Abstract

Daily extreme precipitation events, exceeding a threshold for a 1-in-5-yr occurrence, were identified from a network of 935 Cooperative Observer stations for the period of 1908–2009. Each event was assigned a meteorological cause, categorized as extratropical cyclone near a front (FRT), extratropical cyclone near center of low (ETC), tropical cyclone (TC), mesoscale convective system (MCS), air mass (isolated) convection (AMC), North American monsoon (NAM), and upslope flow (USF). The percentage of events ascribed to each cause were 54% for FRT, 24% for ETC, 13% for TC, 5% for MCS, 3% for NAM, 1% for AMC, and 0.1% for USF. On a national scale, there are upward trends in events associated with fronts and tropical cyclones, but no trends for other meteorological causes. On a regional scale, statistically significant upward trends in the frontal category are found in five of the nine regions. For ETCs, there are statistically significant upward trends in the Northeast and east north central. For the NAM category, the trend in the West is upward. The central region has seen an upward trend in events caused by TCs.

Abstract

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 0.3z _i , 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.

Abstract

Numerical simulations are used to study transitions between boundary layer rolls and more cellular convective structures observed during a lake-effect snow event over Lake Michigan on 17 December 1983. Weak lake-effect nonroll convection was observed near the eastern (downwind) shore preceding passage of a secondary cold front. After frontal passage horizontal wind speeds in the convective boundary layer increased, with subsequent development of linear convective patterns. Thereafter the convective pattern became more three-dimensional as low-level wind speeds decreased. Little directional shear was observed in any of the wind profiles. Numerical simulations with the Advanced Regional Prediction System model were initialized with an upwind sounding and radar-derived wind profiles corresponding to each of the three convective structure regimes. Model-derived reflectivity fields were in good agreement with the observed regimes. These simulations differed primarily in the initial wind speed profiles, and suggest that wind speed and shear in the lower boundary layer are critical in determining the linearity of convection. Simulation with an upwind-overlake wind profile, with strong low-level winds, produced the most linear model reflectivity structure. Fluxes and measures of shear-to-buoyancy ratio for this case were comparable to observations.

Model sensitivity tests were conducted to determine the importance of low-level wind speed and speed shear in determining the linearity of convection. Results are consistent with trends expected from ratios of buoyancy to shear (but not proposed numerical threshold values). Eliminating all directional shear from the initial wind profile for the most linear case did not reduce the degree of linearity, thus showing that directional shear is not a requirement for rolls in lake-effect convection. Elimination of clouds (principally latent heating) reduced the vertical velocities by about 50%. It was found that variations in wind speed shear below 200-m height played a major role in determining the degree of linearity of the convection.

Abstract

This study examines complex flow patterns associated with the Cape Canaveral sea breeze and sea-breeze front using dual-Doppler radar, sounding, and surface data collected on 26 July 1991 during the Convection and Precipitation/Electrification Experiment. This case focuses on (a) the structure of the sea breeze, an associated trailing convergence line, river-induced convergence zones, and thunderstorm outflow boundaries, and (b) the development of convection where these features interacted.

Variations in the direction of the sea breeze in the vicinity of irregular coastlines, such as Cape Canaveral, can lead to persistent zones of convergence within the sea-breeze air. The findings show that these zones of convergence, in turn, can locally increase the depth of the sea-breeze air and create circulations at the top of the sea breeze, which can support the development of convection. The observational study is the first to document the development and evolution of the trailing convergence line over Cape Canaveral and show that its presence can be instrumental in thunderstorm initiation.

Small inland water bodies, such as the Indian River, can have a strong influence on the location where thunderstorms first develop as the sea breeze propagates inland. Divergence over the small, relatively cooler Indian River during daytime was sufficient to maintain a quasi-stationary convergence zone that, when approached and disrupted by the sea-breeze front, triggered thunderstorms. The intersection point between the sea-breeze front and the river-induced convergence zone identified the location where successive thunderstorms developed during the day.

Abstract

Intense lake-effect snowstorms regularly develop over the eastern Great Lakes, resulting in extreme winter weather conditions with snowfalls sometimes exceeding 1 m. The Ontario Winter Lake-effect Systems (OWLeS) field campaign sought to obtain unprecedented observations of these highly complex winter storms.

OWLeS employed an extensive and diverse array of instrumentation, including the University of Wyoming King Air research aircraft, five university-owned upper-air sounding systems, three Center for Severe Weather Research Doppler on Wheels radars, a wind profiler, profiling cloud and precipitation radars, an airborne lidar, mobile mesonets, deployable weather Pods, and snowfall and particle measuring systems. Close collaborations with National Weather Service Forecast Offices during and following OWLeS have provided a direct pathway for results of observational and numerical modeling analyses to improve the prediction of severe lake-effect snowstorm evolution. The roles of atmospheric boundary layer processes over heterogeneous surfaces (water, ice, and land), mixed-phase microphysics within shallow convection, topography, and mesoscale convective structures are being explored.

More than 75 students representing nine institutions participated in a wide variety of data collection efforts, including the operation of radars, radiosonde systems, mobile mesonets, and snow observation equipment in challenging and severe winter weather environments.

A severe 5-day lake-effect storm resulted in eight deaths, hundreds of injuries, and over $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 these collaborative projects. Preliminary findings are given with illustrative examples of new state-of-the-art research observations collected. Analyses associated with Lake-ICE and SNOWBAND hold the promise of greatly improving our scientific understanding of processes involved in these important wintertime phenomena.