Rainfall Concentration Research Articles

Sediment transport model in sewer networks - a new utilisation of the Velikanov model

This paper presents the application of the conceptual total load model of Velikanov through a description of the solids behaviour, its calibration using numerical optimisation and the results on real sites. In order to take into account the full scale measurements, the Velikanov's model has been included in a global model for solid production and transport in sewer system which is divided into four modules: hydrologic, hydraulic (Muskingum model), pollution (build-up, wash off and total load) and calibration functions. The calibration of Velikanov parameters has been made using a procedure of numerical optimisation based on the Powell method. The solids behaviour is characterised by settling velocity (ωs) and density (s) which varies between fine particles (ωs=5.4m/h and s=2.00) essentially transported in suspension and coarse particles (ωs=45m/h and s=2.65). The results of the calibration phase on three real French sites which presented dry weather sediments and the validation phase can be considered of good quality. Simultaneous measurements of rainfall, hydraulic and TSS concentration allowed a large range of calibration and validation. This model provides a satisfying fitting of the real TSS pollutographs and this, considering the global form, maximal value and the temporal position.

Estimation of mean water residence times and runoff generation by 180 measurements in a Pre-Alpine catchment (Rietholzbach, Eastern Switzerland)

Stable isotopes ( 180) are used to investigate hydrological processes in the small Pre-Alpine research catchment Rietholzbach in NE Switzerland. The catchment covers an area of 3.18 km 2 and ranges from 680 to 960 m a.s.l. The 180 concentration in rainfall and in runoff at various sites, including a lysimeter installed within the catchment, over a 2-a period are investigated. First, conceptual flow models are applied to determine the mean water residence times of different systems of the basin and their storage properties. As a second approach, the separation of runoff components using 18O and Mg ions as tracer during some storm flow events was carried out. The following flow mechanisms can be identified from the modelling results: (i) lateral hillslope fluxes, which are faster in the part of the Tertiary Molasse sediments and slower in the part of the Quaternary deposits, and (ii) relatively slow vertical infiltration processes through the soil layer within the lysimeter. Mean residence times of 12.5 months result for the total catchment baseflow component and 24.5 months for the baseflow from the upper part of the catchment, respectively. For the lysimeter seepage water, a mean residence time of about 200 days was obtained. During the intensive storm events, a fast reaction of the stored subsurface water due to heavy precipitation and an expulsion of this storage water amounting to a considerable portion of the total basin runoff can be detected.

Boron water quality for the Plynlimon catchments

Abstract. Boron concentrations in rainfall, throughfall and stemflow for Spruce stands, mist, streamwater and groundwater are compared with chloride to assess atmospheric sources and catchment input-output balances for the Plynlimon catchments. In rainfall, boron concentration averages about 4.5 μg-B l-1 and approximately two thirds of this comes from anthropogenic sources. In through-fall and stemflow, boron concentrations are approximately a factor of ten times higher than in rainfall. This increase is associated with enhanced scavenging of mist and dry deposition by the trees. As the sampling sites were close to a forest edge, this degree of scavenging is probably far higher than in the centre of the forest. The throughfall and stemflow concentrations of boron show some evidence of periodic variations with time with peak concentrations occurring during the summer months indicating some vegetational cycling. In mist, boron concentrations are almost twenty times higher than in rainfall and anthropogenic sources account for about 86% of this. Within the Plynlimon streams, boron concentrations are about 1.4 to 1.7 times higher than in rainfall. However, after allowance for mist and dry deposition contributions to atmospheric deposition, it seems that, on average, about 30% of the boron input is retained within the catchment. For the forested catchments, felling results in a disruption of the biological cycle and a small increase in boron leaching from the catchment results in the net retention by the catchment being slightly reduced. Despite the net uptake by the catchment, there is clear evidence of a boron component of weathering from the bedrock. This is shown by an increased boron concentration in a stream influenced by a nearby borehole which increased groundwater inputs. The weathering component for boron is also observed in Plynlimon groundwaters as boron concentrations and boron to chloride ratios are higher than for the streams. For these Goundwaters, increases in boron concentrations are matched linearly by increases in the concentration of the principal ase cation weathering component in the bedrock, calcium. However, the bedrock weathering term is not uniform as the ratio of boron to calcium concentration varies for the different boreholes sampled.

Atmospheric Gases, Particulates and Rainfall Concentrations During Summer Rain Events

Concentrations of the predominant ions and of hydrogen peroxide in rainwater were measured during two rain events in Hong Kong in August 1995. The ambient nitrogen oxides, sulphur dioxide and ozone concentrations, as well as respirable suspended particulates, were monitored throughout the month, and the concentration and composition of total suspended particulates were also measured before and after the events. Event 1 heralded the commencement of a typhoon. The concentrations of all analytes, with the exception of hydrogen ion, decreased during event 1, due to exhaustive washout of alkaline particulate matter. The pH reached 3.8 at the end of the event, compared with 5.1 at the start. The shorter, more intense event 2 followed a sharp build-up of atmospheric pollutants and rainwater pH varied in a narrower range, between 3.4–3.7. The concentrations of H+, SO4 2- and NO3 − in rainwater were much higher than in event 1, and all analyte concentrations exhibited minima at maximum rainfall intensity. Contributions from in-cloud scavenging and below-cloud gas scavenging to rainwater acidity were both identified, and neutralization by suspended particulate matter also played an important role. The rainwater acidity is attributed to mainly local sources in both events. Both events were characterized by tropical cyclones in the China Seas, which when approaching Hong Kong, lead to a stagnation of local air pollutants. Strengthening winds at the time of event 1 provided more ventilation. Hydrogen peroxide concentrations in rainwater decreased rapidly during both events, and were higher in event 1 than in event 2 because SO2, and NOx levels were lower. Elevated concentrations of nitrogen (II) oxide were found to occur during periods of thunderstonn activity, in conjunction with low levels of ozone.

PCDD/F in ambient air and deposition in Baden-Württemberg, Germany

PCDD/F concentration in deposition and in ambient air was measured at different locations in the state of Baden-Württemberg, Germany. The average dioxin concentration in particulate deposition ranged from 5 to 36 pg I-TEQ/m 2d, while the ambient air concentration ranged from 14 to 77 fg I-TEQ/m 3. In addition, particulate depositions (mg/m 2d), calculated PCDD/F concentration in particulate depositions (ng I-TEQ/kg), suspended particles (< 10 μm) in air (μg/m 3) and the calculated PCDD/F concentration in suspended particulate matter (ng I-TEQ/kg) are reported. Rural areas and urban areas differ in PCDD/F pollution. Connections of PCDD/F concentration vs. temperature, PCDD/F concentration in grass vs. deposition and PCDD/F concentration in deposition vs. rainfall have been measured. The rural area in elevated exposition (Black Forest) reflects the influence of weather and long range transport.

Estimation of rainfall inputs and direct recharge to the deep unsaturated zone of southern Niger using the chloride profile method

An estimate of direct groundwater recharge below a region of natural woodland (tiger bush) has been made in south-west Niger using the solute profile technique. Data has been collected from a 77 m deep well drug within the study area covered by HAPEX-Sahel (Hydrological and Atmospheric Pilot Experiment), an international large-scale energy, water and carbon balance experiment carried out during the summer of 1992. During well construction samples were taken from the unsaturated zone at the following intervals: every 25 cm from 0–10 m, every 50 cm from 10–62.5 m, then every metre to the bottom of the well. Pore water was extracted from each sample either by centrifugation or elutriation and analysed for chloride; moisture contents of samples were obtained gravimetrically. These data have been used to produce depth profiles of pore water chloride concentration and moisture content throughout the unsaturated zone. From these profiles it has been possible to derive an estimate of historic direct recharge at the site. The chloride concentration of rainfall, which is required to make the estimate, was determined from the analysis of 123 rainfall samples collected from five EPSAT (vers une Estimation des Précipitation par Satellite au sahel) rain gauges in 1992. A mean recharge rate of 13 mm year −1 (range 10–19 mm) is estimated for the upper 70m of the profile, with a total residence time of 790 years (range 520–990 years). This is considered to be a representative estimate of the magnitude of direct recharge taking place below tiger bush areas.

EFFECT OF WATER CHEMISTRY AND SOIL AMENDMENTS ON A SILT LOAM SOIL—PART 1: INFILTRATION AND RUNOF

Infiltration, runoff, and soil loss are processes that occur when rainfall and runoff water interact with thestructure and physical and chemical bonds of the soil surface. A well-aggregated soil which is strongly resistant todispersion and aggregate breakdown, and which is fairly porous, will typically have greater infiltration rates and lessrunoff and soil loss than a poorly aggregated soil that is easily dispersed and which seals and crusts. Soil surface sealformation results from: physical breakdown of soil aggregates due to raindrop impact, and/or chemical dispersion whichis dependent upon soil properties and the electrolyte concentration in the surface water solution. This study examined theeffect of electrolyte concentration in rainfall and runoff water, as well as the effect of different soil surface amendmenttreatments on infiltration, runoff, and soil loss from a typical silt loam soil susceptible to aggregate breakdown andsealing. This article presents the infiltration and runoff results, and a companion article presents the soil erosion results.Rain water electrolyte content was found to have no significant effect on final runoff or infiltration rates. The use of afluidized bed combustion bottom ash (a byproduct from coal-fired electric power plant emissions desulfurization) soilsurface amendment significantly increased infiltration on small interrill areas (41.9 mm/h vs 32.2 mm/h for the control),but was less effective on longer rill plots (20.6 mm/h vs 18.4 mm/h for the control, difference not significant). Addition ofa small amount of an anionic polyacrylamide in tap water used as simulated rainfall greatly increased water infiltrationinto the soil, which may have potential application in sprinkler irrigation systems.

Species composition and seasonal cycles of phytoplankton with special reference to the nanoplankton of Lake Mikri Prespa

The phytoplankton of Lake Mikri Prespa was studied atmonthly or biweekly intervals during the period May1990–September 1992. Its species composition,consisting of a great number of cyanophytes and a verysmall number of chrysophytes and desmids, may reflectthe eutrophic character of the lake. Moreover, themean annual biomass values (15.0 and 3.2 g m−3 inthe two years, respectively) and the maximum biomass(38.1, 6.4 and 9.6 g m−3), classify Mikri Prespaas a eutrophic lake. A tendency towards adouble-peaked pattern of biomass distribution in timewith one peak in autumn, composed mainly ofcyanophytes, and another in spring made up of diatoms,was observed. This pattern contrasts with the standardpattern in eutrophic, stratified temperate lakes,which exhibit a third biomass maximum in summer. Cyanophytes were the most important group in terms ofbiomass and were dominated by the species Microcystis aeruginosa, Microcystis wesenbergii,Anabaena lemmermannii var. minor and Aphanocapsa elachista var. conferta. Diatomsconstituted the second most important group, with main representative the species Cyclotellaocellata. Cyanophytes, diatoms, chlorophytes anddinophytes revealed annual periodicity whereas theother algal groups did not show any seasonality atall. The nanoplankton constituted an important part ofalgal biomass (38.9 and 49.9% in the two years,respectively) and revealed annual periodicity withmaximum values in winter and spring, mainly composedof diatoms and cryptophytes. Low temperature,increased rainfall and high DIN concentrations seemedto be the main factors influencing the seasonality.Although the percentage contribution of nanoplanktondecreased with the increase in total biomass,justifying the classification of Lake Mikri Prespaamong the eutrophic lakes, the nanoplankton biomassdid not correlate significantly with totalphytoplankton biomass.

Interpretation of groundwater flow patterns through a reconstruction of the tritium precipitation record in the Cochabamba Valley, Bolivia

Regional behaviour of the groundwater flow system in the Cochabamba Valley, Bolivia, is evaluated through the interpretation of tritium ( 3H) distributions in groundwater samples from wells and springs. In order to interpret groundwater 3H concentrations in Cochabamba Valley, where no historical record of 3H concentrations in rainfall exists, a reconstructed 3H precipitation record is developed. The record of 3H concentrations in precipitation is fairly extensive in the Amazon Basin and this record was extrapolated to the neighbouring Cochabamba Valley. Tritium concentrations in rainfall have been observed to increase under natural conditions with increasing latitude and with increasing distance from the ocean. By considering these trends, a linear relationship for increasing 3H concentration in precipitation is developed, based on data from the Amazon Basin, that realistically predicts regional 3H distributions from the northeast Brazilian coast to Cuzco, Peru. This 3H precipitation record is then extrapolated to the Cochabamba Valley and, after correction for radiogenic decay, is used to interpret trends in groundwater 3H concentrations within the valley. The groundwater flow system in one of the principal alluvial fans, which serves as an important groundwater resource for the city, is studied in detail. Tritium concentrations drop from approximately 8–10 tritium units (TU) in the recharge area to concentrations below the detection limit of 0.8 TU further out in the valley. Groundwater velocities of approximately 0.3 to 0.9 m d −1 are estimated from distributions of groundwater 3H concentrations along the alluvial fan with the use of the reconstructed precipitation 3H record. Regional characteristics of the groundwater flow system are discussed with respect to future development and protection of the groundwater resources.

Wet deposition at the sub-20 km scale in a rural upland area of England

A network of 10 bulk precipitation collectors was established in an upland region of northern England to examine the small-scale variability of precipitation composition in a rural area. Significant spatial variability was shown for non sea-salt sulphate, calcium and hydrogen ion concentrations. Although nitrate concentrations did not show significant variability, a coherent gradient across the study area was observed. The large variability of calcium ion concentration across the area was related to limestone quarrying, which is prevalent in the south of the study area. Calcareous wind-blown soil may have also made a minor contribution to the abundance of calcium in the rain-water samples. It is likely, however, that a large fraction of the calcium was dry deposited to the funnels of the collectors in the form of particles. The network was not designed to study orographic enhancement of deposition, but it is suggested that this mechanism, along with wind-drift of scavenged cap-cloud droplets, cannot explain the spatial variation in ion concentrations. It is speculated that the variability in non-sea-salt sulphate concentrations may also be partially explained by reaction of CaCO 3 particles deposited to the funnels of the collectors with SO 2. Critical load exceedances in terms of deposited acidity from non-sea-salt sulphate were calculated using different spatially resolved deposition data. A better estimate of deposition in the study area was achieved by combining rainfall concentration data from the national network with the 5 km rainfall field, than the current methodology used for mapping of U.K. deposition which utilises a 20 km rainfall field. However, 20 km is considered to be the minimum resolution at which a procedure for parameterising orographic enhancement can be incorporated. Deposition and critical load exceedances in 2005 were calculated using a long-range transport model and significant improvements in the protection of soils were predicted from the projected emissions reductions.

Herbicide and Nitrate Distribution in Central Iowa Rainfall

AbstractHerbicides are detected in rainfall; however, these are a small fraction of the total applied. This study was designed to evaluate monthly and annual variation in atrazine (6‐chloro‐N‐ethyl‐N′‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine), alachlor (2‐chloro‐N‐(2,6‐diethylphenyl)‐N‐(methoxymethyl)acetamide), metolachlor (2‐chloro‐N‐(2‐ethyl‐6‐methylphenyl)‐N‐(2‐methoxy‐1‐methylethyl)acetamide), and NO3‐N concentrations in rainfall over Walnut Creek watershed south of Ames, IA. The study began in 1991 and continued through 1994. Within the watershed, two wet/dry precipitation samplers were positioned 4 km apart. Detections varied during the year with &gt;90% of the herbicide detections occurring in April through early July. Concentrations varied among events from nondetectable amounts to concentrations of 154 μg L−1, which occurred when atrazine was applied during an extremely humid day immediately followed by rainfall of &lt;10 mm that washed spray drift from the atmosphere. This was a local scale phenomenon, because the other collector had a more typical concentration of 1.7 μg L−1 with an 8‐mm rainfall. Variation between the two collectors suggests that local scale meteorological processes affect herbicide movement. Yearly atrazine deposition totals were &gt;100 μg m−2 representing &lt;0.1% of the amount applied. Nitrate‐N concentrations in precipitation were uniformly distributed throughout the year and without annual variation in the concentrations. Deposition rates of NO3‐N were about 1.2 g m−2. Annual loading onto the watershed was about 25% of the amount applied from all forms of N fertilizers. Movement and rates of deposition provide an understanding of the processes and magnitude of the impact of agriculture on the environment.

Effects of controlled drainage on nitrate concentrations in subsurface drain discharge

A water table management field study was conducted on a Bainesville silt loam soil during 1992 and 1993. The water table levels studied were conventional free outlet subsurface drainage (FD), and controlled water tables (CWT) of 0.50 and 0.25 m above the drain level. The three treatments were replicated thrice resulting in nine plots, each measuring 115 m long by 18.69 m wide. A subsurface drain was installed 1.0 m deep in the centre of each plot. Drain discharge, nitrate concentrations in drainage effluent, rainfall and water table elevations were measured during the two growing seasons. The plots were cropped with grain corn ( Zea mays L.) in 1992, and soybean ( Glycine max (L.) Mill.) in 1993. Controlled drainage had a significant effect on drain discharge quantity and quality. In 1992, the 0.25 and 0.50 m CWT treatments reduced drain flow by 58.7% and 65.3% respectively; and in 1993, by 40.9% and 95%, respectively, compared with the FD treatment. In 1992, there was a 75.9% and 68.9% reduction of nitrate concentration in drain flow with the 0.25 and 0.50 m CWT, respectively, compared with FD. In 1993, the reductions were 62.3% and 95.7% for the 0.25 and 0.5 m CWT, respectively. While it was impossible to maintain the water tables consistently at 0.5 and 0.25 m throughout the growing season, these results show that there are significant environmental benefits with controlled drainage.

A method for estimating the probable 1-hr rainfall based on the time concentration of a heavy rainfall

A method for estimating the probable 1-hr rainfall based on the time concentration of a heavy rainfall

Groundwater age analysis using tritium in gneiss of the Samkwang mine in Korea

Abstract Travel times of the groundwater in the gneiss near Samkwang mine in Korea were estimated through the field hydraulic tests, the development and the application of a mathematical model, and the analysis of tritium concentration of the groundwater sampled in the study positions. Results of this study are follows:(1) Through the field hydraulic test, the hydraulic conductivity in the gneiss of the syudied area is 1.7 x 10‐5 x Z‐1,3 m/sec (Z:depth from the surface, (m)) and the longitudinal dispersivity is 9.8 m. (2) The mathematical model to calculate the age of groundwater was developed considering the tritium concentrations of rainfall precipitated in the studied area for period 1961 ‐ 1993. (3) The ages of the groundwater in the tunnel 44, 92, 102, and 205 m below the earth surface were estimated at 2.0, 4.0, 4.5, and 9.0 years, respectively. These results were verified by the data on the tritium concentrations of the groundwater in the tunnel for period 1990 to 1993.

Comparative soil study in a mediterranean ecosystem ofQuercus pyrenaica(Willd.)

A comparison was made of certain elements of the biogeochemical cycle of soils in forest ecosystems of the deciduous species Quercus pyrenaica (Willd.). Four sampling sites were selected in a rainfall gradient, with slight differences in the understory composition. Two of the sites were over schist and the other two over granites; however, the soil type (xerumbrepts) was similar for the four sites. Soil chemical and physicochemical parameters such as carbon, nitrogen, pH, cation exchange capacity (CEC), and exchangeable cations were monitored in the upper soil horizon (0–10 cm). Samples were collected every 3 months for 3 years. The final sample size was n = 96. An inverse relationship between rainfall and exchangeable cation concentration was found, with significant differences (p < .05) among the sites, with the exception of exchangeable sodium. The sum of cation values were 8.15 cmol kg‐1 for site 1 (720 mm mean annual precipitation), 3.07 cmol kg‐1 for site 2 (872 mm mean annual precipitation), 2.02 c...

Atmospheric deposition of organic contaminants to the chesapeake bay

Abstract To estimate loadings of contaminants from the atmosphere to the Chesapeake Bay, ambient atmospheric concentrations and wet deposition fluxes of several polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) were measured at a shore-based station between June 1990 and December 1991. Geometric mean concentrations of individual PAHs in air ranged from 0.003 (dibenz[ah]anthracene) to 1.4 ng m−3 (phenanthrene) and the observed temporal variability in PAH levels in the air ranged from 0.1 to 41 % about the respective geometric mean concentrations. The mean ambient atmospheric PCB concentration over the Chesapeake Bay during the study period was 0.21 ng m−3, with a relative standard deviation of 56%. PCB concentrations in the air were slightly correlated with air temperature. Volume-weighted mean (VWM) concentrations of individual PAHs in precipitation ranged from 1.1 (dibenz[ah]anthracene) to 17 ng l−1 (pyrene). The VWM t-PCB concentration was 1.6 ng l−1. The temporal variability of HOC concentrations in rainfall was as large as one order of magnitude, with no apparent seasonal trend, suggesting highly variable HOC scavenging processes. Assuming a dry deposition velocity of 0.49±0.23 cm s−1 dry particle fluxes of PAHs ranged from 1±0.4 (anthracene) to 16±7 (fluoranthene) μg m−2 yr−1. The dry deposition velocity was derived using monthly average wind speeds and temperature differentials derived for the Chesapeake Bay area from climatological compilations [Quinn T. L., Ondov J. M. and Holland J. Z. (1992) Dependence of deposition velocity on the frequency of meteorological observations for the Chesapeake Bay. J. Aerosol Sci. 23, S973–S976; Wu Z. Y., Ondov J. M., Holland J. Z. and Lin Z. C. (1992) Dry deposition fluxes of elements in Chesapeake Bay aerosol. J. Aerosol Sci. 23, 5969–5972]. Wet fluxes of PAHS ranged from 1 (anthracene) to 20 (pyrene) μg m−2 yr−1. Total (wet + dry) fluxes ranged from 2±0.4 (anthracene) to 35±7 (pyrene) μg m−2 yr−1. The t-PCB wet flux was 1.9 μg m−2 yr−1, with the estimated dry particle flux accounting for 75% of the total flux (3.3 ± 0.6 μg m−2 yr−1). The t-PCB wet flux in Chesapeake Bay is similar to that of rural Minnesota [2 μg m−2 yr−1. Franz T. P., Eisenreich S. J. and Swanson M. B. (1991) Evaluation of precipitation samplers for assessing atmospheric fluxes of trace organic contaminants. Chemo 23, 343–361]. Annual atmospheric loadings of individual PAHs to the surface waters of the Chesapeake Bay were calculated by extrapolating the total flux to the surface area of the Chesapeake Bay. Individual PAH loadings range from 23±5 (anthracene) to 403±81 (pyrene) kg yr−1. and the t-PCB annual atmospheric loading is 38±7 kg yr−1.

The impacts of atmospheric n inputs on throughfall, soil and stream water interactions for different aged forest and moorland catchments in Wales

A study of inorganic-N concentrations in streams, soil waters, throughfall and rainfall was conducted for one year in five moorland and 20 Sitka spruce plantation catchments in upland Wales. The forest ages ranged from 10 to 55 yr. Highly significant positive relationships with forest stand age existed for inorganic-N concentrations in streamwater, B and O horizon soil waters and throughfall. Inorganic-N in streams and B horizon waters was entirely NO3−. Inorganic-N fluxes in throughfall also showed a significant, positive relationship with stand age. Throughfall flux of inorganic-N in the oldest stand was 25.1 kgN ha−1 yr−1, double that in incident rainfall.

Morfogênese quaternária e intemperismo: colúvios do Planalto do Itatiaia

No setor inferior das vertentes do planalto do ltatiaia (2.300 -2.500m) ocorrem duas gerações de colúvios que testemunham fases de intensificação das ações erosivas nas vertentes. Estudo mineralógico e micromorfológico permitiu reconhecer tendência geral laterítica nos materiais de alteração in situ e colúvios. A análise micromorfológica dos colúvios evidencia mistura de materiais com diferentes graus de alteração, mais especialmente em CII, caracterizado por contrastes maiores que o comum num mesmo nível de perfis de alteração. Intensamente argilificados mas pouco pedogeneizados, ricos em fragmentos das rochas subjacentes e com menores contrastes de alteração, os colúvios CI poderiam ter sido originados por movimentos de massa que atingiram o regolito em maior profundidade. Em CII, altero e litorrelíquias bem menos comuns e grãos de esqueleto sempre menores indicariam materiais de origem inicialmente mais evoluídos; mistura de graus extremos de alteração sugere retrabalhamento por processos superficiais de coluviação, com inclusão de fragmentos de rocha menos alterada durante o transporte vertente abaixo. A atuação de diferentes processos nas vertentes permite inferir variações nas condições ambientais do planalto. De climas úmidos mais estáveis, e talvez mais quentes, favoráveis à alteração do regolito, ter-se-ia passado a uma fase de intensificação ou concentração das chuvas, capaz de explicar o desencadeamento das corridas de lama que depositaram CI. Nos últimos 8.000 anos, condições úmidas mas com maiores oscilações da temperatura, incluindo fases de maior intensidade dos processos de gelifração, teriam sido responsáveis pela deposição dos materiais, ora finos ora cascalhentos, das seqüências CII. Estas variações não foram entretanto suficientes para mudar a tendência laterítica da alteração, que teria persistido após o último máximo glacial.

A Case Study of Heavy Rainfall Associated with Weak Cyclogenesis in the Northwest Gulf of Mexico

This paper describes a case of unexpected weak cyclogenesis over the northwestern Gulf of Mexico from 16 to 19 September 1984 based upon manually prepared and European Centre for Medium-Range Weather Forecasts (ECMWF) gridded analyses. Noteworthy aspects of the case include: 1) upward of 50 cm of rain along the extreme southern coast of Texas and 2) the brief occurrence of minimal strength tropical-storm conditions in a weak baroclinic marine environment. A crucial antecedent condition to rainstorm formation was the creation of a low-level baroclinic zone over the northwestern Gulf of Mexico due to the southward advance of drier and slightly cooler air behind a cold front that penetrated into northeastern Mexico. Four factors were responsible for rainfall concentration along the coast: 1) a northward-moving 700-mb trough and embedded vorticity maximum in the easterlies over the western Gulf of Mexico, 2) an eastward-propagating upper-tropospheric disturbance in the midlatitude westerlies over the southern United States to the north of a subtropical ridge line over Texas and Louisiana, 3) the formation of a weak midtropospheric baroclinic zone over the extreme north-western Gulf of Mexico, along which cyclonic-vorticity advection by the thermal wind contributed to a favorable environment for deep convection and cyclogenesis, and 4) the existence and maintenance of a weak north-south-oriented baroclinic zone along the Mexican coast in the lower troposphere. The coastal baroclinic zone was associated with a quasi-stationary axis of ascent that maximized at 700 mb and lay 200–300 km to the cast of a persistent band of frontogenesis along the Sierra Madre Oriental Mountains of Mexico. Frontogenesis (∼2−4×10−10°C m−1 s−1) was dominated by the twisting term as relatively cool air over coastal Mexico was forced to ascend in the 700-mb easterly flow, favoring the northward movement of an area of cyclonic vorticity along the coast. The results from this study are compared and contrasted with a similar September (1979) heavy-rain event that also occurred in southern Texas. In both cases, cyclogenesis occurred in a weakly baroclinic environment with embedded convection concentrated along the boundary of the surface baroclinic zone. The paper concludes with a discussion of tropical-storm formation in a baroclinic environment. It is speculated that the apparent, but short-lived, minimal strength tropical-storm development in this case could not be sustained because of the absence of a significant upstream cyclonic-vorlicity maximum aloft, despite otherwise favorable indicators for tropical cyclogenesis.

On the atmospheric conditions affecting the variation of tritium concentration in rainfall

Atmospheric conditions affecting the variation of tritium concentration in rainfall were investigated by meteorological analysis during 1980 in Pusan, Korea. Tritium concentrations were high in winter, autumn and spring, in comparison with summer. If was found that there is generally a negative correlation between tritium concentration in rainfall and the mixing ratio, and atmospheric thicknesses. In winter, spring and autumn, high tritium concentrations occurred as the southward continental polar air masses from the north merged into the low pressure air mass in southern China; and relatively low concentrations of tritium appeared when the southward polar air masses combined with the northward low pressure air mass from the East China Sea. In the rainy season, extremely low concentrations of tritium were found when a stationary front was located to the south of Korea, allowing a warm and humid southwest airflow to travel toward the Korean peninsula, and here the potential instability and the existence of active convective clouds were major factors affecting the variation of tritium concentration.