Humid Air Masses Research Articles

Early Pliocene vegetation changes forced by eccentricity-precession. Example from Southwestern Romania

Repetitive changes in Early Pliocene marine and continental sediments were forced by astronomical cycles (eccentricity, precession) and related to low amplitude climatic fluctuations as demonstrated at Capo Rossello and Monte San Nicola (Sicily). This work aims to establish if such climatic fluctuations have also controlled changes in the vegetation during the warm Early Pliocene. High-resolution pollen analysis of the Lupoaia section (SW Romania) provides evidence of a clear response owing to its location within a delta system which constitutes a reliable area for pollen deposition near the Carpathians (i.e. capable of registering all the vegetation ecosystems from edaphic coastal associations up to the highest altitudinal belts), and owing to its Southeastern European longitude probably where warm–humid air masses arrived regularly, making the region a highly favourable refuge for thermophilous plants. Eccentricity appears to have controlled fluctuations in temperature (development of thermophilous elements during eccentricity minima, and descent of altitudinal trees during eccentricity maxima) while precession appears to have forced changes in humidity (expansion of cypress swamps related to precession maxima, and spread of herbaceous marshes requiring more humidity related to precession minima) with respect to increased precipitation and run-off.

Forest Fire-Conducive Drought Variability in the Southern Canadian Boreal Forest and Associated Climatology Inferred from Tree Rings

Forest fires in Canada are directly influenced by the state of the climate system. The strong connection between climate and fire, along with the dynamic nature of the climate system, causes the extent, severity and frequency of fires to change over time. For instance, many reconstructions of the history of forest fires across boreal Canada report a general decrease in fire activity since ~1850 which could, in part, result from changes in climate. This paper describes progress in characterizing the variability in fire-conducive droughts in the central and eastern Canadian boreal forests during the past three centuries. An extensive network of drought-sensitive tree-ring records from Manitoba, Ontario and Quebec was used to develop five multi-century reconstructions of the mean July Canadian Drought Code and one reconstruction of mean July and August temperatures. Correlation analyses with regional fire statistics (common period 1959–1998) showed that drought estimates are accurate enough to approximate fire activity and, hence, the estimates are relevant for the study of climate change impacts on Canadian forests. Spatial correlation analysis over the period 1768–1998 revealed that variability between the west and east has increased since the mid–19th century, specifically the decade-to-decade variability and the frequency of extreme events. Based on the synoptic characteristics of recent droughts, we interpret this change in variability as a response to an increasing frequency of upper level ridging and troughing over western and eastern Canada, respectively. The increasing horizontal movement of humid air masses over eastern Canada since ~1850 could have contributed to the creation of moister conditions that are less suitable for fire.

Impact of air traffic on cirrus coverage

TOVS Path‐B satellite data provide information on effective high cloud amount and relative humidity for the period 1987 to 1995. Differences in trends of seasonal mean effective high cloud amount between situations favorable for contrails and for cirrus are analyzed in regions with high and low air traffic density. In regions with especially high air traffic density, a significantly stronger increase of effective high cloud amount is found for situations with sufficiently cold and humid air masses favorable for contrails than for all situations in general or for situations favorable for cirrus. Situations of potential contrails occur in about 5 to 10% of all situations, and their seasonal effective high cloud amount averages lie between about 7% and 22%. Indicators of cirrus increase due to air traffic corresponding to the difference in trends of effective high cloud amount between potential contrail situations and cirrus or all situations are about 2.8%–3.5% and 1.6%–4.7% per decade over Europe and the North Atlantic flight corridor, respectively. Weighted by frequency of potential contrail occurrence, the overall increase amounts to at least 0.20%–0.25% and 0.08%–0.24% per decade over regions with very high air traffic (Europe and the North Atlantic flight corridor, respectively).

MIDDLE HOLOCENE SOIL AND MIGRATION OF CLIMATIC ZONES IN THE GUANZHONG PLAIN OF CHINA

This article discusses the findings from a microtexture determination and a chemical analysis of a field survey of the Holocene paleosol in the Guanzhong Plain part of the Loess Plateau in China. The paleoclimatic characteristics, the type of paleosol, and climatic change are discussed. The materials show that the Holocene paleosol is composed of a transitional horizon (AB) from an organic horizon (A) to a reddish-brown clay horizon (Bt), a weathered-leached parent horizon containing only eluvial soils, and a horizon of CaCO 3 nodules. In the Guanzhong Plain, red ferri-argillans indicating chemical weathering and subtropical climate were well developed in the paleosol. The content of the ferri-argillans in this paleosol is higher than that in a luvic cinnamon soil and is close to that of a subtropical yellow-brown earth developed under forests. Clay minerals in the Bt horizon consist of mixed components of illite-askanite and kaolinite. The pollen assemblage shows that the vegetation was subtropical broadleaf forest, consisting of Quercus, Symplocos, Liquidambar, Pistacia, and so forth. Accordingly, it can be concluded that the soil-forming process was apparently stronger at that time than at the present, that the paleosol was yellow-brown forest earth developed in the northern subtropics, and that the climate was warm and wet. In the Middle Holocene, humid air masses traveling from the Pacific across southeastern China in the summer could frequently reach this area. At that time, a subtropical climate existed both south and north of the Qinling Mountains, and the mountains did not serve as a boundary between the temperate zone and subtropics of China. The mean annual temperature and rainfall were respectively about 15 °C and 750 to 800 mm, which is 3 °C higher and 150 to 200 mm wetter than present.

Aerial measurement of air pollutants and aerosols during 20–22 March 2001 over the East China Sea

Aerial measurement of atmospheric pollutants was carried out over the East China Sea on 20, 21, and 22 March 2001. High concentrations of not only gaseous species (maximum concentrations of SO2 > 10 ppb and O3 > 100 ppb in the marine boundary layer) but also particulate matter (PM) (average concentration of PM2.5 ∼ 30 μg m−3 and PM10 ∼ 1000 μg m−3 in the marine boundary layer) were observed on 21March. Such high concentrations were observed only at low altitude. A cold front passing over the Korean Peninsula and extension of a high‐pressure system from central China to the East China Sea were responsible for the transport of pollutants. High concentrations of pollutants were confined to lower altitude by the high‐pressure system. This result is in marked contrast to our previous observation: A highly polluted air mass was transported from central China when a low‐pressure system originating near Taiwan moved northeast to Japan along the south coast of the main islands. Anthropogenic sulfate aerosols were transported ahead of Kosa particles (fine yellow sand dust particles). A humid air mass containing a high concentration of sulfate (>20 μg m−3) was followed by a dry air mass containing a high concentration of calcium (>1000 μg m−3). Interactions between aerosol components and anthropogenic gaseous species gave insight into the formation process of aerosols over the East China Sea. Nitrate showed a good correlation with calcium except in the highly polluted air mass, indicating that gaseous nitric acid was adsorbed on the surface of coarse particles. No correlation between nitrate and ammonium supported this contention. In contrast, sulfate showed an excellent linear correlation with ammonium, indicating quick neutralization of sulfuric acid by ammonia in the polluted air mass. Exceptions were observed when the calcium concentration was >4 μg m−3, suggesting that SO2 gas was adsorbed on the surface of yellow sand dust particles and that oxidation took place on the surface.

Aerosol spectral optical depths and size characteristics at a coastal industriallocation in India - effect of synoptic and mesoscale weather

Abstract. The aerosol spectral optical depths at ten discrete channels in the visible and near IR bands, obtained from a ground-based passive multi-wavelength solar radiometer at a coastal industrial location, Visakhapatnam, on the east coast of India, are used to study the response of the aerosol optical properties and size distributions to the changes in atmospheric humidity, wind speed and direction. It is observed that during high humidity conditions, the spectral optical depths show about 30% higher growth factors, and the size distributions show the generation of a typical new mode around 0.4 microns. The surface wind speed and direction also indicate the formation of new particles when the humid marine air mass interacts with the industrial air mass. This is interpreted in terms of new particle formation and subsequent particle growth by condensation and self-coagulation. The results obtained on the surface-size segregated aerosol mass distribution from a co-located Quartz Crystal Microbalance during different humidity conditions also show a large mass increase in the sub-micron size range with an increase in atmospheric humidity, indicating new particle formation at the sub-micron size range.

Oxygen-18 variations of rainwater during precipitation: application of the Rayleigh model to selected rainfalls in Southern France

From March 1997 to March 1999, sequential rainwater samples were collected during 12 rainfalls at Avignon, Southern France, with the objective of investigating how the formation conditions affect the isotopic composition of the most frequent rain types occurring in the western Mediterranean region. Three major isotopic trends were identified: (i) a descending δ 18O trend, produced when a cold front penetration causes the rise of warm and humid air masses, accompanied by adiabatic cooling and precipitation. The temperature variation is the predominant feature of frontal rainfalls; (ii) an L-shaped δ 18O trend, which can be produced by convective rains, with an initial rapid decrease followed by a stationary value; (iii) a V-shaped trend, in which the final phase of precipitation obeys a different mechanism from the first phase. Frontal and convective rainfalls may produce this trend. Three heavy rainfalls were selected for applying a numerical Rayleigh condensation model to describe their isotopic trends. Some irregularities in the generally monotonously decreasing isotopic trend were attributed to inputs of new air masses, which were often recorded by chemical variations as well. The numerical model indicates that about 80, 60 and 45% of the vapour initially present was condensed. The lowest fraction corresponds to the most abundant rainfall, and it is due to the convective nature of the latter implying a continuous supply of condensable vapour.

Hydrological impacts of climate change at catchment scale:A case study in the Grand-Duchy of Luxembourg

As a consequence of an increase of days with westerly atmospheric fluxes, bringing humid air masses from the Atlantic Ocean to Western Europe, important changes in the annual and seasonal distribution of rainfall have been observed over the past 150 years. Annual rainfall totals observed during the second half of the 19th century were less important than those observed during the second half of the 20th century. Moreover, during the past 50 years winter rainfall totals have significantly increased, while summer rainfall totals have been decreasing. Streamflow observations through the second half of the 20th century have shown a significant increase of winter maximum daily streamflow, in reaction to the winter rainfall increase. The modelling of the streamflow under the 19th century climatological conditions suggests that since then, the number of winter flood days has increased, while the occurrence of summer flood days has decreased. Moreover, high floods appear to have been more frequent in the second half of the 20th century.

An operational heat/health warning system in Shanghai.

Previous research has noted that high surface temperatures within certain "offensive" air masses can lead to increased mortality. This study assesses the relationship between daily mortality rates and weather within the city of Shanghai, China, while introducing an operational heat/health warning system for the city. Using numerous meteorological observations, the spatial synoptic classification has been used to classify each summer day from 1989 to 1998 into one of eight air mass types for Shanghai. Through the comparative analysis of the daily air mass type and the corresponding Shanghai mortality rate, "moist tropical plus" (MT+), an extremely hot and humid air mass, was identified as an offensive air mass with the highest rates of mortality. Using stepwise regression, an algorithm was produced to help predict the number of excess deaths that will occur with each occurrence of the MT+ airmass. The heat/health warning system was run experimentally in the summer of 2001 and illustrated that the use of a warning system can alert the city's residents of potentially offensive weather situations that can lead to a deterioration in human health.

Composition and size distribution of submicrometer aerosol particles observed on Mt. Fuji in the volcanic plumes from Miyakejima

Abstract An aerosol observation campaign was conducted at the summit of Mt. Fuji (at an altitude of 3776 m), which is located 170 km northwest of Miyakejima. Individual aerosol particles were collected with an electrostatic aerosol sampler and the number–size distribution and composition were examined by electron microscopy. Number–size distributions were measured with the combination of a diffusion battery (DB) and a condensation particle counter. Sulfate and sulfur dioxide concentrations were also determined. During the first half of the campaign (13–15 September 2000), high number fractions of sulfuric-acid containing particles with a mode radius around 0.06 μm were observed in the radius range of 0.02–0.2 μm coincident with a humid maritime air mass originated from the south. Sulfate and sulfur dioxide also showed high concentrations in this period. These results suggested that the volcanic plumes of Miyakejima were transported up to the summit of Mt. Fuji. Number–size distributions determined by electron microscope were consistent with those derived using the DB. During the later half of the experiment (18–20 September), low fractions of sulfuric-acid containing particles represented only a small fraction of the total particle number, coincident with a dry continental air mass originating from the west.

Temporal and spatial variations of fog in the Western Sudety Mts., Poland

The ridges of the Western Sudety are well exposed to the humid maritime air masses that are mainly associated with westerly atmospheric circulation. Fog is the most frequently observed atmospheric phenomenon, being present on average 45% of the time, with 250–300 days with fog per year. This study on temporal variation of fog in both daily and annual cycles is based on 30 years of measurements (1961–1990) made on Mt. Szrenica at 1362 m a.s.l. Based on the data from 51 meteorological stations and the results of fog deposition field measurements, a digital model of fog frequency in the Western Sudety was prepared using a Geographic Information System. The model shows that altitude is not the only important factor controlling fog frequency. Such parameters as slope aspect and position of a particular site in relation to local and regional morphology are also of crucial importance.

Precipitation chemistry in East Asia

The chemical composition of precipitation in East Asia was analyzed on the basis of the results of cooperative monitoring performed during the period from 1992 to 1993. The wet deposition of soluble ions was characterized by two major factors; the geographical distribution of precursor production through natural and anthropogenic activities, and the meteorological conditions such as wind system and rainfall pattern due to the Asian monsoon. Although northeastern China had the largest anthropogenic emission source, precipitation with relatively high pH value was observed. At islands in the northern Pacific far from major anthropogenic emission sources, precipitation with relatively low pH value was observed. It was suggested that ammonia and calcium were constantly produced in the continent, and that the higher concentrations of these bases in the atmosphere were incorporated into precipitation elements and neutralized the acidity due to H 2SO 4 and HNO 3 from fossil fuel combustion. The ratio of [nssCa 2++NH 4 +]/[nssSO 4 2−+NO 3 −] in precipitation was highest on the continent, moderate in the Korean Peninsula and the Japanese Islands, and lowest in the northern Pacific; in other words, this ratio decreased as the distance from the continent increased. In the central region of East Asia, which is affected by both continental cold, dry air masses and subtropical warm, humid air masses, the frequency distribution of pH of precipitation showed a bimodal pattern in which unimodal patterns of northern China and that of the Japanese Islands were superimposed. The wet deposition of nssSO 4 2−, NO 3 −, nssCa 2+, and NH 4 + showed relatively small differences between the sites, while that of Na + and Cl −, relatively large differences between the sites, reflecting the field of ionic concentration and amount of precipitation in East Asia.

Tree-ring evidence for long-term precipitation changes in subtropical South America

In recent years there has been a notable increase in the number of tree-ring chronologies for the temperate and cold regions of the Americas. In comparison, few advances have been reported for the American tropics and subtropics, where the absence of seasonality appears to be the main reason for the lack of well-defined growth bands in most species. Distinct, annually formed tree-rings have recently been reported for subtropical montane trees on the eastern slope of the Andes (22–28°S). Six absolute-dated chronologies from Juglans australis and Cedrela lilloi at the upper treeline (between 1700 and 2000 m) in the montane forest of north-western Argentina were selected to explore the potential of these records to infer decade- to century-scale climatic variations in the subtropics. These tree-rings capture a significant percentage of the variances in regional temperature and precipitation records and appear to be suitable to reconstruct decade-long changes in large-scale circulation over the South American subtropics. In particular, tree-growth at xeric sites has been strongly influenced by precipitation changes, which in turn respond to alternating patterns of zonal versus meridional flows over subtropical South America. The upper treeline records indicate that the increase in precipitation during the past three decades, caused by an enhanced transport of humid air masses from the Brazilian–Bolivian lowland tropics to the semiarid subtropics, has been unprecedented for the past 200 years. Although this precipitation increase may reflect natural variability in the subtropics, it is also consistent with 2×CO2 climatic simulations from five general circulation models. There is a general agreement among model results about a noticeable increase in precipitation in north-western Argentina due to an intensification of the water transport across subtropical South America in response to a southward displacement of the continental low and an increasing warming at these latitudes. A larger network of tree-ring chronologies will aid efforts to understand long-term climatic interactions between tropical and subtropical regions in South America, and shed light on the role of natural versus anthropogenic forcings on regional climatic changes. © 1998 Royal Meteorological Society

Dunes in southeastern Namibia: evidence for Holocene environmental changes in the southwestern Kalahari based on thermoluminescence data

Linear dunes in the southwestern Kalahari provide evidence of the nature of paleoclimates. They have special significance because their alignments indicate a previous circulation pattern with prevailing northwesterly winds. At present, the bases of the dunes are fixed by vegetation and only the dune crests are active. In contrast to previous suggestions final dune stabilization took place before about 9-8 ka. This corresponds to reduced wind velocities accompanied by more humid conditions. This seems to be the last important environmental change recorded by significant landforms in the southwestern Kalahari. Since about 8 ka the region remained more or less semi-arid: The long distance to seasonal south-shifting humid tropic air masses (Innertropical Convergence Zone, ITCZ) and the climatic barrier of the Namib and the Karas Mountains against Atlantic influences allowed only minor fluctuations. The Holocene is a period of reduced pan development. The pans are partly buried by the early Holocene linear dunes as well as by young sand fields. At Pan 1073, southeast of Grootpan, intra-pan dunes have formed during the last 1000 years.

Timberline and alpine vegetation on the tropical and warm-temperate oceanic islands of the world: elevation, structure and floristics

In the oceans of the tropical and warm-temperate zone (40° N–40° S), only a small number of islands are high enough to show timberline and alpine vegetation. Excluding large islands with a more continental climate, only the following oceanic islands are relevant: Pico (Azores), Madeira, Tenerife, Gran Canaria and La Palma (Canary islands), Fogo (Cape Verde islands), Fernando Poo (Bioko) and Tristan da Cunha in the Atlantic Ocean, Reunion and Grande Comore (Ngazidja) in the Indian Ocean, Yakushima (Japan), Maui and Hawaii (Hawaiian islands), and Mas Afuera (Juan Fernandez islands) in the Pacific Ocean. Timberline and alpine vegetation exist here under a unique combination of a highly oceanic climate and a marked geographic isolation which contrasts with the tropical alpine vegetation in the extended mountains of South America, Africa and Southeast Asia. This review seeks to identify common physiognomic patterns in the high elevation vegetation that exist despite the fact that the islands belong to different floristic regions of the world. Based on the existing literature as well as personal observation, an overview of the elevation, physiognomy and floristics of the forest (and tree) line and the alpine vegetation on 15 island peaks is given. The forest line ecosystems are dominated either by conifers (Canary islands, Yakushima), heath woodland (Azores, Madeira, Reunion, Grande Comore, Fernando Poo) or broad-leaved trees (Hawaiian islands, Juan Fernandez islands, Tristan da Cunha). In the subalpine and alpine belts, dry sclerophyllous scrub occurs on island mountains that are exposed to the trade winds (Canary islands, Cape Verde islands, Hawaiian islands, Reunion, Grande Comore). These peaks are more or less arid above the forest line because a temperature inversion restricts the rise of humid air masses further upslope. In the summit regions of the remaining islands, which are located either in the wet equatorial and monsoonal regions or in the temperate westerly zones without an effective inversion layer, mesic to wet vegetation types (such as grassland, alpine heathland and fern scrub) are found. Compared to mountains at a similar latitude in continental areas, the forest line on the islands is found at 1000 to 2000 m lower elevations. The paper discusses four factors that are thought to contribute to this forest line depression: (1) drought on trade-wind exposed island peaks with stable temperature inversions, (2) the absense of well-adapted high-altitude tree species on isolated islands, (3) immaturity of volcanic soils, and (4) an only small mountain mass effect that influences the vertical temperature gradient.

Stable oxygen isotope distribution in the Huanghe (Yellow River) and the Changjiang (Yangtze River) estuarine systems

Water samples from the Huanghe (Yellow River) and Changjiang (Yangtze River) estuarine systems, and China Sea were analysed to determine their stable oxygen isotope composition (18O/16O). The riverine δ18O values range from −8.8to−7.1‰ in these two rivers during the study periods. In the Changjiang estuary, the longitudinal profiles of δ18O show a linear distribution with chlorinity, reflecting the efficiency of mixing processes between river and seawaters, which corresponds to a short water residence time in the estuary, estimated at about 5 days. In the Huanghe estuary, the non-linear distribution of δ18O with chlorinity reveals the occurrence of “old water” masses in the Laizhou Bay, where the water residence time can reach 3–4 months. The difference of δ18O values between these two large Chinese rivers may reflect the “continental/altitude effect” on the isotope fractionation, during the transport of humid air mass from the Pacific and Indian Oceans to the Chinese continent. A rough estimation shows that source region water accounts for about 5–20% of the water discharge in the lower reaches of these two rivers.

Model simulations of an intense meso-<i>ß</i> scale cyclone The role of condensation parameterization

On 23 July 1985, an unusual mesoscale storm struck the Baltic Sea coast of Sweden. Locally, more than 50 mm of precipitation fell in 12 h and mean winds exceeding 25 m s -1 were recorded. The failure of operational models to predict this storm, as well as the damage it caused (2 people were killed), raised important questions. What physical process caused this rare event? Could it be predicted with existing data, and if so, how? Diagnostic studies reveal the following features. The development took place in a very humid air mass, which possessed weak conditional stability. The synoptic-scale environment provided large relative vorticity, which helped to spin up the mesoscale vortex. Calculations show significant adiabatic forcing initially, but then latent heat release takes over as the main driving mechanism. Numerical simulations of this storm show that it can be successfully simulated with a regional model, provided that condensation and latent heat release are treated in a consistent manner. This is demonstrated, e.g., by comparing latent heating distributions in 2 different condensation schemes. The simulations, furthermore, demonstrate the crucial role of good humidity data for this type of simulations. DOI: 10.1034/j.1600-0870.1990.00008.x

Production of condensation nuclei in clean air by nucleation of H 2SO 4

High concentrations (e.g. 10 4–10 5 cm −3) of condensation nuclei are occasionally observed in the polar regions or over the central oceans in air that has been free of passage over continental surfaces for many days. The particles are relatively fresh, being no more than a few days old, as otherwise interparticle coagulation would lower the concentration well below that observed. We show a plausible scheme through which the particles (assumed to be droplets of H 2SO 4) may be produced from the acid vapor phase through the process of homogeneous nucleation, and develop (approximate) analytic expressions to estimate the particle production. A large fraction of pre-existing aerosol, however, must first be scavenged, as they would otherwise serve as surfaces of condensation and inhibit the nucleation. We conclude that the production of substantial (> 10 3 cm −3) numbers of new particles can take place only in humid pre-cleaned air mass systems.

Remote Sensing of Surface and Cloud Temperatures Using the 899-cm Interval

The spectral interval centered at 899 cm(-1) with a bandpass of 8.75 cm(-1) is used for the measurements of surface and cloud-top temperatures from an altitude of 30 km. The continuum and selective absorption contribution to such measurements are discussed. The observations were obtained, using a balloon flight spectrometer, on two occasions: once from Palestine, Texas, in a tropical humid air mass, and once from Sioux Falls, South Dakota, in a cold arctic air mass. The results from these observations are compared with the screen temperatures under the balloon and with special surface temperature measurements made using a thermistor and a radiometer.

Une nouvelle source d'énergie: l'énergie d'instabilité de l'atmosphère

The author summarises his ideas about how to use the latent energy of a humid and unstable air mass, an integral form of solar energy. With a small addition of thermal energy, it should be possible to exert a concerted influence on meteorological phenomena. Artificial convection, either using areas of solar heating or by sudden strong combustion, should permit recovery of part of this latent energy, in the form of rain or in more concentrated forms, of which the limit-type would be ball lightning.