Convective Cells Research Articles

Volcanically hosted venting with indications of ultramafic influence at Aurora hydrothermal field on Gakkel Ridge

The Aurora hydrothermal system, Arctic Ocean, hosts active submarine venting within an extensive field of relict mineral deposits. Here we show the site is associated with a neovolcanic mound located within the Gakkel Ridge rift-valley floor, but deep-tow camera and sidescan surveys reveal the site to be ≥100 m across—unusually large for a volcanically hosted vent on a slow-spreading ridge and more comparable to tectonically hosted systems that require large time-integrated heat-fluxes to form. The hydrothermal plume emanating from Aurora exhibits much higher dissolved CH4/Mn values than typical basalt-hosted hydrothermal systems and, instead, closely resembles those of high-temperature ultramafic-influenced vents at slow-spreading ridges. We hypothesize that deep-penetrating fluid circulation may have sustained the prolonged venting evident at the Aurora hydrothermal field with a hydrothermal convection cell that can access ultramafic lithologies underlying anomalously thin ocean crust at this ultraslow spreading ridge setting. Our findings have implications for ultra-slow ridge cooling, global marine mineral distributions, and the diversity of geologic settings that can host abiotic organic synthesis - pertinent to the search for life beyond Earth.

3D cloud envelope and cloud development velocity from simulated CLOUD (C3IEL) stereo images

Abstract. A method to derive the 3D cloud envelope and the cloud development velocity from high spatial and temporal resolution satellite imagery is presented. The CLOUD instrument of the recently proposed C3IEL mission lends itself well to observing at high spatial and temporal resolutions the development of convective cells. Space-borne visible cameras simultaneously image, under multiple view angles, the same surface domain every 20 s over a time interval of 200 s. In this paper, we present a method for retrieving cloud development velocity from simulated multi-angular, high-resolution top of the atmosphere (TOA) radiance cloud fields. The latter are obtained via the image renderer Mitsuba for a cumulus case generated via the atmospheric research model SAM and via the radiative transfer model 3DMCPOL, coupled with the outputs of an orbit, attitude, and camera simulator for a deep convective cloud case generated via the atmospheric research model Meso-NH. Matching cloud features are found between simulations via block matching. Image coordinates of tie points are mapped to spatial coordinates via 3D stereo reconstruction of the external cloud envelope for each acquisition. The accuracy of the retrieval of cloud topography is quantified in terms of RMSE and bias that are, respectively, less than 25 and 5 m for the horizontal components and less than 40 and 25 m for the vertical components. The inter-acquisition 3D velocity is then derived for each pair of tie points separated by 20 s. An independent method based on minimising the RMSE for a continuous horizontal shift of the cloud top, issued from the atmospheric research model, allows for the obtainment of a ground estimate of the velocity from two consecutive acquisitions. The mean values of the distributions of the stereo and ground velocities exhibit small biases. The width of the distributions is significantly different, with higher a distribution width for the stereo-retrieved velocity. An alternative way to derive an average velocity over 200 s, which relies on tracking clusters of points via image feature matching over several acquisitions, was also implemented and tested. For each cluster of points, mean stereo and ground positions were derived every 20 s over 200 s. The mean stereo and ground velocities, obtained as the slope of the line of best fit to the mean positions, are in good agreement.

Iterative graph deformation for aircraft trajectory planning considering ensemble forecasting of thunderstorms

Convective weather poses a major threat that compromises the safe operation of flights while inducing delay and cost. The aircraft trajectory optimization problem under thunderstorms is addressed, proposing a novel heuristic approach that incorporates uncertainties in the evolution of convective cells. Namely, the Augmented Random Search (ARS) algorithm is used to deform a directed graph iteratively in the search of an optimal path. Flight time and fuel consumption are optimized while avoiding unsafe regions. By means of parallel programming on graphical processing units (GPUs), the computational times are reduced to enable near real-time operation. The suggested method is tested considering a dynamic model of an aircraft flying between two waypoints at constant flight level; the test scenario consists of a real weather forecast described by an ensemble of equiprobable members. The influence of the maximum number of iterations and the relative weights between optimization objectives is analysed. Results show that the methodology is able to avoid dangerous regions and find the balance between total time of flight and fuel consumption.

The effect of spanwise heterogeneous surfaces on mixed convection in turbulent channels

Turbulent mixed convection in channel flows with heterogeneous surfaces is studied using direct numerical simulations. The relative importance of buoyancy and shear effects, characterised by the bulk Richardson number $Ri_b$ , is varied in order to cover the flow regimes of forced, mixed and natural convection, which are associated with different large-scale flow organisations. The heterogeneous surface consists of streamwise-aligned ridges, which are known to induce secondary motion in the case of forced convection. The large-scale streamwise rolls emerging under smooth-wall mixed convection conditions are significantly affected by the heterogeneous surfaces and their appearance is considerably reduced for dense ridge spacings. It is found that the formation of these rolls requires larger buoyancy forces than over smooth walls due to the additional drag induced by the ridges. Therefore, the transition from forced convection structures to rolls is delayed towards larger $Ri_b$ for spanwise heterogeneous surfaces. The influence of the heterogeneous surface on the flow organisation of mixed convection is particularly pronounced in the roll-to-cell transition range, where ridges favour the transition to convective cells at significantly lower $Ri_b$ . In addition, the convective cells are observed to align perpendicular to the ridges with decreasing ridge spacing. We attribute this reorganisation to the fact that flow parallel to the ridges experience less drag than flow across the ridges, which is energetically more beneficial. Furthermore, we find that streamwise rolls exhibit a very slow dynamics for $Ri_b=1$ and $Ri_b=3.2$ when the ridge spacing is of the order of the rolls’ width. For these cases the up- and downdrafts of the rolls move slowly across the entire channel instead of being fixed in space, as observed for the smooth-wall cases.

The role of edge-driven convection in the generation ofvolcanism – Part 2: Interaction with mantle plumes, applied to the Canary Islands

Abstract. In the eastern Atlantic Ocean, several volcanic archipelagos are located close to the margin of the African continent. This configuration has inspired previous studies to suggest an important role of edge-driven convection (EDC) in the generation of intraplate magmatism. In a companion paper (Manjón-Cabeza Córdoba and Ballmer, 2021), we showed that EDC alone is insufficient to sustain magmatism of the magnitude required to match the volume of these islands. However, we also found that EDC readily develops near a step of lithospheric thickness, such as the oceanic–continental transition (“edge”) along the western African cratonic margin. In this work, we carry out 3D numerical models of mantle flow and melting to explore the possible interactions between EDC and mantle plumes. We find that the stem of a plume that rises close to a lithospheric edge is significantly deflected ocean-ward (i.e., away from the edge). The pancake of ponding hot material at the base of the lithosphere is also deflected by the EDC convection cell (either away or towards the edge). The amount of magmatism and plume deflection depends on the initial geometric configuration, i.e., the distance of the plume from the edge. Plume buoyancy flux and temperature also control the amount of magmatism, and influence the style and extent of plume–EDC interaction. Finally, comparison of model predictions with observations reveals that the Canary plume may be significantly affected and deflected by EDC, accounting for widespread and coeval volcanic activity. Our work shows that many of the peculiar characteristics of eastern Atlantic volcanism are compatible with mantle plume theory once the effects of EDC on plume flow are considered.

Effect of inclination angle on heat transport properties in two-dimensional Rayleigh–Bénard convection with smooth and rough boundaries

Using direct numerical simulations, two-dimensional tilted Rayleigh–Bénard convection (RBC) is studied in both smooth and roughness-facilitated convection cells of double aspect ratio ( $\varGamma =2$ ) for air as a working fluid. We investigate the effect of inclination angle ( $0^{\circ } \leq \phi \leq 90^{\circ }$ ) on heat flux ( $Nu$ ), Reynolds number ( $Re$ ) and flow structures. In a Rayleigh number range $10^{6}\leq Ra\leq 10^{9}$ , we address the $Ra$ dependence of $Nu(\phi )$ trend. In the smooth case, while greater tilt results in highest heat flux below $Ra=10^{8}$ , $Nu$ drops with $\phi$ monotonically above it (RBC transports heat most efficiently), which explains the different $Nu(\phi )$ trend observed in the previous studies due to $Ra$ dependence (Guo et al., J. Fluid Mech., vol. 762, 2015, pp. 273–287; Shishkina & Horn, J. Fluid Mech., vol. 790, 2016, R3; Khalilov et al., Phys. Rev. Fluids, vol. 3, 2018, 043503). For the smooth case, we identify the control parameters ( $\phi =75^{\circ }$ and $Ra=10^{7}$ ) that yield maximum heat flux (an increment of $18\,\%$ with respect to the level case). On the other hand, among the three roughness set-ups used in the present study, the tallest roughness configuration yields the maximum increment in heat flux ( $25\,\%$ ) in vertical convection ( $\phi =90^{\circ }$ ) at $Ra=10^{6}$ . With increase in $Ra$ , $Re$ changes with $\phi$ marginally in the smooth case, whereas it shows notable changes in its roughness counterpart. We find that the weakening of thermal stratification is related directly to the height of roughness peaks. While $Ra$ delays the onset of thermal stratification (in terms of inclination angle) in the smooth case, an increase in roughness height plays the same role in roughness-facilitated convection cells.

Differences in Satellite‐Based Latent Heating Profiles Between the 2015/2016 Disruption and Westerly Phase of the Quasi‐Biennial Oscillation

AbstractLatent heating (LH) profiles from the Tropical Rainfall Measuring Mission (TRMM) and the Global Precipitation Measurement mission (GPM) were employed to examine the LH distribution during the quasi‐biennial oscillation (QBO) disruption in the 2015/2016 Northern Hemispheric (NH) winter. We used the LH anomalies from the observation era climatology to compensate for the discontinuity between the TRMM and GPM LH. The difference in the LH anomalies (LHd) between the 2015/2016 winter and the typical winter coinciding with the westerly QBO phase revealed that deeper convective systems in the 2015/2016 winter shifted to the equator, releasing more latent heat relative to the convective systems in the winters coinciding with the westerly QBO phase. Comparison of the LHds calculated for each winter of 1998–2019 shows that the changes in the convective systems in the 2015/2016 winter were statistically exceptional among the changes in other winters. Inside the convective systems, stronger latent heat release and more frequent occurrences of deep convective and deep‐stratiform rain cells made up the total LHd during the 2015/2016 winter. Specifically, convective rain cells mainly derived the LHd of the convective systems in the 2015/2016 winter, except in February 2016. The results suggest that the LH increase from the typical winter with the westerly QBO phase may be considered an unusual equatorial wave source during the 2015/2016 QBO disruption. The signatures of the LHd profiles in the 10°S–10°N zonal band are sourced from the abnormal convective systems that were shifted from the western Pacific to the central‐to‐eastern Pacific.

Temperature statistics in turbulent Rayleigh–Bénard convection with a Prandtl number of Pr = 12.3

The transport of plumes in turbulent convective systems must be understood to study the mantle and various industrial applications. We measured the probability density function P(T) of the temperature at various radial and vertical positions in the bulk of a convection cell. The asymmetric-shaped distribution was decomposed into a turbulent background and plumes. The temperature of the turbulent background was fitted by a Gaussian function according to the peak of P(T). We proposed a simple quantity A ≡ (⟨T⟩ − Tbg) to describe the effective strength of the plume, where ⟨T⟩ is the time-averaged value of the local temperature. The hot plume diminishes as it rises in the cell. The plume strength varies logarithmically with the vertical position. For larger Ra, the plume along the centerline has a longer travel distance in terms of the thermal boundary layer. For a given Ra, the strength and travel distance of the plume increase as the measurements move closer to the sidewall. At the cell center, the temperature fluctuations can be decomposed into fluctuations due to the turbulent background σbg and fluctuations due to the plume. The value of σbg is so small that the relation between σbg and the vertical position can be fitted by a logarithmic function or a power law. The Ra dependence on these two fluctuations was also investigated. The measurements were collected in a cylindrical cell with a unity aspect ratio of 1, and FC72 was used as the working fluid.

First snowflake divertor experiments in MAST-U tokamak

First snowflake (SF) divertor experiments in the MAST-U tokamak demonstrated steady-state snowflake-plus divertor configurations in 450 kA ohmic L-mode plasmas. The SF divertor configuration features a second poloidal field (PF) null in the divertor region close by or overlapping with the main X-point. The resulting low PF region and two additional divertor legs (strike points) may lead to additional power and particle flux sharing via a hypothesized convective cell, and increased plasma-wetted area and radiation. The free-boundary Grad–Shafranov equilibrium code FIESTA was used to design SF configurations with several inter-null distances and orientations. In the experiment, the SF configurations with inter-null distances 0.13–0.20 m and lasting 0.2–0.3 s were obtained. Parallel connection lengths between the midplane and the outer strike point in the SF configurations (evaluated at field lines 1–2 mm from the separatrix in the midplane) were 25–30 m, higher than in the standard divertor (20–25 m) or the Super-X divertor (25 m). Diagnostic measurements highlighted salient SF features. The infra-red video bolometer diagnostic showed that the radiated power peaking in the PF null region was not as pronounces as in the standard divertor. Divertor ion fluxes measured by target Langmuir probes showed increased ion flux in the plate region where a secondary SF strike point landed, concomitantly with the SF formation. These measurements may suggest that some particle and heat redistribution was taking place in the convective SF zone. The first SF experiments provide a basis for future SF studies in MAST-U tokamak with higher input power, improved plasma control and diagnostic measurements, to be compared with the modeling predictions of plasma convective SF mixing and lower density strike point detachment threshold. • First snowflake (SF) divertor experiments in the MAST-U tokamak demonstrated steady-state snowflake-plus divertor configurations with inter-null distances 0.13–0.20 m in 450 kA ohmic L-mode plasmas. • The free-boundary Grad–Shafranov equilibrium code FIESTA was used to design SF configurations with several inter-null distances and orientations. • Divertor geometry (i.e., parallel connection lengths, divertor flux expansion and area expansion) in the SF-plus divertor fvorably comared to the standard divertor • Divertor measurements with the infra-red video bolometer diagnostic and target Langmuir probes may suggest that some particle and heat redistribution took place in the convective SF zone.

Large-scale circulations in a shear-free convective turbulence: Mean-field simulations

It has been previously shown [Elperin et al., “Formation of large-scale semi-organized structures in turbulent convection,” Phys. Rev. E 66, 066305 (2002)] that a non-rotating turbulent convection with nonuniform large-scale flows contributes to the turbulent heat flux. As a result, the turbulent heat flux depends explicitly not only on the gradients of the large-scale temperature, but also on the gradients of the large-scale velocity. This is because the nonuniform large-scale flows produce anisotropic velocity fluctuations, which modify the turbulent heat flux. This effect causes an excitation of a convective-wind instability and formation of large-scale semi-organized coherent structures (large-scale convective cells). In the present study, we perform mean-field numerical simulations of shear-free convection, which take into account the modification of the turbulent heat flux by nonuniform large-scale flows. We use periodic boundary conditions in horizontal direction as well as stress-free or no-slip boundary conditions in vertical direction. We show that the redistribution of the turbulent heat flux by the nonuniform large-scale motions in turbulent convection plays a crucial role in the formation of the large-scale semi-organized coherent structures. In particular, this effect results in a strong reduction of the critical effective Rayleigh number (based on the eddy viscosity and turbulent temperature diffusivity) required for the formation of the large-scale convective cells. We demonstrate that the convective-wind instability is excited when the scale separation ratio between the height of the convective layer and the integral turbulence scale is large. The level of the mean kinetic energy at saturation increases with the scale separation ratio. We also show that inside the large-scale convective cells, there are local regions with the positive vertical gradient of the potential temperature, which implies that these regions are stably stratified.

A non-detection of red supergiant convection in Gaia

ABSTRACT Large-scale surface convection on red supergiants (RSGs) can lead to shifts in the photocentre of the star that might be measured by Gaia and used as a new probe of the surface dynamics of these rare but important stars. Unlike brightness variations, photocentre motions would provide information on the physical scale of the convective cells. The signal would be that RSGs show an excess astrometric noise at the level of a few per cent of the stellar radius. Unfortunately, we find that the excess astrometric noise level of Gaia EDR3 is roughly an order of magnitude too large to detect the predicted motions and that RSGs have excess astrometric noise indistinguishable from other stars of similar magnitude, colour, or parallax. The typical excess astrometric noise steadily decreases with G magnitude (for G < 11 mag), so it is crucial to compare stars of similar brightness.

Contrasts in the Evolution and Microphysical Features of Two Convective Systems during a Heavy Rainfall Event along the Coast of South China

On 1 June 2021, a heavy rainstorm hit the coast of South China (148.6 mm in 1 h, 361 mm over 12 h). The storm process was successively affected by two convective systems (CSs). The initial convection of the two CSs occurred at a similar location; however, they subsequently showed different evolution characteristics. Based on multi-source data, including dual-polarimetric radars, wind profiling radars, sounding, and automatic weather stations, we explored the differences in the key characteristics of these two CSs. It was found that the convection was initially triggered at a similar location for both CSs, closely related to the mesoscale boundary and the hilly terrain. After formation, CS1 moved eastward to the regions with lower surface temperature and weaker lower-level convergence but similar humidity, which means the environmental conditions for sustaining the CS became less favorable. As a result, CS1 dissipated rapidly and only lasted for about 90 min, resulting in 5% of the total precipitation of the overall storm. In contrast, during the lifespan of CS2, the southerly wind over the South China Sea became stronger. This caused an intense lower-level convergence zone along the coastal region of Guangdong Province, which provided favorable dynamic conditions for maintaining CS2. Favored by the strong coastal convergence and abundant moisture, new convective cells (CCs) were generated continuously and merged with CS2, acting as another favorable condition for its sustainment. Overall, CS2 lasted for 8 h, and its precipitation accounted for 95% of the total rainfall. In CS1, CCs showed a notable evaporation process below 4 km, manifested by the large raindrops. However, in CS2, the CCs had a higher concentration of small raindrops and higher ice and liquid water content. Since CS2 was close to the coastal region, the warm local environment promoted convection, leading to intense precipitation. In addition, the riming and melting processes were active, leading to a high precipitation efficiency and strong local precipitation during a short period of time.

Prediction of 3D natural convection heat transfer characteristics in a shallow enclosure with experimental data

This study presents an inverse computational fluid dynamics (CFD) simulation combined with the least-squares method and overdetermined experimental temperature data to predict the unknown heat transfer rate Qh and three-dimensional (3D) natural convection heat transfer characteristics in a closed shallow enclosure. This novel study belongs to the inverse convection-conduction conjugate heat transfer problem. The results show that the zero-equation turbulence model is more suitable for this study compared to other flow models. Thus, the zero-equation model is chosen as the suitable flow model because its estimates are the closest to the experimental data and existing correlation. An important finding is that, for H/L = 1/15 and Ra = 1820, Bénard convection cells and eight vortices appear in 3D and two-dimensional (2D) temperature contours, respectively. This Ra value is close to the existing critical Rayleigh number of 1708. The arrangement of these cells and vortices can be merged with each other and reorganized as H/L increases. The obtained 2D velocity patterns and air temperature contours agree with existing patterns and isotherms. The obtained h‾hi values also agree well with the existing correlation. Thus, the present results have good accuracy. A review of the literature indicates that a few studies have been conducted on using the inverse method along with overdetermined experimental temperature data to study the present problem.

A Journey Through Nonlinear Dynamics: The Case of Temperature Gradients

The overall effect of temperature gradients is stressed for the Earth's core and surface, but also for the Sun's surface. Using Rayleigh–Bénard convection in helium and mercury, we measured all of the scaling properties of the period-doubling cascade and quasiperiodicity. Hard turbulence scaling properties are presented in an experiment using helium gas at low temperature. A [Formula: see text] scaling law is measured and also an exponential distribution for temperature fluctuations is observed. We present a study of a Rayleigh–Bénard convection cell with an open top and a floater. One of the simplest limit cycles is observed for the floater position. It follows a model proposed by Wilson for continent motion. Using the Soret effect, we study how temperature differences lead to strong accumulation of DNA suspensions. Also using polyethylene glycol concentration gradients, we measured local DNA and RNA accumulation. Finally, using thermal convection, we build one of the smallest PCR machines.

A statistical analysis of the distribution of convective windstorms and their radar characteristics in Hubei Province, China. Part 2

A statistical analysis of the distribution of convective windstorms and their radar characteristics in Hubei Province, China. Part 2

Is an NWP-Based Nowcasting System Suitable for Aviation Operations?

The growth of air transport demand expected over the next decades, along with the increasing frequency and intensity of extreme weather events, such as heavy rainfalls and severe storms due to climate change, will pose a tough challenge for air traffic management systems, with implications for flight safety, delays and passengers. In this context, the Satellite-borne and IN-situ Observations to Predict The Initiation of Convection for ATM (SINOPTICA) project has a dual aim, first to investigate if very short-range high-resolution weather forecast, including data assimilation, can improve the predictive capability of these events, and then to understand if such forecasts can be suitable for air traffic management purposes. The intense squall line that affected Malpensa, the major airport by passenger traffic in northern Italy, on 11 May 2019 is selected as a benchmark. Several numerical experiments are performed with a Weather Research and Forecasting (WRF) model using two assimilation techniques, 3D-Var in WRF Data Assimilation (WRFDA) system and a nudging scheme for lightning, in order to improve the forecast accuracy and to evaluate the impact of assimilated different datasets. To evaluate the numerical simulations performance, three different verification approaches, object-based, fuzzy and qualitative, are used. The results suggest that the assimilation of lightning data plays a key role in triggering the convective cells, improving both location and timing. Moreover, the numerical weather prediction (NWP)-based nowcasting system is able to produce reliable forecasts at high spatial and temporal resolution. The timing was found to be suitable for helping Air Traffic Management (ATM) operators to compute alternative landing trajectories.

Relationship Between Convective Storm Properties and Lightning Over the Western Ghats

AbstractX‐band radar observations are integrated with lightning location network observations to investigate the relationship between convective storm properties and lightning over the Western Ghats during a monsoon season (June–September 2014). Convective storms (cells) were identified using an objective‐tracking method and instantaneous lightning strikes within the radar domain were then linked with observed storms. This spatio‐temporal sampling of individual convective cells and lightning has facilitated process‐based study of electrified convection over the Ghats for the first time. Storm and lightning occurrences are typically high during monsoon onset and withdrawal months of June and September, respectively. A spatial correspondence between deep‐intense storms, lightning, and intense convective cores indicated presence of large hydrometeors in the mixed‐phase region of storm supported by strong updrafts and is essential for lightning. The large‐scale instability that peaked during afternoon hours was a key factor in the formation of deep‐intense storms and lightning. Results show that majority of lightning‐producing storms are located on the leeward side as opposed to the windward side. These storms have deeper top‐heights, larger areas and vertically integrated liquid, and an enhanced hail probability than those devoid of lightning. Warm season convection in the study area is accompanied by the preponderance of negative Cloud to Ground (−CG) flashes over positive Cloud to Ground (+CG) lightning. Storms with +CG features exhibited much higher (>2 times) vertical airmass flux in the mid‐troposphere (6–9 km) than storms without +CG features. Furthermore, for majority of +CG storms, intracloud flash occurrences increased significantly above the freezing level.

Optimizing radar scan strategies for tracking isolated deep convection using observing system simulation experiments

Abstract. Optimizing radar observation strategies is one of the most important considerations in pre-field campaign periods. This is especially true for isolated convective clouds that typically evolve faster than the observations captured by operational radar networks. This study investigates uncertainties in radar observations of the evolution of the microphysical and dynamical properties of isolated deep convective clouds developing in clean and polluted environments. It aims to optimize the radar observation strategy for deep convection through the use of high-spatiotemporal cloud-resolving model simulations, which resolve the evolution of individual convective cells every 1 min, coupled with a radar simulator and a cell tracking algorithm. The radar simulation settings are based on the Tracking Aerosol Convection Interactions ExpeRiment (TRACER) and Experiment of Sea Breeze Convection, Aerosols, Precipitation and Environment (ESCAPE) field campaigns held in the Houston, TX, area but are generalizable to other field campaigns focusing on isolated deep convection. Our analysis produces the following four outcomes. First, a 5–7 m s−1 median difference in maximum updrafts of tracked cells is shown between the clean and polluted simulations in the early stages of the cloud lifetimes. This demonstrates the importance of obtaining accurate estimates of vertical velocity from observations if aerosol impacts are to be properly resolved. Second, tracking of individual cells and using vertical cross section scanning every minute capture the evolution of precipitation particle number concentration and size represented by polarimetric observables better than the operational radar observations that update the volume scan every 5 min. This approach also improves multi-Doppler radar updraft retrievals above 5 km above ground level for regions with updraft velocities greater than 10 m s−1. Third, we propose an optimized strategy composed of cell tracking by quick (1–2 min) vertical cross section scans from more than one radar in addition to the operational volume scans. We also propose the use of a single-RHI (range height indicator) updraft retrieval technique for cells close to the radars, for which multi-Doppler radar retrievals are still challenging. Finally, increasing the number of deep convective cells sampled by such observations better represents the median maximum updraft evolution with sample sizes of more than 10 deep cells, which decreases the error associated with sampling the true population to less than 3 m s−1.

Three-dimensional properties of the viscous boundary layer in turbulent Rayleigh–Bénard convection

We report an experimental study of the viscous boundary layer (BL) properties of turbulent Rayleigh–Bénard convection in a cylindrical cell. The velocity profile with all three components was measured from the centre of the bottom plate by an integrated home-made particle image velocimetry system. The Rayleigh number $Ra$ varied in the range $1.82 \times 10^8 \le Ra \le 5.26 \times 10^9$ and the Prandtl number $Pr$ was fixed at $Pr = 4.34$ . The probability density function of the wall-shear stress indicates that using the velocity component in the mean large-scale circulation (LSC) plane alone may not be sufficient to characterise the viscous BL. Based on a dynamic wall-shear frame, we propose a method to reconstruct the measured full velocity profile which eliminates the effects of complex dynamics of the LSC. Various BL properties including the eddy viscosity are then obtained and analysed. It is found that, in the dynamic wall-shear frame, the eddy viscosity profiles along the centre line of the convection cell at different $Ra$ all collapse on a single master curve described by $\nu _t^d / \nu = 0.81 (z / \delta _u^d) ^{3.10 \pm 0.05}$ . The Rayleigh number dependencies of several BL quantities are also determined in the dynamic frame, including the BL thickness $\delta _u^d$ ( ${\sim } Ra^{-0.21}$ ), the Reynolds number $Re^d$ ( ${\sim }Ra^{-0.46}$ ) and the shear Reynolds number $Re_s^d$ ( ${\sim } Ra^{0.24}$ ). Within the experimental uncertainty, these scaling exponents are the same as those obtained in the static laboratory frame. Finally, with the measured full velocity profile, we obtain the energy dissipation rate at the centre of the bottom plate $\varepsilon _{w}$ , which is found to follow $\langle \varepsilon _{w} \rangle _t \sim Ra^{1.25}$ .

Effects of Convective Mergers on the Evolution of Microphysical and Electrical Activity in a Severe Squall Line Simulated by WRF Coupled With Explicit Electrification Scheme

AbstractTo investigate the process of convective merger (CM) and its effect on thunderstorms evolution and corresponding electrical activity, a severe squall line that took place on 27 July 2015 over the Beijing Metropolitan Region (BMR) is simulated and studied using the Weather Research and Forecasting (WRF) model coupled with an explicit electrification lightning scheme (E‐WRF). The model‐simulated radar reflectivity reasonably captured the whole squall line evolution, including the merging of individual cells and storms. The variation of normalized flash frequency simulated by E‐WRF is highly consistent with the occurrence of a prominent flash rate surge during the CM both in the observation and the simulation. Cloud bridge is one of the regions where lightning events increase most. The upstream anvil and the sinking outflow led to the rapid connection and formation of new convective cells between the two older main storms. Subsequently, the mass of graupel and snow increased substantially at the middle level, and the mass of upper‐level ice was horizontally advected from upstream. During the merger, the in‐cloud charge distribution evolved from a staggered charge pocket structure into a vertically stratified five‐layer structure. This study supports the hypothesized role of CM in enhancing lightning activity, and further expands our understanding of CM effects on microphysics and charge‐redistribution.