Maximum Updraft Speed Research Articles

Machine Learning–Derived Severe Weather Probabilities from a Warn-on-Forecast System

Abstract Severe weather probabilities are derived from the Warn-on-Forecast System (WoFS) run by NOAA’s National Severe Storms Laboratory (NSSL) during spring 2018 using the random forest (RF) machine learning algorithm. Recent work has shown this method generates skillful and reliable forecasts when applied to convection-allowing model ensembles for the “Day 1” time range (i.e., 12–36-h lead times), but it has been tested in only one other study for lead times relevant to WoFS (e.g., 0–6 h). Thus, in this paper, various sets of WoFS predictors, which include both environment and storm-based fields, are input into a RF algorithm and trained using the occurrence of severe weather reports within 39 km of a point to produce severe weather probabilities at 0–3-h lead times. We analyze the skill and reliability of these forecasts, sensitivity to different sets of predictors, and avenues for further improvements. The RF algorithm produced very skillful and reliable severe weather probabilities and significantly outperformed baseline probabilities calculated by finding the best performing updraft helicity (UH) threshold and smoothing parameter. Experiments where different sets of predictors were used to derive RF probabilities revealed 1) storm attribute fields contributed significantly more skill than environmental fields, 2) 2–5 km AGL UH and maximum updraft speed were the best performing storm attribute fields, 3) the most skillful ensemble summary metric was a smoothed mean, and 4) the most skillful forecasts were obtained when smoothed UH from individual ensemble members were used as predictors.

The Brettenham storm of 25 July 2021

The <scp>Brettenham</scp> storm of 25 <scp>July</scp> 2021

Lightning Activity Variability with Prevailing Weather Parameters and Aerosol Loading Over Dry and Wet Regions of India

This paper presents the lightning activity, aerosol optical depth (AOD) and climatic parameters (Bowen ratio, relative humidity, rainfall, maximum surface temperature and maximum updraft speed) over wet (Northeast India—NEI) and dry (Northwest India—NWI) land surfaces in a comparative analysis. The analyses are performed on flash counts and weather data of 17 years (1998–2014) retrieved from the Tropical Rainfall Measuring Mission (TRMM) lightning imaging sensors (LIS) and the Moderate Resolution Imaging Spectroradiometer (MODIS), respectively. The first higher peak for flash counts during pre-monsoon (April–May) and second lower peak during September over wet NEI and dry NWI regions indicate the development of strong electrified storms during pre-monsoon and weakly electrified clouds during the withdrawal phase of the southwest summer monsoon. The monthly means of flash counts, Bowen ratio, maximum surface temperature, AOD and maximum updraft speed are higher by 27, 81, 16, 50 and 16%, respectively, for NWI than those of NEI. The relative humidity and rainfall are higher by 12% and 73% for NEI than those of NWI, respectively. The Pearson’s correlation coefficients of lightning activity with climatic parameters and AOD demonstrate the distinctive orographic lifting, moisture content and vertical wind shear in the upper troposphere in the regional climatic zone of dry NWI in comparison to that of wet NEI.

Effect of shallow convection parametrization on cloud resolving NWP forecasts over the Eastern Mediterranean

Numerical Weather Prediction (NWP) models with coarser resolution than ~4 km use convection parametrization to describe both deep and shallow convection. Convection permitting models with grid spacing below ~4 km usually describe deep convection on grid scale, but still parametrize the shallow convection, being sub-grid scale process. Many of these parametrizations are based on the mass-flux approach. The artificial combination of resolved and parametrized convection might deteriorate the model forecast skill, particularly precipitation. In this study, we are using the COSMO 2.5 km resolution limited area NWP model over the eastern Mediterranean region to analyze the effect of the mass-flux shallow convection parametrization (SCP) on the forecasts skill. We show that SCP entrainment rate, as well as restrictions on the shallow convection development height and maximum updraft speed, have a crucial effect on the forecasts of precipitation, temperature and moisture. Physically motivated tuning of these parameters can improve the model forecasts significantly. Finally, we estimate the climatic zones and seasons where SCP is expected to be particularly important for NWP.

A Numerical Study of Aerosol Effects on Electrification with Different Intensity Thunderclouds

Numerical simulations are performed to investigate the effect of varying CCN (cloud condensation nuclei) concentration on dynamic, microphysics, electrification, and charge structure in weak, moderate, and severe thunderstorms. The results show that the response of electrification to the increase of CCN concentration is a nonlinear relationship in different types of thunderclouds. The increase in CCN concentration leads to a significant enhancement of updraft in the weak thunderclouds, while the high CCN concentration in moderate and severe thunderclouds leads to a slight reduction in maximum updraft speed. The increase of the convection promotes the lift of more small cloud droplets, which leads to a faster and stronger production of ice crystals. The production of graupel is insensitive to the CCN concentration. The content of graupel increases from low CCN concentration to moderate CCN concentration, and slightly decreases at high CCN concentration, which arises from the profound enhancement of small ice crystals production. When the intensity of thundercloud increases, the reduction of graupel production will arise in advance as the CCN concentration increases. Charge production tends to increase as the aerosol concentration rises from low to high in weak and moderate thundercloud cases. However, the magnitude of charging rates in the severe thundercloud cases keeps roughly stable under the high CCN concentration condition, which can be attributed to the profound reduction of graupel content. The charge structure in the weak thundercloud at low CCN concentrations keeps as a dipole, while the weak thunderclouds in the other cases (the CCN concentration above 100 cm−3) change from a dipole charge structure to a tripole charge structure, and finally disappear with a dipole. In cases of moderate and severe intensity thunderclouds, the charge structure depicts a relatively complex structure that includes a multilayer charge region.

Land-sea contrasts for climatic lightning activity over Indian region

The land-ocean convective contrasts observed by the satellite-driven data from the Tropical Rainfall Measuring Mission for the year 1998 to 2014 (17 years) are analysed to examine the role of static land surface relative to the dynamic ocean surface for the lightning flash counts (FC). We present the relationship between the lightning flash counts per convective available potential energy (CAPE) (FC/CAPE), the product of CAPE and rainfall (CAPE × RF) and Bowen ratio, FC (land/ocean) and AOD (land/ocean) and FC (land/ocean) and maximum updraft speed (land/ocean) to examine the land-ocean contrasts over the Indian region. The results show that FC/CAPE over Indian land increases by up to 520% with respect to FC/CAPE over Indian oceanic regions. The land-ocean contrast seems to be a result of increase of Bowen ratio and FC over land by up to 800% as compared with Bowen ratio and FC over ocean surface. The increase of CAPE over land is by up to 47% relative to CAPE over ocean with corresponding increase of maximum updraft speed over land by up to 117% with respect to maximum updraft speed over ocean. The temperature profiles over the land and ocean show the same temperature in the lower troposphere, followed by a temperature difference of 8 °C between land and ocean at about 16000 m during pre-monsoon and post-monsoon periods. The product of CAPE with RF over land is higher by 66% to that of oceanic region with aerosol load (AOD) of about 49% more over land as compared with that of ocean. The results demonstrate the warming contrasts of land-ocean in a comparative analysis of FC, CAPE, RF and AOD in the tropics.

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 (&amp;lt;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 Impact of Vertical Wind Shear on Hail Growth in Simulated Supercells

Abstract Severe hailstorms produce over $1 billion in insured losses annually in the United States, yet details of a given storm’s hail threat (e.g., maximum hailstone size and total hailfall) remain challenging to forecast. Previous research suggests that, in addition to maximum updraft speed, the storm-relative airflow could be equally important for hail formation and growth. This study is a first step toward determining how changes in environmental wind shear and subsequent changes in simulated supercell storm structure affect hail growth. Using Cloud Model 1 (CM1) with 500-m horizontal and 250-m vertical grid spacing, 20 idealized simulations are performed in which the thermodynamic profile remains fixed but the environmental hodograph is systematically altered. Hail growth is quantified using the hail mass mixing ratio from composites of storms over the last hour of simulation time. Hailstone growth “pseudotrajectories” are computed from these storm composites to determine favorable embryo source regions. Results indicate that increased deep-layer east–west shear elongates the storm’s updraft in that direction, providing 1) increased volumes over which microphysically relevant hail processes can act, 2) increased hailstone residence times within the updraft, and 3) a larger potential embryo source region; together, these lead to increased hail mass. Increased low-level north–south shear, which results in hodographs with increased 0–3-km storm-relative helicity, also elongates the updraft in the north–south direction. However, hail mass is reduced owing to a separation of favorable embryo source regions (which shift southward) and available hydrometeors to serve as embryos (which shift northward).

The kinematic and microphysical control of lightning rate, extent, and NOX production

AbstractThis study investigates the kinematic and microphysical control of lightning properties, particularly those that may govern the production of nitrogen oxides (NOX = NO + NO2) via lightning (LNOX), such as flash rate, type, and extent. The NASA Lightning Nitrogen Oxides Model (LNOM) is applied to lightning observations following multicell thunderstorms through their lifecycle in a Lagrangian sense over Northern Alabama on 21 May 2012 during the Deep Convective Clouds and Chemistry (DC3) experiment. LNOM provides estimates of flash rate, type, channel length distributions, channel segment altitude distributions (SADs), and LNOX production profiles. The LNOM‐derived lightning characteristics and LNOX production are compared to the evolution of radar‐inferred updraft and precipitation properties. Intercloud, intracloud (IC) flash SAD comprises a significant fraction of the total (IC + cloud‐to‐ground [CG]) SAD, while increased CG flash SAD at altitudes &gt;6 km occurs after the simultaneous peaks in several thunderstorm properties (i.e., total [IC + CG] and IC flash rate, graupel volume/mass, convective updraft volume, and maximum updraft speed). At heights &lt;6 km, the CG LNOX production dominates the column‐integrated total LNOX production. Unlike the SAD, total LNOX production consists of a more equal contribution from IC and CG flashes for heights &gt;6 km. Graupel volume/mass, updraft volume, and maximum updraft speed are all well correlated to the total flash rate (correlation coefficient, ρ ≥ 0.8) but are less correlated to total flash extent (ρ ≥ 0.6) and total LNOX production (ρ ≥ 0.5). Although LNOM transforms lightning observations into LNOX production values, these values are estimates and are subject to further independent validation.

Insight into the Kinematic and Microphysical Processes that Control Lightning Jumps

AbstractA detailed case study analysis of four thunderstorms is performed using polarimetric and multi-Doppler capabilities to provide specificity on the physical and dynamical drivers behind lightning jumps. The main differences between small increases in the total flash rate and a lightning jump are the increases in graupel mass and updraft volumes ≥10 m s−1 between the −10° and −40°C isotherms. Updraft volumes ≥10 m s−1 increased in magnitude at least 3–5 min in advance of the increase in both graupel mass and total flash rate. Updraft volumes ≥10 m s−1 are more robustly correlated to total flash rate than maximum updraft speed over a thunderstorm’s entire life cycle. However, peak updraft speeds increase prior to 8 of the 12 lightning jumps examined. Decreases in mean and median flash footprint size during increases in total lightning are observed in all four thunderstorms and are most notable during development stages within the most intense storms. However, this inverse relationship breaks down on larger storm scales as storms mature and anvils and stratiform regions developed with time. Promisingly, smaller flash sizes are still collocated with the strongest updraft speeds, while larger flash sizes are observed within weaker updraft regions. The results herein emphasize the following for lightning jump applications: both the lightning jump sigma level and the resultant magnitude of the total flash rate must be employed in conjunction to assess storm intensity using lightning data. The sigma-level magnitude of the lightning jump is the early warning that indicates that rapid intensification is occurring, while the magnitude of the total flash rate provides insight into the size and maintenance of the updraft volume and graupel mass. These cases serve as conceptual models for future applications of the lightning jump algorithm for hazardous weather monitoring.

Total lightning activity as an indicator of updraft characteristics

This study investigates the relationship of total lightning activity and updraft characteristics, such as updraft volume and maximum updraft speed, for a number of storms of different types occurring in the High Plains and in Northern Alabama. Ground‐based Doppler and dual polarimetric radar observations were used to compute updraft characteristics. Also, ground‐based total lightning data were available at both locations. Results show that time series of updraft volume in the charging zone (at temperatures colder than −5°C) with vertical velocities greater than either 5 or 10 m s−1 have clear relationships with total lightning activity (r = 0.93). Furthermore, these relationships between updraft volume and lightning activity for the storm types of the two climate regimes tend to converge when considering only the subfreezing layers of the clouds. Neither the maximum nor the mean updraft speed correlate as well with total lightning activity (r = &lt;0.8) as updraft volume. Through expanded study designed to explore further regime variability (or lack thereof) of updraft volume‐lightning flash rate relationships, better or refined parameterizations for the numerical forecasting of lightning and/or detection and prediction of storm intensity could be realized.

Numerically Simulated Electrification and Lightning of the 29 June 2000 STEPS Supercell Storm

Abstract A three-dimensional dynamic cloud model incorporating airflow dynamics, microphysics, and thunderstorm electrification mechanisms is used to simulate the first 3 h of the 29 June 2000 supercell from the Severe Thunderstorm Electrification and Precipitation Study (STEPS). The 29 June storm produced large flash rates, predominately positive cloud-to-ground lightning, large hail, and an F1 tornado. Four different simulations of the storm are made, each one using a different noninductive (NI) charging parameterization. The charge structure, and thus lightning polarity, of the simulated storm is sensitive to the treatment of cloud water dependence in the different NI charging schemes. The results from the simulations are compared with observations from STEPS, including balloon-borne electric field meter soundings and flash locations from the Lightning Mapping Array. For two of the parameterizations, the observed “inverted” tripolar charge structure is well approximated by the model. The polarity of the ground flashes is opposite that of the lowest charge region of the inverted tripole in both the observed storm and the simulations. Total flash rate is well correlated with graupel volume, updraft volume, and updraft mass flux. However, there is little correlation between total flash rate and maximum updraft speed. Based on the correlations found in both the observed and simulated storm, the total flash rate appears to be most representative of overall storm intensity.

Diurnal and spatial variation of convective parameters over Bay of Bengal during BOBMEX 1999

An attempt has been made to study the diurnal and spatial variation of thermo-dynamical parameters viz., Convective Available Potential Energy (CAPE), Convective Inhibition Energy (CINE), Moist Static Energy (MSE), convectively generated maximum updraft speed (Wmax) and non-dimensional kinetic parameter (L) over Bay of Bengal during BOBMEX-1999 period. The study reveals that there is a marked diurnal variation in these thermodynamic parameters over Bay of Bengal. The average value of the CAPE was found to be more during the night than in the day and the negative value of CINE and MSE are found to be more during the day than in the night. The study also shows that during the period of experiment on most of the days, lower troposphere was convectively unstable (moist). The composite analysis of CAPE shows that southwest &amp; adjoining west-central Bay near Sri Lanka - Tamil Nadu coast is more convectively unstable and an alternate rise and fall in it also indicates the presence of inertia gravity waves.

The 1 August 1981 MOPE Storm: Observations and Modeling Results

Abstract Observations made by the T-28 armored research aircraft, radar, and other data sources were used to study an eastern Moutana hailstorm that developed on 1 August 1981 during the Cooperative Convective Precipitation Experiment season. The storm featured a wide, persistent, vertically oriented updraft with speeds exceeding 35 m s−1. Hailstones of over 5 cm diameter were collected at the ground, while the T-28 encountered hail up to about 2.5 cm diameter. Them was no evidence of feeder cells or a weak echo region. The IAS two-dimensional, time-dependent “bulk water” model was run on this case, using a sounding from this day. Some areas of agreement between the simulation and observations include the maximum updraft speed, cloud top height, presence of a rounded cloud dome, and maximum radar reflectivity. The simulation failed to properly model the width and orientation of the updraft, as well as its long lifetime. The IAS hail category model was also run on this case. This model features 20 categori...

Error Sources and Accuracy of Vertical Velocities Computed from Multiple-Doppler Radar Measurements in Deep Convective Storms

Abstract Actual data are used in one case to investigate the nature and source of vertical velocity errors resulting from analyses of multiple-Doppler radar measurements. Consistent with earlier analytical works, larger errors are found than would be expected from previous theoretical studies. It is shown that the reconstructed maximum updraft speed in strong updrafts (>20 m s−1) is accurate, on the average, to within about 10% (standard deviation of 10%). Storm advection, incomplete sampling of low-altitude divergence caused by the radar horizon, top boundary errors, and uneven terrain are studied and all are dismissed as dominant sources of error in the case considered here. The inability to determine a dominant error source has important consequences for the formulation of vertical velocity adjustment schemes.

A Severe Frontal Rainband. Part I. Stormwide Hydrodynamic Structure

Abstract A narrow cold frontal band of intense precipitation is examined by means of triple Doppler radar and supporting observations. As the band passed through the Central Valley of California, it was accompanied by strong gusty winds, electrical activity, tornadoes and pressure jumps. Part I delineates the stormwide kinematic and thermodynamic structure. A highly two-dimensional pre-frontal updraft of 15–20 m s−1 results primarily from intense planetary boundary layer forcing of a low-level jet by the gravity-current propagation mechanism. Maximum updraft speed occurs at 2.1 km and the maximum radar echo depth is 6.6 km. Diabatic cooling, due to melting hydrometers, is proposed as a likely mechanism for control of gravity current depth and maintenance of density contrast together with synoptic-scale cold air advection. Available convective potential energy is shown to be small and kinetic energy of the environmental vertical wind shear is proposed as a likely source of energy on the updraft scale. Torn...

Research on Electrical Properties of Severe Thunderstorms in the Great Plains

In 1978 we began a coordinated effort to study the electrical behavior of large and severe thunderstorms that form over the Great Plains of the central United States. Methods of approach include the study of characteristics of individual phenomena and storm case studies. Our goal is to understand the interrelationships between electrical phenomena and the dynamics and precipitation of storms. Evidence that interrelationships do exist can be seen in the results to date. In one squall-line storm we have studied, 44% of all observed lightning flashes were cloud-to-ground (CG); the total flashing rate averaged 12 min−1 and coarsely followed the changes in Doppler-derived maximum updraft speed. Most of the intracloud (IC) discharge processes in a supercell severe storm were located predominately around the region of the intense updraft of the mesocyclone and near large gradients in reflectivity and horizontal velocity. Both 10 cm and 23 cm wavelength radars have been used to detect lightning radar echoes. The lightning echoes from the 10 cm radar generally had peak signals 10–25 dB greater than the largest precipitation echo in the storm, and they usually were observed where precipitation reflectivities were less than maximum. Comparison of lightning echoes and electric field changes shows that abrupt increases in radar reflectivity often are associated with return strokes and K-type field changes. CG flashes that lower positive charge to earth have been observed to emanate from the wall cloud, high on the main storm tower, and well out in the downwind anvil of severe storms. The percentage of CG flashes that lower positive charge is apparently small.

Results of a Randomized Hail Suppression Experiment in Northeast Colorado. Part VI:Post HocStratification by Storm Intensity and Type

The surface hail and rain data collected during the randomized seeding experiment of the National Hail Research Experiment (NHRE) are stratified according to storm intensity in a search for seeding effects. The problems encountered in attempting to stratify the data according to whether the storm cells exhibited radar vaults are discussed, but no stratification is attempted on this basis. The stratification by storm intensity is accomplished using a one-dimensional steady-state cloud model in conjunction with measured radar echo top heights to estimate the maximum updraft speed of the storm. It is shown that the technique is quite successful in classifying the storm days according to hailfall intensity. Analysis of hail and rain data on seed and control days in the various strata do not show an effect of seeding as assessed by 90% confidence intervals. The confidence intervals are quite wide, however, and a variety of possible effects of seeding are also not inconsistent with the data. The frequency with which vaulted cells were observed during the experiment is documented, and they are shown to constitute only ∼1% of the total. However, the hailfall from vaulted cells, which tend to be very large and long-lived, can be extreme. Three vaulted cells were seeded during the experiment. Analysis of the radar reflectivity structure of these cells did not reveal any obvious effects of seeding.

Height and Temperature of First Echoes in Unseeded and Seeded Convective Clouds in South Dakota

The height and temperature of first radar echoes observed on 49 test cases in a randomized cloud seeding project have been related to seeding treatment and to five cloud parameters, namely, cloud base height, cloud base temperature, maximum radar echo height, and the height and speed of maximum updraft as computed with a cloud model. First echo height above cloud base in unseeded clouds averaged 11,000 ft and was strongly correlated with maximum updraft speed. First echo height in clouds seeded with silver iodide averaged 6900 ft. It was most closely correlated with cloud base temperature, suggesting that first echoes tend to appear at some fixed temperature. First echoes in salt seeded clouds appeared on an average 5300 ft above cloud base and showed no significant correlation with any of the cloud parameters mentioned. It is concluded that both salt and silver iodide seeding can cause first echoes to appear closer to cloud base in unseeded clouds, with salt probably having a more pronounced effect in summertime convective clouds.