Severe Convection Research Articles

Evidence for the Role of the Lid and Underunning in an Outbreak of Tornadic Thunderstorms

Abstract On the last day of May 1985, a series of tornadic thunderstorms devastated pans of Ohio. Pennsylvania, New York, and Ontario, Canada. Analysis of this outbreak reveals that a lid, which moved from the southwestern United States the previous day, played a major role in enabling the latent instability to achieve large values. Severe convection began along the edge of the lid (underrunning) in a region experiencing baroclinically forced ascent associated with a migrating jet streak. This paper discusses the role of the lid in the development of the latent instability, the relationship between underrunning and the migrating jet streak, and the effects of terrain on the evolution of latent instability and on the formation of the lid. Finally, we present a climatological study of the lid in order to show why the convection was so severe, yet so unusual in this part of the country.

Omega Diagnosis of a Cold Vortex with Severe Convection

Abstract A real-time quasi-geostrophic omega diagnostic scheme, based on Hoskins' Q-vector analysis and developed by Barnes, was applied to a cold mesoscale vortex with severe convection over northeast China in summer. The limited area model used at the Beijing Weather Center did not predict this event because the baroclinic forcing was rather weak, but the Q-vector analysis clearly indicated the forcing 12 h before. In addition to Barnes' diagnostics, we estimate divergence tendency in low levels through computation of the rotational component of the Q-vector. Combined with the diagnosed stability tendency, moisture analysis, and low-level wind convergence zone, the convective area can be identified. This microcomputer diagnostic-graphics scheme, when coupled with intelligent use of conventional data, has potential as an aid for local short-range weather forecasting.

A Statistical Evaluation of Lid Strength on Deep Convection

A statistical analysis of the effects of lower-tropospheric inversions (called lids) on deep convection was made using a “logistic” model and reports of deep convection for a six month period over the central two-thirds of the United States during 1982. Observations consist of radar echo intensity, radar echo height and direct reports of severe storm sightings or damage. We find that both the lid strength and buoyancy (lifted index) effects need to be considered jointly. Lid strength appears to exhibit an effective cutoff value, above which deep convection becomes relatively unlikely, even for unstable values of the buoyancy term. On the other hand, deep convection is unlikely for stable buoyancy values. Further, for a given value of buoyancy the probability of severe convection increases with increasing lid strength, although the total probability of deep convection diminished with increasing lid strength. This result seems to support the idea that the lid allows high values of θw to form near the surface and that a favorable location for the release of the latent instability is along the lateral boundary of the lid. The primary value in examining both buoyancy and lid strength separately is that the method helps to eliminate large areas from consideration by the forecaster where the air near the surface is latently unstable but unlikely to produce deep convection due to the presence of a lid.

The Distinction between Large-Scale and Mesoscale Contribution to Severe Convection: A Case Study Example

Abstract Using a case study of a relatively modest severe weather event as an example, a framework for understanding the large-scale-mesoscale interaction is developed and discussed. Large-scale processes are limited, by definition, to those which are quasi-geostrophic. Mesoscale processes are defined to be those which are linked in essence to processes occurring on both larger and smaller scales. It is proposed that convective systems depend primarily on large-scale processes for developing a suitable thermodynamic structure, while mesoscale processes act mainly to initiate convection. The case study is presented not as a “typical” event in its particulars, but rather to suggest the complex ways in which large-scale and mesoscale processes can interact. Implications for forecasting are an essential part of the discussion, since mesoscale systems are so difficult to predict with the present knowledge and technology available in operations.

A Real-Data Numerical Study of the Development of Precipitation along the Edge of an Elevated Mixed Layer

Abstract An 18-h numerical simulation of the weather associated with the severe-storm outbreak in the region of the Texas-Oklahoma panhandles, during the AVE-SESAME IV study period (9–10 May 1979), was performed using the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) mesoscale model. This simulation and the related sensitivity tests provided the four-dimensional data sets that were used to reach a better understanding of the processes that were involved in this case in the development of severe convection along the edge of the elevated mixed layer (EML). The sensitivity studies were performed to isolate the contributions of differential surface forcing, latent heating and the low-level moisture gradient to the development of the underrunning, its intensification, and the heavy rainfall. These studies showed that the differential surface heating at the edge of the EML is the most important single factor responsible for initiating the underrunning, and therefore the preci...

Temperatures of Air Parcels Lifted from the Surface: Background, Application and Nomograms

When the temperature of a parcel lifted adiabatically from the surface to a given pressure level (SLT) is subtracted from the ambient temperature at that level, the result is the surface index (SLI) for that level. This quantity, for the 500-mb pressure level, has been found useful in the very-short-range prediction of deep and often severe convection. We examine the thermodynamics of the lifting process, finding that the SLTs are sensitive mainly to surface dewpoint for very warm humid air and to surface temperature for cold air. Nomograms are presented for determination of SLT's for 850, 700 and 500 mb, given surface temperature and dewpoint and a nominal surface pressure of 950 mb. An approximate correction is given for departures from this pressure. These are intended for use with the routine aviation surface observations. An additional nomogram is provided for use over the sea. An example of application is shown, based on hourly surface observations and on ambient temperatures interpolated between standard rawinsonde analysis times. Severe convection is observed where large parcel buoyancy is indicated by SLI at 500 mb, with no negative buoyancy at 700 or 850 mb. Apparently unreliable measurements of dewpoint in warm humid air constitute a significant analysis problem, and reliable large-scale predictions of ambient temperature are a requirement for optimum real-time use.

Flow Dynamics and Stability in a Severe Rainband

Abstract Observations of a severe form of rainband, which is comprised of a line of strong but shallow convection, suggest that the environment into which it moves is nearly neutrally stratified with respect to moist convection. A simple two-dimensional model of a severe rainband has been developed to explore how the cold air at the base of the rainband modifies local stability as it underruns the saturated surface layer. This dynamic lifting is found to have two distinct effects. First, the lifted fluid contains regions of absolute and conditional instability. Then it also reorganizes the midtroposphere into a sequence of elevated inversions set between unstable layers. This latter effect results from the intense “lee wave” response in the air flow. The elevated inversions would appear to cap the severe moist convection, establishing the rainband character rather than allowing the deep cellular convection of squall lines. The flow disturbances end abruptly at the steering level (a critical level for the ...

Numerical Simulations of a Transverse Indirect Circulation and Low-Level Jet in the Exit Region of an Upper-Level Jet

Uccellini and Johnson present a case study of a severe weather event in Ohio on 10–11 May 1973 to show evidence for coupling between an upper-tropospheric jet streak and a low-level jet within an indirect transverse circulation found in the exit region of the upper-level jet. The differential advection of moisture and temperature created by the shear between the upper- and low-level jets reduced convective stability, thereby enhancing the potential for severe convection. Two 12 h numerical simulations of the 10–11 May 1973 case are studied to determine 1) if a transverse indirect circulation with a low-level jet imbedded in its lower branch can be diagnosed in the exit region of the upper-level jet and studied using the model output at 3 h intervals and 2) if the initial magnitude and structure of the upper-level jet have a significant effect on the subsequent development of the low-level jet and the decrease in convectivc stability due to differential advection. In an adiabatic model simulation, an indirect transverse circulation having a low-level jet within its lower branch occurs in the exit region of the upper-level jet. The simulated vertical distribution of mass divergence and ageostrophic flow in the exit region agree with the diagnoses of Uccellini and Johnson. At upper levels, mass divergence (convergence) occurs on the cyclonic (anticyclonic) side of the exit region, while the opposite occurs at low levels. The, upper branch of the indirect circulation is dominated by the inertial–advective contribution to the ageostrophic wind which is related to the alongstream isotach gradient in the exit region. The lower branch is dominated by the wind tendency contribution to the ageostrophic wind. Ageostrophic shear associated with this circulation contributes to the development of differential moisture and temperature advection, which act to destabilize the preconvective environment. A second simulation using a smoothed, nondivergent initialization with a weaker upper-level jet streak and weaker alongside isotach gradient in the exit region of the upper-level jet produces a weaker indirect transverse circulation even though diabatic heating effects are present. The indirect circulation for this simulation is marked by smaller vertical motions, a weaker low-level return branch, and weaker low-level thermal and moisture advection associated with the low-level flow. Comparison of the two simulations suggests that the indirect circulation in the exit region of the upper-level jet is strongly responsive to dynamical processes associated with the initial structure of the jet streak.

Effects of Diabatic Heating on the Ageostrophic Circulation of an Upper Tropospheric Jet Streak

The interaction between the mass circulations within a mesoscale convective complex (MCC) and the entrance region of an upper tropospheric polar jet streak is examined to investigate mechanisms responsible for linking these two scales of motion. During NASA's fourth Atmospheric Variability Experiment (AVE IV), maximum wind speeds within a jet streak increased nearly 15 m s−1 over three to six hours as the jet streak propagated eastward over the Great Lakes region. Severe convection located on the rear anticyclonic flank of the jet streak within the direct circulation of the entrance region also intensified and increased in areal extent. The results analyzed within isentropic coordinates establish that latent heating in the MCC modified the direct mass circulation in the jet streak entrance region through the forcing of diabatic components of ageostrophic motion. The net isallobaric ageostrophic component in the entrance region, determined through the gradients of differential heating and mass flux, exceeded 4 m s−1. The mass divergence in the upper troposphere was due to the slight excess of the diabatic isallobaric mode over the opposing adiabatic mode, while mass convergence in the lower troposphere was due to the slight excess of the adiabatic isallobaric mode over the diabatic mode. The intensity of the other diabatically forced ageostrophic component, induced through vertical advection of momentum in a sheared environment, ranged from 5 to 10 m s−1 in the middle and upper troposphere of the jet's entrance region. Over much of the convective region, both the total isallobaric and the inertial diabatic ageostrophic components were directed from the anticyclonic to the cyclonic side of the jet streak at jet streak level in the same sense as pre-existing ageostrophic motion in the upper branch of the jet's direct mass circulation. This diabatically forced ageostrophic motion directed along the pressure gradient of the larger scale resulted in additional generation of kinetic energy which ultimately produced stronger winds in the jet streak downstream. A comparison between actual and geostrophic momentum forms for ageostrophic motion revealed discrepancies of 20 m s−1 that were mainly due to differences in the horizontal fields of inertial advective ageostrophic motion. This expected result points out that the rapid evolution of ageostrophic motion within the shorter time scales of MCCs limits the applicability of geostrophic momentum theory in prescribing the structure of ageostrophic motion.

A Subsynoptic Analysis of the Denver Tornadoes of 3 June 1981

On the afternoon of 3 June 1981 a severe thunderstorm spawned two tornadoes which moved across a portion of metropolitan Denver. The tornadoes were classified as strong F2 intensity, and caused damage totaling over $1 5 million. The synoptic-scale setting for this event was similar to that associated with many other occurrences of severe convection in eastern Colorado, with post-frontal moist southeasterly upslope flow at low levels and southwesterly flow aloft in advance of an approaching trough. We chose to study this event in part because of its occurrence within the PROFS (Program for Regional Observing and Forecasting Services) surface mesonetwork. Emphasis is placed on mesoscale evolution culminating in the formation of the tornadic storm. A zone of surface convergence and cyclonic vorticity developed during the early daylight hours over and north of Denver between southeasterly flow over the plains and a region of lighter, generally northerly flow just east of the foothills. The tornadic storm formed from a complex interaction of older thunderstorm cells, and subsequently intensified at the southern end of the convergence-vorticity zone and moved to the north-northeast. Other severe convection also occurred along this zone later in the afternoon. The development of this convergence-vorticity zone under conditions of ambient southeasterly flow appears to be topographically forced by a ridge of higher terrain which extends eastward from south of Denver, but the specific processes involved are unclear. The zone occurs frequently and, for the years 1981 and 1982 for which the PROFS mesonet data were available, was associated with a disproportionate number of severe weather events, especially tornadoes.

An Estimation of the Condensation Rates in Three Severe Storm Systems from Satellite Observations of the Convective Mass Flux

A technique was developed for estimating the condensation rates of convective storms using satellite measurements of cirrus anvil expansion rates and radiosonde measurements of environmental water vapor. Three cases of severe convection in Oklahoma were studied and a diagnostic model was developed for integrating radiosonde data with satellite data. Two methods were used to measure the anvil expansion rates - the expansion of isotherm contours on infrared images, and the divergent motions of small brightness anomalies tracked on the visible images. The differences between the two methods were large as the storms developed, but these differences became small in the latter stage of all three storms. A comparison between the three storms indicated that the available moisture in the lowest levels greatly affected the rain rates of the storms. This was evident from both the measured rain rates of the storms and the condensation rates estimated by the model. The possibility of using this diagnostic model for estimating the intensities of convective storms also is discussed.

Mesoscale Conditions for the Severe Convection of 3 April 1974 in the East-Central United States

Hourly surface observations from more than 200 stations, together with routine radar and rawinsonde data, are used to elucidate the mesoscale conditions for the extraordinarily widespread severe convection in the central United States on 3 April 1974. Strong mesoscale pressure fluctuations on an hourly time scale, particularly intense north of the Ohio River, were superposed on a substantial synoptic-scale pressure fall associated with the northeastward advance of a deep cyclone toward the Great Lakes. Most of this mesoscale variation was associated with 10 wayepackets of considerable spatial and temporal continuity. The character of the accompanying wind and weather fluctuations was highly suggestive of gravity-wave activity. The packets were aligned approximately along the low-level wind flow and propagated to its right at speeds of ∼30 kt. Three major regions of organized convection were present, also oriented approximately parallel to the low-level flow and separated by ∼150 n mi. These regions propagated to the right of this flow at speeds of ∼20 kt and appeared to arise from a source other than the presumed gravity waves. Rather, the convection appeared to initiate the waves, in at least some instances. More generally, the overtaking of a slow-moving convective region by a rapidly moving wave caused an increase in intensity of convection, a reformation of a line of radar echoes within the region, and an increase in the pressure amplitude of the wave.

Wave Generation and Frontal Collapse

Abstract Mid-latitude squall lines are most often observed in the warm sector of a developing or incipient baroclinic wave somewhat in advance of, and parallel to, the surface cold front. An important, yet poorly understood problem in mesoscale dynamics concerns the manner in which the deep convection in such systems is organized. Here we test the idea that the process of frontogenesis may lead naturally to the generation of a packet of internal gravity waves whose group velocity is normal to the frontal surface. We hypothesize that this wave packet may in turn be responsible for providing the convergence necessary to initiate severe convection along its path of propagation. For the most part, the idea is developed analytically within the context of a highly simplified atmospheric model. We employ the non-linear, cross-front geostrophy solutions of Hoskins and Bretherton (1972) to determine the temporal evolution of the unbalanced ageostrophic flow in the region of high baroclinicity. These unbalance term...

Influence of Low-Level Environment on Severity of High-Plains Moist Convection

Abstract Early afternoon environmental conditions preceding hail-producing thunderstorms are statistically compared with conditions for classes of less severe moist convection using only data from individual radiosonde releases collected during the National Hail Research Experiment in northeast Colorado. The ensuing analyses emphasize the thermodynamic characteristics of the mixed layer and immediate overlying free flow. On days with hail-producing thunderstorms, the mixed layer tends to be particularly thin and moist. Energy required to initiate moist convection is found to be somewhat greater than normal, while energy required to further develop moist convection is substantially less than normal. Parcel energies are found to be quite sensitive to the level of parcel origin in the mixed layer.