Allen J. Riordan

Examination of the Mesoscale Features of the GALE Coastal Front of 24-25 January 1986

Abstract The early evolution of a coastal front that formed off the southeast coast of the United States on 24 January 1986 is examined. Satellite and radar imagery together with the intensive surface, upper0air, and aircraft observations available during GALE are combined in an analysis highlighting the low-level mesoscale features associated with the front. The main fonrtal surface appears to form as separate narrow precipitating bands of convection slowly propagate from the east and consolidate near the western edge of the Gulf Stream. The frontal zone to the west is characterized at low levels by alternate axes of convergence and divergence extending roughly parallel to the coast. The role of these semipermanent features in the evolution of the front is discussed and compared with the classical frontal, model. Analysis of the diagnostic frontogenesis terms shows that while confluence reinforces the thermal gradient locally along confluent axes, the differential in diabatic heating, arising principally...

The Genesis of Atlantic Lows Experiment: The Planetary-Boundary-Layer Subprogram of GALE

Abstract The Genesis of Atlantic Lows Experiment (GALE), focused an intensive data-gathering effort along the mid-Atlantic coast of the United States from 15 January through 15 March 1986. Here, the general objectives and experimental layout are described with special emphasis on the planetary-boundary-layer (PBL) component of GALE. Instrumentation is described for buoys, ships, research aircraft, and towers. The networks of the cross-chain long range aid to navigation (LORAN) atmospheric sounding system (CLASS) and the portable automated mesonet (PAM II) are described and their impact on the operation of GALE is outlined. Special use of dual-Doppler radar to obtain detailed wind measurements in the PBL is discussed. Preliminary analyses for a selected observational period are given. Detailed observations of the offshore coastal front reveal direct mesoscale circulations imbedded in the frontal zone. Later in the period, during an intense cold-air outbreak, sensible-heat and latent-heat fluxes over the co...

A WIND TUNNEL STUDY OF THE FLOW FIELD WITHIN AND AROUND OPEN-TOP CHAMBERS USED FOR AIR POLLUTION STUDIES

The EPA Meteorological Wind Tunnel was used to examine the flow field in and around models of open-top field-plant growth chambers used to assess the effects of pollutant gases on plant growth. Baffles designed to reduce the ingress of ambient air into the chamber through the open top were tested; the mean flow and turbulence in the simulated boundary layer with and without the chambers were compared (the chamber was operated with and without the pollutant flow system on); and the effects of surrounding chambers on the concentration field were measured. Results showed that a baffle with a reduced opening vertically above the test area maintained the highest uniform concentration in the test area. The major differences between the three (no chamber and the chamber with flow on and off) mean velocity profiles occurred below z/h = 2.0 (h is chamber height) and at z/h ≤ 4.2. The three Reynolds stress profiles were similar above z/h = 2.0. Downwind of the chamber, the Reynolds stresses in the on-mode were greater than those in the off-mode above z/h = 1.1. The reverse was true below that point. Both longitudinal and vertical intensities above and downwind of the chamber were greater with the mixture flow system on rather than off, below about z/h < 1.5. Lateral variations in the mean wind indicated that the mean velocity was greater with the mixture flow system on except near the centerline where the reverse was true.The concentrations in the downwind wake resembled those for a cube. The location of a cylinder within a regular array had some effect on its internal gas concentration. Locations near the upwind and downwind edges of the array were associated with lower concentrations, although for all locations the highest internal values were always found at the lowest portion of the upwind wall. With active cylinders downwind, the gas plume emitted from a source cylinder at the windward edge of the array was forced 0.5 h higher and the centerline meandered laterally when compared with the single-cylinder case. A cylinder located at z/h = 1.0 downwind from a source cylinder received approximately 3%; of the concentration input to the source, or roughly 10%; of the actual concentration within the source cylinder.

Severe Thunderstorm and Tornado Warnings at Raleigh, North Carolina

Abstract The National Weather Service issues public warnings for severe thunderstorms and tornadoes when these storms appear imminent. A study of the warning process was conducted at the National Weather Service Forecast Office at Raleigh, North Carolina, from 1994 through 1996. The purpose of the study was to examine the decision process by documenting the types of information leading to decisions to warn or not to warn and by describing the sequence and timing of events in the development of warnings. It was found that the evolution of warnings followed a logical sequence beginning with storm monitoring and proceeding with increasingly focused activity. For simplicity, information input to the process was categorized as one of three types: ground truth, radar reflectivity, or radar velocity. Reflectivity, velocity, and ground truth were all equally likely to initiate the investigation process. This investigation took an average of 7 min, after which either a decision was made not to warn or new informat...

Surface Mesoscale Processes during the 1994 Palm Sunday Tornado Outbreak

The tornadic storms that developed in the 27 March 1994 Palm Sunday outbreak were confined to a narrow zone extending from central and northern Alabama to western North Carolina. Analysis of surface observations and soundings is used to examine the mesoscale environment of the region starting 14 h prior to storm development. The evolution of a shallow front that formed the northern boundary of the outbreak region is tied to several diabatic processes including evaporation of precipitation and differential solar heating. The resulting front was found to both limit severe convection and focus supercell development later in the day. During the night before the outbreak, as copious widespread precipitation fell into dry air, evaporation maintained a cold air pool north of the front. By contrast, moderate southerly flow provided warm, moist conditions to the south. Precipitation-enhanced cold air damming along the eastern slopes of the Appalachians also may have provided a source of cold air for subsequent frontogenesis over areas farther west. During the daylight hours, differential solar heating across the front further enhanced frontogenesis. Intensification of convection just prior to the first tornadoes was found to be associated with areas of breaks in the overcast near and upstream of tornadogenesis. Similarly, cells that intensified were moving over a surface that had been thoroughly moistened by previous rainfall. Supercells that intersected and moved along the frontal boundary maintained their tornadic strength for many hours, whereas storms that crossed the boundary disintegrated. Blockage of inflow by upstream storm cells may also have contributed to the rapid reduction of intensity of one of the tornadic cells.

Mesoscale Wind Signatures along the Carolina Coast

Abstract Coastal winds immediately offshore of North and South Carolina often exhibit a mesoscale diffluent–confluent pattern that appears to be governed by the coastal configuration and oceanic thermal field. The stationary pattern roughly parallels the coastline within 50 km of shore during winter when synoptic-scale conditions support northerly winds. Data obtained during the Genesis of Atlantic Lows Experiment (GALE) are statistically analyzed to document the surface winds in this offshore zone. Several specific examples of the mesoscale pattern are presented and compared with results from a simple theoretical model with diabatic heating. For an inviscid flow over an isolated bell-shaped heat source, the air parcels rise in the vicinity of the heating region and descend on both the upstream and downstream sides. A confluent zone is produced in the vicinity of the heat source. With an idealized heat source resembling the observed pattern of sensible heat flux, the flow pattern is similar to that observ...

The Morning Inversion Near the Ground and Its Daytime Transition at Two Rural Sites in the Carolinas

Abstract Six years of tower data from two dissimilar sites in the eastern piedmont of the Carolinas are analyzed to yield a selective climatology of the lower portion of the morning inversion. Its transition to daytime conditions is then described and statistically modeled. Both sites are in clearings surrounded by forest, but one site is in a valley by a lake, while the other, 175 km to the north, is on a low hilltop. Measurements of wind speed and direction, the standard deviation of wind direction, dew point, and temperature at 11 m, temperature difference (ΔT) between 11 and 60 m, plus solar radiation, were analyzed for an 8-h period starting from three hours before local sunrise each day for both locations. Results show that predawn inversions characterize over 70% of the data and strong inversions of over 5°C per 100 m in the tower layer characterize 30% of the mornings at the hilltop site. At the valley site, strong inversions are less common, probably because of the proximity of the lake. There is...

Mesoscale simulations of dynamical factors discriminating between a tornado outbreak and non-event over the southeast US Part I: 84-48 hour precursors

Summary Observational analysis and mesoscale numerical simulations are in agreement concerning key dynamical processes which occurred over Mexico and the Gulf of Mexico 84 hours prior to the 1988 Raleigh (RDU), NC tornado outbreak. The subtropical jet (STJ) over northern Mexico and its associated transverse ageostrophic circulation forced air down the eastern side of the Sierra Madre Mountains creating adiabatic warming due to compressional heating. Along with this warm air, a low-level trough of low pressure formed and a low-level jet (LLJ) developed over the western Gulf of Mexico. This LLJ began the process that transported very warm and potential vorticity (PV) rich air from the Mexican plateau to the Carolina Piedmont.The low-level PV maximum over central NC at the time of the tornado was a coherent entity traceable back 84 hours to the Mexican plateau. Over the Mexican plateau, the STJ transported the PV rich air southward then down to the midlevels. There was substantial heating over the plateau producing a deep well-mixed layer and a mountain-plains solenoid. An area of strong vertical convergence developed in the 500–600 hPa layer which increased the thermal gradient and maintained the PV. This mid-level PV was transported to the low-levels by a hydrostatic mountain wave. As the PV maxima moved down the lee of the mountains it increased due to strong static stability, tilting and frictional effects. Finally, the PV maxima moved along the Gulf Coast and up the East Coast to central NC.

Small-Scale Structure of a Coastal Front as Revealed by Dual-Doppler Radar

Abstract The analysis of the rainband structure and wind fields associated with a coastal front along the North Carolina shoreline is described. Dual-Doppler radar and the augmented GALE (Genesis of Atlantic Lows Experiment) ensemble of in situ stations depict shallow, convective rainbands that overtake the front from the warm-air sector and intensify at the surface front location. Clockwise band rotation is shown to be caused by the difference in alignment between the echo motion and the rainband axes and by new development ahead of the front. Radar measurements depict the circulation systems associated with a portion of one rainband in the cold air ahead of the front. Here shallow precipitation cores are vertically tilted due to the frontal wind shear. Circulation cells and most precipitation cores are centered just above the frontal inversion, as inferred by the wind shift line aloft. This feature is nearly horizontal in the cross-frontal direction but slopes downward in a direction roughly parallel to...

Spectral Analysis of Station Pressure as an Indicator of Climatological Variations in Synoptic-Scale Activity in the Eastern United States

Abstract A substantial decline in North American cyclone and anticyclone activity has been documented by several recent studies based on counts of disturbance tracks. An independent method of assessing long-term trends in synoptic-scale activity based on sequential spectral analysis of station pressure is suggested. The efficacy of this approach is supported by previous studies relating the spatial distribution of variance of band-pass filtered pressures to preferred cyclone tracks. However, examples of a preliminary application of the spectral method to three widely separated stations using approximately 30 years of winter data fail to reveal any significant long-term trends in the variance of pressure for synoptic-scale time periods.