T. D. Davies

Objective Climatology of Cyclones in the Mediterranean Region

Abstract An objective cyclone detection and tracking analysis is performed over an 18-yr period, for the Mediterranean basin. The high-resolution (1.125° × 1.125° grid) European Centre for Medium-Range Weather Forecasts data used in this study proved to be particularly suitable for the detection and tracking techniques and to identify subsynoptic-scale Mediterranean lows, which have often been underestimated in previous studies. The major characteristics of Mediterranean cyclones are examined and compared with other Northern Hemisphere depressions. Both cyclogenesis and cyclolysis regions are identified in the domain of study. In addition, characteristics of Mediterranean depressions, such as cyclone duration and intensity, as well as their persistence throughout the year, are shown to be quite variable for different formation areas. Overall, the regions where cyclogenesis is mainly controlled by topography, like the Gulf of Genoa and south of the Atlas Mountains, seem to generally account for the most in...

Causes of concentration/discharge hysteresis and its potential as a tool for analysis of episode hydrochemistry

Episodic variations in dissolved solutes are frequently complicated by a cyclical relationship between concentration and stream discharge. Established three-component models of runoff generation are used to explain this hysteresis effect and to illustrate how different component concentrations produce different hysteresis forms. It is demonstrated that a two-component model cannot reproduce all the hysteresis forms commonly observed. A method, based on the three-component system, is derived by which C/Q hysteresis can be used to predict relative component concentrations. This may provide a qualitative chemical description of sources supplying runoff for locations where these have not yet been directly established or a validation test where possible components have been sampled. The method has been tested using data collected at streams in the Adirondacks, New York, and the Northern Appalachian Plateau, Pennsylvania, during the Episodic Response Project of the U.S. Environmental Protection Agency. Predicted component compositions were in good agreement with measurements made during other studies and with those subsequently obtained from one of the Pennsylvania watersheds.

Cluster analysis: A technique for estimating the synoptic meteorological controls on air and precipitation chemistry: Method and applications

Abstract Combining the pattern recognition capabilities of cluster analysis with isobaric air trajectory data is a useful way of quantifying the influence of synoptic meteorology on the pollution climatology at a site. A non-hierarchial clustering of 1000 mb isobaric trajectories, using squared Euclidean distance as a similarity measure, leads to the identification of a finite number of distinct synoptic patterns. Typical airbore and aqueous pollutant concentrations associated with each of these patterns may then be established. By considering 3-day air trajectories in this study, the “history” of an air parcel is captured in an improved manner, when compared with attempts to use individual day weather “types” to characterize meterological situations.

Climatology of Cyclogenesis Mechanisms in the Mediterranean

Abstract A general climatology of the main mechanisms involved in Mediterranean cyclogenesis is presented. A diagnostic study of both composite means and case studies is performed to analyze processes occurring in different seasons, and in different cyclogenetic regions within the same season. It is shown that cyclones that developed over the three most active areas in winter—the Gulf of Genoa, the Aegean Sea, and the Black Sea—are essentially subsynoptic lows, triggered by the major North Atlantic synoptic systems being affected by local orography and/or low-level baroclinicity over the northern Mediterranean coast. It is also suggested that cyclones in two, or all three, of these regions often occur consecutively, linked to the same synoptic system. In spring and summer, thermally induced lows become progressively more important, despite the existence of other factors, such as the Atlas Mountains contributing to lee cyclogenesis in northern Africa, or the extension of the Asian monsoon into the eastern ...

Trends of Extreme Temperatures in Europe and China Based on Daily Observations

Ten of the longest daily temperature series presently available in Europe and China are analysed, focusing on changes in extremes since pre-industrial times. We consider extremes in both a relative (with respect to the time of year) and an absolute sense. To distinguish changes in extremes from changes affecting the main part of the temperature distribution, a percentile smaller than 10 (and/or larger than 90) is recommended for defining an extreme. Three periods of changes in temperature extremes are identified: decreasing warm extremes before the late 19th century; decreasing cold extremes since then and increasing warm extremes since the 1960s. The early decreases and recent increases of warm extremes dominate in summer, while the decrease of cold extremes for winter persists throughout the whole period. There were more frequent combined (warm plus cold) extremes during the 18th century and the recent warming period since 1961 at most of the ten stations, especially for summer. Since 1961, the annual frequency of cold extremes has decreased by about 7% per century with warm extremes increasing by more than 10% per century but with large spatial variability. Compared with recent annual mean warming of about 2–3 ° C/century, the coldest winter temperatures have increased atthree times this rate, causing a reduced within-season range and therefore less variable winters. Changes in the warmest summer temperatures since 1961 exhibit large spatial variability, with rates of change ranging from slightly negative to 6 ° C/century. More extensive station observations since 1961 indicate that the single site results are representative of larger regions, implying also that the extremes studied are the result of large-scale changes. Recent circulation changes in daily gridded pressure data, used as an indicator of wind speed changes, support the results by explaining some of the trends.

Episodes of high ozone concentrations at the earth's surface resulting from transport down from the upper troposphere/lower stratosphere: a review and case studies

Abstract When compared with photochemically induced ozone episodes produced in situ near the Earths surface, relatively little attention has been paid to those surface episodes (including mountain sites) which have their origin in transport down from the upper troposphere/lower stratosphere. Although these episodes may be relatively uncommon, they can produce transient peak ozone concentrations of around 100 ppb at sea-level and concentrations in excess of 250 ppb have been reported in mountain regions. The published literature covering such episodes is reviewed, and those synoptic/dynamic features which are common to them are identified. These features are examined in detail through two case studies. The first is a winter 10-day period at a station on the Dutch coast when mean hourly concentrations exceeded 70 ppb on several occasions. The high concentrations over the period appeared to be a consequence of several processes. It is possible that photochemical production from precursors transported from the Greece/Italy region and from central-west Europe made important contributions, although the timing was early in the year for NW Europe. Other components of the episode are more readily explained in terms of mixing down of ozone-rich air from stratospheric intrusions and transport into the planetary boundarylayer, either in association with vigorous cold fronts, or in immediately adjacent high pressure ridges; or in more extensive following anticyclones. Stratospheric intrusions, associated with pronounced cut-off circulations, acted over a number of days to provide a large reservior of ozone-rich air in subsiding, non-dispersive, circulations in the middle/lower troposphere, for eventual transport to the ground. This transport depended on the diurnal cycle of vertical exchange in the lower atmosphere, and so may be in phase with any ozone produced by photochemical reactions near the surface. The second case study was a short-lived ozone “spike” (92 ppb hourly concentration) at a mountain site (3580 m elevation) in Switzerland. In this case, potential vorticity distributions and three-dimensional back-trajectories confirmed that the rapid descent of ozone-rich air, from a tropopause fold, behind a cold front was the mechanism of transfer to the mountain. In both case studies the stratospheric intrusions were associated with tropopause folds on the Western side of upper troughs/cut-off lows.

The composition of snowfall, snowpack and meltwater in the Scottish Highlands ― evidence for preferential elution

Abstract Acidic snows in a small, remote, high-altitude snowpack in the Cairngorms, Scotland, give rise to meltwaters which are proportionally rich in sulphate and nitrate. As a consequence, the within-pack snows become proportionally rich in chloride, even though depleted in solute. Preferential elution appears to be a major process in the chemical evolution of snowfall and snowpack.

Decline in Mediterranean rainfall caused by weakening of Mediterranean cyclones

Wet season (October–March) rainfall over the Northern Mediterranean has decreased over the last four decades. The decline is being forced locally by a decrease in the intensity of cyclogenesis events in the region. The forcing is subtle, since overall cyclone frequency exhibits no significant trend over time. The reduction in strength of the most intense cyclones is driven by recent trends observed in the coupled ocean-atmospheric circulation over the Northern Atlantic, in particular by the northwards migration of the main Atlantic storm-tracks.

Monthly mean pressure reconstructions for Europe for the 1780–1995 period

Monthly grid-point pressure data are reconstructed from station records of pressure for Europe since 1780. The region encompasses 35-70°N to 30°W-40°E. The reconstructions are based on a principal components regression technique, which relates surface pressure patterns to those of the station pressure data. The relationships are derived over a calibration period (1936-1995) and the results tested with independent data (the verification period, 1881-1935). The reconstructions are of excellent quality, although this is slightly lower for regions with poor station coverage in the early years, particularly during the summer months. The reconstructions are compared with other monthly mean pressure maps produced by Lamb and Johnson (1966) for the years 1780-1872 and by Kington (1980, 1988) for 1781-1785. Both of these map series show systematic biases relative to the present reconstructions.

Variability of the surface atmospheric circulation over Europe, 1774-1995

Long series of monthly surface pressure observations are analysed from the 1770s to 1995, using empirical orthogonal function (EOF) analysis and circulation indices. Analyses of the associated principal components show that the variability of surface atmospheric circulation over Europe (35°-70°N and 30°W-40°E) is well captured, using only 20 stations. The first three EOF patterns of the study are the central tendency of European pressure (EOF 1), a zonal flow pattern over Europe (EOF 2), and a blocking/cyclonic pattern in the eastern North Atlantic (EOF 3). EOF analyses on sub-periods of the station records suggest episodes of more intense meridional circulation (EOF 3) from 1822 to 1870, and stronger zonal westerlies from 1947 to 1995. Simple zonal circulation indices were also constructed for a North Atlantic Oscillation (NAO) index using Gibraltar and Reykjavik (1821-1995), a western European zonal index using Madrid, Barcelona, Trondheim and Lund (1786-1995), and a Paris-London index (1774-1995). Correlation analysis suggests that the NAO may be a better indicator of eastern North Atlantic blocking or cyclonicity (EOF 3) than of European zonal flow, especially outside the winter months. Both the western European and Paris-London zonal indices were highly correlated with the time series principal components (PCs) of EOF 2, extending the construction of a reliable monthly index of European surface westerlies to 1774. The zonal flow indicated by these circulation indices appears to have been considerably more variable, with more extreme values, in the late 18th and early 19th centuries than in the 20th century. From the 200-year perspective presented here, the recent positive trend in the NAO does not appear unusual.