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Michael C. Coniglio

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This study uses radiosonde observations obtained during the second phase of the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) to verify base-state variables and severe-weather-related parameters calculated from Rapid Update Cycle (RUC) analyses and 1-h forecasts, as well as those calculated from the operational surface objective analysis system used at the Storm Prediction Center (the SFCOA). The rapid growth in temperature, humidity, and wind errors from 0 to 1 h seen at all levels in a past RUC verification study by Benjamin et al. is not seen in the present study. This could be because the verification observations are also assimilated into the RUC in the Benjamin et al. study, whereas the verification observations in the present study are not. In the upper troposphere, the present study shows large errors in relative humidity, mostly related to a large moist bias. The planetary boundary layer tends to be too shallow in the RUC analyses and 1-h forecasts. Wind speeds tend to be too fast in the lowest 1 km and too slow in the 2–4-km layer. RUC and SFCOA 1-h forecast errors for many important severe weather parameters are large relative to their potential impact on convective evolution. However, the SFCOA significantly improves upon the biases seen in most of the 1-h RUC forecasts for the base-state surface variables and most of the other severe-weather-related parameters, indicating that the SFCOA has a more significant impact in reducing the biases in the 1-h RUC forecasts than on the root-mean-squared errors.

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Michael C. Coniglio and David J. Stensrud

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This study focuses on the progressive derecho, a widespread, convectively induced windstorm produced by a mesoscale convective system that often occurs within a relatively benign synoptic-scale environment. Sounding data from 12 progressive derechos, which occurred in weakly forced large-scale environments, are composited in order to examine important large-scale features in the preconvective environment. This analysis captures many features that are common in warm season derecho environments, such as an upper-level wind maximum, a relatively dry midtroposphere, and low-level warm advection. Initial and boundary conditions for the Pennsylvania State University–National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5) are created using this analysis. A three-dimensional, horizontally nonhomogeneous, explicitly resolved simulation of a progressive derecho is produced and compared to previous, more idealized simulations of similar convective systems that have been used to explain the strength and structure of observed long-lived squall lines and bow echoes.

A subset of previous squall line simulations produced within horizontally homogeneous environments without wind shear above 5 km suggests that a balance between the positive vorticity associated with the environmental low-level shear (Δu) and the negative vorticity created baroclinically at the leading edge of the cold pool (C) is the essential ingredient that determines the strength and time-dependent structure of long-lived squall lines (local balance theory). In the simulation presented here, which occurs in an environment with deep-tropospheric shear but relatively weak low-level shear, the model develops a realistic, rapidly moving squall line with embedded bow echoes that maintains its strength for much longer than the squall lines within previous idealized simulations that develop and evolve within similar less than optimal balance conditions (Cu > 2). Previous simulations of squall lines under similar less than optimal conditions contain updrafts that progressively weaken and become more upshear tilted with time as the cold pool surges ahead of the updrafts within 1–3 h after the system develops. However, the simulated squall line used here contains convective updrafts that remain almost directly above the gust front, maintains a nearly constant upshear tilt for several hours, and produces severe, near-surface winds for over 8 h. Examination of the maximum grid-resolved vertical velocity indicates that the cells are not weakening with time relative to their thermodynamic potential, which contrasts the behavior of the cells within the less than optimal squall lines of the previous, idealized simulations.

These results support the idea that local balance theory, which attempts to explain both the strength and longevity of squall lines, may be incomplete within environments that often favor warm season progressive derechos. In particular, tests with a simple two-dimensional cloud-scale model indicate that both significant upper-tropospheric shear above 5 km (which is found in the composite analysis and in the MM5 solution) and low-level shear play significant roles in maintaining the strength of squall lines over long periods and need to be considered in order to fully understand and forecast these events.

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Diego A. Alfaro and Michael C. Coniglio

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The environmental factors that drive the dissipation of linear severe-wind-producing mesoscale convective systems (MCSs) are investigated. Layer-lifting indices are emphasized, which measure convective instability in forward-propagating MCSs by considering that deep convective latent heating depends on 1) the potential latent heating within the atmospheric column, measured by the integrated CAPE (ICAPE), and 2) the dilution of buoyancy due to midtropospheric inflow, measured by the inflow fraction (IF) of convectively unstable air to total system-relative inflow. These elements are integrated to define the layer-lifting CAPE (CAPEll), which depends on environmental thermodynamics, kinematics, and the MCS’s movement vector. Radar reflectivity plots are used to subjectively identify and classify MCSs in terms of their stage (mature or dissipating) and degree of organization (highly or weakly organized). Nonparametric statistical inferences are performed on several metrics computed at maturity and dissipation from RUC/RAP analysis data, aiming to identify the most skillful indices for diagnosing three different aspects of MCS dissipation: 1) the transition from maturity to dissipation, 2) the stage of an MCS, and 3) the disorganization that characterizes the dissipating stage. In terms of MCS dissipation CAPEll is the best diagnostic. A close approximation to CAPEll is accomplished by estimating an MCS’s movement with Corfidi vectors, providing a potentially useful index in operational settings. ICAPE is the most skillful thermodynamic metric, while IF is the best kinematic discriminator of MCS stage and stage transition, suggesting the fundamental importance of layer-lifting convective instability for MCS maintenance. Layer-lifting indices are not particularly skillful at distinguishing the degree of MCS organization at maturity, which is best diagnosed by deep vertical wind shear.

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Matthew D. Flournoy and Michael C. Coniglio

Abstract

To better understand and forecast nocturnal thunderstorms and their hazards, an expansive network of fixed and mobile observing systems was deployed in the summer of 2015 for the Plains Elevated Convection at Night (PECAN) field experiment to observe low-level jets, convection initiation, bores, and mesoscale convective systems. On 5–6 July 2015, mobile radars and ground-based surface and upper-air profiling systems sampled a nocturnal, quasi-linear convective system (QLCS) over South Dakota. The QLCS produced several severe wind reports and an EF-0 tornado. The QLCS and its environment leading up to the mesovortex that produced this tornado were well observed by the PECAN observing network. In this study, observations from radiosondes, Doppler radars, and aircraft are assimilated into an ensemble analysis and forecasting system to analyze this event with a focus on the development of the observed tornadic mesovortex. All ensemble members simulated low-level mesovortices with one member in particular generating two mesovortices in a manner very similar to that observed. Forecasts from this member were analyzed to examine the processes increasing vertical vorticity during the development of the tornadic mesovortex. Cyclonic vertical vorticity was traced to three separate airstreams: the first from southerly inflow that was characterized by tilting of predominantly crosswise horizontal vorticity along the gust front, the second from the north that imported streamwise horizontal vorticity directly into the low-level updraft, and the third from a localized downdraft/rear-inflow jet in which the horizontal vorticity became streamwise during descent. The cyclonic vertical vorticity then intensified rapidly through intense stretching as the parcels entered the low-level updraft of the developing mesovortex.

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Michael C. Coniglio and Matthew D. Parker

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Hundreds of supercell proximity soundings obtained for field programs over the central United States are analyzed to reconcile differences in recent studies and to refine our knowledge of supercell environments. The large, storm-centric observation-based dataset and high vertical resolution of the sounding data provide an unprecedented look at supercell environments. Not surprisingly, storm-relative environmental helicity (SRH) is found to be larger in tornadic soundings than in nontornadic soundings. The primary finding that departs from previous studies is that storm-relative winds contribute substantially to the larger SRH. Stronger ground-relative winds and more rightward-deviant storm motions contribute to the larger storm-relative winds for the tornadic soundings. Spatial analyses of the soundings reveal lower near-ground pressure perturbations and stronger low- to midlevel cyclonic flow for the tornadic soundings, which suggests stronger mesocyclones, perhaps explaining the more rightward-deviant motions. Differences in the mean critical angle between the tornadic and nontornadic soundings are small and do not contribute to the larger mean SRH, but the tornadic soundings do have fewer instances of smaller (<60°) critical angles. Furthermore, the critical angle is shown to be a function of azimuth from the updraft. Other results include a low-to-the-ground (~250 m on average) hodograph kink for both the tornadic and nontornadic soundings and few notable differences in thermodynamic quantities, except for the expected lower LCLs related to higher RH for the tornadic soundings, somewhat smaller 0–3 km lapse rates in tornadic environments related to weaker/shallower capping inversions, and larger 0–3 km CAPE in near-field environments.

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Michael C. Coniglio and David J. Stensrud

Abstract

Past studies have examined the climatology of derechos and suggest very different distributions of derechos within the United States. This uncertainty in the climatology of derechos is a concern for forecasters, since knowledge of the relevant climatological information is a key piece in the forecast process. A 16-yr dataset from 1986 to 2001 is used to examine the effects that changing the method of identifying derechos may have on the interpretation of the derecho climatology. In addition, an attempt is made to visualize the favored regions of particularly intense derecho events.

The results show aspects seen in earlier climatologies, including a southern axis in the southern plains that is favored in the mid-1980s and early 1990s and a northern axis centered from the upper Mississippi River valley into Ohio that is favored in more recent years. However, altering the criteria to not require three 33 m s−1 gust reports or F1-type damage (low-end events) significantly increases the number of events that are identified in the lower Appalachians, the Ohio valley, and in portions of the southern axis, particularly in the earlier period. To a lesser extent, the inclusion of low-end events also increases the frequency values in the northern axis in the later period. The overall effect of including the low-end events is to create a distribution that still suggests both a southern and northern axis, and a shift of the primary axis from the southern plains in the early period to the upper Mississippi valley in the later period. However, the frequency values of the maxima are noticeably reduced when the low-end events are excluded. Therefore, both the length of the dataset and the criteria used to define derechos can significantly influence the resulting climatology.

High-end derechos, which require three wind gust reports (or comparable damage) exceeding 38 m s−1, appear to be favored in the northern corridor during the warm season, particularly in the later period, and are favored along the lower Mississippi River valley during the colder months in both periods.

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Michael A. VandenBerg, Michael C. Coniglio, and Adam J. Clark

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This study compares next-day forecasts of storm motion from convection-allowing models with 1- and 4-km grid spacing. A tracking algorithm is used to determine the motion of discrete storms in both the model forecasts and an analysis of radar observations. The distributions of both the raw storm motions and the deviations of these motions from the environmental flow are examined to determine the overall biases of the 1- and 4-km forecasts and how they compare to the observed storm motions. The mean storm speeds for the 1-km forecasts are significantly closer to the observed mean than those for the 4-km forecasts when viewed relative to the environmental flow/shear, but mostly for the shorter-lived storms. For storm directions, the 1-km forecast storms move similarly to the 4-km forecast storms on average. However, for the raw storm motions and those relative to the 0–6-km shear, results suggest that the 1-km forecasts may alleviate some of a clockwise (rightward) bias of the 4-km forecasts, particularly for those that do not deviate strongly from the 0–6-km shear vector. This improvement in a clockwise bias also is seen for the longer-lived storms, but is not seen when viewing the storm motions relative to the 850–300-hPa mean wind or Bunkers motion vector. These results suggest that a reduction from 4- to 1-km grid spacing can potentially improve forecasts of storm motion, but further analysis of closer storm analogs are needed to confirm these results and to explore specific hypotheses for their differences.

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Michael C. Coniglio, David J. Stensrud, and Michael B. Richman

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This study identifies the common large-scale environments associated with the development of derecho- producing convective systems (DCSs) from a large number of events. Patterns are identified using statistical clustering of the 500-mb geopotential heights as guidance. The majority of the events (72%) fall into three main patterns that include a well-defined upstream trough (40%), a ridge (20%), and a zonal, low-amplitude flow (12%), which is identified as an additional warm-season pattern. Consequently, the environmental large-scale patterns idealized in past studies only depict a portion of the full spectrum of the possibilities associated with the development of DCSs.

In addition, statistics of derecho proximity-sounding parameters are presented relative to the derecho life cycle as well as relative to the forcing for upward motion. It is found that the environments ahead of maturing derechos tend to moisten at low levels while remaining relatively dry aloft. In addition, derechos tend to decay as they move into environments with less instability and smaller deep-layer shear. Low-level shear (instability) is found to be significantly higher (lower) for the more strongly forced events, while the low-level storm-relative inflow tends to be much deeper for the more weakly forced events. Furthermore, discrepancies are found in both low- level and deep-tropospheric shear parameters between observations and the shear profiles considered favorable for strong, long-lived convective systems in idealized simulations. This study highlights the need to examine DCS simulations within more realistic environments to help reconcile these disparities in observations and idealized models and to provide improved information to forecasters.

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Nicholas A. Engerer, David J. Stensrud, and Michael C. Coniglio

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Cold pools are a key element in the organization of precipitating convective systems, yet knowledge of their typical surface characteristics is largely anecdotal. To help to alleviate this situation, cold pools from 39 mesoscale convective system (MCS) events are sampled using Oklahoma Mesonet surface observations. In total, 1389 time series of surface observations are used to determine typical rises in surface pressure and decreases in temperature, potential temperature, and equivalent potential temperature associated with the cold pool, and the maximum wind speeds in the cold pool. The data are separated into one of four convective system life cycle stages: first storms, MCS initiation, mature MCS, and MCS dissipation. Results indicate that the mean surface pressure rises associated with cold pools increase from 3.2 hPa for the first storms’ life cycle stage to 4.5 hPa for the mature MCS stage before dropping to 3.3 hPa for the dissipation stage. In contrast, the mean temperature (potential temperature) deficits associated with cold pools decrease from 9.5 (9.8) to 5.4 K (5.6 K) from the first storms to the dissipation stage, with a decrease of approximately 1 K associated with each advance in the life cycle stage. However, the daytime and early evening observations show mean temperature deficits over 11 K. A comparison of these observed cold pool characteristics with results from idealized numerical simulations of MCSs suggests that observed cold pools likely are stronger than those found in model simulations, particularly when ice processes are neglected in the microphysics parameterization. The mean deficits in equivalent potential temperature also decrease with the MCS life cycle stage, starting at 21.6 K for first storms and dropping to 13.9 K for dissipation. Mean wind gusts are above 15 m s−1 for all life cycle stages. These results should help numerical modelers to determine whether the cold pools in high-resolution models are in reasonable agreement with the observed characteristics found herein. Thunderstorm simulations and forecasts with thin model layers near the surface are also needed to obtain better representations of cold pool surface characteristics that can be compared with observations.

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Michael C. Coniglio, Stephen F. Corfidi, and John S. Kain

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This work presents an analysis of the vertical wind shear during the early stages of the remarkable 8 May 2009 central U.S. derecho-producing convective system. Comments on applying Rotunno–Klemp–Weisman (RKW) theory to mesoscale convective systems (MCSs) of this type also are provided. During the formative stages of the MCS, the near-surface-based shear vectors ahead of the leading convective line varied with time, location, and depth, but the line-normal component of the shear in any layer below 3 km ahead of where the strong bow echo developed was relatively small (6–9 m s−1). Concurrently, the midlevel (3–6 km) line-normal shear component had magnitudes mostly >10 m s−1 throughout.

In a previous companion paper, it was hypothesized that an unusually strong and expansive low-level jet led to dramatic changes in instability, shear, and forced ascent over mesoscale areas. These mesoscale effects may have overwhelmed the interactions between the cold pool and low-level shear that modulate system structure in less complex environments. If cold pool–shear interactions were critical to producing such a strong system, then the extension of the line-normal shear above 3 km also appeared to be critical. It is suggested that RKW theory be applied with much caution, and that examining the shear above 3 km is important, if one wishes to explain the formation and maintenance of intense long-lived convective systems, particularly complex nocturnal systems like the one that occurred on 8 May 2009.

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