Distribution and content of137+134Cs and90Sr in the water of the North Sea during the years 1982 to 1984

Summary In continuation of the investigations of previous years (Kautsky [1973, 1976, 1985]; Kautsky, Jefferies, and Steele [1980]), besides surface water samples, water samples were also taken at depth for the first time along the entire station grid northwards of 53° 30 N (1982) and 53° N (1984).The fundamental distribution pattern of the^137+134Cs in the North Sea, within certain limits was practically analogous with that of the previous years. In the South and westwards of Jutland, on the basis of steep activity concentration gradients, one can again clearly recognize the boundary between the water coming out of the Channel from the South and the water coming from the North along the English coast. In the surface water, this boundary between the two water masses — as in previous years — lies between 6° E and 7° E.However, in deep water westwards of Jutland, clear differences of the distribution are observed between the years 1982 and 1984. In August/September 1982, a clear front in the entire water column between surface and bottom is present (Fig. 11). A comparable pattern, in principle, is indicated by the salinity distribution (Fig. 13). In comparison with it, in May/June 1984, a clearly stronger advance of the deep water (on the basis of the activity concentration differences) in the direction of Jutland compared with the surface water is recognizable (Fig. 12). Here also this effect appeared in outline to a certain extent in the salinity distribution (Fig. 14). Altogether, the content of radio caesium in the water of the North Sea from 1982 to 1984 has decreased by 30%. In 1984, it totalled, approximately 2400 TBq^137+134Cs, only 0.5% of the circa 481000 TBq⁴⁰K present in nature in the North Sea.

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Verteilung und Gehalt von^137+134Cs und⁹⁰Sr im Wasser der Nordsee in den Jahren 1982 bis 1984

Zusammenfassung In Weiterführung der Untersuchungen vorhergehender Jahre (Kautsky [1973, 1976, 1985]; Kautsky, Jefferies, und Steele [1980]) wurden erstmalig neben Oberflächenwasserproben auf dem gesamten Stationsnetz nördlich von 53° 30 N (1982) und 53° N (1984) auch Tiefenwasserproben entnommen.Das grundsätzliche Verteilungsmuster des^137+134Cs in der Nordsee gleicht im Rahmen einer gewissen Variationsbreite praktisch dem der vorhergehenden Jahre. Im Süden und westlich von Jütland ist anhand der steilen Aktivitätskonzentrationsgradienten wieder deutlich die Grenze zwischen dem aus dem Kanal von Süden und dem entlang der englischen Küste von Norden kommenden Wasser zu erkennen. Im Oberflächenwasser leigt diese Grenze zwischen den beiden Wassermassen — wie auch in den Vorjahren — zwischen 6° E und 7° E.Im Tiefenwasser westlich Jütland sind zwischen den Jahren 1982 und 1984 aber deutliche Unterschiede der Verteilung zu beobachten. Im August/September 1982 ist eine klare Front in der gesamten Wassersäule zwischen Oberfläche und Boden vorhanden (Abb. 11). Ein im Prinzip vergleichbares Muster zeigt die Salzgehaltsverteilung (Abb. 13). Dagegen ist im Mai/Juni 1984 ein deutlich stärkeres Vordringen des Tiefenwassers (anhand der Aktivitätskonzentrationsunterschiede) in Richtung Jütland gegenüber dem Oberflächenwasser erkennbar (Abb. 12). Auch hier zeichnet sich dieser Effekt bis zu einem gewissen Grad in der Salzgehaltsverteilung ab (Abb. 14). Insgesamt hat der Gehalt des Radiocaesium im Wasser der Nordseee von 1982 bis 1984 um 30% abgenommen. Er beträgt 1984 mit rund 2400 TBq^137+134Cs nur 0,5% der in der Nordsee von Natur aus vorhandenen rund 481 000 TBq⁴⁰K.

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Distribution et concentration du caesium 137 et 134 et du strontium 90 dans les eaux de la Mer du Nord au cours des années 1982 et 1984

Résumé Pendant la continuation des recherches des années antérieures (Kautsky [1973, 1976, 1985]; Kautsky, Jefferies et Steele [1980]) et en plus des prélèvements d'eau de surface, on a aussi prélevé pour la première fois, des échantillons d'eau profonde à toutes les stations du réseau au Nord du parallèle 53° 30 N (1982) et du parallèle 53° N (1984).Le modèle fondamental de distribution du caesium 137 et 134 en Mer du Nord est pratiquement analogue, à l'intérieur de certaines limites de latitude, à celui des années antérieures. Dans le Sud et à l'Ouest du Jutland, on peut encore reconnaître clairement, sur la base de forts gradients de concentration d'activité, la frontière entre les eaux venant de la Manche par le Sud et les eaux pénétrant par le Nord le long de la côte anglaise. Pour l'eau de surface, cette frontière entre les deux masses d'eau s'étend, comme dans les années antérieures, entre les méridiens 6° E et 7° E.Cependant, en eau profonde à l'Ouest du Jutland, de réelles différences de distributions ont été observées entre les années 1982 et 1984. En août-septembre 1982, un front net est présent dans toute la colonne d'eau comprise entre la surface et le fond (Fig. 11). Un modèle comparable, dans le principe, est indiqué par la distribution de la salinité (Fig. 13). Par contre en mai–juin 1984 on peut reconnaître, une progression significativement plus forte de l'eau profonde (sur la base des différences de concentration d'activité comparée à celle de l'eau de surface en direction du Jutland (Fig. 12). Ici aussi cet effet est apparu jusqu'à un certain degré dans la distribution de la salinité (Fig. 14). Tout compte fait, la concentration de caesium radioactif a diminué de 30% dans la Mer du Nord entre 1982 et 1984. En 1984 le caesium 137 et 134 totalisait approximativement 2400 TBq ce qui représente seulement 0,5% des 481 000 TBq environ de potassium 40 présent au naturel dans la Mer du Nord.

     

Spectral energy contributions of quasi-periodic oscillations (2–35 days) to the variability of the f oF2

The relative contributions of quasi-periodic oscillations from 2 to 35 days to the variability of f_oF2 at middle northern latitudes between 42°N and 60°N are investigated. The f_oF2 hourly data for the whole solar cycle 21 (1976–1986) for four European ionospheric stations Rome (41.9°N, 12.5°E), Poitiers (46.5°N, 0.3°E), Kaliningrad (54.7°N, 20.6°E) and Uppsala (59.8°N, 17.6°E) are used for analysis. The relative contributions of different periodic bands due to planetary wave activity and solar flux variations are evaluated by integrated percent contributions of spectral energy for these bands. The observations suggest that a clearly expressed seasonal variation of percent contributions exists with maximum at summer solstice and minimum at winter solstice for all periodic bands. The contributions for summer increase when the latitude increases. The contributions are modulated by the solar cycle and simultaneously influenced by the long-term geomagnetic activity variations. The greater percentage of spectral energy between 2 to 35 days is contributed by the periodic bands related to the middle atmosphere planetary wave activity.     

Regional energetics over the North Pacific,South China Sea and the Indonesian seas during winter

The energy equation was applied to four limited regions to investigate the basic mechanisms through which area-averaged eddy kinetic energy is maintained during the northern winter. The regions selected for this study are as follows: extratropical North Pacific (24.2°N–44.6°N, 130°E–150°W), tropical eastern North Pacific (0°–19.6°N, 170°W–110°W), South China Sea and. Bay of Bengal (0°–19.6°N, 80°E–140°E), and Timor Sea and eastern Indian Ocean (0°–19.6°S, 80°E–140°E). The zonally averaged upper flows over the first region were found to be barotropically stable. In contrast, they were barotropically unstable over the second region; namely, eddy motions over the tropical eastern North Pacific are maintained by receiving energy from zonal flows via barotropic interaction. The third and fourth regions are characterized by the importance of the conversion process between eddy available potential and eddy kinetic energy.Contribution No. 77-5, Department of Meteorology, University of Hawaii, USA.     

Long term ionospheric electron content variations over Delhi

Ionospheric electron content (IEC) observed at Delhi (geographic co-ordinates: 28.63°N, 77.22°E; geomagnetic co-ordinates: 19.08°N, 148.91E; dip Latitude 24.8°N), India, for the period 1975/80 and 1986/89 belonging to an ascending phase of solar activity during first halves of solar cycles 21 and 22 respectively have been used to study the diurnal, seasonal, solar and magnetic activity variations. The diurnal variation of seasonal mean of IEC on quiet days shows a secondary peak comparable to the daytime peak in equinox and winter in high solar activity. IEC_max (daytime maximum value of IEC, one per day) shows winter anomaly only during high solar activity at Delhi. Further, IEC_max shows positive correlation with F_10.7 up to about 200 flux units at equinox and 240 units both in winter and summer; for greater F_10.7 values, IEC_max is substantially constant in all the seasons. IECmax and magnetic activity (A_p) are found to be positively correlated in summer in high solar activity. Winter IEC_max shows positive correlation with A_p in low solar activity and negative correlation in high solar activity in both the solar cycles. In equinox IEC_max is independent of A_p in both solar cycles in low solar activity. A study of day-to-day variations in IEC_max shows single day and alternate day abnormalities, semi-annual and annual variations controlled by the equatorial electrojet strength, and 27-day periodicity attributable to the solar rotation.     

Radar observations of prevailing winds and waves in the Southern Hemisphere mesosphere and lower thermosphere

HF radar stations (utilizing the spaced-antenna partial-reflection technique) located at Adelaide (35°S, 138°E) and Mawson Station (67°S, 63°E) have observed horizontal mesospheric winds continuously since mid-1984. Observations in the period 1984–87 are compared with the Northern Hemisphere [latitude conjugate] stations of Kyoto (35°N, 136°E) and Poker Flat (65°N, 147°W), and with satellite-derived circulation models. Particular reference is made to the equinoctial changeovers in zonal flow and to the temporal and altitude variations in the planetary wave activity at Mawson and Adelaide.     

Diurnal Variation of Gravity Wave Activity at Midlatitudes in the Ionospheric F Region

We investigate the short-term fluctuations in the period range from 15 to 180 minutes in the electron density variations of the F region ionosphere. Electron density profiles obtained at the ionospheric stations of Pruhonice (49.9° N, 14.5° E) and Ebro (40.8° N, 0.5° E) at five minute time sampling have been used for this analysis. The diurnal changes of the activity of the acoustic gravity wave fluctuations (AGW) show a clear enhancement during and several hours after sunrise. The periods of such AGW's are about 60 to 75 minutes and these waves propagates vertically through the ionosphere from a source located at an altitude of 180-220 km. The most likely source for these events seems to be passage of the Solar terminator.     

Note on the azimuth deviations ofP-waves recorded at Djakarta station

Summary Differences between epicenter azimuths and the direction of arrival of firstP-waves are determined from the horizontal seismograms of Djakarta station. Systematic azimuth deviations of more than 15° are encountered in the NW through N to SE azimuths. Azimuth deviations caused by a discontinuity dipping 5, 10 and 15° and with a ratio of velocity of longitudinal waves at either side of the discontinuity of 1.23, 1.38 and 1.52 are calculated. The most simple model suitable to the observed data is that of a discontinuity with a velocity ratio of about 1.5, dipping 8–10° in a N 10° E direction. In the S and W directions of Djakarta no dipping discontinuity is needed to account for the few observed data in these quadrants.     

A high-resolution satellite-derived sea surface temperature climatology for the western North Atlantic Ocean

Long-term and high-resolution (∼1.2 km) satellite-derived sea surface temperature (SST) fields of a monthly mean time series for the 1985–1999 period, and a daily climatology have been calculated for the North West Atlantic Ocean. The SST fields extend from 78°W to 41°W in longitude, and 30°N to 56°N in latitude, encompassing the region off Cape Hatteras, North Carolina, to the southern Labrador Sea. The monthly mean time series, consists of 180 cloud-masked monthly mean SST fields, derived from a full-resolution NOAA/NASA Pathfinder SST data set for the 1985–1999 period. The satellite-derived monthly mean SST fields, as compared with in situ monthly mean near-surface ocean temperatures from buoys located in the western North Atlantic, yield an overall RMS difference of 1.15 °C. The daily climatology, which consists of 365 fields, was derived by applying a least-squares harmonic regression technique on the monthly mean SST time series for the full study period. The monthly mean and daily climatological SST fields will be useful for studying inter-annual variability related to climate variability of SST over the study domain.     

Paleomagnetism of the Djebel Reouina Namurian Formation (Tindouf Basin,Algeria)

The paleomagnetic study of the Namurian of Reouina (28.9°N, 08.0°W) revealed the existence of two magnetization components, either juxtaposed or superimposed, besides a viscous component. The high blocking temperature component, carried by hematite, has a mean direction defined by D = 126.9° and I = 10.8°. It provides a Namurian paleomagnetic pole located at 28.4°S and 56.9°E (K = 642, A ₉₅=1.7°). The second component is carried at least in part, by grains with blocking temperatures lower than 550°C. Though well defined, it consists of two superimposed components, the high unblocking temperature component with a likely Permian overprint.     

Global distribution of gaseous mercury in the troposphere

Atmospheric mercury concentrations were measured during a nautical expedition on the Atlantic Ocean between Hamburg (54°N, 10°E) and Santo Domingo (20°N, 67°W). In addition, samples were taken during flights on a commerical aircraft in the upper and middle troposphere between 60°N and 55°S, mostly over the Pacific Ocean. The data obtained in the lower troposphere over the Northern Atlantic show considerable variation in the Hg concentrations, with values ranging between 1 and 11 ng/m³; the average concentration was found to be 2.8 ng/m³. The upper tropospheric data show an interhemispheric difference with average values of 1.45 ng/m³ and 1.08 ng/m³ in the Northern and Southern Hemisphere, respectively. This suggests that mercury production occurs predominantly over the continents both by natural and anthropogenic processes. The mercury content in aerosols was found to be 0.3 ng/m³, or one-tenth of the atmospheric concentration. The data indicate a mean residence time of mercury in the atmosphere of a few months to one year.     

Fracture system in Phlegraean Fields (Naples,southern Italy)

During the 1983 seismic crisis in the Phlegraean Fields bradyseismic region (southern Italy), a structural analysis of the area was carried out.With a detailed field survey based on a net of 34 measure stations, a total of 536 fractures (mainly joints and a few normal faults) were measured on a 10 × 10 km area in volcanites capable of memorizing post depositional stress activity by fracturing.The analysis of the collected data was performed with the data bank of the University of Rome computer facilities. The azimuthal analysis of total fractures showed a nonrandom distribution with 5 major sets: N13°E, N45°E, N14°W, N55°W and E-W. These preferential orientations have been detected with an automatic fitting of gaussian curves (bell curves) on the azimuthal histograms. The areal distribution showed that all these fracture sets are in general present in the main collapse area. An azimuthal analysis performed by selecting the data collected for rocks older than 4,600 y BP showed a possible youngest age for the N14°W set (domain) (E-W extension). Fractures with an «opening» wider than 1 cm presented the same 5 azimuthal sets and fit fairly well with a concentric distribution around the main collapse area. The presence of an analogous radial pattern is not evident. A tentative interpretation model relates the superficial fracture sets to two possible causes: volcanic activity, including doming and collapsing, and propagation of active tensile deformations in the sedimentary basement due to regional stress trajectories.Contribution of «Centro di Studio per la Geologia dell'Italia Centrale», CNR, Roma.     

Changes of Ozone Content at Middle Latitudes During Forbush Decreases in Cosmic Rays

The variations of total ozone at Alma-Ata (43°N, 76 °E) and ozone profiles obtained by balloon sounding at Tateno (36°N, 140°E), Wallops Island (38°N, 75°W) and Cagliari (39°N, 9°E) in the periods of Forbush decreases (FD) in galactic cosmic rays have been analysed. A decrease of total ozone was observed in the initial stage of the FD and an increase 10–11 days later. The average total deviations calculated using the superposed epoch method for 9 FD events are equal to 30 D. U. in the positive and to –18 D. U. in the negative phase. The changes of average ozone profiles, associated with 26 FD events, are more significant in the lower stratosphere and upper troposphere. The decrease of the partial ozone pressure at a height of 12–15 km is about 30 mb. These vertical variations of ozone coincide with the average changes of the respective temperature profiles. A cooling, on the average, of 3°C was observed at 12–15 km, and a heating of 4°C below this level.     

Rupture process of large earthquakes in the northern Mexico subduction zone

The Cocos plate subducts beneath North America at the Mexico trench. The northernmost segment of this trench, between the Orozco and Rivera fracture zones, has ruptured in a sequence of five large earthquakes from 1973 to 1985; the Jan. 30, 1973 Colima event (M _s 7.5) at the northern end of the segment near Rivera fracture zone; the Mar. 14, 1979 Petatlan event (M _s 7.6) at the southern end of the segment on the Orozco fracture zone; the Oct. 25, 1981 Playa Azul event (M _s 7.3) in the middle of the Michoacan gap; the Sept. 19, 1985 Michoacan mainshock (M _s 8.1); and the Sept. 21, 1985 Michoacan aftershock (M _s 7.6) that reruptured part of the Petatlan zone. Body wave inversion for the rupture process of these earthquakes finds the best: earthquake depth; focal mechanism; overall source time function; and seismic moment, for each earthquake. In addition, we have determined spatial concentrations of seismic moment release for the Colima earthquake, and the Michoacan mainshock and aftershock. These spatial concentrations of slip are interpreted as asperities; and the resultant asperity distribution for Mexico is compared to other subduction zones. The body wave inversion technique also determines theMoment Tensor Rate Functions; but there is no evidence for statistically significant changes in the moment tensor during rupture for any of the five earthquakes. An appendix describes theMoment Tensor Rate Functions methodology in detail.The systematic bias between global and regional determinations of epicentral locations in Mexico must be resolved to enable plotting of asperities with aftershocks and geographic features. We have spatially shifted all of our results to regional determinations of epicenters. The best point source depths for the five earthquakes are all above 30 km, consistent with the idea that the down-dip edge of the seismogenic plate interface in Mexico is shallow compared to other subduction zones. Consideration of uncertainties in the focal mechanisms allows us to state that all five earthquakes occurred on fault planes with the same strike (N65°W to N70°W) and dip (15±3°), except for the smaller Playa Azul event at the down-dip edge which has a steeper dip angle of 20 to 25°. However, the Petatlan earthquake does prefer a fault plane that is rotated to a more east-west orientation—one explanation may be that this earthquake is located near the crest of the subducting Orozco fracture zone. The slip vectors of all five earthquakes are similar and generally consistent with the NUVEL-predicted Cocos-North America convergence direction of N33°E for this segment. The most important deviation is the more northerly slip direction for the Petatlan earthquake. Also, the slip vectors from the Harvard CMT solutions for large and small events in this segment prefer an overall convergence direction of about N20°E to N25°E.All five earthquakes share a common feature in the rupture process: each earthquake has a small initial precursory arrival followed by a large pulse of moment release with a distinct onset. The delay time varies from 4 s for the Playa Azul event to 8 s for the Colima event. While there is some evidence of spatial concentration of moment release for each event, our overall asperity distribution for the northern Mexico segment consists of one clear asperity, in the epicentral region of the 1973 Colima earthquake, and then a scattering of diffuse and overlapping regions of high moment release for the remainder of the segment. This character is directly displayed in the overlapping of rupture zones between the 1979 Petatlan event and the 1985 Michoacan aftershock. This character of the asperity distribution is in contrast to the widely spaced distinct asperities in the northern Japan-Kuriles Islands subduction zone, but is somewhat similar to the asperity distributions found in the central Peru and Santa Cruz Islands subduction zones. Subduction of the Orozco fracture zone may strongly affect the seismogenic character as the overlapping rupture zones are located on the crest of the subducted fracture zone. There is also a distinct change in the physiography of the upper plate that coincides with the subducting fracture zone, and the Guerrero seismic gap to the south of the Petatlan earthquake is in the wake of the Orozco fracture zone. At the northern end, the Rivera fracture zone in the subducting plate and the Colima graben in the upper plate coincide with the northernmost extent of the Colima rupture zone.     

Is there an active subduction beneath the Gibraltar orogenic arc? Constraints from Pliocene to present-day stress field

We present a new set of brittle microtectonic measurements carried out in the Pliocene and Quaternary rocks outcropping in several key sectors of the western Betic and Rif orogen, the so-called Gibraltar orogenic arc. This data set, along with available earthquake focal mechanisms and borehole breakouts, allowed us to compile the Pliocene and Quaternary stress map of this area. This map provides new constraints for tectonic models and the present-day tectonic activity of the proposed active eastward subduction of oceanic lithosphere beneath the Gibraltar Arc and roll-back. The horizontal maximum compressive stress (S_Hmax) is NW-SE in the Betic Orogen and N-S/NNW-SSE in the southern Rif Cordillera. There is a significant consistency between S_Hmax and the displacement field deduced from GPS measurements with respect to the African plate: both appear to reflect the NW-SE convergence between the African and the European plates that is perturbed in the Rif. We propose that part of the eastern Rif behaves as a quasi-rigid block welded to the stable African plate. This block is bounded by important faults that localized most of the deformation disturbing the stress and surface displacement field. Pliocene to Quaternary N-S to NW-SE Africa-Europe plate convergence seem to be associated to the reorganization of the remnant Early Miocene subduction system in a continental–continental collision framework. Three-dimensional reconstruction of available seismic tomography plotted against the intermediate seismicity shows that only part of the old subduction system, whose orientation ranges from N20°E to N100°E, remains active: the portion ranging from N30°E to N40°E, orthogonal to the regional convergence.     

Long-term variation of solar UVB (290–330 nm) observed at the earth's surface

During solar cycle 21 (1976–86), the primary solar irradiance at 300 nm was steady during 1980–82 and thereafter decreased until 1986 by only 2–3%. The stratospheric ozone in middle latitudes had a QBO of 3–4% in this interval but the long-term ozone trend was less than 3% per decade, which could result in a UVB increase of only 5–6% per decade. Thus, the combined effect of changes in primary solar irradiance and ozone changes could be an increase of 5–6% in UVB, observed at ground during 1977–81 and a steady level during 1981–86. During 1976–86, the average cloudiness changed by less than 5% indicating UVB changes of 5% or less on this count. The aerosol level was almost constant during 1976–82 and increased abruptly in 1982 due to the E1 Chichon eruption and decayed slowly unitl 1986. Thus, due to aerosols only, the UVB was expected to be constant during 1976–82, to decrease sharply in 1982 and to recoup slowly thereafter.Measurements of clear-sky solar UVB at ground made at Jungfraujoch (Swiss Alps, 47°N, 8°E) during 1981–89 and at Rockville, USA (39°N, 77°W) were not comparable between themselves and did not follow the above expected patterns. Neither did the all-day R-B meter UVB measurements at Philadelphia, USA (40°N, 75°W) and Minneapolis, USA (45°N, 93°W). We suspect that some of these measurements are erroneous. This needs further detailed scrutiny.     

Night-to-night changes in the characteristics of gravity waves at stratospheric and lower-mesospheric heights

Observations made with the co-located Rayleigh lidar and MST radar systems at Aberystwyth (52.4°N, 4.1°W) in Wales and radiosondes from Valentia (51.9°N, 10.2°W) in Eire are used to investigate the changes in the vertical propagation of gravity waves during periods of 4 days in June 1995 and February 1993. In each month, the lidar observations show that the wave activity in the upper stratosphere and lower mesosphere changes between two pairs of days. The radar and radiosonde measurements indicate that mountain waves make no contribution to the changes in intensity. Instead, the changes seem to arise largely from the presence or absence of long-period waves with vertical wavelengths near 8 and 10 km in June and February, respectively. The influence of such waves on the vertical wavenumber spectra is examined and related to the evidence for convective instabilities provided by the temperature profiles.     

Bemerkungen zu den Wellenbeobachtungen der Wetterschiffe im östlichen Nordatlantik

Zusammenfassung Mittelwerte und Häufigkeitsverteilungen von Wellengrößen nach niederländischen Beobachtungen werden verglichen: auf Station A (62° N, 33° W) mit Werten nach amerikanischen Beobachtungen, auf Station J (52°30N, 20° W) mit Werten nach britischen Beobachtungen, auf Station K (45° N, 16° W) mit Werten nach französischen Beobachtungen, auf Station M (66° N, 2° E) mit Werten nach norwegischen Beobachtungen.Auf jeder dieser Stationen ergeben sich deutliche Hinweise für bestimmte nationale Unterschiede bei den Wellenbeobachtungen. Hierdurch wird das Auffinden von regionalen Unterschieden in den statistischen Wellengrößen im Nordatlantischen Ozean merklich erschwert.Es wird wahrscheinlich gemacht, daß man in großen Teilen des offenen Nordatlantischen Ozeans bei geringen Windstärken (0–3 Bft) mit einer Dünung von im Mittel 7,5–8,5 sec (signifikanter) Periode und 1,6–1,8 m (signifikanter) Höhe rechnen kann.

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On the wave observations by weather ships in the Eastern North Atlantic

Summary Average values and frequency distributions of wave data according to Netherlands observations are compared, on Station A (62° N, 33° W), with data according to USA observations, on Station J (52°30N, 20° W), with data according to British observations, on Station K (45° N, 16° W), with data according to French observations, on Station M (66° N, 2° E), with data according to Norwegian observations.On each of these stations, national differences in the observation of waves are shown to exist. In consequence of this, it is difficult to prove the reality of regional differences between wave data in different parts of the North Atlantic.Evidence is supplied that in large parts of the open North Atlantic the average (significant) period and the average (significant) height of the swell occurring at low local winds (0–3 Beaufort) are 7,5–8,5 sec and 1,6–1,8 m, respectively.

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Sur les observations de vagues par les navires météorologiques dans la partie orientale de l'Atlantique Nord

Résumé On a comparé des valeurs moyennes et des fréquences de caractéristiques de vagues d'après les observations néerlandaises avec des caractéristiques, d'après des observations américaines, à la position A (62° N, 33° W), d'après des observations britanniques, à la position J (52°30N, 20° W), d'après des observations françaises, à la position K (45° N, 16° W), d'après des observations norvégiennes, à la position M (66° N, 2° E).Sur chaque position on a signalé l'existence de différences nationales dans l'observation des vagues. Par conséquence, il est difficile de démontrer la réalité de différences régionales entre les caractéristiques de vagues dans des parties différentes de l'Atlantique Nord.Les observations fournissent l'indication que dans la plus grande partie de l'Atlantique Nord, la période (significante) moyenne et la hauteur (significante) moyenne de la houle, présente pendant des vents locales faibles (0–3 Beaufort), sont de 7,5 à 8,5 sec et de 1,6 à 1,8 m respectivement.

     

Evidence of more efficient whistler-mode transmission during periods of increased magnetic activity

In a previous study it was reported that whistler- mode signals received at Faraday, Antarctica (65°S,64°W) and Dunedin, New Zealand (46°S, 171°E) with entry regions in Pacific longitudes (typically from the VLF transmitter NLK, Seattle, USA) showed an increase in transmission of wave energy as magnetic activity increased. However, signals with entry regions in Atlantic longitudes (typically from the NSS transmitter, Annapolis, USA) did not appear to show such a relationship. This paper reports the results of a study of the same two longitude ranges but with the opposite transmitter providing additional whistler-mode signal information, with L-values in the range 1.8–2.6. Transmissions from NLK once again indicate a relationship between the transmission of wave energy and magnetic activity even though the signals were propagating in Atlantic longitudes, not Pacific. Any trend in NSS events observed at Dunedin was obscured by a limited range of magnetic activity, and duct exit regions so close to the receiver that small-scale excitation effects appeared to be occurring. However, by combining data from both longitudes, i.e Pacific and Atlantic, and using only ducts with exit regions that were > 500 km from the receiver, NSS events were found to show the same trend as NLK events. No significant longitude-dependent or transmitter-dependent variations in duct efficiency could be detected. Duct efficiency increases by a factor of about 30 with K_p = 2–8 and this result is discussed in terms of changes in wave-particle interactions and duct size.     

Cloud tracking using satellite data for predicting the probability of heavy rainfall events in the Mediterranean area

The use of cloud tracking techniques and storm identification procedures is proposed in this paper with the aim of predicting the evolution of cloud entities associated with the highest rainfall probability within a given meteorological scenario. Suitable algorithms for this kind of analysis are based on the processing of digital images in the thermal infrared (IR) band from geostationary satellites: a selection of such algorithms is described in some detail together with a few real case applications. Three heavy rainfall events have been selected for this purpose with reference to the extreme meteorological situation observed during Fall 1992 and 1993 over the Mediterranean area. A window from 30 to 60 °N and from 20 °W to 30 °E has been identified for the analysis of data from the radiometer on board the ESA Meteosat platform. In conclusion, the suitability of cloud tracking techniques for predicting the probability of heavy rainfall events is discussed provided that the former are associated with proper modeling of small scale rainfall distribution.