7 May and a continuous experiment from May to August 2018. Firstly, a series of sensitivity experiments was carried out to adjust the configuration of the assimilation system based on the squall line event. The results show that LDN expanded the spatial distribution of the positive vertical velocity (>1 m s−1) at 700 hPa by about 2%, and thus enlarged the spatial distribution of severe rainfall (>40 mm h−1). It should be noted that LDN did not significantly change the intensity of the updraft (i.e., the maximum vertical velocity) of the squall line relative to the simulation without assimilation. Overall, the equitable threat score (ETS) and fractions skill score (FSS) were increased by 0.04 and 0.13, respectively, and the positive effect lasted for 2–3 h after the data were assimilated. In view of the data assimilation frequency, using two successive nudging procedures with an interval of 12 min had the best effect on the forecast of severe precipitation. For the assimilation of both lightning and radar observations, the asynchronous assimilation of lightning and radar measurements performed slightly better than the synchronous assimilation. LDN improved the accuracy of severe precipitation forecast (>40 mm h−1 and > 20 mm h−1 for the squall line case and the continuous run, respectively), whereas assimilating radar data improved weak precipitation forecast (1–20 mm h−1 in the squall line case and 1–10 mm h−1 in the continuous run).

Inner, meso, and outer scales in a differentially heated vertical channel

Reynolds shear stress and Reynolds normal stress in a differentially heated vertical channel were shown in a previous paper to scale by a mixed scale of the friction velocity uτ and the maximum mean vertical velocity Umax. This scale was empirically determined; this work presents a physical understanding of the new scaling. A new dimensional analysis is developed that involves a simple modification of the traditional inner–outer scaling. The functional dependencies in the new dimensional analysis are determined using direct numerical simulation (DNS) data. The DNS data confirm the new physical reasoning and reveal, as follows: (i) The velocity fluctuation variance at the centerline scales with an outer scale, which is the mixed scale uτUmax. (ii) The location of the maximum mean vertical velocity scales as an Obukhov-style length scale, which, in turn, scales as the geometric mean of the inner length scale and the outer length scale. (iii) The temperature fluctuation variance at the centerline scales with an outer temperature scale. (iv) The temperature fluctuation variance peaks in the inner layer, and its peak value scales with an inner temperature scale. The new outer scales obtained from the dimensional analysis are shown to be consistent with the properties of the mean momentum balance equation.

Azimuthally Averaged Wind and Thermodynamic Structures of Tropical Cyclones in Global Climate Models and Their Sensitivity to Horizontal Resolution

AbstractCharacteristics of tropical cyclones (TCs) in global climate models (GCMs) are known to be influenced by details of the model configurations, including horizontal resolution and parameterization schemes. Understanding model-to-model differences in TC characteristics is a prerequisite for reducing uncertainty in future TC activity projections by GCMs. This study performs a process-level examination of TC structures in eight GCM simulations that span a range of horizontal resolutions from 1° to 0.25°. A recently developed set of process-oriented diagnostics is used to examine the azimuthally averaged wind and thermodynamic structures of the GCM-simulated TCs. Results indicate that the inner-core wind structures of simulated TCs are more strongly constrained by the horizontal resolutions of the models than are the thermodynamic structures of those TCs. As expected, the structures of TC circulations become more realistic with smaller horizontal grid spacing, such that the radii of maximum wind (RMW) become smaller, and the maximum vertical velocities occur off the center. However, the RMWs are still too large, especially at higher intensities, and there are rising motions occurring at the storm centers, inconsistently with observations. The distributions of precipitation, moisture, and radiative and surface turbulent heat fluxes around TCs are diverse, even across models with similar horizontal resolutions. At the same horizontal resolution, models that produce greater rainfall in the inner-core regions tend to simulate stronger TCs. When TCs are weak, the radial gradient of net column radiative flux convergence is comparable to that of surface turbulent heat fluxes, emphasizing the importance of cloud–radiative feedbacks during the early developmental phases of TCs.

Prandtl number scalings for unsteady natural convection boundary-layer flow on an evenly heated vertical plate in a homogeneous Pr > 1 fluid

Scalings for the thicknesses of the thermal and velocity boundary layers, the plate temperature, and the maximum vertical velocity in a homogeneous Pr > 1 fluid are validated against numerical results for unsteady natural convection boundary-layer flow on an evenly heated vertical plate. Results have been obtained for Rayleigh number and Prandtl number and the scaling relations are shown to provide a good description of the flow for both the initial conduction dominated, one-dimensional, unsteady phase and the fully developed convection dominated, two-dimensional, steady phase of the flow. Scaling constants have been obtained for all relations.

具有水平流动的对流斑图成长和动力学特性

The growth and dynamic characteristics of convection patterns with horizontal flow for Prandtl number Pr=0.72 were numerically simulated with 2D basic equations of fluid mechanics. The results show that, for given relative Rayleigh number Ra_r=5(Rayleigh number Ra=8 540) and Reynolds number Re=22.5, the growth of the traveling wave convection pattern can be divided into 3 stages: the convection development stage, the exponential growth stage and the periodic variation stage (including the transition adaptation region and the stable periodic variation region). The average wave number in the traveling wave convection decreases with time or with the growth of the convection pattern. The exponential growth stage length in traveling wave convection becomes shorter and the growth rate of the maximum vertical velocity of convection increases with relative Rayleigh number Ra_r.For Reynolds number Re=5,the growth rate of the maximum vertical velocity of convection relates with the variation of relative Rayleigh number Ra_r in the form of γ_m=0.004 8Ra^{6.065 3}_r.In the periodic variation stage, after the transition adaptation region of the convection pattern and parameters in traveling wave convection, the convection enters the stable periodic variation region of the convection pattern and parameters. For given relative Rayleigh number Ra_r=5,dimensionless period T_t of traveling wave convection varying with Re can be expressed as T_t=0.001 4Re^{2.363 5}.

Convection partition and dynamics in inclined Poiseuille-Rayleigh-Bénard flow

Since Henri Bénard first carried out experiments on convection in the fluid layer heated from below at the beginning of last century, and Lord Rayleigh first analyzed small disturbance theoretically, Rayleigh-Bénard convection has received much attention from many researchers, and has become one of the models to study the spatiotemporal structure, flow stability and dynamic characteristics of convection. The methods of studying the Rayleigh-Bénard convection are divided into experimental research, theoretical analysis and numerical simulation. With the development of computer, the research of numerical simulation has made great progress. Because the Rayleigh-Bénard convection can be accurately described by continuity equation, momentum equation and energy equation of hydrodynamics. Therefore, the numerical simulation based on hydrodynamics equations has aroused a lot of research interest. Based on the classical Rayleigh-Bénard convection, the influence of horizontal flow on the Rayleigh-Bénard convection can be studied by applying horizontal flow to one end of the horizontal cavity. On the other hand, the influence of cavity inclination on Rayleigh-Bénard convection can be studied by considering the variation of inclined angles in the cavity. Some valuable convective properties have been obtained. In order to reveal some new convection structures or phenomena, the effects of cavity inclination and through-flow on Rayleigh-Bénard convection are considered at the same time in this paper.By using the numerical simulation of the basic equations of hydrodynamics, the convection partition and dynamic characteristics of the fluid with Prandtl number &lt;i&gt;Pr&lt;/i&gt; = 6.99 in the inclined cavity with through-flows are discussed. The results show that for the reduced Rayleigh number &lt;i&gt;r&lt;/i&gt; = 9, the system presents uniform traveling wave convection, non-uniform traveling wave convection and single roll convection pattern at the through-flow Reynolds number &lt;i&gt;Re&lt;/i&gt; = 1.5 with the increase of the inclined angle &lt;i&gt;θ&lt;/i&gt; in the cavity, that for the through-flow Reynolds number &lt;i&gt;Re&lt;/i&gt; = 12.5, the system presents the localized traveling wave convection, parallel flow and localized single roll convection pattern with the increase of the inclined angle &lt;i&gt;θ&lt;/i&gt; in the cavity, that furthermore, the numerical simulation of different values of through-flow Reynolds number &lt;i&gt;Re&lt;/i&gt; and inclined angle &lt;i&gt;θ&lt;/i&gt; in the cavity shows that on the plane composed of through flow Reynolds number &lt;i&gt;Re&lt;/i&gt; and inclined angle &lt;i&gt;θ&lt;/i&gt; in the cavity, the convection in the inclined cavity with through-flow can be divided into six kinds of pattern regions, namely, uniform traveling wave convection region, non-uniform traveling wave convection region, single roll convection region, localized traveling wave convection region, parallel flow region, and localized single roll convection region. The characteristics of the maximum vertical velocity &lt;i&gt;w&lt;/i&gt;&lt;sub&gt;max&lt;/sub&gt; and Nusselt number &lt;i&gt;Nu&lt;/i&gt; of convection varying with time in different convection regions are studied. The dynamic properties of convective amplitude &lt;i&gt;A&lt;/i&gt; and Nusselt number &lt;i&gt;Nu&lt;/i&gt; in different convective regions varying with inclined angle &lt;i&gt;θ&lt;/i&gt; in the cavity are discussed.

Numerical Simulations of Hydraulic Characteristics of A Flow Discharge Measurement Process with A Plate Flowmeter in A U-Channel

The use of a flow discharge measuring device in irrigated areas is the key to utilizing water in a planned and scientific manner and to developing water-saving irrigation techniques. In this study, a new type of flow discharge measuring device for a U-channel—a plate flowmeter—was designed, and then the hydraulic characteristics of the flow discharge measurement process using the plate flowmeter were simulated and experimentally verified by adopting an RNG (Renormalization Group) k-ε turbulence model based on Flow-3D software. The results showed that in the process of measuring flow discharge with the plate flowmeter, the transverse flow velocity, the vertical flow velocity, and the relationship between the measured flow discharge and the deflection angle of the angle-measuring plate were basically consistent with the experimental results. The maximum relative errors were 5.3%, 6.2%, and 6.8% respectively, proving that it was feasible to use Flow-3D software to simulate the hydraulic characteristics of the flow discharge measurement process using the plate flowmeter. The vertical flow velocities at the center of the upstream section of the channel increased gradually from the bottom of the channel to the free water surface. The vertical flow velocities at the center of the downstream section of the channel first increased and then decreased from the bottom of the channel to the free water surface, and the maximum vertical flow velocity was located at a position below the free water surface. The maximum range of influence of the plate flowmeter on the flow disturbance in the channel was from 0.75 m upstream to 1.24 m downstream of the plate flowmeter. These results can provide a theoretical basis for optimizing the structural parameters of a plate flowmeter.

Thermo-Convective Flow of a Yield-Stress Fluid Between Two Horizontal Plates for Rayleigh Numbers Greater Than the Critical Value

Abstract In this paper, two-dimensional unsteady simulations of yield-stress fluids in the Poiseuille–Rayleigh–Bénard flow between two horizontal plates were performed. The laminar flow of these fluids is characterized by the existence of high- and low-viscous zones. The thermal and hydrodynamic behaviors were studied in the case where the convective instability was developed, and the high-viscous zones were minimized. The commercial computational fluid dynamics (CFD) package fluent was adopted to perform the different simulations. The average Nusselt number over time of the two plates was chosen to characterize the thermal transport, while the average of the maximum vertical velocity component over time in the horizontal mid-plane described the hydrodynamics of the flow. It was found that the fluid behaved according to a range of viscosities with the same order of magnitude as the plastic viscosity. The effect of the dimensionless numbers on the flow showed that the yield-stress fluid could mimic a Newtonian behavior in the pre-described conditions. Although, the fluid still held the non-Newtonian character distinguished by the dependency on the Bingham number Bn. This returned to the increase of the apparent viscosity with Bn which contributed to the weakening of the convective streams, and consequently, reduced convective thermal transport.

Tobac 1.2: towards a flexible framework for tracking and analysis of clouds in diverse datasets

Abstract. We introduce tobac (Tracking and Object-Based Analysis of Clouds), a newly developed framework for tracking and analysing individual clouds in different types of datasets, such as cloud-resolving model simulations and geostationary satellite retrievals. The software has been designed to be used flexibly with any two- or three-dimensional time-varying input. The application of high-level data formats, such as Iris cubes or xarray arrays, for input and output allows for convenient use of metadata in the tracking analysis and visualisation. Comprehensive analysis routines are provided to derive properties like cloud lifetimes or statistics of cloud properties along with tools to visualise the results in a convenient way. The application of tobac is presented in two examples. We first track and analyse scattered deep convective cells based on maximum vertical velocity and the three-dimensional condensate mixing ratio field in cloud-resolving model simulations. We also investigate the performance of the tracking algorithm for different choices of time resolution of the model output. In the second application, we show how the framework can be used to effectively combine information from two different types of datasets by simultaneously tracking convective clouds in model simulations and in geostationary satellite images based on outgoing longwave radiation. The tobac framework provides a flexible new way to include the evolution of the characteristics of individual clouds in a range of important analyses like model intercomparison studies or model assessment based on observational data.

The Role of Vertical Wind Shear in Modulating Maximum Supercell Updraft Velocities

AbstractObserved supercell updrafts consistently produce the fastest mid- to upper-tropospheric vertical velocities among all modes of convection. Two hypotheses for this feature are investigated. In the dynamic hypothesis, upward, largely rotationally driven pressure gradient accelerations enhance supercell updrafts relative to other forms of convection. In the thermodynamic hypothesis, supercell updrafts have more low-level inflow than ordinary updrafts because of the large vertical wind shear in supercell environments. This large inflow makes supercell updrafts wider than that of ordinary convection and less susceptible to the deleterious effects of entrainment-driven updraft core dilution on buoyancy. These hypotheses are tested using a large suite of idealized supercell simulations, wherein vertical shear, CAPE, and moisture are systematically varied. Consistent with the thermodynamic hypothesis, storms with the largest storm-relative flow have larger inflow, are wider, have larger buoyancy, and have faster updrafts. Analyses of the vertical momentum forcing along trajectories shows that maximum vertical velocities are often enhanced by dynamic pressure accelerations, but this enhancement is accompanied by larger downward buoyant pressure accelerations than in ordinary convection. Integrated buoyancy along parcel paths is therefore a strong constraint on maximum updraft speeds. Thus, through a combination of processes consistent with the dynamic and thermodynamic hypotheses, supercell updrafts are able to realize a larger percentage of CAPE than ordinary updrafts.

Budget Analyses of a Record‐Breaking Rainfall Event in the Coastal Metropolitan City of Guangzhou, China

AbstractTo investigate the mechanisms for the record‐breaking rainfall in the coastal metropolitan city of Guangzhou, China during 6–7 May 2017, budget analyses of advection and source/sink terms of the water vapor, potential temperature, and vertical momentum equations were conducted using the model output of a nested very large eddy simulation with the Weather Research and Forecasting model. Results show that the warm and moist air flows from the south and east onshore in the lower troposphere provided the main moisture source for the heavy rainfall. The structure of vertical velocity and hydrometeors (low‐echo centroid structure), in which the heavy rainfall was separated from the low‐level updraft, was favorable for the formation and maintenance of a heavy precipitation rate. The removal of the heat due to the advection (cooling tendency) in the upper troposphere increased the convective available potential energy of parcels rising from the lower troposphere, maintaining the development of updrafts. Although the total buoyancy forcing was the main contribution term for maintaining the updrafts, total dynamic acceleration played an important role in the vertical acceleration below the maximum vertical velocity core. In particular, the nonlinear dynamic perturbation pressure gradient force in the lower troposphere induced by the rotations aloft maintained the strong updrafts.

Numerical simulation study on the influences of the secondary-tertiary air proportion on the airflow mixing effects and pulverized coal combustion characteristics in a 300-MW down-fired boiler

In this paper, based on the MIMSC (multi-injection and multi-stage combustion) technology, new burner arrangement and air distribution parameter settings were proposed for a 300-MW subcritical down-fired boiler originally using MBEL (Mitsui Babcock Energy Limited) combustion technology. Numerical simulations were conducted to study the influences of the secondary-tertiary air proportion on the airflow mixing effect in the furnace, the ignition and pulverized coal combustion characteristics. The airflow mixing effect in the furnace is characterized by the size of the dimensionless vertical velocity decay area (Vda) and the fluctuation of the maximum vertical velocity decay curve (Vdc). During the research, the sum of the secondary and tertiary air rate remained constant, and the secondary air rate was set to 30.54%, 33.54%, 36.54%, 39.54%, and 42.54%. It was found that under the condition of using new type burner, with the secondary air rate increased from 30.54% to 42.54%, the ignition distance of pulverized coal decreased from 1.30m to 0.84m, the dimensionless penetration depth decreased from 1.46 to 1.27, and the NOx emission and carbon in the fly ash decreased first and then increased. The variation of Vdc showed the following regulation. At the secondary air rate of 30.54% to 33.54%, increasing the secondary air rate decreased the fluctuation amplitude. While at the secondary air rate of 39.54% to 42.54%, increasing the secondary air rate increased the fluctuation amplitude. Only at the secondary air rate of 36.54%, Vdc in the tertiary air mixing area was smooth. When Vdc was smooth, Vda was small at 0.043, the carbon in fly ash at the furnace outlet was the lowest at 4.19%, and the NOx emission was low at 675.9 mg/m3 at 6% O2. An optimal secondary air rate of 36.54% is recommended. For the subsequent design of burner structure and parameters, it is suggested that Vdc in the tertiary air mixing area should be smooth.

Effects of Sea Spray on Microphysics and Intensity of Deep Convective Clouds Under Strong Winds

AbstractDeep convective clouds similar to those arising in the tropical cyclone eyewall are simulated using a parcel model and 2‐D slab symmetric cloud model with spectral bin microphysics (the Hebrew University Cloud Model). The size distribution of sea spray particles (SSPs) at cloud base is calculated using the Lagrangian‐Eulerian bin microphysics model. The model describes the SSP production, advection, and formation of the size distribution of SSP in the hurricane atmospheric boundary layer at different strong wind speeds. The SSP distributions calculated by the Lagrangian‐Eulerian bin microphysics model are used in the parcel model and the Hebrew University Cloud Model to investigate the microphysical and dynamical effects of SSP on clouds. The SSPs ascending in cloud updrafts dramatically increase the number concentration of cloud drops within a wide range of drop sizes. As a result, sea spray creates clouds with unique property combinations of both maritime and continental types. These clouds have droplet size distributions characterized by a high drop concentration and a low effective radius, as in continental clouds. At the same time, the presence of SSP of a few hundred microns in radii triggers intense rain just above the cloud base, which is typical of extreme maritime clouds. In the presence of large sea spray drops, the smallest cloud condensational nuclei, including the smallest SSP, are activated, giving rise to the permanent in‐cloud nucleation of small droplets, which produce a high concentration of small ice crystals above the level of homogeneous freezing. We showed that the SSP substantially increased the maximum vertical velocity, cloud water content, and mass contents of ice particles. The results are compared with available observed data.