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.

A “climatology” (climatological description of spatial, temporal, and synoptic characteristics) of snow events with thunder is presented for the contiguous United States. Based upon 30 yr of 3-hourly reports from 204 stations in the 48 contiguous United States, 229 reports are extracted from the 3-hourly observations (consistently bearing the present-weather group in each surface observation) that featured thunder with snow only. When these reports are plotted spatially, the central United States, the intermountain west, and the Great Lakes region emerge as the preferred regions for thundersnow occurrence. A thundersnow event is then defined. Isolated thundersnow reports clearly constitute a thundersnow event. Also, multiple thundersnow reports that are not separated spatially by over 1100 km or temporally by 6 h are considered part of one event. The location reporting the first occurrence of thunder with snow in such a collection of stations then carries the representative time and location for ... Abstract A “climatology” (climatological description of spatial, temporal, and synoptic characteristics) of snow events with thunder is presented for the contiguous United States. Based upon 30 yr of 3-hourly reports from 204 stations in the 48 contiguous United States, 229 reports are extracted from the 3-hourly observations (consistently bearing the present-weather group in each surface observation) that featured thunder with snow only. When these reports are plotted spatially, the central United States, the intermountain west, and the Great Lakes region emerge as the preferred regions for thundersnow occurrence. A thundersnow event is then defined. Isolated thundersnow reports clearly constitute a thundersnow event. Also, multiple thundersnow reports that are not separated spatially by over 1100 km or temporally by 6 h are considered part of one event. The location reporting the first occurrence of thunder with snow in such a collection of stations then carries the representative time and location for ...

The precipitation efficiencies for mesoscale convective systems (MCS) over the central United States are calculated. During July–September 2000 and June–September 2001, 24 MCS for which sufficient data were available occurred over or near Missouri. Precipitable water fields from the hourly Rapid Update Cycle (RUC) and radar-derived precipitation grids are used to calculate the precipitation efficiency. Geostationary Operational Environmental Satellite soundings and RUC winds are used to assess the pre-MCS environment. Statistical analysis reveals that precipitation efficiency has a relatively strong positive correlation with the relative humidity in the layer between the surface and the lifting condensation level; significant negative correlations are found between the precipitation efficiency and both the convective inhibition and the environmental wind shear. The latter, inverse relationship between shear and precipitation efficiency supports the findings of previous investigators. Abstract The precipitation efficiencies for mesoscale convective systems (MCS) over the central United States are calculated. During July–September 2000 and June–September 2001, 24 MCS for which sufficient data were available occurred over or near Missouri. Precipitable water fields from the hourly Rapid Update Cycle (RUC) and radar-derived precipitation grids are used to calculate the precipitation efficiency. Geostationary Operational Environmental Satellite soundings and RUC winds are used to assess the pre-MCS environment. Statistical analysis reveals that precipitation efficiency has a relatively strong positive correlation with the relative humidity in the layer between the surface and the lifting condensation level; significant negative correlations are found between the precipitation efficiency and both the convective inhibition and the environmental wind shear. The latter, inverse relationship between shear and precipitation efficiency supports the findings of previous investigators.

Long-term urban and rural climate data spanning January 1995 through October 2013 were analyzed to investigate the Urban Heat Island (UHI) effect in a representative mid-sized city of the central US. Locally distributed climate data were also collected at nested low density urban, recently developed, and high density urban monitoring sites from June through September 2013 to improve mechanistic understanding of spatial variability of the UHI effect based upon urban land use intensity. Long-term analyses (1995–2013) indicate significant differences (p < 0.001) between average air temperature (13.47 and 12.89 °C, at the urban and rural site respectively), relative humidity (69.11% and 72.51%, urban and rural respectively), and average wind speed (2.05 and 3.15 m/s urban and rural respectively). Significant differences (p < 0.001) between urban monitoring sites indicate an urban microclimate gradient for all climate variables except precipitation. Results of analysis of net radiation and soil heat flux data suggest distinct localized alterations in urban energy budgets due to land use intensity. Study results hold important implications for urban planners and land managers seeking to improve and implement better urban management practices. Results also reinforce the need for distributed urban energy balance investigations.