Long-term Characteristics of Seasonal Rainfall at Katherine,Northern Territory

Rainfall at Katherine, Northern Territory, is concentrated into a few months, mainly December-March. For the 101 seasons, 1888–89 to 1988–89, the average rainfall is 947 mm and it is relatively reliable with a coefficient of variation of 24 per cent. Within this period use of the 15 seasons moving average shows two major spans of seasons in which average seasonal rainfall has increased and two others of decreasing average seasonal rainfall Also, the rain-causing mechanisms that operate in the region produce markedly more variable rainfall within seasons. This paper characterises the types of growing seasons at Katherine on the basis of the proportion of the seasonal total that falls within each of the seventeen fortnightly periods from 9 October to 3 June. Overall, 31 different groups of seasons are identified with 13 groups consisting of a single season. Conversely, the largest group contains 35 seasons. The analyses identify groups of seasons in which rainfall has occurred early or late in the season and others which have featured dry spells or wet spells in the core of the season. This variability helps to explain some of the challenges for farming systems research and for the development of sustainable farming systems.     

Synoptic weather patterns and wave conditions,Surfers Paradise Beach,south-eastern Queensland, 1988

Wave conditions experienced along the beaches of south-eastern Australia are controlled by winds generated by synoptic scale weather systems over the Tasman and Coral Seas. A synoptic climatological approach provides a framework to establish relationships between these synoptic weather map patterns and daily wave conditions recorded at Surfers Paradise beach. This establishes frequently recurring synoptic weather map patterns (synoptic types) and wave conditions at Surfers Paradise associated with each type. Eight synoptic types, differentiated by isobar orientation and pressure gradient, are identified for a 12 month period. Moderate wave power conditions occurred most frequently and were produced by two anticyclonic types. Less frequent, low wave power conditions were produced by synoptic types with offshore, south-west to westerly surface airflow over the southern Coral and northern Tasman Seas. Synoptic situations dominated by a tropical cyclone or extra-tropical cyclone produced the highest, but least frequent wave power conditions. The results indicated that the synoptic climatological approach provides a viable framework to establish and examine links between weather systems and wave conditions.     

Meteorological Sub‐divisional Scale Rainfall Monitoring Using Kalpana‐1 VHRR Measurements

Geostationary satellites provide measurements over a wider geographical area with high temporal sampling, while microwave measurements are more accurate but sparse. For continuous monitoring of the Indian monsoon, geostationary platform would be ideal. In this study, INSAT (Indian National Satellite) Multi‐spectral Rainfall Algorithm (IMSRA) has been used for the estimation of rainfall from Kalpana‐1 very high resolution radiometer (VHRR) measurements. IMSRA benefits from the relative advantages of infrared and microwave sensors and is operational at the India Meteorological Department (IMD). In this paper, rainfall is estimated over India at meteorological sub‐divisional scale during the south‐west monsoon season of 2009 using Kalpana‐1 satellite measurements. This is the first experimental attempt to generate meteorological sub‐divisional scale rainfall maps using Kalpana‐1 satellite measurements. The rainfall maps for the south‐west monsoon season over the Indian land region are successfully utilised as a space input for the drought monitoring of the year 2009. The results have been compared with the IMD gauge‐based accumulated rainfall maps at monthly and seasonal time scales. The qualitative comparison suggests that rainfall maps generated using the present methodology is in good agreement with the IMD rainfall maps. The quantitative comparison of the sub‐divisional monthly accumulated rainfall shows a correlation of 0.77 and standard error of 71 mm over the non‐orographic regions, whereas a correlation of 0.60 and standard error of 117 mm is observed over the orographic regions. The present study shows that Kalpana‐1 satellite‐based rainfall estimates (IMSRA technique) can act as a complementary tool for the monsoon monitoring over the Indian meteorological sub‐divisions and can be used for various meteorological and hydrological applications.     

The effects of neutral winds on the propagation of medium-scale atmospheric gravity waves at mid-latitudes

The influence of neutral winds on the propagation of medium-scale atmospheric gravity waves at mid-latitudes is investigated. A 3-dimensional neutral wind model is developed and used together with an atmospheric model in a gravity-wave ray-tracing analysis. It is demonstrated that the thermospheric wind can act as a filter for waves travelling at unfavorable angles to the mean flow, via the mechanisms of reflection and critical coupling. This wind filtering action rotates clockwise diurnally through 360° in the northern hemisphere. Observational evidence is presented which supports these predictions. Extensive modelling indicates that (a) faster and longer period waves are least affected by the neutral winds and (b) fixed-height (e.g. HF Doppler) observations of medium scale gravity waves is only likely to be possible for waves generated locally (within 500–1000 km). Waves generated at greater distances are probably dissipated before reaching the observation region.     

降雨和空气污染对城市居民公共自行车使用的影响研究——以西安市为例

公共自行车交通（以下简称公自）作为近年来快速发展的新型公共交通模式,逐渐成为国内外相关学科的研究热点。国内外大量学者探索了公自的使用特征及影响因素,但关注天气条件对公自使用影响的研究不足,来自中国城市的实证证据更是缺乏。本文以西安市为案例,利用西安市公自系统运营数据,探索降雨和空气污染对城市居民公自使用的影响。研究结果表明：①降雨和空气污染条件下公自使用规模减少,但降雨带来的影响大于空气污染。②降雨和空气污染条件下公自站点中平衡站点比例减少,且公自借出和归还为主站点的冷热点区域空间分布发生显著差异。③降雨和空气污染对公自使用强度有显著影响,一级站点数量明显减少,且降雨的影响大于空气污染。降雨条件下公自使用高强度站点中心化和沿走廊分布的空间特征弱化。但空气污染条件下,无显著变化。研究结果有助于准确评估降雨和空气污染对于城市居民社会经济活动的影响程度,也能为更准确的研究城市居民公自使用的影响机制提供参考依据。     

Dynamics and the ozone distribution in the winter stratosphere : modelling the inter-hemispheric differences

Results from a two year simulation of a General Circulation Model are used to illustrate the main differences found in the lower stratosphere dynamics and the ozone distribution between the Southern and the Northern Hemispheres in winter.The model extends from ground to mesospheric levels with a spectral horizontal resolution up to isotropic wavenumber 42. It incorporates a fully interactive scheme for the ozone mixing ratio which accounts for photochemical sources and sinks, advection by the model winds and coupling with radiative calculations.The model reproduces the large scale inter hemispheric differences quite well, with a very stable and cold vortex in the Southern Hemisphere and a warmer vortex often distorted in the Northern Hemisphere. It is concluded that due to interactions between dynamics, polar stratospheric cloud formation and chemistry, there is a possibility that some stratospheric ozone depletion could be effective in late winter near the night terminator in the Northern Hemisphere, whereas significant ozone depletion only occurs in early spring in the Southern Hemisphere.The importance of synoptic scale dynamics on the ozone transport between the high latitudes and the equator is also stressed. The model develops tongues of ozone-rich air from the high latitudes which are irreversibly mixed at mid-latitudes with tongues of ozone-poor air from the low latitudes. Similar tongues or filaments are clearly visible in the TOMS satellite data. They result from the activity of medium scale-waves in the Southern Hemisphere, whereas in the Northern Hemisphere the larger scale planetary waves play a major role in their development, and their size and extension are larger. It is concluded that transport of the ozone depletion to the mid-latitudes could be more effective in the Northern than in the Southern Hemisphere.     

Winter Daily Precipitation Variability over Northern Scotland

Daily winter (December, January and February) precipitation variability over mainland Northern Scotland (1994–2004) is analysed using 99 rain gauges. Principal component analysis (PCA) is used to identify four main patterns of rainfall variability that collectively account for nearly 75% of the variance. The most important pattern (PC 1) shows a west to east decline in rainfall; and is associated with a westerly wind flow, orographic enhancement and rain shadow effects. PC 2 shows a south to north decline in rainfall, a distribution that is generated disproportionately by southerly winds and a more southerly storm track than PC 1's pattern. PC 3 produces the most rainfall in Aberdeenshire and the least in Inverness-shire and Argyll. This spatial distribution is caused mostly by south-easterly flows, with the lowest pressure occurring in eastern Scotland or over the North Sea. PC 4 exhibits a localised precipitation maximum in Nairnshire and Moray, which tends to occur on days with a north to north-easterly wind. Cluster analysis is used to identify six rainfall regions, with regionalisations being derived for each wind direction. The most important sub-division in the region's rainfall climatology is between Aberdeen and Banff and the remainder of the mainland; rainfall variability over time in these two eastern counties thus differs considerably from other parts of Northern Scotland. The Caithness Plain also emerges as a distinct region despite its small area. Distinct rainfall regions are most evident on days when the wind blows from the westerly quadrant.     

Simulations of the behaviour of the polar thermosphere and ionosphere for northward IMF

The dynamics and structure of the polar thermosphere and ionosphere within the polar regions are strongly influenced by the magnetospheric electric field. The convection of ionospheric plasma imposed by this electric field generates a large-scale thermospheric circulation which tends to follow the pattern of the ionospheric circulation itself. The magnetospheric electric field pattern is strongly influenced by the magnitude and direction of the interplanetary magnetic field (IMF), and by the dynamic pressure of the solar wind. Previous numerical simulations of the thermospheric response to magnetospheric activity have used available models of auroral precipitation and magnetospheric electric fields appropriate for a southward-directed IMF. In this study, the UCL/Sheffield coupled thermosphere/ionosphere model has been used, including convection electric field models for a northward IMF configuration. During periods of persistent strong northward IMF B_z, regions of sunward thermospheric winds (up to 200 m s⁻¹) may occur deep within the polar cap, reversing the generally anti-sunward polar cap winds driven by low-latitude solar EUV heating and enhanced by geomagnetic forcing under all conditions of southward IMF B_z. The development of sunward polar cap winds requires persistent northward IMF and enhanced solar wind dynamic pressure for at least 2–4 h, and the magnitude of the northward IMF component should exceed approximately 5 nT. Sunward winds will occur preferentially on the dawn (dusk) side of the polar cap for IMF B_y negative (positive) in the northern hemisphere (reverse in the southern hemisphere). The magnitude of sunward polar cap winds will be significantly modulated by UT and season, reflecting E-and F-region plasma densities. For example, in northern mid-winter, sunward polar cap winds will tend to be a factor of two stronger around 1800 UT, when the geomagnetic polar cusp is sunlit, then at 0600 UT, when the entire polar cap is in darkness.     

Estimates of gravity wave momentum fluxes in the winter and summer high mesosphere over northern Scandinavia

As part of the MAP/WINE campaign (winter 1983–1984) and the MAC/SINE campaign (summer 1987) high resolution wind profiles were obtained in the upper mesosphere using the foil cloud technique. Vertical winds were derived from the fall rate of the foil clouds and are used for estimating the momentum fluxes associated with vertical wavelengths shorter than about 10 km. From the ensemble average of 15 observations over an altitude range of 74–89 km we calculate a zonal net momentum flux of +12.6 ± 4.5 m²s⁻² in summer. The average of 14 measurements in winter between 73 and 85 km indicates a zonal net momentum flux of −3.7 ± 2.4 m²2 s⁻².     

Sediment Loss from a Waste Rock Dump,ERA Ranger Mine,Northern Australia

During the 1994/95 wet season, runoff, suspended load and bedload loss from large scale erosion plots under natural rainfall events were measured at three sites with different treatments on the Energy Resources of Australia Ranger waste rock dump (cap, soil, fire). The fire site has well established trees, the soil site smaller shrubs and the cap site minimal vegetation. All three sites are located on the flat areas of the waste rock dump with similar slopes and cap, fire and soil are local names for the sites. The quantity of bedload eroded from the soil and fire sites decreased during the monitoring period, however, the same trend was not observed on the cap site. Bedload loss from the fire and soil sites is significantly log‐linear with time and reduces at a decreasing rate until becoming statistically constant toward the later part of the first half of the wet season. For corresponding storms at each site, bedload erosion was highest from the unvegetated and unripped cap site and lowest from the well vegetated fire site. Site specific linear relationships between bedload and total sediment load have been derived and can be used to derive total sediment load in the absence of measured suspended load.     

Variability in cementum deposition in springbok from the South African Cape

Over the past two decades, cementum increment analysis has played an important role in determining the season of death of ungulates at archaeological sites in many parts of the world, but its potential to address questions of seasonality in South African prehistory has remained largely unexplored. The ability to determine the seasonality of hunting practices would prove an important asset to understanding the foraging strategies and mobility patterns of Middle and Later Stone Age people in this region. A sample of springbok (Antidorcas marsupialis) of known date-of-death was collected from the South African Cape, a region across which three different rainfall regimes are represented. This study documents a very general correspondence between rainfall patterns and cementum development in springbok but also a level of variability that exceeds that observed by studies in other regions. In two sub-samples in which the sex of all individuals is known, cementum deposition among females is more strongly correlated with season of death than it is among males. This difference may reflect the social organization of springbok. The findings for springbok highlight the complexity of applying cementum increment analysis to archaeological studies not only on the Cape but also in regions of similar climatic variability.     

Observed and calculated F2 peak heights and derived meridional winds at mid-latitudes over a full solar cycle

The peak height of the F2 layer, hmF2, has been calculated using the ‘servo’ model of Rishbeth et al. [(1978), J. atmos. terr. Phys. 40, 767], combined with the hedin et al. [(1988), J. geophys. Res. 93, 9959] neutral wind model. The results are compared with observed values at noon and midnight derived from ionosonde measurements at two mid-latitude stations, Boulder and Wallops Island, over a full solar cycle. The reduced height of the F2 layer, zmF2, is also computed for the same period using the observed hmF2 values and the MSIS-86 model. Day-night, seasonal, and solar cycle variations in zmF2 are attributed to neutral composition changes and winds. Anomalously low values of hmF2 and zmF2 during summer both at solar minimum and during the solar cycle maximum in magnetic activity may be associated with increases in the molecular to atomic ion concentration ratio. Under these circumstances the F2 peak may lie significantly below the O⁺ peak height calculated by the servo model. Neutral meridional winds at Wallops Island are derived from the servo model using the observed hmF2 values and the calculated O⁺ ‘balance height’. It is shown that if the anomalously low hmF2 values are used, unrealistically large poleward winds are derived, which are inconsistent with both theory and observations made using other techniques. For most conditions the F2 peak is clearly an O⁺ peak, and daily mean winds at hmF2 derived from the servo model are consistent with the hedin et al. (1988) wind model. Unexpectedly, the results do not show an abrupt transition in the thermospheric circulation at the equinoxes. Diurnal curves of the servo model winds reveal a larger day-night difference at solar minimum than at solar maximum.     

The deserts of the Kashmir‐Tibetan frontier

Abstract

Semi-arid landscapes are vulnerable to cultivation, overgrazing and climate variability, although it is difficult to identify the relative significance of these three factors. In the South African Karoo, the ‘desertification debate' seeks to explain a change to more shrubby vegetation in heavily grazed areas. We examine these issues in catchments where farm reservoirs provide sediment stores with 137Cs, 210Pb, geochemical and mineral magnetic signatures. Rainfall data and stocking numbers are reported and current erosion rates are estimated. Sediment accumulation in the reservoirs increases between 1935 and 1940, probably due to rises in the frequency of rainfall events of >25mm day. Significant increases in sedimentation rates (×8) occur during rain-fed wheat cultivation. In an uncultivated catchment, sediment yields remain relatively high and reflect increases in erosion from hillslopes, colluvial storage, and cultivated land sources. Gully systems have acted as transport routes rather than sediment sources over the last ～70 years. Badland erosion rates average ca. 50t ha^?1 yr^?1. At Ganora, their development in the 1920s strongly influences peak sedimentation between 1970 and 1980. Any delay between badland initiation and increased sediment yield appears to be a function of landscape connectivity. Recovery following disturbance is occurring slowly and is likely to take >100 years. De-stocking and better management systems are reducing erosion rates but may be offset by increases in rainfall intensity.     

Changing Fire Management in the Pastoral Lands of Cape York Peninsula of northeast Australia, 1623 to 1996

Accounts of European explorers between 1623 and 1880 indicate that fires were lit by Aboriginal people on Cape York Peninsula in northeast Australia throughout the dry season (May–October). Diaries kept by three generations of pastoralists in the Musgrave area (1913–1952, 1953–1974 and 1976–1992) show that burning activities were largely confined to a two to six week period between May and early August. The timing of burning depended on the amount and date of cessation of wet season rainfall. More rarely, ‘storm‐burning’, burning under hot conditions within a few days of the first heavy rains of the wet season, was undertaken. Long‐term pastoralists felt a responsibility to use fire wisely and had a detailed knowledge of the role of fire in land management. Their decisions to burn were based on the extent of grass curing, and soil and weather conditions, all of which affected the extent of each burn. They used early dry season fires mainly to maintain forage and control cattle movements. Storm‐burns were reputed to control woody weeds, but were used infrequently because of difficulty in controlling their spread and uncertainty as to when the next rains would stimulate new grass growth.     

High Resolution Stratigraphic Distribution of Coprolites within Eneolithic Middens,a Case Study: Hârsova-Tell (Constanta County,Southeast Romania)

Abstract

Quantitative distribution of coprolites, fish bones and mammal bones recovered by wet screening from an Eneolithic midden at Hârsova-tell (southeast Romania) is surveyed within a high resolution stratigraphic framework. The coprolites, all containing bone fragments, are attributed to carnivores/omnivores: dog, pig, man. Highly significant correlation (Spearman's coefficient of rank correlation r_s = 0.46, P < 0.0001) between coprolite distribution and the seasonality-related fish bone distribution suggests that seasonality controls the stratigraphic distribution of coprolites recovered by wet screening within middens. Periods of high fish bone input into the midden, correlated with frequent coprolite occurrences, correspond to the warm season (late spring to early autumn). Assessment of potential factors controlling coprolite distribution within the midden suggests that sections of high coprolite concentration reflect preservation of faeces until embedding in sediments, and/or rapid embedding of faeces. Preservation of faeces is favored by dry periods, characteristic of the summer season in the highly continental climate of the region. Rain, freeze-thaw cycles or other wet season processes favor faeces disintegration. Our results suggest that the stratigraphic distribution of coprolites obtained by wet screening may represent an indicator of seasonality in middens. Additional studies are needed to better characterise this relationship and refine interpretations.     

The trouble with thermospheric vertical winds: geomagnetic,seasonal and solar cycle dependence at high latitudes

The vertical wind component is frequently used to determine the zero-velocity baseline for measurements of thermospheric winds by Fabry-Perot and other interferometers. For many of the upper atmospheric emission lines from which Doppler shifts are determined, for example for the OI 630 nm emission, available laboratory sources are not convenient for long-term use at remote automatic observatories. Therefore, the assumption that the long-term average vertical wind is zero is frequently used to create a baseline from which the Doppler shifts corresponding with the line-of-sight wind from other observing directions can then be calculated. A data base consisting of 1242 nights of thermospheric wind measurements from Kiruna (68°N, 20°E), a high-latitude site, has been analysed. There are many interesting short-term fluctuations of the vertical wind which will be discussed in future papers. However, the mean vertical wind at Kiruna also has a systematic variation dependent on geomagnetic activity, season and solar cycle. This means that the assumption that the average value of the vertical wind is zero over the observing period cannot be used in isolation to determine the instrument reference or baseline. Despite this note of caution, even within the auroral oval, the assumption of a zero mean vertical wind can be used to derive a baseline which is probably valid within 5 ms⁻¹ during periods of quiet geomagnetic activity (K_p < 2), near winter solstice. During other seasons, and during periods of elevated geomagnetic activity, a systematic error in excess of 10 ms⁻¹ may occur.     

MIDDLE HOLOCENE PERIODICITIES IN RAINFALL INFERRED FROM OXYGEN AND CARBON ISOTOPIC FLUCTUATIONS IN PREHISTORIC TROPICAL ESTUARINE MOLLUSC SHELLS

Stable oxygen and carbon isotopic ratios in modern and archaeological estuarine mollusc shells, Polymesoda radiata, change in accordance with seasonal salinity fluctuations in the Acapetahua estuary located on the Pacific coast of southern Mexico. This region receives ?3000 mm of precipitation annually, most during a wet season between April and October. The changing flux of fresh water and organic detritus into the estuary causes large changes in the oxygen and carbon isotopic composition of the estuarine waters and in the carbonate precipitated by P. radiata. Oxygen isotopic ratios in the shells of molluscs collected by late Archaic period populations (5000–4000 BP) in this region indicate that patterns of rainfall were similar to today. Modern shells, however, exhibit much more negative carbon isotopic values than observed in prehistoric shells. This change may be associated with the input of modern fertilizers into the estuary.     

Mean winds in the winter middle atmosphere above northern Scandinavia

Wind measurements which were carried out during the MAP/WINE Campaign in northern Scandinavia between 2 December 1983 and 24 February 1984 are used to derive background winds and monthly as well as winter mean values from the ground up to 90 km altitude. These mean winds compare favourably to the wind field proposed for the revised CIRA 86, which is deduced from satellite measurements. The vertical structure of the zonal monthly means is similar in both data sets during January and February. The winter mean zonal winds are observed to be slightly stronger in the stratosphere and lower mesosphere during the MAP/WINE winter than the satellite winds proposed for CIRA 86. The long term mean meridional winds are in good agreement up to 60 km. They indicate a dominant influence of quasistationary planetary waves up to 90 km and an ageostrophic poleward flow between 60 km and 85 km over northern Scandinavia, which maximizes at 76 km at about 8 m s⁻¹. The observed short term variability of the wind is discussed with respect to a possible impact of saturating gravity waves on the momentum budget of the middle atmosphere.     

A theoretical study of the distribution of ionization in the high-latitude ionosphere and the plasmasphere: first results on the mid-latitude trough and the light-ion trough

A model of the O⁺ and H⁺ distributions in the plasmasphere and high-latitude ionosphere is described and first results are presented. The O⁺ and H⁺ continuity and momentum equations are solved from the F-region to the equatorial plane in the inner plasmasphere, and to an altitude of 1400 km in the outer plasmasphere and high-latitude ionosphere. Account is taken of high-latitude convection, departure from corotation inside the plasmasphere, and neutral air winds. The neutral air winds are consistent with the assumed convection pattern. For equinox and magnetically quiet conditions the calculations show that a mid-latitude trough in F-layer electron concentration is present from 1600 to 0600 LT and the trough may occur either inside or outside the plasmasphere. The movement of the trough in this period is from higher to lower latitudes and is in qualitative agreement with AE-C and ESRO-4 data. A light-ion trough feature is apparent in the H⁺ distribution in the topside ionosphere at all local times. During the day the upward H⁺ flow increases with latitude to produce the light-ion trough. At night the H⁺ trough may be directly produced by the occurrence of the mid-latitude O⁺ trough. The relationships between the position of the plasmapause and the trough are discussed. Also discussed are the influence of particle ionization in the auroral zone and the effect of the neutral air wind.     

Variations of mean winds and tides in the upper middle atmosphere over a solar cycle,Saskatoon, Canada, 52°N, 107°W

Saskatoon (52 N, 107 W) medium frequency (MF) radar data from 1979 to 1990 have been analyzed to investigate the solar activity effects on upper middle atmospheric winds and tidal amplitudes. The period of study covers two solar maxima and a solar minimum; the continuous data allow a systematic analysis of solar cycle dependence on mean winds and tides. The height region of 79–97 km sampled in the study shows an apparent but very weak dependence of mean winds and tidal amplitudes on solar activity variation. The observed features are fairly consistent with the early results reported by Sprenger and Schmindkr [(1969) J. atmos. terr. Phy. 31, 217). The mean zonal wind and the semidiurnal tidal amplitudes appear to exhibit positive and negative correlations with the solar activity, respectively; the statistical significances of these correlations are generally low. There is a biennial periodicity evident in the zonal wind oscillations but this docs not have a consistent phase relationship with the equatorial stratospheric wind oscillations (QBO). The meridional winds and the tidal amplitudes are characterized with different and quite irregular periods of oscillations (2–5 yr). The diurnal tidal variations over the solar cycle are small and irregular, although amplitudes are slightly larger during the solar minimum years.