Charles E. Konrad

Relationships between the intensity of cold-air outbreaks and the evolution of synoptic and planetary-scale features over North America

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 relat...

An Examination of Extreme Cold Air Outbreaks over Eastern North America

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 intensifi...

Synoptic Climatology of 500 mb Circulation Changes during Explosive Cyclogenesis

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 conne...

MOISTURE TRAJECTORIES ASSOCIATED WITH HEAVY RAINFALL IN THE APPALACHIAN REGION OF THE UNITED STATES

Synoptic data associated with a sample of 554 heavy rainfall events is utilized to carry out a trajectory analysis that identifies the movements of moisture towards regions of heavy rain. Both seasonal and regional variations are found in the moisture trajectories associated with heavy rain events occurring in three regions in the Appalachian study area. Numerous events in the region west of the mountain range are tied to westerly and south-southwesterly circulations that bring moisture from the Mississippi River Valley and the Gulf of Mexico during the warm and cool seasons, respectively. Many events southeast of the mountain range are associated with southerly to southeasterly circulations that advect moisture from the Atlantic Ocean. Because of orographic precipitation enhancement and a good exposure to the Gulf of Mexico and the Atlantic Ocean, the southern and southeastern slopes of the Appalachian mountains display high frequencies of heavy rainfall, particularly during the cool season. The interior...

Spatial and temporal patterns of thunderstorm events that produce cloud-to-ground lightning in the interior southeastern United States

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 calculat...

Diurnal Variations in the Landfall Times of Tropical Cyclones over the Eastern United States

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).

Numerical Prediction of a Cold-Air Outbreak: A Case Study with Ensemble Forecasts

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 attribu...