This paper presents analyses of the finescale structure of convection in the comma head of two continental winter cyclones and a 16-storm climatology analyzing the distribution of lightning within the comma head. A case study of a deep cyclone is presented illustrating how upper-tropospheric dry air associated with the dry slot can intrude over moist Gulf air, creating two zones of precipitation within the comma head: a northern zone characterized by deep stratiform clouds topped by generating cells and a southern zone marked by elevated convection. Lightning, when it occurred, originated from the elevated convection. A second case study of a cutoff low is presented to examine the relationship between lightning flashes and wintertime convection. Updrafts within convective cells in both storms approached 6–8 m s−1, and convective available potential energy in the cell environment reached approximately 50–250 J kg−1. Radar measurements obtained in convective updraft regions showed enhanced spectral width within the temperature range from −10° to −20°C, while microphysical measurements showed the simultaneous presence of graupel, ice particles, and supercooled water at the same temperatures, together supporting noninductive charging as an important charging mechanism in these storms. A climatology of lightning flashes across the comma head of 16 winter cyclones shows that lightning flashes commonly occur on the southern side of the comma head where dry-slot air is more likely to overrun lower-level moist air. Over 90% of the cloud-to-ground flashes had negative polarity, suggesting the cells were not strongly sheared aloft. About 55% of the flashes were associated with cloud-to-ground flashes while 45% were in-cloud flashes.

Data from airborne W-band radar are used in conjunction with thermodynamic fields from the Weather Research and Forecasting Model and air-parcel back trajectories from the HYSPLIT model to investigate the finescale reflectivity, vertical motion, and airmass structure of the comma head of a winter cyclone in the vicinity of the Great Lakes. Cloud-top generating cells formed along an upper-level frontal boundary vertically separating dry air, which 48 h earlier was located in the upper troposphere over south-central Canada, from moist air, which was located in the lower troposphere over the southeast United States. The stronger updrafts within the generating cells had vertical velocities ranging from 1 to 3 m s 21 . The generating cells were important to precipitation production within the comma head. Precipitation trails formed within the generating cells could sometimes be followed to the boundary layer before merging.Boundary layer air beneath the cyclone's comma head exhibited convective circulations and was turbulent. Gravity waves were sometimes observed at the base of the stable layer atop the convective boundary layer. Trajectory analyses showed that boundary layer air sampled by radar beneath the aircraft path had a history of crossing the Great Lakes. The magnitude of updrafts and downdrafts in the boundary layer were 1-2 m s 21 , while wave circulations exhibited maximum updrafts and downdrafts of ;3 m s 21 . The tops of some boundary layer convective circulations and gravity waves exhibited enhancements in radar reflectivity. The data presented illustrate the impact of the Great Lakes on cyclone mesostructure during the passage of a cyclone through the region.

The objective of the research presented is to assess the impact of sensor response and aircraft airspeed on the accuracy of in situ observations collected by small unmanned aircraft systems profiling the convective boundary layer or transecting airmass boundaries. Estimates are made using simulated aircraft flown within large-eddy simulations. Both instantaneous errors (differences between observed temperature, which include the effects of sensor response and airspeed, and actual temperature) and errors in representation (differences between serial observations and representative snapshots of the atmospheric state) are considered. Synthetic data are retrieved assuming a well-aspirated first-order sensor mounted on rotary-wing aircraft operated as profilers in a simulated CBL and fixed-wing aircraft operated through transects across a simulated airmass boundary. Instantaneous errors are found to scale directly with sensor response time and airspeed for both CBL and airmass boundary experiments. Maximum errors tend to be larger for airmass boundary transects compared to the CBL profiles. Instantaneous errors for rotary-wing aircraft profiles in the CBL simulated for this work are attributable to the background lapse rate and not to turbulent temperature perturbations. For airmass boundary flights, representation accuracy is found to degrade with decreasing airspeed. This signal is most pronounced for flights that encounter the density current wake. When representation errors also include instantaneous errors resulting from sensor response, instantaneous errors are found to be dominant for flights that remain below the turbulent wake. However, for flights that encounter the wake, sensor response times generally need to exceed ~5 s before instantaneous errors become larger than errors in representation.

Cloud-to-ground (CG) lightning, radar, and radiosonde data were examined to determine how frequently lake-effect storms (rain/snow) with lightning occurred over and near the lower Great Lakes region (Lakes Erie and Ontario) from September 1995 through March 2007. On average, lake-effect lightning occurred on 7.9 days and with 5.8 storm events during a particular cool season (September-March). The CG lightning with these storms had little inland extent and was usually limited to a few flashes per storm. Some storms had considerably more, with the most intense storm (based on National Lightning Detection Network observations) producing 1551 CG flashes over a 4-day period. Thundersnow events were examined in more detail because of the rarity of this phenomenon across the United States. Most lake-effect thundersnow events (75%) occurred in November and December. An analysis of model sounding data using the Buffalo Toolkit for Lake Effect Snow (BUFKIT) software package in which lower boundary conditions can be modified by lake surfaces showed that thundersnow events had an 82% increase in the mean height of the 2108C level when compared with nonelectrified lake-effect snowstorms (1.2 vs 0.7 km AGL), had higher lake-induced equilibrium levels (EL; above 3.6 km AGL) and convective available potential energy (CAPE; .500 J kg 21 ), had low wind shear environments, and were intense, single-band storms. A nomogram of the altitude of the 2108C isotherm and EL proved to be useful in predicting lake-effect thundersnowstorms.

Predictions of lake and sea breezes are particularly important in large coastal population centers because of the circulations’ influence on heat-wave relief, energy use, precipitation, and dispersion of pollutants. While recent numerical modeling studies have suggested that sea or lake breezes should move more slowly through urban areas than in the surrounding suburbs because of urban heat island (UHI) circulations, there have been few quantitative observational studies to evaluate these results. This study utilizes high-resolution Weather Surveillance Radar-1988 Doppler (WSR-88D) observations to determine the effect of the UHI on lake-breeze frontal movement through Chicago, Illinois, and nearby suburban areas. A total of 44 lake-breeze cases from the April–September 2005 period were examined. The inland movement of the lake-breeze front (LBF) was calculated by tracking “fine lines” of radar reflectivity along several cross sections perpendicular to the Lake Michigan shoreline. The average inland propagation speed of the LBF was 5.0 km h−1; there was substantial spatial and temporal variability in LBF propagation, however. Chicago’s UHI magnitude on lake-breeze days exhibited an average nighttime maximum urban–rural temperature difference near 4.5°C and an afternoon minimum near 0°C. The observed daytime UHI magnitude did not have a significant relationship with lake-breeze frontal movement through Chicago. However, the maximum magnitude of the nighttime UHI preceding lake-breeze development was found to be strongly related to a decrease in speed of LBF movement through Chicago’s southwest (inland) suburbs. This relationship is consistent with previous studies of the diurnal evolution of UHI circulations and may represent a useful method for predicting lake-breeze inland movement.