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Abstract
An area of intense thunderstorms occurred within the special rawinsonde network collecting data on 20–21 May 1979, the fifth day of the Atmospheric Variability Experiment-Severe Environmental Storms and Mesoscale Experiment (AVE-SESAME). The data are at the meso β-scale, i.e., 75 km spacing and 3 or 1.5 h intervals. They are used to perform a kinetic energy analysis of the near storm environment. The mesoscale storm environment is characterized by cross-contour generation of kinetic energy, transfers of energy to nonresolvable scales of motion (negative dissipation), horizontal flux divergence and upward transport of energy. These processes are maximized within the upper troposphere and are greatest during times of strongest convection. Current mesoscale values are much larger than previous results based on synoptic-scale data.
Energy budgets are obtained at 3 h intervals from the routine National Weather Service rawinsonde network. A comparison of results from the same analysis region, but derived from the two different resolutions, reveals several common features. Complex vertical variations in winds (energy) over southeastern Oklahoma are also examined in detail. Motions not detected by the meso β-scale input data appera to play an important role in the energy balance of some layers. A sensitivity analysis is presented to quantify uncertainties in the energy budget terms.
Abstract
An area of intense thunderstorms occurred within the special rawinsonde network collecting data on 20–21 May 1979, the fifth day of the Atmospheric Variability Experiment-Severe Environmental Storms and Mesoscale Experiment (AVE-SESAME). The data are at the meso β-scale, i.e., 75 km spacing and 3 or 1.5 h intervals. They are used to perform a kinetic energy analysis of the near storm environment. The mesoscale storm environment is characterized by cross-contour generation of kinetic energy, transfers of energy to nonresolvable scales of motion (negative dissipation), horizontal flux divergence and upward transport of energy. These processes are maximized within the upper troposphere and are greatest during times of strongest convection. Current mesoscale values are much larger than previous results based on synoptic-scale data.
Energy budgets are obtained at 3 h intervals from the routine National Weather Service rawinsonde network. A comparison of results from the same analysis region, but derived from the two different resolutions, reveals several common features. Complex vertical variations in winds (energy) over southeastern Oklahoma are also examined in detail. Motions not detected by the meso β-scale input data appera to play an important role in the energy balance of some layers. A sensitivity analysis is presented to quantify uncertainties in the energy budget terms.
Abstract
The kinetic energy balance during the Red River Valley tornado outbreak (10–11 April 1979) is examined using mesa a-scale rawinsonde data from the first regional-scale day of AVE-SESAME 1979. Computational procedures account for non-simultaneous sonde releases, sonde drift downwind, and the substitution for some missing data. When the entire area is considered for the composite 2A h period, horizontal flux convergence due to jet intrusion is the primary kinetic energy source to the region. Cross-contour destruction of kinetic energy is the primary sink. Using the 3 h data, the energy balance is seen to change considerably throughout the experiment. Horizontal maps and limited area budgets are used to examine the energetics of a low-level jet stream and an upper-level wind maximum. Both features are maintained primarily by cross-contour kinetic energy generation that may be related to feedback mechanisms from the severe storm outbreak. Random error analyses are used to quantify confidence limits in the energy budget parameters.
Abstract
The kinetic energy balance during the Red River Valley tornado outbreak (10–11 April 1979) is examined using mesa a-scale rawinsonde data from the first regional-scale day of AVE-SESAME 1979. Computational procedures account for non-simultaneous sonde releases, sonde drift downwind, and the substitution for some missing data. When the entire area is considered for the composite 2A h period, horizontal flux convergence due to jet intrusion is the primary kinetic energy source to the region. Cross-contour destruction of kinetic energy is the primary sink. Using the 3 h data, the energy balance is seen to change considerably throughout the experiment. Horizontal maps and limited area budgets are used to examine the energetics of a low-level jet stream and an upper-level wind maximum. Both features are maintained primarily by cross-contour kinetic energy generation that may be related to feedback mechanisms from the severe storm outbreak. Random error analyses are used to quantify confidence limits in the energy budget parameters.
Abstract
Seasonal, regional, and storm-scale variations of cloud-to-ground (CG) lightning characteristics in Florida are presented. Strong positive CG (+CG) and negative CG (−CG) flashes (i.e., having large peak current) are emphasized since they often are associated with strong storms, structural damage, and wildfire ignitions. Although strong −CG flashes are most common during the warm season (May–September) over the peninsula, the greatest proportion of strong +CG flashes occurs during the cool season (October–April) over the panhandle. The warm season exhibits the smallest +CG percentage but contains the greatest +CG flash densities, due in part to more ambiguous +CG reports (15–20 kA). The more frequent occurrence of ambiguous +CG reports helps explain the unusually small average +CG peak current during the warm season, whereas strong +CG reports (>20 kA) appear to be responsible for the greater average warm season +CG multiplicity. The −CG flash density, multiplicity, and peak current appear to be directly related, exhibiting their greatest values during the warm season when deep storms are most common. A case study examines the atmospheric conditions and storm-scale processes associated with two distinct groups of storms on 13–14 May 2007. Although these groups of storms form in close proximity, several factors combine to produce predominately strong +CG and −CG flashes in the northern (south Georgia) and southern (north Florida) regions, respectively. Results suggest that heat and smoke very near preexisting wildfires are key ingredients in producing reversed-polarity (+CG dominated) storms that often ignite subsequent wildfires.
Abstract
Seasonal, regional, and storm-scale variations of cloud-to-ground (CG) lightning characteristics in Florida are presented. Strong positive CG (+CG) and negative CG (−CG) flashes (i.e., having large peak current) are emphasized since they often are associated with strong storms, structural damage, and wildfire ignitions. Although strong −CG flashes are most common during the warm season (May–September) over the peninsula, the greatest proportion of strong +CG flashes occurs during the cool season (October–April) over the panhandle. The warm season exhibits the smallest +CG percentage but contains the greatest +CG flash densities, due in part to more ambiguous +CG reports (15–20 kA). The more frequent occurrence of ambiguous +CG reports helps explain the unusually small average +CG peak current during the warm season, whereas strong +CG reports (>20 kA) appear to be responsible for the greater average warm season +CG multiplicity. The −CG flash density, multiplicity, and peak current appear to be directly related, exhibiting their greatest values during the warm season when deep storms are most common. A case study examines the atmospheric conditions and storm-scale processes associated with two distinct groups of storms on 13–14 May 2007. Although these groups of storms form in close proximity, several factors combine to produce predominately strong +CG and −CG flashes in the northern (south Georgia) and southern (north Florida) regions, respectively. Results suggest that heat and smoke very near preexisting wildfires are key ingredients in producing reversed-polarity (+CG dominated) storms that often ignite subsequent wildfires.
Abstract
The authors develop a statistical guidance product, the tropical cyclone tornado parameter (TCTP), for forecasting the probability of one or more tornadoes during a 6-h period that are associated with landfalling tropical cyclones affecting the coastal Gulf of Mexico and the southern Atlantic coast. TCTP is designed to aid forecasters in a time-limited environment. TCTP provides a “quick look” at regions where forecasters can then conduct detailed analyses. The pool of potential predictors included tornado reports and tropical cyclone data between 2000 and 2008, as well as storm environmental parameters. The original pool of 28 potential predictors is reduced to six using stepwise regression and logistic regression. These six predictors are 0–3-km wind shear, 0–3-km storm relative helicity, azimuth angle of the tornado report from the tropical cyclone, distance from the cyclone’s center, time of day, and 950–1000-hPa convective available potential energy. Mean Brier scores and Brier skill scores are computed for the entire TCTP-dependent dataset and for corresponding forecasts produced by the Storm Prediction Center (SPC). TCTP then is applied to four individual cyclone cases to qualitatively and quantitatively assess the parameter and compare its performance with SPC forecasts. Results show that TCTP has skill at identifying regions of tornado potential. However, tornadoes in some tropical systems are overpredicted, but underpredicted in others. TCTP 6-h forecast periods provide slightly poorer statistical performance than the 1-day tornado probability forecasts from SPC, probably because the SPC product includes forecaster guidance and because their forecasts are valid for longer periods (24 h).
Abstract
The authors develop a statistical guidance product, the tropical cyclone tornado parameter (TCTP), for forecasting the probability of one or more tornadoes during a 6-h period that are associated with landfalling tropical cyclones affecting the coastal Gulf of Mexico and the southern Atlantic coast. TCTP is designed to aid forecasters in a time-limited environment. TCTP provides a “quick look” at regions where forecasters can then conduct detailed analyses. The pool of potential predictors included tornado reports and tropical cyclone data between 2000 and 2008, as well as storm environmental parameters. The original pool of 28 potential predictors is reduced to six using stepwise regression and logistic regression. These six predictors are 0–3-km wind shear, 0–3-km storm relative helicity, azimuth angle of the tornado report from the tropical cyclone, distance from the cyclone’s center, time of day, and 950–1000-hPa convective available potential energy. Mean Brier scores and Brier skill scores are computed for the entire TCTP-dependent dataset and for corresponding forecasts produced by the Storm Prediction Center (SPC). TCTP then is applied to four individual cyclone cases to qualitatively and quantitatively assess the parameter and compare its performance with SPC forecasts. Results show that TCTP has skill at identifying regions of tornado potential. However, tornadoes in some tropical systems are overpredicted, but underpredicted in others. TCTP 6-h forecast periods provide slightly poorer statistical performance than the 1-day tornado probability forecasts from SPC, probably because the SPC product includes forecaster guidance and because their forecasts are valid for longer periods (24 h).
Abstract
This study presents a new method for assimilating lightning data into numerical models that is suitable at convection-permitting scales. The authors utilized data from the Earth Networks Total Lightning Network at 9-km grid spacing to mimic the resolution of the Geostationary Lightning Mapper (GLM) that will be on the Geostationary Operational Environmental Satellite-R (GOES-R). The assimilation procedure utilizes the numerical Weather Research and Forecasting (WRF) Model. The method (denoted MU) warms the most unstable low levels of the atmosphere at locations where lightning was observed but deep convection was not simulated based on the absence of graupel. Simulation results are compared with those from a control simulation and a simulation employing the lightning assimilation method developed by Fierro et al. (denoted FO) that increases water vapor according to a nudging function that depends on the observed flash rate and simulated graupel mixing ratio. Results are presented for three severe storm days during 2011 and compared with hourly NCEP stage-IV precipitation observations. Compared to control simulations, both the MU and FO assimilation methods produce improved simulated precipitation fields during the assimilation period and a short time afterward based on subjective comparisons and objective statistical scores (~0.1, or 50%, improvement of equitable threat scores). The MU generally performs better at simulating isolated thunderstorms and other weakly forced deep convection, while FO performs better for the case having strong synoptic forcing. Results show that the newly developed MU method is a viable alternative to the FO method, exhibiting utility in producing thunderstorms where observed, and providing improved analyses at low computational cost.
Abstract
This study presents a new method for assimilating lightning data into numerical models that is suitable at convection-permitting scales. The authors utilized data from the Earth Networks Total Lightning Network at 9-km grid spacing to mimic the resolution of the Geostationary Lightning Mapper (GLM) that will be on the Geostationary Operational Environmental Satellite-R (GOES-R). The assimilation procedure utilizes the numerical Weather Research and Forecasting (WRF) Model. The method (denoted MU) warms the most unstable low levels of the atmosphere at locations where lightning was observed but deep convection was not simulated based on the absence of graupel. Simulation results are compared with those from a control simulation and a simulation employing the lightning assimilation method developed by Fierro et al. (denoted FO) that increases water vapor according to a nudging function that depends on the observed flash rate and simulated graupel mixing ratio. Results are presented for three severe storm days during 2011 and compared with hourly NCEP stage-IV precipitation observations. Compared to control simulations, both the MU and FO assimilation methods produce improved simulated precipitation fields during the assimilation period and a short time afterward based on subjective comparisons and objective statistical scores (~0.1, or 50%, improvement of equitable threat scores). The MU generally performs better at simulating isolated thunderstorms and other weakly forced deep convection, while FO performs better for the case having strong synoptic forcing. Results show that the newly developed MU method is a viable alternative to the FO method, exhibiting utility in producing thunderstorms where observed, and providing improved analyses at low computational cost.
Abstract
An aircraft prototype of the High-Resolution Interferometer Sounder (HIS) was flown over Tennesse and northern Alabama during summer 1986. HIS temperature and dewpoint soundings were examined on two flight days to determine their error characteristics and utility in mesoscale analyses. Random errors were calculated from structure functions while total errors were obtained by pairing the HIS soundings with radiosonde-derived profiles. Random temperature errors were found to be less than 1°C at most levels, but random dewpoint errors ranged from 1° to 5°C. Total errors of both parameters were considerably greater, with dewpoint errors especially large on the day having a pronounced subsidence inversion.
Cumulus cloud cover on 15 June limited HIS mesoscale analyses on that day. Previously undetected clouds were found in many HIS fields of view, and these probably produced the low-level horizontal temperature and dewpoint variations observed in the retrievals. HIS dewpoints at 300 mb indicated a strong moisture gradient that was confirmed by GOES 6.7-µm imagery.
HIS mesoscale analyses on 19 June revealed a tongue of humid air stretching across the study area. The moist region was confirmed by radiosonde data and imagery from the Multispectral Atmospheric Mapping Sensor (MAMS). Convective temperatures derived from HIS retrievals helped explain the cloud formation that occurred after the HIS overflights. Crude estimates of Bowen ratio were obtained from HIS data using a mixing-line approach. Values indicated that areas of large sensible heat flux were the areas of first cloud development. These locations were also suggested by GOES visible and infrared imagery. The HIS retrievals indicated that areas of thunderstorm formation were regions of greatest instability.
Local landscape variability and atmospheric temperature and humidity fluctuations were found to be important factors in producing the cumulus clouds on 19 June. HIS soundings were capable of detecting some of this variability. The authors were impressed by HIS's performance on the two study days.
Abstract
An aircraft prototype of the High-Resolution Interferometer Sounder (HIS) was flown over Tennesse and northern Alabama during summer 1986. HIS temperature and dewpoint soundings were examined on two flight days to determine their error characteristics and utility in mesoscale analyses. Random errors were calculated from structure functions while total errors were obtained by pairing the HIS soundings with radiosonde-derived profiles. Random temperature errors were found to be less than 1°C at most levels, but random dewpoint errors ranged from 1° to 5°C. Total errors of both parameters were considerably greater, with dewpoint errors especially large on the day having a pronounced subsidence inversion.
Cumulus cloud cover on 15 June limited HIS mesoscale analyses on that day. Previously undetected clouds were found in many HIS fields of view, and these probably produced the low-level horizontal temperature and dewpoint variations observed in the retrievals. HIS dewpoints at 300 mb indicated a strong moisture gradient that was confirmed by GOES 6.7-µm imagery.
HIS mesoscale analyses on 19 June revealed a tongue of humid air stretching across the study area. The moist region was confirmed by radiosonde data and imagery from the Multispectral Atmospheric Mapping Sensor (MAMS). Convective temperatures derived from HIS retrievals helped explain the cloud formation that occurred after the HIS overflights. Crude estimates of Bowen ratio were obtained from HIS data using a mixing-line approach. Values indicated that areas of large sensible heat flux were the areas of first cloud development. These locations were also suggested by GOES visible and infrared imagery. The HIS retrievals indicated that areas of thunderstorm formation were regions of greatest instability.
Local landscape variability and atmospheric temperature and humidity fluctuations were found to be important factors in producing the cumulus clouds on 19 June. HIS soundings were capable of detecting some of this variability. The authors were impressed by HIS's performance on the two study days.
Abstract
A three-dimensional numerical simulation of land–water circulations near Cape Canaveral, Florida, is performed using the Advanced Regional Prediction System. The role of Kelvin–Helmholtz instability (KHI) in determining the time and location of convection behind the sea-breeze front is examined. The model configuration attempts to improve upon limitations of previous work (e.g., resolution, surface characteristics, initial state). It provides a detailed and realistic simulation of the desired features.
The simulation exhibits a single precipitating storm that forms behind the sea-breeze front. This postfrontal storm develops when an outflow boundary intersects a deep layer of upward motion above the marine air. The region of ascent initially is the remnant of a cell that formed along the sea-breeze front, but before the cell decays, a portion of its upward motion is intensified and displaced. The modification of the ascent is a product of KHI that is occurring on top of the sea-breeze interface in the form of billows. The region of enhanced ascent then moves backward with the billow until the outflow boundary arrives. Thus, KHI can be critical in determining the location and time of storm development that occurs behind the Cape Canaveral sea-breeze front.
Abstract
A three-dimensional numerical simulation of land–water circulations near Cape Canaveral, Florida, is performed using the Advanced Regional Prediction System. The role of Kelvin–Helmholtz instability (KHI) in determining the time and location of convection behind the sea-breeze front is examined. The model configuration attempts to improve upon limitations of previous work (e.g., resolution, surface characteristics, initial state). It provides a detailed and realistic simulation of the desired features.
The simulation exhibits a single precipitating storm that forms behind the sea-breeze front. This postfrontal storm develops when an outflow boundary intersects a deep layer of upward motion above the marine air. The region of ascent initially is the remnant of a cell that formed along the sea-breeze front, but before the cell decays, a portion of its upward motion is intensified and displaced. The modification of the ascent is a product of KHI that is occurring on top of the sea-breeze interface in the form of billows. The region of enhanced ascent then moves backward with the billow until the outflow boundary arrives. Thus, KHI can be critical in determining the location and time of storm development that occurs behind the Cape Canaveral sea-breeze front.
Abstract
Contibutions of divergent and rotational wind components to the synoptic-scale kinetic energy balance are described using rawinsonde data at 3 and 6 h intervals from NASA’s fourth Atmospheric Variability Experiment (AVE 4). Two intense thunderstorm complexes occurred during the period. Energy budgets am described for the entire computational region and for limited volumes that enclosed storm-induced, upper-level wind maxima touted poleward of the convection.
Although small in magnitude, the divergent wind component played an important role in the cross-contour generation and horizontal flux divergence of kinetic energy. The importance of V D appears directly related to the presence and intensity of convection. Although K D usually comprised lm than 10% of the total kinetic energy content generation of kinetic energy by V D was a major factor in the creation of upper-level wind maxima to the north of the storm complexes. Omission of the divergent wind apparently would lead to serious misrepresentations of the energy balance. A random error analysis is presented to assess confidence limits in the various energy parameters.
Abstract
Contibutions of divergent and rotational wind components to the synoptic-scale kinetic energy balance are described using rawinsonde data at 3 and 6 h intervals from NASA’s fourth Atmospheric Variability Experiment (AVE 4). Two intense thunderstorm complexes occurred during the period. Energy budgets am described for the entire computational region and for limited volumes that enclosed storm-induced, upper-level wind maxima touted poleward of the convection.
Although small in magnitude, the divergent wind component played an important role in the cross-contour generation and horizontal flux divergence of kinetic energy. The importance of V D appears directly related to the presence and intensity of convection. Although K D usually comprised lm than 10% of the total kinetic energy content generation of kinetic energy by V D was a major factor in the creation of upper-level wind maxima to the north of the storm complexes. Omission of the divergent wind apparently would lead to serious misrepresentations of the energy balance. A random error analysis is presented to assess confidence limits in the various energy parameters.
Meso β-scale rawinsonde data from the Atmospheric Variability Experiment-Severe Environmental Storms and Mesoscale Experiment (AVE-SESAME) V period (20–21 May 1979) are used to diagnose atmospheric variability in the environment of a convective area. As the storms developed, temperatures increased in the upper stratosphere; however, cooling was observed nearer to the surface and in the lower stratosphere. Height rises above 400 mb produced a mesohigh over the convective area that was most pronounced near 200 mb. Weaker height falls occurred in the lower troposphere.
Wind patterns underwent especially interesting fluctuations. North of the convective area, upper-level winds increased significantly during storm development. Southeast of the convection, however, winds near 200 mb decreased approximately 50% during a 3 h period coinciding with the most active storms. On the other hand, winds at 400 mb almost doubled during the same 3 h period. Strong low-level convergence, upper-level divergence, and ascending motion developed after storm initiation.
Much more detailed study is required to understand this fascinating case. However, many of the current findings about the meso β-scale storm environment are consistent with those previously attributed to feedback mechanisms from severe thunderstorms.
Meso β-scale rawinsonde data from the Atmospheric Variability Experiment-Severe Environmental Storms and Mesoscale Experiment (AVE-SESAME) V period (20–21 May 1979) are used to diagnose atmospheric variability in the environment of a convective area. As the storms developed, temperatures increased in the upper stratosphere; however, cooling was observed nearer to the surface and in the lower stratosphere. Height rises above 400 mb produced a mesohigh over the convective area that was most pronounced near 200 mb. Weaker height falls occurred in the lower troposphere.
Wind patterns underwent especially interesting fluctuations. North of the convective area, upper-level winds increased significantly during storm development. Southeast of the convection, however, winds near 200 mb decreased approximately 50% during a 3 h period coinciding with the most active storms. On the other hand, winds at 400 mb almost doubled during the same 3 h period. Strong low-level convergence, upper-level divergence, and ascending motion developed after storm initiation.
Much more detailed study is required to understand this fascinating case. However, many of the current findings about the meso β-scale storm environment are consistent with those previously attributed to feedback mechanisms from severe thunderstorms.
Abstract
The preconvective environment of summer thunderstorms over the Florida Panhandle is investigated. Geostationary satellite imagery as well as surface and radiosonde data were examined during the summers of 1990 and 1991. Days were classified either as synoptically disturbed or undisturbed based on the imagery. The undisturbed days then were subjectively subdivided into those having strong, weak, or no convection. Composite sounding profiles of various meteorological parameters were constructed for each category of the undisturbed days. Composites of various stability indexes also were calculated.
Midtropospheric moisture (particularly from 700 to 500 mb) and low-level instability were the best thermodynamic parameters for forecasting convection over the Florida Panhandle. The surface-based lifted index was the most useful stability index for predicting convective development. Wind direction also was related to the degree of convective activity in the Florida Panhandle. The strong convection days tended to have low-level winds from the south or southwest. Low-level winds on the driest days generally had northerly and easterly components.
Abstract
The preconvective environment of summer thunderstorms over the Florida Panhandle is investigated. Geostationary satellite imagery as well as surface and radiosonde data were examined during the summers of 1990 and 1991. Days were classified either as synoptically disturbed or undisturbed based on the imagery. The undisturbed days then were subjectively subdivided into those having strong, weak, or no convection. Composite sounding profiles of various meteorological parameters were constructed for each category of the undisturbed days. Composites of various stability indexes also were calculated.
Midtropospheric moisture (particularly from 700 to 500 mb) and low-level instability were the best thermodynamic parameters for forecasting convection over the Florida Panhandle. The surface-based lifted index was the most useful stability index for predicting convective development. Wind direction also was related to the degree of convective activity in the Florida Panhandle. The strong convection days tended to have low-level winds from the south or southwest. Low-level winds on the driest days generally had northerly and easterly components.
