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The first broad program of scientific shallow drilling on the U.S. Atlantic continental shelf has delineated rocks of Pleistocene to Late Cretaceous age, including phosphoritic Miocene strata, widespread Eocene carbonate deposits that serve as reflective seismic markers, and several regional unconformities. Two sites, off Maryland and New Jersey, showed light hydrocarbon gases having affinity to mature petroleum. Pore fluid studies showed that relatively fresh to brackish water occurs beneath much of the Atlantic continental shelf, whereas increases in salinity off Georgla and beneath the Florida-Hatteras slope suggest buried evaporitic strata. The sediment cores showed engineering properties that range from good foundation strength to a potential for severe loss of strength through interaction between sediments and man-made structures.

In the wake of the eastern United States cyclone of 12-14 March 1993, a cold surge, originating over Alaska and western Canada, brought northerlies exceeding 20 m s Ϫ1 and temperature decreases up to 15ЊC over 24 h into Mexico and Central America. This paper addresses the multiscale aspects of the surge from the planetary scale to the mesoscale, focusing on 1) the structure and evolution of the leading edge of the cold surge, 2) the reasons for its extraordinary intensity and equatorward extent, and 3) the impact of the surge on the Tropics, specifically, on the strength of the trade winds and on the sea surface temperature in the eastern Pacific. The cold surge was initiated as a developing cyclone over the Gulf of Mexico, and an upstream anticyclone east of the Rockies caused an along-barrier pressure gradient to form, forcing topographically channeled northerlies along the Rocky and Sierra Madre Mountains to transport cold air equatorward. On the mesoscale, the leading edge of the cold surge possessed nonclassical frontal structure. For example, as the cold surge entered Mexico, the coldest air and the strongest wind arrived at about 900 hPa before affecting the surface, suggestive of a tipped-forward leading edge to the surge. Also, satellite imagery and surface observations indicate that the leading edge appeared to be successively regenerated in the warm presurge air. The cold surge had characteristics reminiscent of a Kelvin wave, a tipped-forward cold front, a pressure-jump line, a bore, and a gravity current, but none of these conceptual/dynamical models was fully applicable. Associated with the cold surge, gap winds up to 25 m s Ϫ1 were observed in the Gulfs of Tehuantepec (a tehuantepecer), Fonseca, Papagayo, and Panama, owing to the strong cross-mountain pressure gradient. In the case of the tehuantepecer, a rope cloud emanated from the Isthmus of Tehuantepec and turned anticyclonically, consistent with an inertial oscillation. On the synoptic and planetary scales, the extraordinary equatorward extent of the cold surge was aided by topographic channeling similar to cold-air damming, by a low-latitude upper-tropospheric trough, and by the lower branch of the secondary circulation associated with a confluent jet-entrance region aloft. The cold surge also impacted the tropical atmosphere and ocean, by contributing to the strengthening of the northeast trade winds over the eastern Pacific Ocean and by inducing local cooling of the sea surface temperature in the Gulfs of Tehuantepec and Papagayo by about 4Њ-8ЊC.

Motivated by outstanding issues from a previous case study of a midlatitude cold surge that affected Mexico and Central America, the climatology of Central American cold surges is examined in this paper. An independently derived listing of 177 cold-surge events is employed for which the following properties are tabulated: onset date, duration, time between cold-surge events, latitude of maximum equatorward penetration (min), and 48-h maximum surface temperature change at Merida, Mexico (⌬T). These data show that 75% of the cold surges have durations of 2-6 days, the same timescale as mobile disturbances in the westerlies. Also, there does not appear to be any relationship between ⌬T and the duration of the event, although cold surges that penetrate to low latitudes (min ϭ 7Њ-10ЊN) have a weak tendency to persist longer than those that do not penetrate to low latitudes (min ϭ 15Њ-20ЊN). In addition, the Reding data indicate that the cold surges tend to reach their most equatorward extent where topographic features impede the progress of equatorward-moving cold air; the temperature decrease in the postsurge air (as measured by ⌬T) does not appear to be related to the most equatorward extent. To examine the planetary-and synoptic-scale patterns associated with different categories of cold surges, events with similar characteristics from this database were composited: COLD (min Յ 10ЊN and ⌬T Ն 9ЊC), COOL (min Յ 10ЊN and ⌬T ϭ 4Њ-5ЊC), and LONG (events lasting at least 8 days). COLD surges are characterized by a persistent upper-level ridge over the western United States, 200-hPa confluence over the Gulf of Mexico, and the migration of a Canadian lower-tropospheric anticyclone equatorward along the Rocky Mountains and the Sierra Madre. In contrast, COOL surges are associated with a progressive, upper-level ridge over the western United States, weak 200-hPa confluence over the Gulf of Mexico, and the migration of a North Pacific anticyclone over the intermountain west and into the southeast United States. LONG surges are associated with a slowermoving planetary-scale pattern; 200-hPa confluence over the Gulf of Mexico; the occurrence of multiple cold surges, which reinforce the anticyclone over Mexico; and the absence of low-latitude, upper-tropospheric, mobile shortwave troughs to prematurely weaken the anticyclone. Cold surges (especially COLD) can be associated with an acceleration of the trade winds over the eastern North Pacific Ocean and play a role in El Niño-Southern Oscillation. The results in this paper are compared to the results of previous studies of North American, Central American, and east Asian cold surges.

Observational and modeling studies documented in the literature indicate that the large-scale flow has an important effect on the structure and evolution of low-level fronts in midlatitude cyclones. The purpose of this paper is to address the role of the large-scale flow on low-level cyclone/frontal structure and evolution through a combined observational and idealized modeling approach. Analyses of two observed cyclone cases embedded in large-scale diffluence and confluence, respectively, are presented to illustrate two possible cyclone/frontal structures and evolutions. Specifically, the cyclone moving into a diffluent, high-amplitude ridge becomes meridionally elongated and possesses a strong meridionally oriented cold front and a weak warm front. The cold front rotates into the warm front, forming an occluded front in the manner of the Norwegian cyclone model, as indicated by the narrowing of the thermal ridge connecting the warm sector to the cyclone center. In contrast, the cyclone moving into confluent, low-amplitude zonal flow becomes zonally elongated and possesses strong zonally oriented warm and bent-back fronts and a weak cold front. The frontal structure in this case is reminiscent of the Shapiro-Keyser cyclone model, exhibiting a fracture between perpendicularly oriented cold and warm fronts (i.e., the so-called frontal T-bone structure). The idealized simulations employ a nondivergent barotropic model in which potential temperature is treated as a passive tracer. When a circular vortex acts on an initially zonally oriented baroclinic zone, cold and warm fronts, a frontal fracture, a bent-back front, and eventually a Norwegian-like occlusion develop. When a circular vortex is placed in a diffluent background flow, the vortex and frontal zones become meridionally elongated, and the evolution resembles the Norwegian occlusion with a narrowing thermal ridge. When a circular vortex is placed in a confluent background flow, the vortex and frontal zones become zonally elongated, and the evolution resembles the Shapiro-Keyser model with a frontal fracture, frontal T-bone, and bent-back front. Although the idealized model qualitatively reproduces many of the frontal features found in the observed cyclones analyzed in the present study, one significant difference is that the maximum potential temperature gradient and frontogenesis along the cold and warm fronts may differ by a factor of 2 or more in the observed cases, but remain equal along the cold and warm fronts throughout the idealized model simulations. Possible reasons for this asymmetry in the strength of the observed cold and warm fronts are discussed.

The European Severe Storms Laboratory (ESSL) was founded in 2006 to advance the science and forecasting of severe convective storms in Europe. ESSL was a grassroots effort of individual scientists from various European countries. The purpose of this article is to describe the 10-yr history of ESSL and present a sampling of its successful activities. Specifically, ESSL developed and manages the only multinational database of severe weather reports in Europe: the European Severe Weather Database (ESWD). Despite efforts to eliminate biases, the ESWD still suffers from spatial inhomogeneities in data collection, which motivates ESSL’s research into modeling climatologies by combining ESWD data with reanalysis data. ESSL also established its ESSL Testbed to evaluate developmental forecast products and to provide training to forecasters. The testbed is organized in close collaboration with several of Europe’s national weather services. In addition, ESSL serves a central role among the European scientific and forecast communities for convective storms, specifically through its training activities and the series of European Conferences on Severe Storms. Finally, ESSL conducts wind and tornado damage assessments, highlighted by its recent survey of a violent tornado in northern Italy.

Mammatus clouds are an intriguing enigma of atmospheric fluid dynamics and cloud physics. Most commonly observed on the underside of cumulonimbus anvils, mammatus also occur on the underside of cirrus, cirrocumulus, altocumulus, altostratus, and stratocumulus, as well as in contrails from jet aircraft and pyrocumulus ash clouds from volcanic eruptions. Despite their aesthetic appearance, mammatus have been the subject of few quantitative research studies. Observations of mammatus have been obtained largely through serendipitous opportunities with a single observing system (e.g., aircraft penetrations, visual observations, lidar, radar) or tangential observations from field programs with other objectives. Theories describing mammatus remain untested, as adequate measurements for validation do not exist because of the small distance scales and short time scales of mammatus. Modeling studies of mammatus are virtually nonexistent. As a result, relatively little is known about the environment, formation mechanisms, properties, microphysics, and dynamics of mammatus.This paper presents a review of mammatus clouds that addresses these mysteries. Previous observations of mammatus and proposed formation mechanisms are discussed. These hypothesized mechanisms are anvil subsidence, subcloud evaporation/sublimation, melting, hydrometeor fallout, cloud-base detrainment instability, radiative effects, gravity waves, Kelvin-Helmholtz instability, Rayleigh-Taylor instability, and Rayleigh-Bénard-like convection. Other issues addressed in this paper include whether mammatus are composed of ice or liquid water hydrometeors, why mammatus are smooth, what controls the temporal and spatial scales and organization of individual mammatus lobes, and what are the properties of volcanic ash clouds that produce mammatus? The similarities and differences between mammatus, virga, stalactites, and reticular clouds are also discussed. Finally, because much still remains to be learned, research opportunities are described for using mammatus as a window into the microphysical, turbulent, and dynamical processes occurring on the underside of clouds.