Updraft Intensity Research Articles

Cumulonimbus clouds convert a smaller fraction of CAPE into kinetic energy in a warmer atmosphere

Abstract This study investigates how entrainment’s diluting effect on cumulonimbus updraft buoyancy is affected by the temperature of the troposphere, which is expected to increase by the end of the century. A parcel model framework is constructed that allows for independent variations in the temperature (T), the entrainment rate ε, the free-tropospheric relative humidity (RH), and the convective available potential energy (CAPE). Using this framework, dilution of buoyancy is evaluated with T and RH independently varied, and with CAPE either held constant or increased with temperature. When CAPE is held constant, buoyancy decreases as T increases, with parcels in warmer environments realizing substantially smaller fractions of their CAPE as kinetic energy (KE). This occurs because the increased moisture difference between an updraft and its surroundings at warmer temperatures drives greater updraft dilution. Similar results are found in midlatitude and tropical conditions when CAPE is increased with temperature. With the expected 6-7 % increase in CAPE per degree K of warming, KE only increases at 2-4 % per degree in narrow updrafts but tracks more closely with CAPE at 4-6 % in wider updrafts. Interestingly, the rate of increase in the KE with T becomes larger than that of CAPE when the later quantity increases at more than 10 % per K. These findings emphasize the importance of considering entrainment in studies of moist convection’s response to climate change, as the entrainment-driven dilution of buoyancy may partially counteract the influence of increases in CAPE on updraft intensity.

Investigating Temporal Characteristics of Polarimetric and Electrical Signatures in Three Severe Storms: Insights from the VORTEX-Southeast Field Campaign

Abstract The dual-polarization radar characteristics of severe storms are commonly used as indicators to estimate the size and intensity of deep convective updrafts. In this study, we track rapid fluctuations in updraft intensity and size by objectively identifying polarimetric fingerprints such as ZDR and KDP columns, which serve as proxies for mixed-phase updraft strength. We quantify the volume of ZDR and KDP columns to evaluate their utility in diagnosing temporal variability in lightning flash characteristics. Specifically, we analyze three severe storms that developed in environments with low-to-moderate instability and strong 0–6-km wind shear in northern Alabama during the 2016–17 VORTEX-Southeast field campaign. In these three cases (a tornadic supercell embedded in stratiform precipitation, a nontornadic supercell, and a supercell embedded within a quasi-linear convective system), we find that the volume of the KDP columns exhibits a stronger correlation with the total flash rate. The higher covariability of the KDP column volume with the total flash rate suggests that the overall electrification and precipitation microphysics were dominated by cold cloud processes. The lower covariability with the ZDR column volume indicates the presence of nonsteady updrafts or a less prominent role of warm rain processes in graupel growth and subsequent electrification. Furthermore, we observe that the majority of cloud-to-ground (CG) lightning strikes a carried negative charge to the ground. In contrast to findings from a tornadic supercell over the Great Plains, lightning flash initiations in the Alabama storms primarily occurred outside the footprint of the ZDR and KDP column objects. Significance Statement This study quantifies the correlation between mixed-phase updraft intensity and total lightning flash rate in three severe storms in northern Alabama. In the absence of direct updraft velocity measurements, we use polarimetric signatures, such as ZDR and KDP columns, as proxies for updraft strength. Our analysis of polarimetric radar and lightning mapping array data reveals that the lightning flash rate is more highly correlated with the KDP column volume than with the ZDR column volume in all three storms examined. This contrasts with previous findings in storms over the central Great Plains, where the ZDR column volume showed higher covariability with flash rate. Interestingly, lightning initiation in the Alabama storms mainly occurred outside the ZDR and KDP column areas, contrary to previous findings.

Evaluation of Lightning Flash Rate Parameterizations in a Cloud‐Resolved WRF‐Chem Simulation of the 29–30 May 2012 Oklahoma Severe Supercell System Observed During DC3

AbstractEighteen lightning flash rate parameterization schemes (FRPSs) were investigated in a Weather Research and Forecasting model coupled with chemistry cloud‐resolved simulation of the 29–30 May 2012 supercell storm system observed during the Deep Convective Clouds and Chemistry (DC3) field campaign. Most of the observed storm's meteorological conditions were well represented when the model simulation included both convective damping and lightning data assimilation techniques. Newly‐developed FRPSs based on DC3 radar observations and Lightning Mapping Array data are implemented in the model, along with previously developed schemes from the literature. The schemes are based on relationships between lightning and various kinematic, structural, and microphysical thunderstorm characteristics (e.g., cloud top height, hydrometeors, reflectivity, and vertical velocity) available in the model. The results suggest the model‐simulated graupel and snow/ice hydrometeors require scaling factors to more closely represent proxy observations. The model‐simulated lightning flash trends and total flashes generated by each scheme over the simulation period are compared with observations from the central Oklahoma Lightning Mapping Array. For this supercell system, 13 of the 18 schemes overpredicted flashes by >100% with the group of FRPSs based on storm kinematics and structure (particularly updraft volume) performing slightly better than the hydrometeor‐based schemes. During the storm's first 4 hr, the upward cloud ice flux FRPS, which is based on the combination of vertical velocity and hydrometeors, well represents the observed total flashes and flash rate trend; while, the updraft volume scheme well represents the observed flash rate peak and subsequent sharp decline in flash rate.

Study of the correlation between electrical activity and the microphysics and dynamsics of a real severe event using the WRF‐ELEC model

AbstractAtmospheric electrical activity is one of the most damaging meteorological phenomena. Studies suggest that storm electrical discharges are correlated with severe weather events such as hail, strong surface winds, etc. To study these correlations, in addition to real data, numerical simulations can be used. In this work, we investigated the electrical activity of simulated thunderstorms using the WRF‐ELEC model over a case study of the RELAMPAGO field campaign (11 December 2018) associated with the occurrence of generalized deep moist convection in central Argentina including some supercell storms. The WRF‐ELEC model successfully reproduced the convective event. A tracking algorithm was used to individualize different convective cells and to study the relationship between their electric activity and different microphysical and kinematic variables. The model reproduced the expected correlations and time lags between these variables. In particular, the model showed that the best correlation occurred between the graupel and hail mass and the electric activity of the storms; this is expected given the key role of these microphysical species in the charge separation processes. Also, 5 m·s−1 updraft volume and cloud top temperature (CTT) temporal evolutions presented good correlations with the electrical activity. Maximum updraft show a less but still good correlation with the electrical activity. We also investigated the correlation between the electrical activity and the hail and graupel concentration near the surface (as an indicator of hail precipitation) and the near‐surface winds and we found that correlation is strong. In particular, for near‐surface ice, we determined that peaks in electrical activity precede peaks in hail and graupel precipitation and that electrical activity could be used as a proxy of this high‐impact weather event.

Impacts of an aerosol layer on a midlatitude continental system of cumulus clouds: how do these impacts depend on the vertical location of the aerosol layer?

Abstract. Effects of an aerosol layer on warm cumulus clouds in the Korean Peninsula when the layer is above or around the cloud tops in the free atmosphere are compared to effects when the layer is around or below the cloud bases in the planetary boundary layer (PBL). For this comparison, simulations are performed using the large-eddy simulation framework. When the aerosol layer is in the PBL, aerosols absorb solar radiation and radiatively heat up air enough to induce greater instability, stronger updrafts and more cloud mass than when the layer is in the free atmosphere. Hence, there is a variation of cloud mass with the location (or altitude) of the aerosol layer. It is found that this variation of cloud mass is reduced as aerosol concentrations in the layer decrease or aerosol impacts on radiation are absent. The transportation of aerosols by updrafts reduces aerosol concentrations in the PBL. This in turn reduces the aerosol radiative heating, updraft intensity and cloud mass.

Assessing the impact of the recent warming in the East China Sea on a torrential rain event in northern Kyushu (Japan) in early July 2017

Sea surface temperature (SST) in the East China Sea (ECS) has undergone a rapid rise in recent decades, but the associated impact on extreme weather remains under debate. Here, using a cloud-permitting model, we assess the impact of the ECS warming observed since the 1980s on a torrential rain event that caused devastating floods and landslides in the Kyushu Island, western Japan, in July 2017. Without the increasing trends of SST and air temperature, the model cannot reproduce the observed extremely high amount of precipitation during the event, i.e., >700 mm/12-h. The SST increase is found more influential in determining the precipitation amount. Without the ocean warming, increases in precipitable water and horizontal moisture transport due to the atmospheric warming would not lead to precipitation increase during this event. The change in the amount of precipitation can be largely explained by the change in the updraft intensity of the convective system. Higher SST suppresses downward surface sensible heat flux and enhances upward latent heat flux along the paths of air parcels flowing into the convective system in this case. This increases the equivalent potential temperature in the lower troposphere, which enhances the convective available potential energy in the lower troposphere, leading to intensification of the convective system and thereby the increase of precipitation. The findings of this case study suggest an important role of the warming ECS in the intensification of torrential rain events around Japan and the necessity of further assessment of the role of the ocean warming in the torrential rains.

Convective Updraft and Downdraft Characteristics of Continental Mesoscale Convective Systems in the Model Gray Zone

AbstractThe “gray zone” of convective modeling is defined as the range of horizontal grid spacings (Δx) at which turbulent transport processes are only partially resolved by the dynamics of the numerical model. This zone typically covers Δx from a few kilometers to several hundred meters, wherein the realistic representation of convective cloud processes can be challenging. This study characterizes the convective draft behaviors at multiple Δx across the gray zone and determines the appropriate Δx that can reliably capture these salient convective properties. We perform an ensemble of idealized simulations of mesoscale convective systems (MCS) using the Weather Research and Forecasting model at various Δx from 4 km to 250 m over the central U.S. An evaluation of key MCS kinematic properties is constrained using unique, long‐term vertical velocity estimates obtained by radar wind profilers deployed by the Department of Energy Atmospheric Radiation Measurement user facility. MCS simulations for all Δx tested overestimate (underestimate) the probabilities of convective updrafts (downdrafts) compared to the observations. In terms of the convective draft intensity, finer‐Δx models overestimate the updraft intensity, while the opposite is found for downdrafts. Moving from Δx = 4 km to 250 m, downdrafts become stronger and more frequent especially at middle and upper levels, attributed to additional drag from an increasing graupel frequency and compensation of enhanced updraft velocity. Simulated draft characteristics, including core size, intensity, and probability of occurrence, exhibit pronounced changes at Δx greater than 500 m, but suggest less sensitivity when Δx is reduced below 500 m.

Polarimetric Size Sorting Signatures in the Convective Regions of Mesoscale Convective Systems in PECAN: Implications on Kinematics, Thermodynamics, and Precipitation Pathways

An object-based technique was utilized to identify hydrometeor size-sorting signatures at lower levels in the convective regions of 10 mesoscale convective systems (MCSs) during the 2015 Plains Elevated Convection at Night (PECAN) field campaign. Composite statistical analysis indicates that the magnitudes of size-sorting signatures (the separation distances between rain diameter maxima and concentration maxima) are nonlinearly correlated to the echo-top height, rain mass beneath the melting level, and precipitation rates at higher percentiles. To explore this correlation, the weather forecasting and research model was used to simulate the 20 June 2015 MCS during PECAN. Statistical analysis of the model outputs indicates more active riming growth and quicker graupel fallout at warmer temperatures near areas with larger separation distances. While updraft intensity above the melting level was also positively correlated to separation distances, this correlation was only statistically significant within certain temperature ranges. Additional analyses reveal that the higher intense precipitation potential near signatures with large separation distances could be attributed to precipitation production from the melted graupel. Finally, spatial correspondence between graupel distribution at the melting level and rain distribution at the lowest model level illustrates the critical role of graupel sedimentation and sorting in creating size-sorting signatures in MCSs during the PECAN field experiment.

How Does Vertical Wind Shear Influence Entrainment in Squall Lines?

AbstractThe influence of vertical wind shear on updraft entrainment in squall lines is not well understood. To address this knowledge gap, a suite of high-resolution idealized numerical model simulations of squall lines were run in various vertical wind shear (hereafter “shear”) environments to study the effects of shear on entrainment in deep convective updrafts. Low-level horizontal mass flux into the leading edge of the cold pool was strongest in the simulations with the strongest low-level shear. These simulations consequently displayed wider updrafts, less entrainment-driven dilution, and larger buoyancy than the simulations with comparatively weak low-level shear. An analysis of vertical accelerations along trajectories that passed through updrafts showed larger net accelerations from buoyancy in the simulations with stronger low-level shear, which demonstrates how less entrainment-driven dilution equated to stronger updrafts. The effects of upper-level shear on entrainment and updraft vertical velocities were generally less pronounced than the effects of low-level shear. We argue that in addition to the outflow boundary-shear interactions and their effect on updraft tilt established by previous authors, decreased entrainment-driven dilution is yet another beneficial effect of strong low-level shear on squall-line updraft intensity.

Impacts of Varying Concentrations of Cloud Condensation Nuclei on Deep Convective Cloud Updrafts—A Multimodel Assessment

AbstractThis study presents results from a model intercomparison project, focusing on the range of responses in deep convective cloud updrafts to varying cloud condensation nuclei (CCN) concentrations among seven state-of-the-art cloud-resolving models. Simulations of scattered convective clouds near Houston, Texas, are conducted, after being initialized with both relatively low and high CCN concentrations. Deep convective updrafts are identified, and trends in the updraft intensity and frequency are assessed. The factors contributing to the vertical velocity tendencies are examined to identify the physical processes associated with the CCN-induced updraft changes. The models show several consistent trends. In general, the changes between the High-CCN and Low-CCN simulations in updraft magnitudes throughout the depth of the troposphere are within 15% for all of the models. All models produce stronger (~+5%–15%) mean updrafts from ~4–7 km above ground level (AGL) in the High-CCN simulations, followed by a waning response up to ~8 km AGL in most of the models. Thermal buoyancy was more sensitive than condensate loading to varying CCN concentrations in most of the models and more impactful in the mean updraft responses. However, there are also differences between the models. The change in the amount of deep convective updrafts varies significantly. Furthermore, approximately half the models demonstrate neutral-to-weaker (~−5% to 0%) updrafts above ~8 km AGL, while the other models show stronger (~+10%) updrafts in the High-CCN simulations. The combination of the CCN-induced impacts on the buoyancy and vertical perturbation pressure gradient terms better explains these middle- and upper-tropospheric updraft trends than the buoyancy terms alone.

Controls of Quasi-Linear Convective System Tornado Intensity

AbstractAlthough tornadoes produced by quasi-linear convective systems (QLCSs) generally are weak and short lived, they have high societal impact due to their proclivity to develop over short time scales, within the cool season, and during nighttime hours. Precisely why they are weak and short lived is not well understood, although recent work suggests that QLCS updraft width may act as a limitation to tornado intensity. Herein, idealized simulations of tornadic QLCSs are performed with variations in hodograph shape and length as well as initiation mechanism to determine the controls of tornado intensity. Generally, the addition of hodograph curvature in these experiments results in stronger, longer-lived tornadic-like vortices (TLVs). A strong correlation between low-level mesocyclone width and TLV intensity is identified (R2 = 0.61), with a weaker correlation in the low-level updraft intensity (R2 = 0.41). The tilt and depth of the updraft are found to have little correlation to tornado intensity. Comparing QLCS and isolated supercell updrafts within these simulations, the QLCS updrafts are less persistent, with the standard deviations of low-level vertical velocity and updraft helicity approximately 48% and 117% greater, respectively. A forcing decomposition reveals that the QLCS cold pool plays a direct role in the development of the low-level updraft, providing the benefit of additional forcing for ascent while also having potentially deleterious effects on both the low-level updraft and near-surface rotation. The negative impact of the cold pool ultimately serves to limit the persistence of rotating updraft cores within the QLCS.

Evaluation of Different Storm Parameters as the Proxies for Gridded Total Lightning Flash Rates: A Convection-Allowing Model Study

Lightning simulation is important for a variety of applications, including lightning forecast, atmospheric chemical simulation, and lightning data assimilation. In this study, the potential of five storm parameters (graupel volume, precipitation ice mass, radar echo volume, maximum updraft, and updraft volume) to be used as the proxy for the diagnosis of gridded total lightning flash rates has been investigated in a convection-allowing model. A mesoscale convective system occurred in the Guangdong province of China was selected as the test case. Radar data assimilation was used to improve the simulation accuracy of the convective clouds, hence providing strong instantaneous correlations between observed and simulated storm signatures. The areal coverage and magnitude of the simulated lightning flash rates were evaluated by comparing to those of the total lightning observations. Subjective and the Fractions Skill Score (FSS) evaluations suggest that all the five proxies tested in this study are useful to indicate general tendencies for the occurrence, region, and time of lightning at convection-allowing scale (FSS statistics for the threshold of 1 flash per 9 km2 per hour were around 0.7 for each scheme). The FSS values were decreasing as the lightning flash rate thresholds used for FSS computation increased for all the lightning diagnostic schemes with different proxies. For thresholds from 1 to 3 and 16 to 20 flashes per 9 km2 per hour, the graupel contents related schemes achieved higher FSS values compared to the other three schemes. For thresholds from 5 to 15 flashes per 9 km2 per hour, the updraft volume related scheme yielded the largest FSS. When the thresholds of lightning flash rates were greater than 13 flashes per 9 km2 per hour, the FSS values were below 0.5 for all the lightning diagnostic schemes with different proxies.

Evaluating Ensemble Kalman Filter Analyses of Severe Hailstorms on 8 May 2017 in Colorado: Effects of State Variable Updating and Multimoment Microphysics Schemes on State Variable Cross Covariances

Abstract Ensemble Kalman filter (EnKF) analyses of the storms associated with the 8 May 2017 Colorado severe hail event using either the Milbrandt and Yau (MY) or the NSSL double-moment bulk microphysics scheme in the forecast model are evaluated. With each scheme, two experiments are conducted in which the reflectivity (Z) observations update in addition to dynamic and thermodynamic variables: 1) only the hydrometeor mixing ratios or 2) all microphysical variables. With fewer microphysical variables directly constrained by the Z observations, only updating hydrometeor mixing ratios causes the forecast error covariance structure to become unreliable, and results in larger errors in the analysis. Experiments that update all microphysical variables produce analyses with the lowest Z root-mean-square innovations; however, comparing the estimated hail size against hydrometeor classification algorithm output suggests that further constraint from observations is needed to more accurately estimate surface hail size. Ensemble correlation analyses are performed to determine the impact of hail growth assumptions in the MY and NSSL schemes on the forecast error covariance between microphysical and thermodynamic variables. In the MY scheme, Z is negatively correlated with updraft intensity because the strong updrafts produce abundant small hail aloft. The NSSL scheme predicts the growth of large hail aloft; consequently, Z is positively correlated with storm updraft intensity and hail state variables. Hail production processes are also shown to alter the background error covariance for liquid and frozen hydrometeor species. Results in this study suggest that EnKF analyses are sensitive to the choice of MP scheme (e.g., the treatment of hail growth processes).

An Evaluation of Relationships between Radar-Inferred Kinematic and Microphysical Parameters and Lightning Flash Rates in Alabama Storms

Lightning flash rate parameterizations based on polarimetric and multi-Doppler radar inferred microphysical (e.g., graupel volume, graupel mass, 35 dBZ volume) and kinematic (e.g., updraft volume, maximum updraft velocity) parameters have important applications in atmospheric science. Although past studies have established relations between flash rate and storm parameters, their expected performance in a variety of storm and flash rate conditions is uncertain due to sample limitations. Radar network and lightning mapping array observations over Alabama of a large and diverse sample of 33 storms are input to hydrometeor identification, vertical velocity retrieval and flash rate algorithms to develop and test flash rate relations. When applied to this sample, prior flash rate linear relations result in larger errors overall, including often much larger bias (both over- and under-estimation) and root mean square errors compared to the new linear relations. At low flash rates, the new flash rate relations based on kinematic parameters have larger errors compared to those based on microphysical ones. Sensitivity of error to the functional form (e.g., zero or non-zero intercept) is also tested. When considering all factors (e.g., low errors including at low flash rate, consistency with past linear relations, and insensitivity to functional form), the flash rate parameterization based on graupel volume has the best overall performance.

Relationships between Deep Convection Updraft Characteristics and Satellite-Based Super Rapid Scan Mesoscale Atmospheric Motion Vector–Derived Flow

Abstract Rapid acceleration of cloud-top outflow near vigorous storm updrafts can be readily observed in Geostationary Operational Environmental Satellite-14 (GOES-14) super rapid scan (SRS; 60 s) mode data. Conventional wisdom implies that this outflow is related to the intensity of updrafts and the formation of severe weather. However, from an SRS satellite perspective, the pairing of observed expansion and updraft intensity has not been objectively derived and documented. The goal of this study is to relate GOES-14 SRS-derived cloud-top horizontal divergence (CTD) over deep convection to internal updraft characteristics, and document evolution for severe and nonsevere thunderstorms. A new SRS flow derivation system is presented here to estimate storm-scale (<20 km) CTD. This CTD field is coupled with other proxies for storm updraft location and intensity such as overshooting tops (OTs), total lightning flash rates, and three-dimensional flow fields derived from dual-Doppler radar data. Objectively identified OTs with (without) matching CTD maxima were more (less) likely to be associated with radar-observed deep convection and severe weather reports at the ground, suggesting that some OTs were incorrectly identified. The correlation between CTD magnitude, maximum updraft speed, and total lightning was strongly positive for a nonsupercell pulse storm, and weakly positive for a supercell with multiple updraft pulses present. The relationship for the supercell was nonlinear, though larger flash rates are found during periods of larger CTD. Analysis here suggests that combining CTD with OTs and total lightning could have severe weather nowcasting value.

The Role of Cold Pools in Tropical Oceanic Convective Systems

Abstract The processes governing organized tropical convective systems are not completely understood despite their important influences on the tropical atmosphere and global circulation. In particular, cold pools are known to influence the structure and maintenance of midlatitude systems via Rotunno–Klemp–Weisman (RKW) theory, but cold pools may interact differently with tropical convection because of differences in cold pool strength and environmental shear. In this study, the role of cold pools in organized oceanic tropical convective systems is investigated, including their influence on system intensity, mesoscale structure, and propagation. To accomplish this goal, high-resolution idealized simulations are performed for two different systems that are embedded within a weakly sheared cloud population approaching radiative–convective equilibrium. The cold pools are altered by changing evaporation rates below cloud base in a series of sensitivity tests. The simulations demonstrate surprising findings: when cold pools are weakened, the convective systems become more intense. However, their propagation speeds and mesoscale structure are largely unaffected by the cold pool changes. Passive tracers introduced into the cold pools indicate that the convection intensifies when cold pools are weakened because cold pool air is entrained into updrafts, thereby reducing updraft intensity via the cold pools’ initial negative buoyancy. Gravity waves, rather than cold pools, appear to be the important modulators of system propagation and mesoscale structure. These results reconfirm that RKW theory does not fully explain the behavior of tropical oceanic convective systems, even those that otherwise appear consistent with RKW thinking.

Evaluation of Modeled Lake Breezes Using an Enhanced Observational Network in Southern Ontario: Case Studies

AbstractCanadian Global Environmental Multiscale (GEM) numerical model output was compared with the meteorological data from an enhanced observational network to investigate the model’s ability to predict Lake Ontario lake breezes and their characteristics for two cases in the Greater Toronto Area—one in which the large-scale wind opposed the lake breeze and one in which it was in the same direction as the lake breeze. An enhanced observational network of surface meteorological stations, a C-band radar, and two Doppler wind lidars were deployed among other sensors during the 2015 Pan and Parapan American Games in Toronto. The GEM model was run for three nested domains with grid spacings of 2.5, 1, and 0.25 km. Comparisons between the model predictions and ground-based observations showed that the model successfully predicted lake breezes for the two events. The results indicated that using GEM 1 and 0.25 km increased the forecast accuracy of the lake-breeze location, updraft intensity, and depth. The accuracy of the modeled lake breeze timing was approximately ±135 min. The model underpredicted the surface cooling caused by the lake breeze. The GEM 0.25-km model significantly improved the temperature forecast accuracy during the lake-breeze circulations, reducing the bias by up to 72%, but it mainly underpredicted the moisture and overpredicted the surface wind speed. Root-mean-square errors of wind direction forecasts were generally high because of large biases and high variability of errors.

A Numerical Investigation of the Potential Effects of Aerosol-Induced Warming and Updraft Width and Slope on Updraft Intensity in Deep Convective Clouds

Abstract The effects of aerosol perturbations on deep convective clouds have received considerable attention in the recent literature, especially from a modeling perspective. The published responses in precipitation amount and updraft strength vary in both sign and magnitude and may be the result of different models and parameterizations. Here, a simple numerical framework is employed to determine the potential effects of warming both below the freezing level (warm invigoration) and above the freezing level (mixed-phase invigoration) due to increased aerosol loading. The role of updraft width and slope in the same framework is also examined, highlighting the relative importance of each factor on the resulting updraft strength. The results show that the potential effects of warm invigoration are 2–3 times larger than for mixed-phase invigoration. However, a relatively small response in updraft velocity to warming is found, especially in comparison with the predicted changes in updraft velocity due to small differences in system slope and width, with 87.7% and 96.4% of the subadiabatic and adiabatic realizations, respectively, showing changes in updraft velocity of less than 15% for warmings of no more than 2°C. This result suggests that observations of the aerosol effect will be largely muddled by the natural variability of convective updraft width and slope (which are related to environmental wind shear).

Relationships between Electrification and Storm-Scale Properties Based on Idealized Simulations of an Intensifying Hurricane-Like Vortex

AbstractThis study investigates relationships between storm-scale properties and the electrification and lightning of two simulations of an intensifying idealized tropical cyclone (TC) using the cloud-resolving Collaborative Model for Multiscale Atmospheric Simulation (COMMAS). To produce an intensifying storm, an initial weak TC is subjected to a linear increase in sea surface temperature.As the TC intensifies, lightning flash rates increase in both the inner core (r ≤ 100 km) and outer region (100 < r ≤ 300 km). As time progresses, lightning in the outer region gradually decreases, while the inner-core lightning remains relatively steady. Bootstrapped correlation statistics using 1000 random samples between the pressure trace and time series of lightning rates shows a statistically significant negative correlation between inner-core lightning and TC intensification. Lightning rates in the outer bands were found to lag minimum surface pressure by 12 h.The increases in lightning in both the inner core and outer region coincided well with increases in 0.5 g kg−1 graupel and 5 m s−1 updraft volumes in each respective region. Correlation statistics with selected kinematic and microphysical variables known to be associated with lightning in thunderstorms, such as the ice water path, integrated updraft volume, and graupel volume, revealed that their increase in the inner core indicated an ongoing deepening, similar to the lightning. Trends in these proxy variables in the outer bands were also found to lag TC intensification by 12 h.Overall, the best linear relationships with lightning in either the inner core or the outer region were obtained with the 0.5 g kg−1 graupel volume and total graupel mass.

Numerical simulation of 23 June 2016 Yancheng City EF4 tornadic supercell and analysis of lightning activity

Based on the Weather Research Forecasting (WRF) model that features charging and discharging parameterization, relationships between tornado, hail and lightning were investigated for a tornado-producing (EF4 intensity) supercell thunderstorm over Yancheng City in Jiangsu Province, China, on 23 June 2016. Based on a sounding at 0800, there was a low lifting condensation level, substantial convective available potential energy (CAPE), and strong vertical wind shear near Yancheng City, which promote supercell development. At 1400, observations revealed that hail production and a dramatic increase of positive cloud-to-ground flash rates occurred simultaneously, maximizing five minutes later. The tornado occurred 30 min after the hail production. The time of minimum positive cloud-to-ground flash rates was 15 min later. The simulation indicated that the tornadic supercell moved eastward and that positive cloud-to-ground flash rates increased dramatically at 1400, the same as observed, but their maximum was 5 min later than observed. The simulated updraft volume peaked at 1425 and the simulated downdraft volume maximized 5 min later, when the mesocyclone formed. Simulated reflectivities showed no hook echo and horizontal winds for different height at mid-low levels had a different cyclonic shear at 1430, favorable to mesocyclone formation. Based on the simulated results, the region of positively charged graupel ascended resulting from the region of high liquid water content was lifted by the strong updraft, forming a mid-level strong positive charge region. A lower negative charge region formed by the inductive charging mechanism of collisions between graupel and droplets at the bottom of the cloud, conducive to positive cloud-to-ground flashes.