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Allen J. Riordan

Students enrolled in a satellite meteorology course at North Carolina State University, Raleigh, recently had an unusual opportunity to apply their forecast skills to predict wind and weather conditions for a remote site in the Southern Hemisphere. For about 40 days starting in early February 2001, students used satellite and model guidance to develop forecasts to support a research team stationed on Bouvet Island (54°26′S, 3°24′E). Internet products together with current output from NCEP's Aviation (AVN) model supported the activity. Wind forecasts were of particular interest to the Bouvet team because violent winds often developed unexpectedly and posed a safety hazard.

Results were encouraging in that 24-h wind speed forecasts showed reasonable reliability over a wide range of wind speeds. Forecasts for 48 h showed only marginal skill, however. Two critical events were well forecasted—the major February storm with wind speeds of over 120 kt and a brief calm period following several days of strong winds in early March. The latter forecast proved instrumental in recovering the research team.

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Allen J. Riordan

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 from differences the underlying surface, promotes the westward displacement of the northern half of the frontal zone. The subsequent inland propagation of the front is found to be complex. The northernmost section of the frontal boundary initially moves steadily inland, leaving a small frontal remnant at the coast, then the entire northern half of the front jumps inland discontinuously. By 1800 UTC on 25 January, the norther portion of the front has become a broad zone established over 200 km inland, while the southern portion has maintained a very sharp boundary offshore, coincident with an oceanic thermal discontinuity.

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Allen J. Riordan and Yuh-Lang Lin

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 observed. Frictional effects are shown to be negligible. Thus, results from the simple theoretical model suggest that diabatic heating is largely responsible for the observed flow pattern.

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Raymond B. Kiess and Allen J. Riordan

Abstract

The association between the synoptic-scale sea level pressure field and the behavior of the morning inversion at two 60-m tower sites in North and South Carolina is investigated. Daily gridded pressure data for 1976 through 1982 for the eastern third of the United States are objectively classified according to their similarity with one of four basic eigenvector patterns developed for the dataset.

The occurrence and site-to-site correlation of predawn inversion strength at the two tower locations, as well as the transition time between sunrise and inversion breakdown are then compared and contrasted for the different synoptic types. Results show a statistically significant relationship between inversion strength and synoptic type. As expected, strong inversions are most common when anticyclones are centered over the region. However, strong inversions are also found when anticyclones are centered southwest of the sites. The synoptic association is very similar for both tower sites expect for conditions when local care of a lake near one site lead to obvious modifications there. Transition times are only occasionally related to the synoptic type and relate best when the pressure gradient is weakest (long transition) or strongest (short transition).

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Sethu Raman and Allen J. Riordan

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 coastal ocean each attain values of about 500 W · m−2. Coordinated aircraft operations are outlined for this case and a few early findings are given.

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Adam H. Langmaid and Allen J. Riordan

Abstract

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.

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Allen J. Riordan, J. Thomas Anderson, and S. Chiswell

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 the front.

Ahead of the front, main updrafts in and above the cold air are found near the upwind portion of precipitation cores and along two well-defined lines aligned roughly perpendicular to the front. These lines propagate northward and affect several nearby surface sites prior to frontal passage. The speed of northward propagation is consistent with gravity wave theory, while on the larger scale the front appears to behave as the leading edge of a density current. The major features found in this case are compared and contrasted with those of a synoptic-scale warm front.

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David A. Barber, Jerry M. Davis, and Allen J. Riordan

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.

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Andrew P. Holland, Allen J. Riordan, and E. C. Franklin

Abstract

An analytical model is presented to describe patterns of downed trees produced by tornadic winds. The model uses a combined Rankine vortex of specified tangential and radial components to describe a simple tornado circulation. A total wind field is then computed by adding the forward motion of the vortex. The lateral and vertical forces on modeled tree stands are then computed and are compared with physical characteristics of Scots and loblolly pine. From this model, patterns of windfall are computed and are compared to reveal three basic damage patterns: cross-track symmetric, along-track asymmetric, and crisscross asymmetric. These patterns are shown to depend on forward speed, radial speed, and tree resistance. It is anticipated that this model will prove to be useful in assessing storm characteristics from damage patterns observed in forested areas.

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Allen J. Riordan, Jerry M. Davis, and Raymond B. Kiess

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 a correlation of 0.71 in daily site-to-site ΔT at dawn. This suggests strong overall synoptic control of the local inversion frequency.

The transition to well-mixed conditions after sunrise depends chiefly on ΔT prior to sunrise. Analysis of mean trends in variables during the transition shows it is a remarkably well-ordered process. The time from sunrise to a mean isothermal state (between 11 and 60 m only) takes about 1 to 2 h.

Daily transition is predicted by a linear regression scheme based on predawn conditions and developed and tested separately at each site. Chief predictors are inversion intensity, dew point and 60 m wind speed. For cloudy mornings the rms error for the prediction time from sunrise to mean isothermal conditions is 0.3 h. For days with variable cloudiness, a rather unspectacular R 2 value of 0.3 to 0.4 is, nevertheless, statistically significant. A similarity in models at both sites is noted. In cloudless conditions the models are, in fact, nearly interchangeable.

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