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Cold Front (cold + front)
Selected AbstractsDemographic Characteristics of Lytechinus variegatus (Echinoidea: Echinodermata) from Three Habitats in a North Florida Bay, Gulf of MexicoMARINE ECOLOGY, Issue 1 2000Steven D. Beddingfield Abstract. The population densities, spatial distributions, size frequencies, growth rates, longevity and reproductive activities of sub-populations of the sea urchin Lytechinus variegatus were investigated over a two-year period. Sea urchins were examined in three habitats in Saint Joseph Bay, Florida, which is within the northern limits of their distribution. Densities of sea urchins, which ranged as high as 35 individuals ·,2, fluctuated seasonally at all sites and were higher in seagrass beds comprised of Thalassia testudinum than Syringodium filiforme or on a sand flat. A cold front caused large-scale, catastrophic mortality among adult, and especially juvenile, sea urchins in nearshore habitats of the Bay in the spring of 1993, leading to a dramatic decline in sea urchin densities at the Thalassia seagrass site. The population recovered over 6 months at this site and was attributable to immigration of new adults. Juvenile recruitment displayed both interannual and site-specific variability, with recruitment being highest in seagrass habitats in fall and spring. The most pronounced recruitment event occurred in fall 1993 at the Thalassia site. Spatial distributions of adult individuals ascertained monthly never varied from random in the seagrass beds (T. testudinum and S. filiforme) or during spring, summer or fall months on the sand flat. Nonetheless, aggregations of adult sea urchins were observed on the sand flat in the winter months and were associated with patchy distributions of plant food resources. Juvenile sea urchins (< 25 mm test diameter) exhibited aggregations at all sites and 67 % of all juveniles under 10 mm test diameter (91 of 165 individuals observed) were found under the spine canopies of adults. Measurements of the inducibility of spawning indicated peak gametic maturity in all three sub-populations in spring and summer. Gonad indices varied between habitats and years, but distinct maxima were detected, particularly in spring 1993 and late summer 1994. The mean gonad index of individuals at the Syringodium seagrass site was 2- to 4-fold higher than the other sites in spring 1993 and gonad indices were much higher at all sites in spring of 1993 than 1994. Estimates of growth based on changes in size frequency cohorts coupled with measurements of growth bands on lantern demipyramids indicated that L. variegatus in three habitats of Saint Joseph Bay have similar growth rates and attain a mean test diameter of approximately 35 mm in one year. In contrast to populations within the central biogeographical range of the species, which may attain test diameters up to 90 mm, the largest individuals recorded in Saint Joseph Bay were 60 mm in test diameter, and almost all individuals were no more than 45 mm in test diameter or two years of age. The demographics of L. variegatus in the northern limits of their distribution appear to be strongly influenced by latitudinally driven, low-temperature events and secondarily by local abiotic factors, especially springtime low salinities, which may negatively impact larval development and recruitment. [source] Use of METEOSAT water-vapour images for the diagnosis of a vigorous stratospheric intrusion over the central MediterraneanMETEOROLOGICAL APPLICATIONS, Issue 3 2000K Lagouvardos The diagnosis of a vigorous dry intrusion over the central Mediterranean is performed using water-vapour images from METEOSAT. This dry intrusion was located on the rear side of a cold front (propagating from Italy to Greece) and played an important role in the onset of thunderstorms over the western Greek coasts. A combination of satellite imagery and potential vorticity analyses showed that the dry air originated in the lower-stratospheric and higher-tropospheric layers. The interaction of the dry air with the moist air masses within the warm conveyor belt ahead of the cold front (overrun of warm air by low equivalent potential temperature air) produced a potentially unstable region over the area of reported thunderstorms. Copyright © 2000 Royal Meteorological Society [source] Non-thermal X-rays, a high-abundance ridge and fossil bubbles in the core of the Perseus cluster of galaxiesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2005J. S. Sanders ABSTRACT Using a deep Chandra observation of the Perseus cluster of galaxies, we find a high-abundance shell 250 arcsec (93 kpc) from the central nucleus. This ridge lies at the edge of the Perseus radio mini-halo. In addition we identify two H, filaments pointing towards this shell. We hypothesize that this ridge is the edge of a fossil radio bubble, formed by entrained enriched material lifted from the core of the cluster. There is a temperature jump outside the shell, but the pressure is continuous indicating a cold front. A non-thermal component is mapped over the core of the cluster with a morphology similar to the mini-halo. Its total luminosity is 4.8 × 1043 erg s,1, extending in radius to ,75 kpc. Assuming the non-thermal emission to be the result of inverse Compton scattering of the cosmic microwave background and infrared emission from NGC 1275, we map the magnetic field over the core of the cluster. [source] Baroclinic development within zonally-varying flowsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 626 2007David M. Schultz Abstract Previous idealized-modelling studies have shown the importance of across-jet barotropic shear to the resulting evolution of cyclones, anticyclones, surface-based fronts, and upper-level fronts. Meanwhile, many observational studies of cyclones have shown the importance of along-jet variations in the horizontal wind speed (i.e. confluence and diffluence). This study investigates the importance of these along-jet (zonal, for zonally-oriented jets) variations in the horizontal wind speed to the resulting structures and evolutions of baroclinic waves, using idealized models of growing baroclinic waves. An idealized primitive-equation channel model is configured with growing baroclinic perturbations embedded within confluent and diffluent background flows. When the baroclinic perturbations are placed in background confluence, the lower-tropospheric frontal structure and evolution initially resemble the Shapiro,Keyser cyclone model, with a zonally-oriented cyclone, strong warm front, and bent-back warm front. Later, as the baroclinic wave is amplified in the stronger downstream baroclinicity, the warm sector of the cyclone narrows, becoming more reminiscent of the Norwegian cyclone model. The upper-level frontal structure develops with a southwest,northeast orientation, and becomes strongest at the base of the trough, where geostrophic cold advection is occurring. In contrast, when the baroclinic perturbations are placed in background diffluence, the lower-tropospheric frontal structure and evolution resemble the Norwegian cyclone model, with a meridionally-oriented cyclone, strong cold front, and occluded front. The upper-level frontal structure is initially oriented northwest,southeast on the western side of the trough, before becoming zonally oriented. Weak geostrophic temperature advection occurs along its length. These results are compared to those from previous observational and idealized-modelling studies. Copyright © 2007 Royal Meteorological Society [source] Simulations of low-level convergence lines over north-eastern AustraliaTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 616 2006Gerald L. Thomsen Abstract We describe high-resolution numerical model simulations of low-level convergence lines over north-eastern Australia using the Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model (MM5). The simulations are for selected events that were documented during the Gulf Lines Experiment, held in September,October 2002. The calculations provide further insights into the dynamics of the convergence lines and the mechanisms involved in their formation. In particular they show two clearly distinct convergence lines, one that corresponds to the morning glory and one which corresponds to the North Australian Cloud Line; the former originates from the east-coast sea breeze over Cape York Peninsula south of about 14°S, while the latter originates from the east-coast sea breeze north of this latitude. They support also a recently proposed conceptual model for the generation of southerly morning glories and show for the first time the separation of a bore-like disturbance following the collision of a nocturnal cold front to the south of the inland trough with a sea-breeze front to the north of the trough. Moreover, they show the progressive transition of the east-coast sea-breeze front and the inland cold front from gravity-current-like flows to bore-like disturbances overnight to form north-easterly and southerly morning glories, respectively. Copyright © 2006 Royal Meteorological Society. [source] Convective mixing in a tropopause foldTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 599 2004H. J. Reid Abstract We present a case study of the passage of a tropopause fold over the UK behind a cold front, with emphasis on the mixing caused by convection extending into the fold. The event took place on 15,16 January 1999, and was the subject of intensive observations using the Met Office C130 aircraft and the mesosphere,stratosphere,troposphere (MST) radar at Aberystwyth. Here we concentrate on radar and satellite observations during the afternoon of 16 January, when the surface cold front had passed over the UK. A tongue of moist air moved north-eastwards over Wales at 700 hPa at this time, which, because of the very dry air in the fold above, resulted in potential instability. The resulting convection was clearly observed in NOAA satellite images. The MST radar depicted the passage of the cold front and tropopause fold as a layer of high-echo power and vertical wind shear ascending with time. Spectral widths showed the fold to be free of turbulence until 1200 UTC on 16 January, when convection was observed reaching into the frontal zone and generating turbulence. Eddy dissipation and diffusivity rates of 8.6 mW kg,1 and 8.5 m2s,1, respectively, were derived for this event. To place these figures in context, they are compared with corresponding rates derived for sixteen other passages of tropopause folds over the radar, each resulting from shear rather than convective instability. The convective event is found to be comparable to the strongest shear events, and to correspond to moderate turbulence as experienced by an aircraft. This process is of potential importance for atmospheric chemistry because it mixes boundary layer air directly with stratospheric air over a timescale of 1,2 hours. Copyright © 2004 Royal Meteorological Society [source] The impact of mergers on relaxed X-ray clusters , I. Dynamical evolution and emergent transient structuresMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2006Gregory B. Poole ABSTRACT We report on the analysis of a suite of smoothed particle hydrodynamics simulations (incorporating cooling and star formation) of mergers involving idealized X-ray clusters whose initial conditions resemble relaxed clusters with cool compact cores observed by Chandra and XMM. The simulations sample the most-interesting, theoretically plausible, range of impact parameters and progenitor mass ratios. We find that all mergers evolve via a common progression. We illustrate this progression in the projected gas density, X-ray surface brightness, Sunyaev,Zel'dovich, temperature, and gas-entropy maps. Several different classes of transient ,cold front' like features can arise over the course of a merger. Each class is distinguished by a distinct morphological signature and physical cause. We find that all these classes are present in Chandra and XMM observations of merging systems and propose a naming scheme for these features: ,comet-like' tails, bridges, plumes, streams and edges. In none of the cases considered do the initial cool compact cores of the primary and the secondary get destroyed during the course of the mergers. Instead, the two remnant cores eventually combine to form a new core that, depending on the final mass of the remnant, can have a greater cooling efficiency than either of its progenitors. We quantify the evolving morphology of our mergers using centroid variance, power ratios and offset between the X-ray and the projected mass maps. We find that the centroid variance best captures the dynamical state of the cluster. It also provides an excellent indicator of how far the system is from virial and hydrostatic equilibrium. Placing the system at z= 0.1, we find that all easily identified observable traces of the secondary disappear from a simulated 50-ks Chandra image following the second pericentric passage. The system, however, takes an additional ,2 Gyr to relax and virialize. Observationally, the only reliable indicator of a system in this state is the smoothness of its X-ray surface brightness isophotes, not temperature fluctuations. Temperature fluctuations at the level of ,T/T, 20 per cent, can persist in the final systems well past the point of virialization, suggesting that the existence of temperature fluctuations, in and of themselves, does not necessarily indicate a disturbed or unrelaxed system. [source] Larval transport and retention of the spiny lobster, Panulirus argus, in the coastal zone of the Florida Keys, USAFISHERIES OCEANOGRAPHY, Issue 5 2002Cynthia Yeung Abstract The spiny lobster Panulirus argus is of ecological and commercial importance in the South Florida coast of the continental USA and throughout the Intra-Americas Sea. Essential spiny lobster habitat in South Florida is primarily located in the Florida Keys coastal zone (including the Dry Tortugas), where the dynamic regional circulation coupled with the long planktonic larval duration (6,12 months) of P. argus raises questions of larval retention and recruitment. Locally spawned phyllosomata entrained in the Florida Current are likely to be expatriated out of the Straits of Florida, which implies that the local spiny lobster population is sustained by the transport of larval recruits from upstream locations. We examined the physical processes that may influence recruitment. Transport processes in the Keys coastal zone are spatially variable. Observed and modelled data suggest that the upper Keys is a point of onshore larval transport via the inshore meandering of the Florida Current, and the lower Keys to Dry Tortugas region apoint of retention through wind-driven onshore/countercurrents and eddy recirculation. Eddies that propagate between the Dry Tortugas and the lower Keys facilitate the exchange of larvae between the Florida Current and the coastal zone. Northerly wind events associated with cold fronts can enhance recirculation of larvae in the upper Keys. The association of older larvae with the Florida Current front supports the hypothesis that spiny lobster larval recruits come from upstream sources in the Caribbean. [source] Ages and inferred causes of Late Pleistocene glaciations on Mauna Kea, Hawai'i,JOURNAL OF QUATERNARY SCIENCE, Issue 6-7 2008Jeffrey S. Pigati Abstract Glacial landforms on Mauna Kea, Hawai'i, show that the summit area of the volcano was covered intermittently by ice caps during the Late Pleistocene. Cosmogenic 36Cl dating of terminal moraines and other glacial landforms indicates that the last two ice caps, called Older Makanaka and Younger Makanaka, retreated from their maximum positions approximately 23,ka and 13,ka, respectively. The margins and equilibrium line altitudes of these ice caps on the remote, tropical Pacific island were nearly identical, which would seem to imply the same mechanism for ice growth. But modelling of glacier mass balance, combined with palaeotemperature proxy data from the subtropical North Pacific, suggests that the causes of the two glacial expansions may have been different. Older Makanaka air atop Mauna Kea was likely wetter than today and cold, whereas Younger Makanaka times were slightly warmer but significantly wetter than the previous glaciation. The modelled increase in precipitation rates atop Mauna Kea during the Late Pleistocene is consistent with that near sea level inferred from pollen data, which suggests that the additional precipitation was due to more frequent and/or intense tropical storms associated with eastward-moving cold fronts. These conditions were similar to modern La Niña (weak ENSO) conditions, but persisted for millennia rather than years. Increased precipitation rates and the resulting steeper temperature lapse rates created glacial conditions atop Mauna Kea in the absence of sufficient cooling at sea level, suggesting that if similar correlations existed elsewhere in the tropics, the precipitation-dependent lapse rates could reconcile the apparent difference between glacial-time cooling of the tropics at low and high altitudes. Copyright © 2008 John Wiley & Sons, Ltd. [source] Detection and climatology of fronts in a high-resolution model reanalysis over the AlpsMETEOROLOGICAL APPLICATIONS, Issue 1 2010J. Jenkner Abstract The identification of low-level thermal fronts is particularly challenging in high-resolution model fields over complex terrain. Firstly, direct model output often contains numerical noise which spuriously influences the high-frequency variability of thermal parameters. Secondly, the boundary layer interferes via convection and consequently leaves its thermal marks on low levels. Here, an automated objective method for the detection of frontal lines is introduced which is designed to be insusceptible to consequences of small grid spacings. To this end, existing algorithms are readopted and combined in a novel way. The overall technique subdivides into a basic detection of fronts and a supplemental division into local fronts and synoptic fronts. The fundamental parts of the detection are: (1) a smoothing of the initial fields, (2) a definition of the frontal strength, and, (3) a localisation with the thermal front parameter. The local fronts are identified by means of a classification of open and closed thermal contours. The resulting data comprise the spatial outline of the frontal structures in a binary field as well as their type and movement. The novel methodology is applied to a 3 year high-resolution reanalysis over central Europe computed with the COSMO model using a grid spacing of 7 km. Grid-point based climatologies are derived for the Alpine region. Frequencies of occurrence and characteristics of motion are analysed for different frontal types. The novel climatology also provides quantitative evidence of dynamical properties such as the retardation of cold fronts ahead of mountains and the dissolution of warm fronts over mountains. Copyright © 2009 Royal Meteorological Society [source] A Chandra observation of the disturbed cluster core of Abell 2204MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2005J. S. Sanders ABSTRACT We present results from an observation of the luminous cluster of galaxies Abell 2204 using the Chandra X-ray Observatory. We show the core of the cluster has a complex morphological structure, made up of a high-density core (ne, 0.2 cm,3) with flat surface brightness, a surrounding central plateau, a tail-like feature, wrapping around to the east, and an unusual radio source. A temperature map and deprojected profile shows that the temperature rises steeply outside these regions, until around ,100 kpc where it drops, then rises again. Abundance maps and profiles show that there is a corresponding increase in abundance at the same radius as where the temperature drops. In addition, there are two cold fronts at radii of ,28 and 54.5 kpc. The disturbed morphology indicates that the cluster core may have undergone a merger. However, despite this disruption, the mean radiative cooling time in the centre is short (,230 Myr) and the morphology is regular on large scales. [source] Isothermal shocks in Abell 2199 and 2A 0335+096?MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2006J. S. Sanders ABSTRACT We report on a partially circular X-ray surface brightness discontinuity found at about 55 kpc from the centre of Abell 2199 with Chandra X-ray Observatory observations. Unlike cold fronts found in other clusters, the feature shows no significant temperature change across it but has an apparent density jump. We therefore identify it as a weak isothermal shock associated with the central active galactic nucleus and the inflation of its radio bubbles, as found in the Perseus cluster. We examine a similar feature at 40 kpc radius found by Mazzotta et al. in 2A 0335+096, and conclude that it too may be an isothermal shock. The change in density if these are shocks implies a Mach number of ,1.5. If the isothermal nature of these features is confirmed by deeper observations, the implication is that such shocks are common in clusters of galaxies, and are an important mechanism for the transport of energy from a central supermassive black hole into the cluster core. [source] The sting at the end of the tail: Damaging winds associated with extratropical cyclonesTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 597 2004K. A. Browning Abstract Strong surface winds often accompany the low-level jets that occur along the cold fronts of extratropical cyclones, but there is evidence that the strongest surface winds occur in a distinctly different part of a certain class of cyclone. The most damaging extratropical cyclones go through an evolution that involves the formation of a bent-back front and cloud head separated from the main polar-front cloud band by a dry slot. When the cyclone attains its minimum central pressure, the trailing tip of the cloud head bounding the bent-back front forms a hook which goes on to encircle a seclusion of warm air. The most damaging winds occur near the tip of this hook,the sting at the end of the tail. Observations of the Great Storm of October 1987 in south-east England are re-examined in some detail to study this phenomenon. The cloud head is shown to have a banded structure consistent with the existence of multiple mesoscale slantwise circulations. Air within these circulations leaves the hooked tip of the cloud head (and enters the dry slot) much faster than the rate of travel of the cloud-head tip, implying rapid evaporation and diabatic cooling immediately upwind of the area of damaging surface winds. The circumstantial evidence from the observational study leads one to hypothesize that the mesoscale circulations and the associated evaporative heat sinks may play an active role in strengthening the damaging winds. Regardless of how important this role may be, the evolution of the cloud pattern seen in satellite imagery is a useful tool for nowcasting the occurrence and location of the worst winds. Copyright © 2004 Royal Meteorological Society [source] | |||||||||||||