Mark R. Hjelmfelt

Structure and Life Cycle of Microburst Outflows Observed in Colorado

Abstract Intense, small-scale divergent outflows known as microbursts are held responsible for a number of aircraft accidents. This paper describes the morphology of microburst outflows observed in Colorado. Outflows are categorized into morphological types based on analysis of observation by Doppler radars and a surface meteorological network. Outflow life cycle is discussed, and the vertical and horizontal structure is described. Basic characteristics of microburst outflows are summarized from statistics compiled using both single and multiple Doppler analyses. The microburst outflows are classified into two types: individual microbursts and microburst lines. Examples of observations of each type are shown. Organization of microbursts into microburst lines results in much longer-lasting wind shear than exists with isolated microbursts. The greater lifetime of microburst lines, combined with the much larger area of divergence, can create a much greater potential for hazard to aircraft than is the case fo...

Rolls, Streets, Waves, and More: A Review of Quasi-Two-Dimensional Structures in the Atmospheric Boundary Layer

Abstract The atmospheric boundary layer is home to a number of horizontally elongated quasi-two-dimensionalphenomena including cloud streets, roll vortices, thermal waves, and surface layer streaks. These phenomena, their dynamics, and their interactions are explored via a review of the literature. Making a clear distinction betweenthe various quasi-two-dimensional phenomena allows improved synthesis of previous results and a betterunderstanding of the interrelationships between phenomena.

Numerical Study of the Influence of Environmental Conditions on Lake-Effect Snowstorms over Lake Michigan

Abstract Numerical simulations are used to examine the influence of environmental parameters on the morphology of lake effect snowstorms over Lake Michigan. A series of model sensitivity studies are performed using the Colorado State University mesoscale model to examine the effects of lake–land temperature difference, surface roughness, atmospheric boundary layer stability, humidity, and wind speed and direction on the morphology of simulated storms. Four morphological types of lake effect snowstorms have been identified: (i) Broad area coverage, which may become organized into wind parallel bands or cellular convection; (ii) shoreline bands with a line of convection roughly parallel to the lee shore and a well developed land breeze on the lee shore; (iii) midlake band with low-level convergence centered over the lake; and (iv) mesoscale vortices with a well-developed cyclonic flow pattern in the boundary layer. The model is able to reproduce all four morphological types. Simulations varying environmenta...

Numerical Simulation of the Effects of St. Louis on Mesoscale Boundary-Layer Airflow and Vertical Air Motion: Simulations of Urban vs Non-Urban Effects

Abstract A three-dimensional mesoscale computer model is used to assess the importance of urban effects, relative to non-urban effects, on mesoscale boundary-layer vertical air motion and on the height of the boundary layer downwind of St. Louis, Missouri. Simulations are made for south, southwest, west and northwest winds, with urban land uses replaced by rural land uses, both with and without topography. Simulations including urban effects indicated mesoscale upward air motion downwind of the city for all wind directions, strongest for southwest winds and weakest for northwest winds. With urban effects excluded, much weaker upward motion was found downwind for south, southwest and west winds, and downward vertical velocities occurred in the downwind areas for northwest winds. The results of this study imply that mesoscale boundary-layer upward air motion occurs downwind of St. Louis, primarily as a result of urban effects. Local geographic influences may tend to enhance or suppress this upward air motio...

Numerical Simulation of the Airflow over Lake Michigan for a Major Lake-Effect Snow Event

Abstract A mesoscale model is used to simulate the airflow over Lake Michigan for the major lake-effect snowstorm of 10 December 1977. This storm was characterized by a land breeze circulation and a narrow shore-parallel radar reflectivity band. The model successfully simulated the major atmospheric circulation features including a mesoscale low pressure center and a land breeze front. The model also captured the general character of the observed precipitation pattern which was typified by a narrow band of heavy precipitation along the eastern shore of Lake Michigan. Further simulations were made to examine the effects of latent heat release, lake surface temperature distribution and model grid resolution upon the simulation. Latent heat release was found to have an important effect in strengthening convection. However, the basic land-breeze circulation was found to develop for the simulated conditions even without latent beating. For a given mean lake-land temperature difference, details of the lake surf...

The Lake—Induced Convection Experiment and the Snowband Dynamics Project

Abstract A severe 5—day lake—effect storm resulted in eight deaths, hundreds of injuries, and over

An Observational Examination of Long-Lived Supercells. Part II: Environmental Conditions and Forecasting

3 million in damage to a small area of northeastern Ohio and northwestern Pennsylvania in November 1996. In 1999, a blizzard associated with an intense cyclone disabled Chicago and much of the U.S. Midwest with 30—90 cm of snow. Such winter weather conditions have many impacts on the lives and property of people throughout much of North America. Each of these events is the culmination of a complex interaction between synoptic—scale, mesoscale, and microscale processes. An understanding of how the multiple size scales and timescales interact is critical to improving forecasting of these severe winter weather events. The Lake—Induced Convection Experiment (Lake—ICE) and the Snowband Dynamics Project (SNOWBAND) collected comprehensive datasets on processes involved in lake—effect snowstorms and snowbands associated with cyclones during the winter of 1997/98. This paper outlines the goals and operations of thes...

An Observational Examination of Long-Lived Supercells. Part I: Characteristics, Evolution, and Demise

Abstract The local and larger-scale environments of 184 long-lived supercell events (containing one or more supercells with lifetimes ≥4 h; see Part I of this paper) are investigated and subsequently compared with those from 137 moderate-lived events (average supercell lifetime 2–4 h) and 119 short-lived events (average supercell lifetime ≤2 h) to better anticipate supercell longevity in the operational setting. Consistent with many previous studies, long-lived supercells occur in environments with much stronger 0–8-km bulk wind shear than what is observed for short-lived supercells; this strong shear leads to significant storm-relative winds in the mid- to upper levels for the longest-lived supercells. Additionally, the bulk Richardson number falls into a relatively narrow range for the longest-lived supercells—ranging mostly from 5 to 45. The mesoscale to synoptic-scale environment can also predispose a supercell to be long or short lived, somewhat independent of the local environment. For example, long...

Transitions in Boundary Layer Meso-γ Convective Structures:An Observational Case Study

Observations of supercells and their longevity across the central and eastern United States are examined, with the primary focus on understanding the properties of long-lived supercells (defined as supercells lasting 4 h). A total of 224 long-lived supercells, occurring in 184 separate events, are investigated. These properties are compared with those of short-lived supercells (lifetimes 2 h) to determine the salient differences between the two classifications. A key finding is that long-lived supercells are considerably more isolated and discrete than short-lived supercells; as a result, the demise of a long-lived supercell (i.e., the end of the supercell phase) is often signaled by a weakening of the storm’s circulation and/or a rapid dissipation of the thunderstorm. In contrast, short-lived supercells commonly experience a demise linked to storm mergers and convective transitions (e.g., evolution to a bow echo). Also noteworthy, 36% of the long-lived supercell events were associated with strong or violent tornadoes (F2–F5), compared with only 8% for the short-lived supercell events. Evolutionary characteristics of long-lived supercells vary geographically across the United States, with the largest contrasts between the north-central United States and the Southeast. For example, 86% of the long-lived supercells across the north-central United States were isolated for most of their lifetime, whereas only 35% of those in the Southeast displayed this characteristic. Not surprisingly, the convective mode was discrete for 70% of the long-lived supercell events across the north-central United States, compared with 39% for the Southeast.

A Numerical Case Study of Convection Initiation along Colliding Convergence Boundaries in Northeast Colorado

Boundary layer rolls over Lake Michigan have been observed in wintertime conditions predicted by many past studies to favor nonroll convective structures (such as disorganized convection or cellular convection). This study examines mechanisms that gave rise to transitions between boundary layer rolls and more cellular convective structures observed during a lake-effect snow event over Lake Michigan on 17 December 1983. The purposes of this study are to better understand roll formation in marine boundary layers strongly heated from below and examine the evolution of snowfall rate and mass overturning rate within the boundary layer during periods of convective transition. A method of quantifying the uniformity of convection along the roll axes, based on dual-Doppler radar-derived vertical motions, was developed to quantify changes in boundary layer convective structure. Roll formation was found to occur after (within 1 h) increases in low-level wind speeds and speed shear primarily below about 0.3zi, with little change in directional shear within the convective boundary layer. Roll convective patterns appeared to initiate upstream of the sample region, rather than form locally near the downwind shore of Lake Michigan. These findings suggest that either rolls developed over the upwind half of Lake Michigan or that the convection had a delayed response to changes in the atmospheric surface and wind forcing. Mass overturning rates at midlevels in the boundary layer peaked when rolls were dominant and gradually decreased when cellular convection became more prevalent. Radar-estimated aerial-mean snowfall rates showed little relationship with changes in convective structure. However, when rolls were dominant, the heaviest snow was more concentrated in updraft regions than during more cellular time periods.