SODAR-based estimation of TKE and momentum flux profiles in the atmospheric boundary layer: Test of a parameterization model

Summary A simple parameterization for the estimation of turbulent kinetic energy (TKE) and momentum flux profiles under near-neutral stratification based on sodar measurements of the vertical velocity variance has been tested using data from the LINEX-2000 experiment. Measurements included operation of a phased-array Doppler sodar DSDPA.90 and of a sonic anemometer USA-1 mounted at a meteorological tower at a height of 90m. Good agreement has been found between the TKE and momentum flux values derived from the sonic and sodar data (with correlation coefficients r>0.90 and a slope of the regression lines of about 1.01.1) suggesting the possible use of sodar measurements of _w ² to derive turbulence parameter profiles above the tower range.     

Effect of land management on ecosystem carbon fluxes at a subalpine grassland site in the Swiss Alps

Summary The influence of agricultural management on the CO₂ budget of a typical subalpine grassland was investigated at the Swiss CARBOMONT site at Rigi-Seebodenalp (1025m a.s.l.) in Central Switzerland. Eddy covariance flux measurements obtained during the first growing season from the mid of spring until the first snow fall (17 Mai to 25 September 2002) are reported. With respect to the 10-year average 1992–2001, we found that this growing season had started 10 days earlier than normal, but was close to average temperature with above-normal precipitation (100–255% depending on month). Using a footprint model we found that a simple approach using wind direction sectors was adequate to classify our CO₂ fluxes as being controlled by either meadow or pasture. Two significantly different light response curves could be determined: one for periods with external interventions (grass cutting, cattle grazing) and the other for periods without external interventions. Other than this, meadow and pasture were similar, with a net carbon gain of –128±17g Cm^–2 on the undisturbed meadow, and a net carbon loss of 79±17g Cm^–2 on the managed meadow, and 270±24g Cm^–2 on the pasture during 131 days of the growing season, respectively. The grass cut in June reduced the gross CO₂ uptake of the meadow by 50±2% until regrowth of the vegetation. Cattle grazing reduced gross uptake over the whole vegetation period (37±2%), but left respiration at a similar level as observed in the meadow.     

Propagation of the sea-surface temperature anomalies in the Tropical Atlantic

Summary The evolving modes of the sea-surface temperature (SST) in the Tropical Atlantic on the short interannual (IA) timescale were obtained by performing the extended empirical orthogonal function (EEOF) analyses on this variable separately for the 106-year (1871–1976) and 20-year (1881–1900; 1901–1920; 1921–1940; 1941–1960) periods. The equatorial and inter-hemispheric patterns manifest in the first EEOF mode of each analysis as part of the short IA evolution of the SST anomalies in the Tropical Atlantic. Another outstanding feature of the first EEOF mode of each analysis concerns the propagations of the SST anomalies in the meridional direction within the 20°N–20°S band and in the zonal direction in the sector 40°W–20°W. For all analyses, the SST anomalies propagate northward from the equator to 15°N and southward from 20°N to 15°N, with the same sign anomalies merging approximately at 15°N. On the other hand, the SST anomalies propagate westward in the sector 40°W–20°W with a propagation rate close to that of the phase speed of the fastest baroclinic Rossby wave in the ocean. So, the observed propagations of the SST anomalies in the 20°N–20°S band might result from the combined effect of the surface oceanic currents in this band and the baroclinic Rossby waves in the ocean.     

Environmental aerosols and their effect on the Earth’s local fair-weather electric field

Summary The Earths local fair-weather electric field is significantly affected by small ions present in the atmosphere. These ions are typically smaller than 0.001µm and occur in concentrations from 500 to 600cm^–3 in air. Attachment to larger aerosol particles may severely decrease the mobility of these atmospheric ions resulting in an increased local electric field. The number concentration of environmental aerosol particles in the size range 0.1 to 5.0µm was measured with two automatic laser scattering particle counters. The Earths electric field was monitored with an electric fieldmeter. Measurements were made in clean air and in an environment highly polluted by wood smoke. The electric field was found to be positively correlated to the aerosol number concentration. During one 24-hour period of measurement, the electric field increased from 180 to about 280Vm^–1 as the number concentration of aerosols larger than 0.1µm increased from about 2000 to 9000cm^–3. The number concentrations of aerosols larger than 0.1 and 0.3µm were both found to be positively correlated with the Earths electric field with correlation coefficients of 70% and 61%, respectively.Present address: School of Physical Sciences, Queensland University of Technology, Brisbane 4001, Australia.     

A study on the number of snow days over Eurasia, Indian rainfall and seasonal circulations

Summary In this paper, the relationship between seasonal mean (June, July, August and September) monsoon circulation features and the midlatitude circulations in winter and spring seasons have been examined during contrasting years of more (less) number of snow days in winter/spring followed by deficient (excess) Indian Summer Monsoon Rainfall (ISMR) using NCEP/NCAR reanalyzed data for the period 1966–1994. The Historical Soviet Daily Snow Depth (HSDSD) version II data set has been used to calculate the number of days of snow over west and east Eurasia separately under three classes: class 1 for SD>5cm, class 2 for SD>10cm and class 3 for SD>50cm where SD stands for snow depth. Correlation coefficients are computed between the anomaly in the number of days of snow depth under the above three classes during winter/spring over west and east Eurasia and the subsequent ISMR. HSDSD data show that difference in the number of days of SD>10cm in two extreme years is most prominent in the west Eurasia in the months of January and April. Also the anomaly in the number of days of snow in January and April over west Eurasia has correlation coefficients of –0.69 and –0.56 with the following ISMR, respectively at 0.1% significance level when the SD is more than 10cm at all the stations. Results also show that low-level atmospheric temperature difference between two extreme years of snow days in winter is up to 10°C and the cooling persists up to spring season with a difference of 2°C. This cooling persistence may give rise to anomalous cyclonic circulations over the midlatitudes and tropics which may be responsible for weakening the monsoon circulation over India during the year of more snow days over west Eurasia.     

Application of a “Big-Tree” model to regional climate modeling: a sensitivity study

Summary ¶In order to better understand land-atmosphere interactions and increase the predictability of climate models, it is important to investigate the role of forest representation in climate modeling. Corresponding to the big-leaf model commonly employed in land surface schemes to represent the effects of a forest, a so called big-tree model, which uses multi-layer vegetation to represent the vertical canopy heterogeneity, was introduced and incorporated into the National Center for Atmospheric Research (NCAR) regional climate model RegCM2, to make the vegetation model more physically based. Using this augmented RegCM2 and station data for China during 1991 Meiyu season, we performed 10 experiments to investigate the effects of the application of the big-tree model on the summer monsoon climate.With the big-tree model incorporated into the regional climate model, some climate characteristics, e.g. the 3-month-mean surface temperature, circulation, and precipitation, are significantly and systematically changed over the model domain, and the change of the characteristics differs depending on the area. Due to the better representation of the shading effect in the big-tree model, the temperature of the lower layer atmosphere above the plant canopy is increased, which further influences the 850hPa temperature. In addition, there are significant decreases in the mean latent heat fluxes (within 20–30W/m²) in the three areas of the model domain.The application of the big-tree model influences not only the simulated climate of the forested area, but also that of the whole model domain, and its impact is greater on the lower atmosphere than on the upper atmosphere. The simulated rainfall and surface temperature deviate from the originally simulated result and are (or seem to be) closer to the observations, which implies that an appropriate representation of the big-tree model may improve the simulation of the summer monsoon climate.We also find that the simulated climate is sensitive to some big-tree parameter values and schemes, such as the shape, height, zero-plane displacement height and mixing-length scheme. The simulated local/grid differences may be very large although the simulated areal-average differences may be much lower. The area-average differences in the monthly-mean surface temperature and heat fluxes can amount to 0.5°C and 4W/m², respectively, which correspond to maximum local/grid differences of 3.0°C and 40W/m² respectively. It seems that the simulated climate is most sensitive to the parameter of the zero-plane displacement among the parameters studied.     

Estimates of snow accumulation and volume in the Swiss Alps under changing climatic conditions

Summary ¶Snow is a key feature of mountain environments in terms of the controls it exerts on hydrology, vegetation, and in terms of its economic significance (e.g. for the ski industry). Its quantification in a changing climate is thus important for various environmental and economic impact assessments. Based on observational analysis, surface energy balance modeling, and the latest data from high-resolution regional climate models, this paper investigates the possible changes in snow volume and seasonality in the Swiss Alps. An average warming of 4°C as projected for the period 2071–2100 with respect to current climate suggests that snow volume in the Alps may respond by reductions of at least 90% at altitudes close to 1000m, by 50% at 2000m, and 35% at 3000m. In addition, the duration of snow cover is sharply reduced in the warmer climate, with a termination of the season 50–60 days earlier at high elevations above 2000–2500m and 110–130 days earlier at medium elevation sites close to the 1000m altitude. The shortening of the snow season concerns more the end (spring) rather than the beginning (autumn), so that it should be expected that snow melt will intervene much earlier in the season than under current conditions. The results of this study are of relevance to the estimations of the impacts that the projected warming may have on the amount and timing of water in hydrological basins, on the start of the vegetation season, and on the financial status of many mountain resorts.     

Tropical cyclone environments over the northeastern Pacific, including mid-level dry intrusion cases

Summary This study uses a 1°×1° lat/long dataset, extracted from ECMWF re-analyses for the 15-year period 1979–1993 (ERA-15), to diagnose the synoptic-scale kinematic, thermodynamic and moisture environments in the vicinity of named tropical cyclones (TCs) in the eastern North Pacific. Based on the NCDC best track dataset, TCs are partitioned into one of three categories: weak (W), strong (S) or intensifying (I). In total, 63TCs are examined: 8Ws and 20Is at point A (maximum intensification) and 11Ws, 13Ss and 11Is at point B (maximum frequency). Composite maps are compiled for all five groups, and six individual case studies are examined, four for extreme TC cases and two for cases involving dry air intrusions.For the most part, peak values and patterns of composited ERA-15 variables display circulation, thermodynamic and moisture characteristics that are compatible with the strength represented by a groups classification. Intercomparison between Ws and Is at points A and B yielded larger conditional instability of low-level air parcels and upper-level outflow within the region of maximum intensification (point A).The intrusions of dry versus moist mid-level air are addressed for each storm with the assistance of 72-hour backward trajectories. Trajectory density maps indicate two preferred paths of air parcels that reach the environment of W storms at point A on the 700 and 500hPa levels. The first one crossed Central America in the region of the Isthmus of Tehuantepec and the second one south of the Central American mountains. Several storms revealed that these trajectories were associated with dry air intrusions into the larger storm area, and this might be one reason for their weak status at point A. One documented example is Kevin (1985). By the time it reached point B, the dry air was replaced by air that was moist and Kevin intensified, although it remained a W system. In contrast, Narda (1989) received a dry air intrusion from Central Mexico at 500hPa as a weak storm at point B and did not intensify. Despite possible analyses problems, the documentation in this study of mid-level dry air intrusions into eastern Pacific TCs from the Mexican-Central American region suggests a hitherto unexploited forecast potential. Received January 15, 2002; revised November 28, 2002; accepted December 19, 2002 Published online: May 8, 2003     

Remote sensing of the dynamic stability of the atmospheric boundary layer

Summary The relative strength of the stabilizing effect of buoyancy and the destabilizing effect of velocity shear in a stratified shear flow, such as a stable atmospheric boundary layer, is measured by the gradient Richardson number, Ri_g. The boundary layer static stability, as described by the buoyancy frequency, N, can be calculated from the virtual potential temperature gradient derived from RASS temperature profiles. The mean wind profiles from a sodar can be used to calculate the mean vertical velocity shear. In combination these profilers are potentially a powerful tool for the remotely sensing the dynamic stability of the boundary layer. However, experience shows that the combinations of two experimentally derived quantities, like N and shear, may give highly variable results. On the other hand, a simple sensitivity analysis shows that reasonable estimates of Ri_g are achievable over a range of conditions in the stable nocturnal boundary layer. To test this conclusion, high spatial and temporal resolution temperature and velocity soundings were obtained above 50m in the stable nocturnal boundary layer using a 920MHz continuous wave Radio Acoustic Sounding System (RASS) and 1.875kHz and 5.00kHz Doppler sodars. Examples of the evolution of Ri_g are presented from 24 hours of observations of the boundary layer in Canberra, on the tablelands in south- eastern Australia. Most of the boundary layer had Ri_g between 0.1 and 1. Thus, it was marginally dynamically stable, even with the gradient Richardson number calculated from finite differences over a vertical interval of 68m. A comparison of the results from the two sodars showed that the velocity shear increased significantly when the vertical differencing interval was decreased from 68m to 20m.     

Transition from the Southern to the Northern Hemisphere summer of zones of active convection over the Congo Basin

Summary This study used monthly rainfall totals for the period 1961 to 1988 and pentad OLR values for the period 1974 to 1991 to study the structure and transition of active convention across the Congo Basin (10°S–5°N, 15°E–35°E) from the southern to the northern hemisphere summer. This involved the examination of map patterns and cross-sections of monthly rainfall and pentad OLR data.The results from the study indicated that there were two seasons observed over the Congo Basin; one is the wet season lasting from September to April and the other a dry season covering the rest of the year. The onset of the wet season takes place rapidly with active convection spreading very quickly to the south near latitude 20°S. This is due to the formation of the meridional (north–south) branch of the ITCZ over this region.This study has confirmed that the annual rainfall over the Congo Basin is reliable with the coefficient of variation of less than 30%. The wet seasons (e.g., SON and DJF) also show reliable rainfall occurrence but the dry season (e.g., JJA) has low reliability.The anomalously wet seasons are characterised by a relatively slow transition rate (1° latitude per pentad) of zones of active convection resulting in a late withdrawal of the rainy season while the dry seasons show a rapid transition rate with an early withdrawal of zones of active convection.High-rainfall months (>200mm) are concentrated within the Southern Hemisphere summer months. These high-rainfall months progress from the equator to the southern latitude following generally the movement of the overhead sun.The results further revealed that the years 1987/1984 had the lowest/highest mean OLR values over the Congo Basin within the period 1979 to 1991. The rates of transition of the zones of low OLR values were 0.9/5.0 degrees of latitude during 1987/1984, respectively.Received June 18, 2002; revised September 30, 2002; accepted November 21, 2002 Published online: June 12, 2003     

A diagnostic study on the environmental influence of a mesoscale convective system over southern China in Meiyu season

Summary ¶During the Post-TAMEX forecast experiment of Taiwan in 1992, a mesoscale convective system (MCS) developed on June 5–6 over southern China. As this system matured, it produced readily apparent cirrus outflow on satellite imageries while the upper level flow also exhibited a diffluent pattern. The purpose of the current study is to examine the possible changes in its environment associated with the development of this MCS.By using 12-h data from 1200 UTC June 5 to 1200 UTC June 6, objective analyses were performed for a 1°×1° latitude/longitude grid using sounding data and a low-pass filter. To facilitate the diagnosis, a band-pass filter was further applied to separate mesoscale features from macroscale ones, while the apparent heat source and apparent moisture sink defined by Yanai et al (1973) were also calculated.Results suggest that the MCS exerted clearly discernable effects on its environment. The latent heat release led to the development of a warm core and mesoscale high-pressure disturbance at upper levels when the system matured. Ageostrophic winds and diffluent flow patterns together with strong anticyclonic vorticity at 200hPa near the MCS were associated with the mesohigh. After the mature stage, weak cooling occurred above 350hPa, likely due to radiative emission from the cloud top. However, a mid-level cyclonic vortex, often present in MCSs over the North America, was not apparent here due to weak environmental vorticity and small Coriolis parameter f. The level of maximum divergence was initially located at 500hPa, but rose to 200hPa as the MCS matured. In response, the upward motion not only intensified, but the level at which strongest rising occurred also ascended from 700 to 350hPa. Results from the apparent heat source and moisture sink calculation suggest that this slow ascent of maximum heating was partially due to vertical transport of sensible heat by updrafts.During the MCSs mature stage, under the stratiform clouds to the west of the strongest convection, a cold mesohigh formed at the surface due to evaporative cooling in downdrafts, and a gust front appeared along the leading edge of the outflow boundary. A trailing mesolow was also observed, likely due to near-adiabatic warming in drier downdrafts since no precipitation was associated with it.Received April 11, 2002; revised May 27, 2002; accepted July 14, 2002 Published online: April 10, 2003     

Recent warming in a small region with semi-oceanic climate, 1949–1998: what is the ground truth?

Summary Trends of monthly air temperature extremes were investigated in five meteorological stations of the Grand-Duchy of Luxembourg during the period 1949–1998. The application of an innovative homogenization method based on the concept of relative homogeneity to climatic time series allows identifying multiple break points, as well as correcting data series in an objective and robust statistical way. The rise of maximum temperature (Tmax) has occurred at a rate of 1.5 times that of the minimum temperature (Tmin) in winter (+1.4°C versus +0.9°C) and summer (+1.4°C versus +0.8°C). No trend in temperature extremes was found in autumn, while spring was affected by a small warming (+0.3°C) of Tmin and no change in Tmax resulting in a decrease of the diurnal temperature range (DTR) (–0.3°C). In spring, a strong positive linear relationship between Tmin warming and local terrain slope could be found. Comparison to new-gridded large-scale climatologies indicates generally close agreement to temperature trends during the 1949–1998 period, while a lower local warming was observed in summer during the post-1975 period following the changing-point year of atmospheric circulation over North-western Europe. This study shows that the question of data homogeneity is not trivial and should receive careful attention before quantifying historical temperature trends and identifying their spatial patterns at regional scale.     

Rain height in relation to 0°C isotherm height for satellite communication over the Indian Subcontinent

Summary ¶The 0°C isotherm height, a parameter needed for the estimation of attenuation of microwave and millimetre wave for earth-space communication, has been estimated for different stations spread over India. The variations of 0°C isotherm height for different seasons over these stations are presented. Attenuations of radio wave due to rain at frequencies 10GHz and above have also been estimated for few stations using the 0°C isotherm height so derived. The results are useful for radio systems designers.     

Amazonian forest dieback under climate-carbon cycle projections for the 21st century

Summary The first GCM climate change projections to include dynamic vegetation and an interactive carbon cycle produced a very significant amplification of global warming over the 21st century. Under the IS92a business as usual emissions scenario CO₂ concentrations reached about 980ppmv by 2100, which is about 280ppmv higher than when these feedbacks were ignored. The major contribution to the increased CO₂ arose from reductions in soil carbon because global warming is assumed to accelerate respiration. However, there was also a lesser contribution from an alarming loss of the Amazonian rainforest. This paper describes the phenomenon of Amazonian forest dieback under elevated CO₂ in the Hadley Centre climate-carbon cycle model.     

The convective boundary layer over pasture and forest in Amazonia

Summary The coupling between different types of surface (tropical forest or grass) and the Convective Boundary Layer (CBL) has been investigated using observational (rawinsoundings) data collected over Rondônia in southwest Amazonia. The data reported here support the notion that deforestation may modify the dynamics of the boundary layer, in particular during the dry season. In this period the sensible heat fluxes are very high over pasture, creating a CBL around 550m deeper compared to that over the forest. The measurements showed the height of the fully developed CBL for pasture to be 1650m, compared to around 1100m for forest. During the wet season the height of the CBL is lower than during the dry season and has the same height (around 1000m) for forest and pasture sites. The CBL over pasture is hotter and drier than over forest during the dry season, but during the wet season the air temperatures and humidities are similar. Comparing the CBL growth during the dry and wet season, there is evidence that the CBL properties over the forest are not dependent on the surface characteristics, but over the pasture they are.     

Hailstorms in Northern Greece: synoptic patterns and thermodynamic environment

Summary ¶Two cyclonic vortices close to each other, a binary cyclone or binary system, tend to rotate cyclonically relative to one another and to merge, i.e. the Fujiwhara effect. The point vortex model that represents barotropic binary cyclones predicts their rotation features as follows. The rotation rate is proportional linearly to the sum of the cyclones intensities and inversely to the square of their separation distance while the more intense cyclone rotates slower. Our earlier observational analysis of 1423 mid-latitude binary cyclones (Ziv and Alpert, 1995) showed a reasonable fit to theory, except for the absence of a correlation between individual speeds and intensities within the binary systems, and a reversal of the inverse rotation-separation relationship at the range of 1400–1800km.This study is the first attempt to describe the mid-latitude binary systems using potential vorticity concepts (PV thinking), which implies that a binary interaction takes place between the 3-D flow patterns induced by upper-PV or surface-thermal anomalies rather than by the surface cyclones alone. It is argued that the upper-anomalies dominate the rotation process, and hence the rotational speeds of the interacting surface cyclones are more closely correlated with the relative intensities of their corresponding upper-level anomalies rather than with their own intensities, as reflected in weather charts. Data analysis indicates that mid-latitude binary cyclones are normally associated with at least one upper-PV anomaly. This explains the absence of a correlation between the rotation speed and the intensity of the surface cyclones there.A unique type of a mid-latitude binary system is identified, in which one cyclone coincides with an upper major PV-anomaly and the other moves along the periphery of the former. Such a binary system is entitled here the Contact Binary System (CBS), in contrast with remote interacting systems implied by the point vortex theory.Analytical considerations yield an increase in the rotation rate with separation for CBSs of separation smaller than 1000–1500km, in contrast to the normal decrease with R ². The contribution of CBSs is suggested here to explain the abnormal increase in rotation rate at 1400–1900km range.     

Atmospheric response to soil-frost and snow in Alaska in March

Summary A hydro-thermodynamic soil-vegetation model including soil freezing/thawing (soil-frost) and snow-metamorphism has been integrated into the PennState/NCAR Mesoscale Meteorological Model MM5 in a two-way coupled mode. A hierarchy of simulations with and without the soil-frost module, each combined with and without the snow module, shows the influence of snow-cover and soil-frost on weather in Alaska. Herein the landscape is featured as it is typically by mesoscale models.Theoretical considerations suggest that organic soil types should be considered in mesoscale modeling because of their different thermal and hydrological behavior as compared to mineral soils. The Ludwig-Soret and Dufour effects are small, but increase appreciably during freezing/thawing and snow-melt.The snow and soil-frost processes have a demonstrable impact on the surface thermal and hydrological regimes and on the near-surface atmospheric conditions even on the short (synoptic) timescales. The presence of snow-cover results in a highly stable stratification. In cloud-free areas, the enhanced loss of radiant energy and cooling of the air over snow-cover lead to a positive feedback to relatively colder, drier conditions. In cloudy areas, a positive feedback to warmer, moister conditions develops over snow-cover. As the changes in atmospheric humidity and temperature caused by snow-cover propagate into the pressure field, sea level pressure is lower by more than 1hPa in the simulations with snow-cover. Although the effect of soil-frost alone is an order of magnitude smaller, the soil-frost snow system leads to an increase of the pressure difference to 1.2hPa. The changes in the pressure field alter wind speed and direction slightly.Soil-frost results in soil temperature differences of 2–5K in the upper soil layers, while snow results in differences of 3–10K. Soil-frost has a notably greater impact in cloud-free than cloudy areas. When a snow-cover is present, frozen soil enhances the insulating effect of a snow-cover in cloudy areas, but reduces it in cloud-free areas. In cloudy areas, soil-frost without snow-cover leads to cooler, drier atmospheric conditions relative to no frost. In cloudy areas, soil-frost under a snow-cover reduces the water supply to the atmosphere as compared to snow-covered conditions without soil-frost. The combined effects of soil-frost and snow increase precipitation locally by as much as 12.2mm/ 48h. If mesoscale modeling does not consider the soil-frost snow system, predicted water vapor fluxes will be too high in cloud-free areas, and too low in cloudy areas.     

Seasonal variations of anthropogenic sulfate aerosol and direct radiative forcing over China

Summary Regional climate model (RegCM2) and sulfur transport model (NJUADMS) were combined to simulate the distribution of anthropogenic sulfate aerosol burden over China, where a look up table method was applied to illustrate sulfate formation from SO₂-oxidation. Direct radiative forcing of sulfate aerosol was further estimated using the scheme suggested by Charlson et al (1991). Investigations show that the annual average total sulfate column over mainland China is 2.01mg/m² with high value in East and Central areas (more than 7mg/m²). The annual average direct radiative forcing of China is about –0.85W/m². The forcing can reach –7W/m² in Central and East China during the winter season. Total sulfate column shows significant seasonal variations with winter maximum-summer minimum in the Southern part of China and spring maximum-autumn minimum in the northern part of China. Strong seasonal cycles of direct radiative forcing are also found due to the influence of total sulfate column, cloud, relative humidity and the reflectivity of underlying surfaceReceived May 16, 2001; accepted August 5, 2002 Published online: May 8, 2003     

Stable Isotope Ratios: Hurricane Olivia

The oxygen and hydrogen isotopic compositions of rains from HurricaneOlivia (1994) in the eastern Pacific were measured. The rains werecollected on 24 and 25 September during airplane flights conducted at anelevation of 3 km. Hurricane Olivia peaked in intensity to a category-4storm between the two dates. Isotope ratios of rains from HurricaneOlivia were markedly lower ( ¹⁸O = –13.9to –28.8) than that of rain collected from a thunderstormat an elevation of 2.3 km outside the influence of Olivia (¹⁸O = –3.8). A distinct decrease in isotoperatios from the first day to the next ( ¹⁸O =–18.4 to –21.9) in Hurricane Olivia wasattributed to decreased updraft velocities and outflow aloft. Thisshifted the isotopic water mass balance so that fewer hydrometeors werelifted and more ice descended to flight level. A decrease in the averagedeuterium excess from the first day to the next (d = 15.5 to 7.1)was attributed to an increase in the relative humidity of the watervapor `source' area. We hypothesize that the `source' region for therain was in the boundary layer near the storm center and that becausethe hurricane was at peak intensity prior to the second day the relative humidity was higher.     

Analysis of the Saharan dust regional transport

Summary The Balkan Peninsula is situated in the impact zone of Saharan dust storms. The case of Saharan dust transport to Belgrade in the period of 14–17 April 1994 is analyzed using the Eta model for synoptic and meso scale processes. Air back trajectories are calculated at six model levels from 434 up to 5129m with horizontal grid resolutions of 1°×1° and 10×10. Following cyclonic circulation the dust was picked up from North Africa, and transported over Mediterranean. Simultaneously, according to the analysis of the three lowest trajectories, transport of trace metals from Macedonia and southern Serbia by the Koshava wind might be dominant in the observed episode. Turbulent flow enhanced the coagulation process of initially clean dust particles with particles containing Pb and Cd. The coagulation and scavenging processes below and in clouds increased deposition rates of Pb and Cd in Belgrade in the course of wet removal, and consequently trough resuspension processes. Dry deposition samples contained characteristic particles up to 30µm in diameter with Fe content of 11 to 15 atomic% and significant ratio Si/Fe of 3 to 5, determined for selected single particles by the SEM/EDX method. Following dry and wet deposition of Cd and Pb, a residual effect of dustfall is noticed throughout the vegetational period.