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Storm Tracks (storm + tracks)
Selected AbstractsRainfall patterns and critical values associated with landslides in Povoação County (São Miguel Island, Azores): relationships with the North Atlantic OscillationHYDROLOGICAL PROCESSES, Issue 4 2008Rui Marques Abstract São Miguel Island (Azores) has been affected by hundreds of destructive landslide episodes in the last five centuries, triggered either by earthquakes, volcanic eruptions or rainfall episodes, which were responsible for many deaths and very important economic losses. Among the instability causes, meteorological factors are of primary importance on Povoação County, namely the high recurrence rate of calamitous rainfall triggering landslides. The most recent catastrophic episode took place on the 31st October 1997 when almost 1000 soil slips and debris flows were triggered, and 29 people died in the Ribeira Quente village. The role of rainfall on regional landslide activity was analysed applying cumulative rainfall methods. The method comprises the reconstruction of both absolute and calibrated antecedent rainfalls associated with each major landslide event. The critical rainfall combination (amount-duration) responsible for each landslide event was assessed and a rainfall critical threshold for landslide occurrence was calculated. Rainfall-triggered landslides in the study area are ruled by the function I = 144·06 D,0·5551, and they are related both to short duration precipitation events (1,3 days) with high average intensity (between 78 and 144 mm/day) and long-lasting rainfall episodes (1,5 months) with a lower intensity (between 9 and 22 mm/day). The impact of the North Atlantic Oscillation (NAO) on the regional precipitation regime was evaluated. It is shown that the monthly precipitation of São Miguel is largely modulated by the NAO mode presenting a significant negative correlation with the NAO index. This result arises from the NAO control on the travelling latitude of most storm tracks that cross the Northern Atlantic Ocean. Copyright © 2007 John Wiley & Sons, Ltd. [source] An event-based jet-stream climatology and typologyINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 3 2006Patrick Koch Abstract A novel climatology is developed for upper-tropospheric jet streams, which is complementary to and an alternative for the traditional depictions of the time-mean jets. It entails identifying the occurrence of a jet event at a given location and then compiling the spatial frequency distribution of such events. The resulting climatology, derived using the ERA-15 reanalysis data set of the ECMWF for the period 1979,1993 indicates that (1) in both hemispheres the annual cycle of jet events takes the form of comparatively smooth transition from a quasi-annular structure in summer to a more spiral-like structure in winter with a temporally asymmetric return to the summer pattern; (2) the hemispheres differ primarily in the amplitude of the frequencies and the longitudinal overlap of the spiral portion of the pattern. In addition, the jet events are subdivided using a two-class typology comprising shallow and deep jets whose vertical shear (sic. baroclinicity) are/are not confined principally to the upper troposphere. This provides a conceptually simple and dynamically meaningful classification since deep jets are more likely to spawn tropospheric-spanning cyclones. The accompanying climatology displays important longitudinal variations and significant inter-hemispheric differences. A comparison is drawn between these new and conventional climatologies and typologies. Also, comments are proffered on the relationship between, on the one hand, the patterns of jet frequency including the differing distributions of the shallow and deep types and, on the other hand, the location of the time-mean jets and the downstream storm tracks. Copyright © 2006 Royal Meteorological Society. [source] Dependence between extreme sea surge, river flow and precipitation in eastern BritainINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 10 2002Cecilia Svensson Abstract Flooding in estuaries may be caused by both high river flows and by high sea levels. In order to investigate whether these tend to occur simultaneously in eastern Britain, the dependence between high sea surge (observed sea level minus predicted astronomical tide), river flow and precipitation was studied using a measure of dependence specially developed for extremal dependence. Extreme events were interpreted using meteorological maps. This new analysis found that the strongest flow,surge dependence occurs between river flow on the north shore of the Firth of Forth and sea surge at Aberdeen, Wick and Lerwick. In contrast to most other catchments in eastern Britain, the area to the north of the Firth of Forth is not sheltered from south-westerly winds by any major topographical barrier. Therefore, precipitation from this direction may be orographically enhanced as it encounters the hills on the northern side of the firth, and high river flows may ensue. Events resulting in both high river flow and surge in the northern part of the study area were found to be caused by cyclones travelling north-eastward to the north of Scotland. High surge events, only, were associated with similar storm tracks, but without much precipitation from the fronts. High river flows, only, were associated with rain-bearing east,west-directed fronts over northern Britain, with slow-moving depressions located over or to the west of the British Isles where they are unable to generate a strong surge in the North Sea. The dependence between river flow and surge was found to be stronger during winter than summer, and a lagged analysis revealed that the dependence is strongest when flow and surge occur on the same day, but was also strong for lags of plus and minus 1 day. For precipitation, the dependence with both flow and surge is strongest when precipitation precedes them by 1 day. Copyright © 2002 Royal Meteorological Society. [source] Adjustment of the atmospheric circulation to tropical Pacific SST anomalies: Variability of transient eddy propagation in the Pacific,North America sector,THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 647 2010R. Seager Abstract El Niño,Southern Oscillation (ENSO) related precipitation anomalies in North America are related to changes in the paths of storm systems across the Pacific Ocean, with a more southern route into southwestern North America during El Niños and a more northern route into the Pacific Northwest during La Niñas. Daily reanalysis data are analyzed to confirm these changes. Seasonal mean upper tropospheric eddy statistics show, for El Niños (La Niñas), a pattern that is shifted southward (northward) compared with climatology. Paths of coherent phase propagation of transient eddies and of the propagation of wave packets are analyzed. A coherent path of propagation across the Pacific towards North America is identified that is more zonal during El Niño winters and, during La Niñas, has a dominant path heading northeastward to the Pacific Northwest. A second path heading southeastward from the central Pacific to the tropical east Pacific is more accentuated during La Niñas than El Niños. These changes in wave propagation are reproduced in an ensemble of seasonal integrations of a general circulation model forced by a tropical Pacific sea-surface temperature pattern, confirming that the changes are forced by changes in the mean atmospheric state arising from changes in tropical sea-surface temperature. A simplified model with a specified basic state is used to model the storm tracks for El Niño and La Niña winters. The results suggest that the changes in transient eddy propagation and the eddy statistics can be understood in terms of the refraction of transient eddies within different basic states. Copyright © 2010 Royal Meteorological Society [source] Southern hemisphere winter ozone fluctuationsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 572 2001P. K. Vigliarolo Abstract In this paper the relationship between ozone and atmospheric variability is explored over the southern hemisphere during the austral winter season, with special emphasis on synoptic transient fluctuations. The analysis of ozone tracks (or high-frequency ozone variability) shows that they have a significant correspondence with storm tracks at middle and high latitudes. Moreover, ozone tracks maximize over the Indian Ocean slightly downstream of the storm-track maximum, while over the Pacific region both ozone and storm tracks show decreased amplitudes. In particular, over southern South America (a region of climatological winter ozone minima and moderate to high ozone variability) the leading winter synoptic-scale variability mode was identified through a rotated extended empirical orthogonal function analysis applied to the meridional-wind perturbation at 300 hPa. The resulting mode is characterized by a baroclinic wave travelling eastward along subpolar latitudes, which maximizes near the tropopause level. Composite ozone fields based on this mode confirm, from a statistical point of view, the classical relationship between ridges (troughs) and minimum (maximum) ozone content. Furthermore, it is shown that dynamical processes in the upper troposphere and lower stratosphere associated with subpolar waves are responsible for the observed ozone distribution. This happens due to the barotropic equivalent vertical structure of the wave, together with the fact that ozone partial pressure maximizes near the level where the waves attain maximum amplitudes. [source] | ||||||||||