Cellular Convection Research Articles

Shallow-to-Deep Transition of Continental Moist Convection: Cold Pools, Surface Fluxes, and Mesoscale Organization

Abstract Large-eddy simulation is used to investigate the effects of cold pools driven by rain evaporation on the shallow-to-deep convection transition over land. The physically consistent methodologies are developed to obtain a time-dependent reference ensemble without cold pools and to apply interactive surface heat fluxes without modeling of surface energy and water budgets. Three different simulation ensembles are contrasted. The reference ensemble, in the spirit of one-dimensional single-column models, eliminates cold pools by horizontally homogenizing negative buoyancy production due to rain evaporation. The additional ensembles complement the reference cold-pool-free ensemble by including cold pools and by applying either interactive or prescribed surface fluxes. Contrasting these ensembles suggests possible improvements of convection parameterization in large-scale models of weather and climate. Without cold pools, the reference ensemble preserves key features of buoyancy-driven cellular convection associated with a field of convective plumes, as assumed in a typical convection parameterization. With cold pools, a significant enhancement of surface heat and moisture fluxes and about an hour delay of their daily maximum is simulated. Cold pools enhance near-surface temperature and moisture standard deviations as well as maxima of the near-surface updraft velocity. They also lead to the reduction of cloud lateral entrainment, deeper vertical development of the cloud layer, and a few-times-larger accumulated surface precipitation. Interactive surface fluxes provide a damping mechanism that noticeably suppresses all these effects. Perhaps surprisingly, cold pools do not significantly change the cloud-base convective mass flux that approximately follows the evolution of surface heat fluxes.

A Comparison of Cloud Microphysical Properties Derived From MODIS and CALIPSO With In Situ Measurements Over the Wintertime Southern Ocean

AbstractIn situ observations of cloud effective radius (reff), droplet number concentration (Nd), and thermodynamic phase from 11 wintertime flights over the Southern Ocean (43–45°S, 145–148°E) are compared to products from MODerate‐resolution Imaging Spectroradiometer (MODIS) and Cloud‐Aerosol Lidar with Orthogonal Polarization. The in situ observations were in close alignment with A‐train overpasses for a 30‐min window. For open mesoscale cellular convection, which was predominantly observed, clouds were commonly found to be intermittently drizzling, patchy, and mixed phase. Compared to the in situ observations of the cloud thermodynamic phase, the Cloud‐Aerosol Lidar with Orthogonal Polarization and MODIS cloud phase optical property products consistently underestimated the occurrence of mixed‐phase clouds, whereas the MODIS infrared‐based phase product showed a better qualitative agreement despite a frequent classification of uncertainty. The MODIS reff_2.1 overestimated the in situ reff for nondrizzling clouds (by ~13 μm on average) and, to a lesser extent, for lightly drizzling cases. Conversely, MODIS reff_2.1 underestimated the in situ reff for heavily drizzling cases by ~10 μm on average. The overestimation of reff is much greater than that for the stratocumulus over the Southeast Pacific shown in other studies. An examination on subpixel heterogeneity, droplet size variability, a bimodal distribution, and solar zenith angle suggests that all of these factors have measurable impacts on the MODIS reff bias. The MODIS Nd is largely consistent with the in situ observations. However, the Nd of the two high Nd cases (closed mesoscale cellular convection) are highly underestimated. An error analysis suggests that the Nd biases are likely a result of a compensating error effect.

Simulation of mesoscale cellular convection in marine stratocumulus.: Part I: Drizzling conditions.

This study uses eddy-permitting simulations to investigate the mechanisms that promote mesoscale variability of moisture in drizzling stratocumulus-topped marine boundary layers. Simulations show that precipitation tends to increase horizontal scales. Analysis of terms in the prognostic equation for total water mixing ratio variance indicates that moisture stratification plays a leading role in setting horizontal scales. This result is supported by simulations in which horizontal mean thermodynamic profiles are strongly nudged to their initial well-mixed state, which limits cloud scales. It is found that the spatial variability of sub-cloud moist cold pools surprisingly tends to respond to, rather than determine, the mesoscale variability, which may distinguish them from dry cold pools associated with deeper convection. Simulations also indicate that moisture stratification increases cloud scales specifically by increasing latent heating within updrafts, which increases updraft buoyancy and favors greater horizontal scales.

Identifying Meteorological Controls on Open and Closed Mesoscale Cellular Convection Associated with Marine Cold Air Outbreaks

AbstractMesoscale cellular convective (MCC) clouds occur in large‐scale patterns over the ocean and have important radiative effects on the climate system. An examination of time‐varying meteorological conditions associated with satellite‐observed open and closed MCC clouds is conducted to illustrate the influence of large‐scale meteorological conditions. Marine cold air outbreaks (MCAO) influence the development of open MCC clouds and the transition from closed to open MCC clouds. MCC neural network classifications on Moderate Resolution Imaging Spectroradiometer (MODIS) data for 2008 are collocated with Clouds and the Earth's Radiant Energy System (CERES) data and ERA‐Interim reanalysis to determine the radiative effects of MCC clouds and their thermodynamic environments. Closed MCC clouds are found to have much higher albedo on average than open MCC clouds for the same cloud fraction. Three meteorological control metrics are tested: sea‐air temperature difference (ΔT), estimated inversion strength (EIS), and a MCAO index (M). These predictive metrics illustrate the importance of atmospheric surface forcing and static stability for open and closed MCC cloud formation. Predictive sigmoidal relations are found between M and MCC cloud frequency globally and regionally: negative for closed MCC cloud and positive for open MCC cloud. The open MCC cloud seasonal cycle is well correlated with M, while the seasonality of closed MCC clouds is well correlated with M in the midlatitudes and EIS in the tropics and subtropics. M is found to best distinguish open and closed MCC clouds on average over shorter time scales. The possibility of a MCC cloud feedback is discussed.

Feedback control and heat transfer measurements in a Rayleigh-Bénard convection cell

We report experimental results on the heat transfer and instability onset of a Rayleigh-Bénard convection cell of aspect ratios 6:3:1 filled with a high Prandtl aqueous solution of glycerol under feedback control. We investigate the transient and stationary response of both local temperature readings and heat transfer fluxes on the Rayleigh Bénard cell in both conductive and convective states when we perform two independent feedback control actions on both hot and cold walls. We evaluate the performance of both controllers to maintain a temperature gradient independently if the system is below or above the convection threshold. As the convection cell can be rotated at 180° about the shorter axis of the cell, it was possible to perform transitions between thermal conduction and convection regimes and vice versa under a constant temperature difference maintained by both independent controllers. The experimental setup provided an accurate measurement of the critical Rayleigh number and the evolution of the Nusselt number as a function of the Rayleigh number in the moderately supercritical regime (Ra<104). Flow visualizations show a steady cellular convection pattern formed by 6 transverse rolls throughout the range of Rayleigh numbers.

Finite amplitude cellular convection under the influence of a vertical magnetic field

At the onset of stationary convection, the effect of a vertical magnetic field on the heat transfer of Rayleigh-Benard convection for an electrically conducting fluid is studied. The nonlinear governing equations describing the motion, temperature and magnetic fields are expanded as the sequence of non-homogeneous linear equations, which depend on the solutions of the linear stability problem. Infinite number of steady state with finite amplitude solutions are obtained for the stress-free boundary conditions. The perturbation method proposed by Kuo (1961) is used for the first time to highlight the heat transfer features of magnetoconvection. An explicit expression at the onset of convection in terms of parameters of the system is obtained. The dependence of heat transfer rate on Rayleigh number (R), Chandrasekhar number, thermal and magnetic Prandtl numbers is extensively examined until sixth order using an expansion of R as proposed by Kuo (1961). The results show that the magnetic field dampens the heat flow for stationary convection, i.e., the onset of convection shifts to higher values of R as the vertical magnetic field increases. Under the uniform magnetic field, heat flow gets enhanced as the thermal Prandtl number increases, whereas heat flow diminishes for the increase in magnetic Prandtl number. The results of flow field and heat transfer characteristics are depicted in the form of streamlines and isotherms, respectively. The presence of magnetic field changes the flow structure of streamlines from unicellular to multicellular patterns. This is due to the magnetic susceptibility of colder fluid flow towards the magnetic field. The flow field is analyzed with respect to the topological invariant relation. To trace the path of convective heat transport, the concept of Heatfunction has been employed. This methodology explains the comprehensive interpretation of energy distribution in terms of heatlines.

Convection driven by temperature and composition flux with the same diffusivity

Temperature, pressure, and composition determine density of fluids within the earth, the ocean, our atmosphere, stars and planets. In some cases, variation of composition component C competes equally with temperature T to determine buoyancy-driven flow. Properties of two-dimensional cellular convection are calculated with density difference between top and bottom boundaries determined by difference of temperature T (Dirichlet boundary conditions, quantified by Rayleigh number Ra that is positive destabilising), fluxes of C (Neumann boundary conditions quantified by Raf that is positive stabilising), and Prandtl number Pr. Numerical solutions in a 2-dimensional rectangular chamber are analysed for Prandtl numbers . For Ra and and Raf above approximately 300, subcritical instability separates T-driven convection from C-dominated stagnation. The flow is steady but a sudden change in Ra or Raf produces decaying pulsations to the new flow. A boundary layer solution for rapid flow exists in which T, which has the Dirichlet condition, is more sensitive to flow speed than C with the Neumann condition. A new type of pulsating flow occurs for Ra and . The pulsations are characterised by slow flow with gradually strengthening compositional plumes in a thermally stratified flow interrupted by rapid flow with gradually weakening compositional plumes. In this slow speed range, C is more sensitive to speed than T.

In situ observations of wintertime low‐altitude clouds over the Southern Ocean

Cloud droplet concentration (Nd), effective radius (reff) and liquid water content (LWC) measured by a DMT CAPS and an SEA WCM‐2000 of wintertime low‐altitude clouds over the Southern Ocean (SO) are presented for 20 flights taken over 3 years (June–October, 2013–2015). Such clouds have been reported to have the lowest Nd on record (10–40 cm−3) from the Southern Ocean Cloud Experiment (SOCEX I) field campaign in 1993. Of the total 20 357 one‐second records spent in cloud, 38.5% were found to contain ice crystals, primarily in mixed‐phase clouds (36.7%). Ice was observed at some point during 19 of the 20 missions. The droplet spectra and temperature range suggest these clouds were often ideal for the Hallett–Mossop ice multiplication process.The average Nd and reff for liquid clouds were 28 (±30) cm−3 and 12.5 (±2.9) µm, which are consistent with those from SOCEX I. Forty‐nine percent of all liquid cloud samples were observed to be drizzling with an average drizzle rate of 0.733 mm h−1. As drizzle samples were commonly in the neighbourhood of mixed‐phase or non‐drizzling clouds, it was rare to observe solid patches of drizzle of greater than 10 s. On average, drizzling clouds had lower Nd and greater reff and LWC than those of non‐drizzling clouds. Distinct observations of non‐drizzling clouds with relatively high Nd (∼89 cm−3), small reff (∼8.5 µm) and low LWC (∼0.173 g kg−1) were noted for two flights. An initial examination of the local environment and synoptic meteorology for these flights failed to identify any particular forcing that may have led to these unique microphysical properties, although these were the only observations of closed mesoscale cellular convection. This research highlights that greater variability exists in the microphysics of wintertime clouds over the SO, when a wider range of synoptic meteorology is investigated.

Evidence for secondary ice production in Southern Ocean open cellular convection

Ice particles present at temperatures warmer than −9 °C were encountered in unexpectedly high number concentrations (up to 54 L−1) by an instrumented aircraft over the Southern Ocean (SO), off the southwest coast of Tasmania, Australia, on 7 September 2013. The sampled clouds were precipitating, characterized by mixed‐phase, open‐cellular shallow convection. These clouds were present within a large‐scale environment characterized by cold air advection, in a pristine air mass for over 72 h. Using a Cloud and Aerosol Spectrometer, aerosol particles (diameters > 0.6 µm) size and number concentrations were measured and ice nucleating particle (INP) number concentrations were estimated with a recognized ice nuclei parametrization scheme. The estimated INP number concentrations were in the range of 10−5–10−1 L−1 at temperature above −9 °C, which is up to three orders of magnitude less than the ice number concentrations typically observed. The high ice number concentrations are largely consistent with the theoretical values when ice crystals are produced via a splinter production. The evidence suggests that secondary ice processes (likely the Hallett–Mossop mechanism) were playing a key role in generating the high ice number concentrations observed. Satellite observations from an A‐Train overpass in the neighbourhood during the flight period reveal a qualitatively consistent story, with patchy, mixed‐phase (but predominantly supercooled liquid water) clouds observed at cloud‐top temperatures around −6 °C. Using back trajectory calculations, these clouds are tracked over 23 and 46 h with A‐Train observations. The presence of these clouds is found to be common over the SO during this period of time. This suggests that the ice particles present in a relatively warm temperature range could potentially be commonplace, within the widespread (up to thousands of kilometres) shallow convective cloud fields over the SO. These clouds may have important implication for the energy budget and precipitation production over this climatically important region.

Convection-driven kinematic dynamos at low Rossby and magnetic Prandtl numbers

Most large-scale planetary magnetic fields are thought to be driven by low Rossby number convection of a low magnetic Prandtl number fluid. Here kinematic dynamo action is investigated with an asymptotic, rapidly rotating dynamo model for the plane layer geometry that is intrinsically low magnetic Prandtl number. The thermal Prandtl number and Rayleigh number are varied to illustrate fundamental changes in flow regime, ranging from laminar cellular convection to geostrophic turbulence in which an inverse energy cascade is present. A decrease in the efficiency of the convection to generate a dynamo, as determined by an increase in the critical magnetic Reynolds number, is observed as the buoyancy forcing is increased. This decreased efficiency may result from both the loss of correlations associated with the increasingly disordered states of flow that are generated, and boundary layer behavior that enhances magnetic diffusion locally. We find that the spatial characteristics of $\alpha$, and thus the large-scale magnetic field, is dependent only weakly on changes in flow behavior. However, our results are limited to the linear, kinematic dynamo regime, and future simulations including the Lorentz force are therefore necessary to assess the robustness of this result. In contrast to the large-scale magnetic field, the behavior of the small-scale magnetic field is directly dependent on, and therefore shows significant variations with, the small-scale convective flow field.

A Numerical Study of Convection in a Condensing CO2 Atmosphere under Early Mars-Like Conditions

Abstract Cloud convection of a CO2 atmosphere where the major constituent condenses is numerically investigated under a setup idealizing a possible warm atmosphere of early Mars, utilizing a two-dimensional cloud-resolving model forced by a fixed cooling profile as a substitute for a radiative process. The authors compare two cases with different critical saturation ratios as condensation criteria and also examine sensitivity to number mixing ratio of condensed particles given externally. When supersaturation is not necessary for condensation, the entire horizontal domain above the condensation level is continuously covered by clouds irrespective of number mixing ratio of condensed particles. Horizontal-mean cloud mass density decreases exponentially with height. The circulations below and above the condensation level are dominated by dry cellular convection and buoyancy waves, respectively. When 1.35 is adopted as the critical saturation ratio, clouds appear exclusively as intense, short-lived, quasi-periodic events. Clouds start just above the condensation level and develop upward, but intense updrafts exist only around the cloud top; they do not extend to the bottom of the condensation layer. The cloud layer is rapidly warmed by latent heat during the cloud events, and then the layer is slowly cooled by the specified thermal forcing, and supersaturation gradually develops leading to the next cloud event. The periodic appearance of cloud events does not occur when number mixing ratio of condensed particles is large.

Gas–liquid mass transfer in a falling film microreactor: Effect of reactor orientation on liquid-side mass transfer coefficient

Microreactors offer a unique platform for chemical syntheses and have been applied to numerous reaction types including nitrations, fluorinations and hydrogenations. A key feature of falling film microreactors is the comparably large specific surface area they afford compared to conventional reactors. The enhanced heat and mass transfer characteristics can be exploited for rapid and exothermic reactions. Adequate understanding of the mass transfer processes occurring within microchannels is necessary for proper reactor design and optimization. In the current study the influence of reaction plate orientation and gas flowrate on liquid-side mass transfer coefficient was investigated via CO2 absorption experiments. Lower plate angles resulted in lower liquid-side mass transfer coefficients. At higher film velocities the rate of mass transfer was greater. The experimentally determined mass transfer coefficients were at least twice as high as those predicted either by film or penetration theory. The enhancement in mass transfer is suggested to be due to cellular convection in the microchannels. For inclined reaction plates, increasing the gas flowrate had a positive effect on the mass transfer characteristics due to induced fluctuations of the gas–liquid interface.

On the correlation of spatial wind speed and solar irradiance variability above the North Sea

Abstract. Mesoscale wind fluctuations on a time scale of tens of minutes to several hours lead to high wind power fluctuations. Enhanced mesoscale wind variability emerges during cold air outbreaks and resulting cellular convection. The study investigates spatial wind and solar variability and their correlation during cellular convection. Cellular convection leads to simultaneous high solar and wind variability, but the highest solar or wind variability occurs due to other meteorological phenomena.

Experimental validation of thermal design of top shield for a pool type SFR

An Integrated Top Shield Test Facility towards validation of thermal design of top shield for a pool type SFR has been conceived, constructed & commissioned. Detailed experiments were performed in this experimental facility having full-scale features. Steady state temperature distribution within the facility is measured for various heater plate temperatures in addition to simulating different operating states of the reactor. Following are the important observations (i) jet cooling system is effective in regulating the roof slab bottom plate temperature and thermal gradient across roof slab simulating normal operation of reactor, (ii) wire mesh insulation provided in roof slab-main vessel annulus is effective in obtaining gradual thermal gradient along main vessel top portion and inhibiting the setting up of cellular convection within annulus and (iii) cellular convection with four distinct convective cells sets in the annular gap between roof slab and small rotatable plug measuring ∼ϕ4m in diameter & gap width varying from 16mm to 30mm. Repeatability of results is also ensured during all the above tests.The results presented in this paper is expected to provide reference data for validation of thermal hydraulic models in addition to serving as design validation of jet cooling system for pool type SFR.

Exploiting cellular convection in a thick liquid layer to pattern a thin polymer film

A method to shape thin polymer films into periodic array of lenses is presented. A liquid layer placed on top of a much thinner polymer film is exposed to a transverse temperature gradient. The upper liquid layer undergoes the short-wavelength Bénard-Marangoni instability, which leads to periodic shear stresses at the liquid-liquid interface and corresponding interfacial deformations. The structures formed are solidified by curing with ultraviolet light. In comparison to previously reported single-layer patterning techniques, the presented method achieves structures of very high horizontal planform symmetry (highly ordered structures). Furthermore, other shapes of technical interest can be easily fabricated by engineering the temperature distribution at the liquid-gas interface.

On the reversibility of transitions between closed and open cellular convection

Abstract. The two-way transition between closed and open cellular convection is addressed in an idealized cloud-resolving modeling framework. A series of cloud-resolving simulations shows that the transition between closed and open cellular states is asymmetrical and characterized by a rapid ("runaway") transition from the closed- to the open-cell state but slower recovery to the closed-cell state. Given that precipitation initiates the closed–open cell transition and that the recovery requires a suppression of the precipitation, we apply an ad hoc time-varying drop concentration to initiate and suppress precipitation. We show that the asymmetry in the two-way transition occurs even for very rapid drop concentration replenishment. The primary barrier to recovery is the loss in turbulence kinetic energy (TKE) associated with the loss in cloud water (and associated radiative cooling) and the vertical stratification of the boundary layer during the open-cell period. In transitioning from the open to the closed state, the system faces the task of replenishing cloud water fast enough to counter precipitation losses, such that it can generate radiative cooling and TKE. It is hampered by a stable layer below cloud base that has to be overcome before water vapor can be transported more efficiently into the cloud layer. Recovery to the closed-cell state is slower when radiative cooling is inefficient such as in the presence of free tropospheric clouds or after sunrise, when it is hampered by the absorption of shortwave radiation. Tests suggest that recovery to the closed-cell state is faster when the drizzle is smaller in amount and of shorter duration, i.e., when the precipitation causes less boundary layer stratification. Cloud-resolving model results on recovery rates are supported by simulations with a simple predator–prey dynamical system analogue. It is suggested that the observed closing of open cells by ship effluent likely occurs when aerosol intrusions are large, when contact comes prior to the heaviest drizzle in the early morning hours, and when the free troposphere is cloud free.

The continental drift convection cell

AbstractContinents on Earth periodically assemble to form supercontinents and then break up again into smaller continental blocks (the Wilson cycle). Previous highly developed numerical models incorporate fixed continents while others indicate that continent movement modulates flow. Our simplified numerical model suggests that continental drift is fundamental. A thermally insulating continent is anchored at its center to mantle flow on an otherwise stress‐free surface for infinite Prandtl number cellular convection with constant material properties. Rayleigh numbers exceed 107, while continent widths and chamber lengths approach Earth's values. The Wilson cycle is reproduced by a unique, rugged monopolar “continental drift convection cell.” Subduction occurs at the cell's upstream end with cold slabs dipping at an angle beneath the moving continent (as found in many continent/subduction regions on Earth). Drift enhances vertical heat transport up to 30%, especially at the core‐mantle boundary, and greatly decreases lateral mantle temperature differences.

On the relationship between open cellular convective cloud patterns and the spatial distribution of precipitation

Abstract. Precipitation is thought to be a necessary but insufficient condition for the transformation of stratocumulus-topped closed cellular convection to open cellular cumuliform convection. Here we test the hypothesis that the spatial distribution of precipitation is a key element of the closed-to-open cell transition. A series of idealized 3-D simulations are conducted to evaluate the dependency of the transformation on the areal coverage of rain, and to explore the role of interactions between multiple rainy areas in the formation of the open cells. When rain is restricted to a small area, even substantial rain (order few mm day−1) does not result in a transition. With increasing areal coverage of the rain, the transition becomes possible provided that the rain rate is sufficiently large. When multiple small rain regions interact with each other, the transition occurs and spreads over a wider area, provided that the distance between the rain regions is short. When the distance between the rain areas is large, the transition eventually occurs, albeit slowly. For much longer distances between rain regions the system is anticipated to remain in a closed-cell state. These results suggest a connection to the recently hypothesized remote control of open-cell formation. Finally it is shown that this transition occurs along a consistent path in the phase space of the mean vs. coefficient of variation of the liquid water path, droplet number and optical depth. This could be used as a diagnostic tool for global analyses of the statistics of closed- and open-cell occurrence and transitions between them.

Impact of resolution on simulation of closed mesoscale cellular convection identified by dynamically guided watershed segmentation

AbstractOrganized mesoscale cellular convection (MCC) is a common feature of marine stratocumulus clouds that forms in response to interactions among dynamic, microphysical, and radiative processes at the mesoscale. Cloud resolving models begin to resolve some of these processes, but using high resolutions is extremely costly. To understand the impact of limited resolution on MCC, we use the Weather Research and Forecasting model with chemistry and fully coupled cloud‐aerosol interactions to simulate MCC over the southeast Pacific during the Variability of the American Monsoon Systems Ocean‐Cloud‐Atmosphere‐Land Study Regional Experiment (VOCALS‐REx). A suite of experiments with 3 and 9 km grid spacing indicates that the simulations with finer grid spacing have smaller liquid water paths and cloud fractions, while cloud tops are higher. The observed diurnal cycle is reasonably well simulated. To isolate organized MCC characteristics, we develop a new automated method that uses the watershed segmentation combining the detection of cloud boundaries with coincident vertical velocities. This ensures that the detected cloud fields are dynamically consistent for closed MCC, a common feature within the VOCALS‐REx region. We demonstrate that the 3 km simulation is able to reproduce the scaling between horizontal cell size and boundary layer depth seen in satellite observations for the conditions of 900–1400 m deep boundary layers. However, the 9 km simulation is unable to resolve smaller circulations corresponding to shallower boundary layers, instead producing MCC with an invariant horizontal scale for all simulated boundary layers depths. The results demonstrate that the grid spacing needed for proper simulation of the MCC structure in marine stratocumulus regions depends on the boundary layer depth.

Synoptic scale outbreak of self‐initiated upward lightning (SIUL) from tall structures during the central U.S. blizzard of 1–2 February 2011

AbstractA major central U.S. winter cyclone on 1–2 February 2011 produced a band of high winds, up to 75 cm of snow, and numerous reports of thundersnow from Oklahoma into Ontario over a 26 h period. The National Lightning Detection Network (NLDN) recorded 282 flashes comprised of 1153 events which were >96% negative polarity. Hopes of imaging winter sprites associated with energetic positive cloud‐to‐ground events that sometimes accompany such winter storms did not materialize. However, the lack of lightning over the Great Lakes waters, plus media reports of numerous thundersnow events in downtown Chicago, prompted a detailed analysis of the NLDN data. This revealed that >93% of all lightning in the snow band was likely or possibly associated with self‐initiated upward lightning (SIUL) events from a variety of tall, and some not so tall, structures. In addition to 43 events from two Chicago skyscrapers, many shorter structures were involved, including wind turbines (13.1% of the total) and transmission line towers (6.7%). Wind speeds for all events exceeded the 8 m s−1 minimum threshold associated with SIULs in Japanese winter lightning storms. Radar reflectivities at the event locations had a mean of 28 dBZ and were almost always <35 dBZ. While conventional radar displays suggested stratiform precipitation in the thundersnow region, detailed analysis of 3‐D‐gridded NMQ (National Mosaic and Multi‐Sensor Quantitative Precipitation Estimation) radar reflectivity data confirmed elevated embedded cellular convection spanning the −10°C region associated with isentropic lifting above a frontal surface, evidence of noninductive charge generation sufficient to allow upward leader initiation from tall objects.