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Charles E. Konrad II

Abstract

Much of the previous work on cold-air outbreaks has examined the synoptic development associated with small samples of strong outbreaks. In this study, a synoptic climatology of cold-air outbreaks is developed from a large outbreak sample displaying a wide range of intensities over the southeastern United States. Relationships are developed between the intensity of cold-air outbreaks and the magnitude of planetary- and synoptic-scale surface temperature, pressure, and 500-mb height anomalies over North America. These atmospheric fields are passed through a Gaussian nine-point filter in order to distinguish between planetary- and synoptic-scale features. Lag correlation fields are constructed to identify patterns in the strength of the relationship between the outbreak intensity and the magnitude of the planetary- and synoptic-scale atmospheric fields over North America during a 12-day window preceding the cold-air outbreak.

Planetary-scale circulation anomalies are found to be more strongly related to the intensity of cold-air outbreaks than the synoptic-scale anomalies. Incipient outbreak intensity is most strongly related to persistent, positive surface pressure anomalies over western Canada 6–12 days before the outbreak. Several days before the outbreak onset, outbreak intensity is more strongly associated with negative 500-mb height and surface temperature anomalies over the Great Lakes and the Midwest, respectively.

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Charles E. Konrad II

Abstract

Best track tropical cyclone data are examined for the period 1950–96 to estimate the landfall times of all tropical storms and hurricanes over the coastal margins of the eastern United States. The analysis reveals a marked diurnal pattern with tropical cyclones making landfall more frequently during the evening and midmorning hours. Lulls in cyclone landfall are identified during the afternoon and early in the morning. Weak hurricanes display the strongest diurnal cycle of landfall. Category 3 and stronger hurricanes display little diurnal variation in landfall time. An examination of the diurnal pattern of cyclone passages within 300 km of the coast reveals that the pattern is most coherent and displays the greatest statistical significance at the coastline (i.e., points of cyclone landfall).

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Charles E. Konrad II

Abstract

The intensity or magnitude of a given heavy precipitation event is typically associated with the greatest point precipitation total. The scale or size of the heavy precipitation region, however, is important because it affects the scale of the flooding potential (e.g., local- vs regional-scale basins). In this study, a heavy-rain climatological description is constructed that identifies all precipitation events for the period of 1950–96 and estimates the heaviest mean 2-day precipitation totals over a range of spatial scales (i.e., circular regions from 2500 to 500 000 km2). Ranks of the most extreme precipitation events are provided for four regions of the study area for each of the 10 spatial scales. To develop the dataset, daily precipitation totals from the cooperative observer network are spatially interpolated onto a finescale (10 km by 10 km) grid over the eastern two-thirds of the United States. An automated algorithm is developed 1) to identify regions displaying the greatest mean 2-day precipitation totals over each spatial scale and 2) to link nearby regions of different scales together to form precipitation events. Precipitation events with regional recurrence intervals of approximately 1 yr or greater at each spatial scale are examined and compared across four subregions. The geographical and seasonal distributions of these events are provided. The extreme events are also tied to the occurrence of tropical cyclones and 500-hPa cyclones. Secular trends are identified in the frequency of extreme events over the different spatial scales.

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Charles E. Konrad II

Abstract

Previous research has established links between heavy rainfall and a wide variety of synoptic features and parameters. In this study, 312 heavy rainfall events are identified over the southeastern United States and used to construct a synoptic climatology that relates 14 synoptic features and parameters to the occurrence and amounts of heavy rainfall. To carry this out, an automated synoptic typing scheme is employed to classify the heavy rainfall sample according to several characteristics of the synoptic regime. This classification provides five distinct synoptic patterns that can be associated with heavy rainfall over the southeastern United States. Commonly occurring synoptic features in each synoptic pattern are highlighted and discussed. Correlation analysis is then used to relate the occurrence and strength of these features to the heavy precipitation totals.

Heavy rainfall in four out of the five identified synoptic patterns is most frequently associated with high levels of moisture at the 700-mb level. Ridging in the 850-mb warm air advection field is common over the heavy rain area as well. Numerous relationships are identified between the heavy rainfall amounts and the character of synoptic features; however, the nature of these relationships is found to vary strongly according to the synoptic pattern.

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Fan Chen and Charles E. Konrad II

Abstract

The synoptic patterns and boundary layer conditions over a range of antecedent periods associated with the summertime hot events for the years 1951–93 are examined. A hot event is defined as a single day with the highest average daily temperature within a surrounding 5-day window. Among these events, four event subtypes were determined on the basis of extreme values of temperature and/or dewpoint. Composite synoptic maps and vertical profiles of atmospheric variables are produced to distinguish the hottest and moistest events. The hot events, including the extreme categories, are influenced by similar large-scale circulation features. The region is under the control of the Bermuda high, which is centered off the coast of Florida and in the Atlantic Ocean. An upstream 500-hPa ridge produces subsidence and adiabatic warming in the midlevels of the troposphere. Composite patterns of the hottest and moistest events indicate stronger upstream 500-hPa ridging and upper-level subsidence, which suggest greater suppression of local convection and reduction in the upward turbulence transfer of surface sensible heat and water vapor. The moistest events are tied to considerably greater antecedent precipitation, which suggests increased evapotranspiration and accumulation of water vapor near the ground. The extreme hot and humid events are also associated with greater accumulated precipitation hours in the antecedent periods, especially on a 30-day scale. The hottest events also have less sky cover in the 30-day antecedent period, allowing more insolation and surface heating. The extreme events also have greater atmospheric thickness, lighter winds, and greater westerly component in the winds. Synoptic analysis shows that low-level thermal and moisture advection are not significant contributors to the heat and moisture in the extreme events of the Piedmont region.

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Christopher M. Fuhrmann and Charles E. Konrad II

Abstract

Winter storms, namely snowstorms and ice storms, are a major hazard and forecasting challenge across central North Carolina. This study employed a trajectory approach to analyze the ingredients (i.e., temperature, moisture, and lift) associated with heavy snowstorms and ice storms that occurred within the Raleigh, North Carolina, National Weather Service forecast region from 2000 to 2010. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) tool was used to calculate 72-h backward (i.e., upstream) air parcel trajectories from three critical vertical pressure levels at the time and location of heaviest precipitation for each storm. Analysis of composite trajectories revealed the source regions and meteorological properties of air parcels associated with heavy winter storms. Adiabatic and diabatic contributions to air parcel temperature and moisture content were also estimated along each trajectory to assess the physical processes connected with heavy winter precipitation in the region. Results indicate that diabatic warming and cooling contribute significantly to the vertical temperature profile during heavy winter storms and therefore dictate the resulting precipitation type. The main source of diabatic warming is fluxes of sensible and latent heat within the marine atmospheric boundary layer over the Gulf Stream. These fluxes contribute to a warming and moistening of air parcels associated with heavy ice storms. In contrast, heavy snowstorms are characterized by diabatic cooling in the lower troposphere above the marine atmospheric boundary layer. The most significant moisture source for heavy snowfall is the Caribbean Sea, while heavy ice storms entrain moisture from the Gulf of Mexico and Gulf Stream region near the Carolina coast.

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Charles E. Konrad II and Stephen J. Colucci

Abstract

The sequence of development and thermodynamic aspects of two strong cold air outbreaks over eastern North America during January 1977 are described. In the first outbreak, surface cyclogenesis occurs prior to the outbreak onset (local 850 mb temperature decreases). Regions of strong cold air advection and adiabatic warming are found immediately upstream of the cyclone over the cold air outbreak area. Since the two regions are nearly superimposed, the effect of advective cooling is partially opposed by adiabatic warming. In the second outbreak, surface cyclogenesis follows the outbreak onset. In this case, local and advective cooling is observed over a larger region as a cold air pool over central Canada is transported southeastward. Initially, adiabatic warming is weak or replaced by adiabatic cooling over eastern North America as cold air advection dominates the thermodynamic energy balance. As downstream cyclogenesis proceeds during the latter stages of the outbreak, adiabatic warming intensifies over the cold air advection region, reducing the cooling effect. An inspection of 15 other strong cold air outbreaks in a ten-winter sample reveals a qualitatively similar relationship among the size of the cold air advection region, timing of the surface cyclogenesis and areal averaged 850 mg temperatures. It is hypothesized that the intensity of cold air outbreaks over eastern North America is proportional to the areal coverage of the cold air advection region and timing, relative to cold air advection onset, of surface cyclogenesis.

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Mark S. Murphy and Charles E. Konrad II

Abstract

Cloud-to-ground (CG) lightning data are used in this study to trace the daily patterns of thunderstorms in time and space across the topographically diverse southeastern United States. Four reoccurring patterns of thunderstorms (i.e., local, multilocal, regional, and widespread) are identified on the basis of the size of the region of CG lightning as well as the spatial pattern of the flashes within this region. To identify these patterns, hourly maps of CG flashes are produced over five summer seasons (June–August) and used to identify thunderstorm events on all days in which at least one CG lightning is observed. Thunderstorm events are defined by a temporally and spatially clustered hourly pattern of lightning flashes. The spatial pattern of lightning associated with each event is examined during the hour in which the flash density is the highest and is used to classify the event. The geographical and temporal patterns of each thunderstorm type are described. Also, flash densities are calculated at spatial scales ranging from 1- to 100-km radial distance. Over half of the identified thunderstorm events in the study were confined to the local scale and contained relatively few flashes. They were most common early in the morning and in the mountainous portions of the study area. Widespread events, on the other hand, showed a dense coverage of flashes within a given hour over a majority of the area. Although they occurred much less frequently (i.e., once every 8 days across most locales), they were responsible for the highest number of CG lightning flashes in the study region; furthermore, they produced the highest flash densities, both at the local and regional scale. A radar echo classification revealed that these events were not tied to mesoscale convective systems, but rather to the early afternoon development of numerous convective cell clusters and lines across the study area.

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Charles E. Konrad II and Stephen J. Colucci

Abstract

Changes in the 500 mb circulation are examined near and during each of 141 explosive surface cyclogenesis cases during the period 1 September 1980 through 31 May 1987 (June through August excluded) over the western half of the Northern Hemisphere. Two types of circulation changes evident in the geopotential height fields are investigated: 1) amplification of 500 mb troughs and/or ridges in which the explosively deepening cyclones (bombs) are imbedded and 2) formation of 500 mb cyclones (closed lows) or anticyclones (closed highs) near the bombs. By either stratification, most bombs in the sample are found not to be associated with nearby 500 mb circulation changes on the time scale of the explosive deepening. Relatively weak bombs are more likely to be associated with identifiable circulation changes than those which are moderate or strong. Antecedent conditions for the bomb-related circulation changes are examined. For example, bombs associated with 500 mb trough (ridge) amplifications are connected with southeastward (northeastward) moving disturbances at 500 mb imbedded in the larger scale 500 mb trough (ridge). Conversely, all persistent 500 mb cyclone-anticyclone pairs (blocking patterns) identified in the sample were preceded in their formations by nearby explosive surface cyclogenesis.

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Stephen J. Colucci, David P. Baumhefner, and Charles E. Konrad II

Abstract

The forecastability of a cold-air outbreak over eastern North America during January 1985 has been studied with ensemble forecasts from the NCAR Community Climate Model version 2 run at T42 horizontal resolution. The cold-air outbreak case was characterized by a pool of very cold air (T < −35°C at 850 mb) that moved southward into the central United States and intensified. The ensemble’s 10 member forecasts were initialized at 0000 UTC 15 January 85, a few days before the cold-air pool began its southward movement and reached its peak intensity. The ensemble members predicted the southward passage of the cold air but faster and weaker than analyzed. The predicted weakening of the cold-air pool was consistent with the model’s systematic error. Quasi-Lagrangian diagnosis of the 850-mb temperature tendency budget revealed that the analyzed intensification of the cold-air pool was due to residual rather than adiabatic effects. These residual effects could have been diabatic in origin but also attributable to observational errors. Similar diagnoses applied to selected ensemble members indicated that diabatic cooling, specifically longwave radiative cooling, contributed to the forecast cooling of the cold-air pool by one ensemble member but was overwhelmed by adiabatic warming in a weakening cold-air pool predicted by another ensemble member. These results suggest that the forecast details of a cold-air outbreak may depend upon the subtle balance between diabatic and adiabatic processes. Furthermore, forecasts constructed from ensemble predictions must account for model biases as well as information from the ensembles.

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