Continental Stations Research Articles

Evaluation of the boundary layer dynamics of the TM5 model over Europe

Abstract. We evaluate the capability of the global atmospheric transport model TM5 to simulate the boundary layer dynamics and associated variability of trace gases close to the surface, using radon (222Rn). Focusing on the European scale, we compare the boundary layer height (BLH) in the TM5 model with observations from the National Oceanic and Atmospheric Admnistration (NOAA) Integrated Global Radiosonde Archive (IGRA) and also with ceilometer and lidar (light detection and ranging) BLH retrievals at two stations. Furthermore, we compare TM5 simulations of 222Rn activity concentrations, using a novel, process-based 222Rn flux map over Europe (Karstens et al., 2015), with harmonised 222Rn measurements at 10 stations. The TM5 model reproduces relatively well the daytime BLH (within 10–20 % for most of the stations), except for coastal sites, for which differences are usually larger due to model representation errors. During night, however, TM5 overestimates the shallow nocturnal BLHs, especially for the very low observed BLHs (< 100 m) during summer. The 222Rn activity concentration simulations based on the new 222Rn flux map show significant improvements especially regarding the average seasonal variability, compared to simulations using constant 222Rn fluxes. Nevertheless, the (relative) differences between simulated and observed daytime minimum 222Rn activity concentrations are larger for several stations (on the order of 50 %) than the (relative) differences between simulated and observed BLH at noon. Although the nocturnal BLH is often higher in the model than observed, simulated 222Rn nighttime maxima are actually larger at several continental stations. This counterintuitive behaviour points to potential deficiencies of TM5 to correctly simulate the vertical gradients within the nocturnal boundary layer, limitations of the 222Rn flux map, or issues related to the definition of the nocturnal BLH. At several stations the simulated decrease of 222Rn activity concentrations in the morning is faster than observed. In addition, simulated vertical 222Rn activity concentration gradients at Cabauw decrease faster than observations during the morning transition period, and are in general lower than observed gradients during daytime. Although these effects may be partially due to the slow response time of the radon detectors, they clearly point to too fast vertical mixing in the TM5 boundary layer during daytime. Furthermore, the capability of the TM5 model to simulate the diurnal BLH cycle is limited by the current coarse temporal resolution (3 h/6 h) of the TM5 input meteorology.

Rainouts over the Arabian Sea and Western Ghats during moisture advection and recycling explain the isotopic composition of Bangalore summer rains

AbstractVariations in the isotope ratios (18O/16O and D/H) of meteoric water at continental stations serve as valuable tracers for the hydrological cycle. In the present study, we investigated the role of sea surface temperature, wind speed, relative humidity, and rainout processes on the stable isotopic composition of the 2010 monsoon rainwater of Bangalore. The wind over the ocean influences the sea surface temperature and humidity which in turn influence the vapor isotopic composition. The rainout over the Arabian Sea and the land mass (Western Ghats) during advection of the air parcel to Bangalore and its recycling further modify the vapor composition. The isotopic ratios (δ18O, δD, and d‐excess) of the precipitation at Bangalore was estimated following a Rayleigh fractionation model involving rainout and recycling processes yield values consistent with our observation. In some samples, however, the observed isotopic ratios are higher (by 2 to 5‰), and a few of them are associated with high d‐excess values. These discrepancies could be due to limitation of the model assumptions.

Characteristics of the atmospheric electric field and correlation with CO2 at a rural site in southern Balkans

In the current work, 4 years of atmospheric electric field observations at a rural site near Xanthi, Greece, are presented for 2011–2014. The site is situated in an area with very high radon fluxes and high thunderstorm activity. The annual variation is consistent with that at other Northern Hemisphere continental stations, with maxima (minima) occurring during the cold (warm) months. The diurnal variation of the atmospheric electric field both for fair weather (FW) and all weather is found to exhibit a double-peak structure corresponding to local effects and global thunderstorm activity. Comparison with the Carnegie curve shows that nighttime hours and winter months are preferable for observing the Global Electric Circuit at the Xanthi site. Finally, it is shown that atmospheric CO2 measurements can be effectively utilized as a stratification proxy, indicating conditions of potential radon trapping, whereas CO2 was found to anticorrelate with the atmospheric electric field during such conditions.

Monitoring of Greenhouse Gases in the Atmosphere – A Polish Perspective

Abstract An overview of systematic observations of the trace-gas composition of the atmosphere over southern Poland is presented, against the background of data available for other greenhouse gas (GHG) monitoring stations in Europe. The results of GHG monitoring for three major greenhouse gases (CO2, CH4, N2O) are discussed. Measurements were performed at two locations of contrasting characteristics, i.e. (i) the high-altitude mountain station of Kasprowy Wierch in the High Tatras, representing atmospheric conditions relatively free of local influences, and (ii) an urban station located in the Krakow agglomeration. The GHG data available for the Kasprowy Wierch station were compared with relevant data available for two marine reference stations (Mace Head, Ireland and Terceira Island, Azores), and two continental stations (Hohenpeissenberg, Germany and Pallas-Sammaltunturi, Finland). The growth rates for the CO2 mole fraction recorded at these five stations reveal only small temporal changes that almost coincide, leading to a quasi-linear increase of the CO2 mixing ratio over the European continent over the past 20 years. While N2O observations also reveal a steady increase over this time period, the mole fraction accounted for by CH4 is increasing again, after a period of stagnation in the years 2001–2007. The impact of continental sources of CH4 and N2O is seen clearly in the Kasprowy Wierch records. The mean departure between the CH4 mixing ratios recorded at Kasprowy Wierch and at the marine reference stations in the period 1994–2014 is of 27.3 ppb, and stems from continental emissions of this gas originating mainly from anthropogenic activities (leaking natural-gas distribution networks, landfills and livestock). For N2O, a departure of 1 ppb was observed for the period 2009–2014. Comparison of quasi-continuous measurements of CO2, CH4 and N2O mixing ratios made in the urban atmosphere of Krakow and at the regional reference site Kasprowy Wierch (located approximately 100 km away), allows for a deeper insight into the mechanisms controlling daily variations in atmospheric mixing ratios of these gases at the two sites. The development of a nocturnal inversion layer in the atmosphere above the city leads to local enhancements of CO2, CH4 and N2O mole fractions in the Krakow atmosphere during the night hours, with these exceeding the baseline level significantly.

Polychaete diversity in the Scotia Arc benthic realm: Are polychaetes tracers for faunal exchange?

The Scotia Arc is the only shallow-water and island bridge linking nowadays Patagonia and the Antarctic. The Antarctic Circumpolar Current as an oceanographic peculiarity makes this region an interesting biogeographic transition zone, because this frontal system traditionally is said to isolate the Antarctic fauna from that of the adjacent northern ecosystems. Based on benthos samples from three expeditions onboard R/V Polarstern, we studied distribution patterns of 200 polychaete species and 34 major benthic taxa in order to evaluate the role of polychaetes in the benthic realm of this part of the Southern Ocean. ANOSIM test distinguished three station groups: the central eastern Scotia Sea, the continental shelf off South America and stations at the tip of the Antarctic Peninsula. These station groups differed in organism densities and diversities with stations at the tip of the Antarctic Peninsula hosting the most diverse and dense community. The polychaete diversity patterns in the three assemblages evidenced closer connectivity between the tip of the Antarctic Peninsula and the central eastern Scotia Sea than between the continental shelf off South America with either the stations off the tip of the Peninsula or the central eastern Scotia Sea. This is probably supported by the Polar Front, which divides the island chain into two branches. Species distribution and community patterns of polychaetes appear to be associated with oceanographic and sediment conditions in this region. Most of the shared species showed the capability to tolerate differences in hydrostatic pressure. We suggest that the islands of the Scotia Sea may constitute a bridge for exchange of benthic species, particularly for polychaetes with eurybathic distribution and high dispersal capabilities.

On the Long-Term Climate Memory in the Surface Air Temperature Records over Antarctica: A Nonnegligible Factor for Trend Evaluation

AbstractIn this study, observed temperature records of 12 stations from Antarctica island, coastline, and continental areas are analyzed by means of detrended fluctuation analysis (DFA). After Monte Carlo significance tests, different long-term climate memory (LTM) behaviors are found: temperatures from coastal and island stations are characterized by significant long-term climate memory whereas temperatures over the Antarctic continent behave more like white noise, except for the Byrd station, which is located in the West Antarctica. It is argued that the emergence of LTM may be dominated by the interactions between local weather system and external slow-varying systems (ocean), and therefore the different LTM behaviors between temperatures over the Byrd station and that over other continental stations can be considered as a reflection of the different climatic environments between West and East Antarctica. By calculating the trend significance with the effect of LTM taken into account, and further comparing the results with those obtained from assumptions of autoregressive (AR) process and white noise, it is found that 1) most of the Antarctic stations do not show any significant trends over the past several decades, and 2) more rigorous trend evaluation can be obtained if the effect of LTM is considered. Therefore, it is emphasized that for air temperatures over Antarctica, especially for the Antarctica coastline, island, and the west continental areas, LTM is nonnegligible for trend evaluation.

Directionality of ambient noise on the Juan de Fuca plate: implications for source locations of the primary and secondary microseisms

Based on cross-correlations of ambient seismic noise computed using 61 ocean bottom seismometers (OBSs) within the Juan de Fuca (JdF) plate from the Cascadia Initiative experiment and 42 continental stations near the coast of the western United States, we investigate the locations of generation of the primary (11–20 s period) and secondary (5–10 s period) microseisms in the northern Pacific Ocean by analysing the directionality and seasonality of the microseism (Rayleigh wave) signals received in this region. We conclude that (1) the ambient noise observed across the array is much different in the primary and secondary microseism bands, both in its azimuthal content and seasonal variation. (2) The principal secondary microseism signals propagate towards the east, consistent with their generation in deep waters of the North Pacific, perhaps coincident both with the region of observed body wave excitation and the predicted wave–wave interaction region from recent studies. (3) The primary microseism, as indicated by observations of the azimuthal dependence of the fundamental mode Rayleigh wave as well as observations of precursory arrivals, derives significantly from the shallow waters of the eastern Pacific near to the JdF plate but also has a component generated at greater distance of unknown origin. (4) These observations suggest different physical mechanisms for generating the two microseisms: the secondary microseisms are likely to be generated by non-linear wave–wave interaction over the deep Pacific Ocean, while the primary microseism may couple directly into the solid earth locally in shallow waters from ocean gravity waves. (5) Above 5 s period, high quality empirical Green's functions are observed from cross-correlations between deep water OBSs and continental stations, which illustrates that microseisms propagate efficiently from either deep or shallow water source regions onto the continent and are well recorded by continental seismic stations.

Pycnogonida from the Bellingshausen and Amundsen seas: taxonomy and biodiversity

The Bellingshausen and Amundsen Seas are among the least studied Antarctic areas. Pycnogonids constitute a common and conspicuous component of the Antarctic marine fauna. Antarctic pycnogonids have been widely studied and are usually more abundant than elsewhere. Therefore, they represent a key taxon to understand the zoogeographic and bathymetric distributions of the fauna from these two poorly sampled seas. Furthermore, we aim to compare the diversity and composition of the pycnogonids in these areas with those in other Antarctic zones. Three main surveys were carried out in these regions (Bentart 2003 and 2006 and Biopearl II 2008). In total, 879 pycnogonids belonging to 65 species were recorded in 49 stations. Two new species are described: Heteronymphon krappi n.sp. and Nymphon nakamurai n.sp. Ammothea magniceps and A. hesperidensis are recorded for the second time in the Antarctic. The most abundant family is the Nymphonidae (60.5 %), and Nymphon australe is the most abundant species (25.5 %). The biogeographic analysis revealed 39 species in the Bellingshausen Sea (16 new records) and 19 species in the Amundsen Sea (18 new records). The circumpolar pattern is the most common found. The Bellingshausen Sea seems to be a poor area in terms of abundance and species richness compared to the Amundsen Sea and other Antarctic zones. Faunal similarity was clustered into three main groups: the shallow-water stations, the outer continental shelf stations and the slope stations. The abundance of individuals likely responds to the varying amount of organic matter that reaches each bathymetric zone.

Water limitations on potato yield in Estonia assessed by crop modelling

Soil moisture is one the most limiting factors for crop production. In temperate climates yields are reduced by both water deficit and excess. To calculate the impact of water limitations, alternative water balance schemes were added to the potato production model POMOD (POtato MODel) to keep the soil moisture content within its optimal range. The effects of irrigation, drainage and both together were investigated. Calculations were carried out for three Estonian locations (Tallinn, Tartu and Kuressaare) and two varieties.For the reference run the yields were higher for the late variety and lowest in dry Kuressaare. The average irrigation requirement was 30–45mm for the early and 47–58mm for the late variety in continental locations, but over two times higher in insular Kuressaare. In continental stations the need for drainage was 30–40mm per growing period in addition to natural outflow (about 20–25mm more without outflow). In Kuressaare the mean amount of excess precipitation was only 9–20mm and its removal had a mostly negative effect on yield due to increasing water deficit during the following dry period. In over half of the years the crops suffered from both water shortage and excess. The greatest increase in yield was achieved when both drainage and irrigation were applied. The benefit from irrigation alone was also considerable, more for the late variety and in Kuressaare. The effect of drainage was most evident in conditions of restricted outflow The negative effects of drainage on yield, as observed in years when drainage led to increasing water deficit in subsequent periods, lowered the average impact of drainage. However, in some cases the highest yield losses were observed in years with excess water.

Stable isotopic composition of atmospheric water vapor in Patras, Greece: A concentration weighted trajectory approach

This paper presents the results of a three year sampling of the isotopic composition (δ18O and δ2Η) of atmospheric water vapor in Patras, Greece. Since the distribution of stable isotopes in precipitation/water vapor is strongly affected by the climatic conditions occurring over a certain area, the link between the isotopic composition of water vapor and meteorological parameters is examined. Air temperature partly modulates the isotopic composition explaining only a small amount of the overall isotopic variation. The temperature effect for δ18Ο is more remarkable in the vapor phase providing a higher isotope-temperature slope for monthly water vapor (0.5‰/°C) in comparison to that of precipitation (0.32‰/°C). The seasonality effect of the vapor δ18Ο data is determined by applying a sinusoidal model. The modeled peak of the seasonal effect is observed in the beginning of June with amplitude equal to 1.22‰ fluctuating around −13‰. Furthermore the mean monthly isotopic composition of water vapor in Patras is compared against those of several sampling stations of the Northern Hemisphere. The mean monthly isotopic variation reflects the overall characteristics of the coastal locations (i.e. Rehovot) providing a lower seasonal amplitude in comparison to continental stations. Finally, in order to identify the potential sources affecting the D-excess of water vapor a Concentration Weighted Trajectory model was used. Back trajectory analysis combined with the isotopic information was used to calculate the concentration fields. Air parcels coming from the north-westerly direction affect the region of Patras implying that this north-western flow contributes to the observed D-excess variability.

Monitoring of Southwest Monsoon Using Isotope Analysis of Ground Level Vapour (Glv) in Indian Sub-Continent

This paper presents the work carried out by National Institute of Hydrology, Roorkee to understand the role of water vapor isotopes in understanding and monitoring the southwest monsoons. For this study, a network of stations was established all over India for collection of ground level vapor (Glv). The stations were established at Roorkee in foot hills of Shiwalik (Uttarakhand), Sagar in Central India (Madhya Pradesh), Jammu in Sub-Himalayan region (Jammu & Kashmir), Kakinada in coastal region (Andhra Pradesh), Tezpur in North-Eastern region (Assam), Kanpur in Gangetic plains (Uttar Pradesh) and Manali in Himalayan region (Himachal Pradesh). A stronger isotopic depletion in the Glv with higher latitude, altitude, distance from coast and rainfall has been noticed which clearly indicate ‘latitude effect’, ‘altitude effect’, ‘continental effect’ and ‘amount effect’. The Glv received during the SW monsoon period is always depleted as compared to the Glv received during non-monsoon period in the continental stations and the extent of depletion in isotopic composition of Glv and period over this depletion continues is directly linked with monsoon strength (intensity, episodes and duration), showing a possibility of using isotopes to monitor movement of monsoon vapors.

Recordings of Long-Period Fluctuations Associated with the Passage of Three Distinct Tsunamis at Broadband Seismometers Made at the International Monitoring System (IMS) Hydroacoustic T-station H06 (Socorro Island, Mexico)

In the wake of the very large Indian Ocean tsunami on 26 December 2004, and several subsequent events in the Pacific Ocean, scientific and disaster mitigation initiatives have taken place to better understand and observe tsunami propagation. This paper describes observations made on nearshore long‐period seismometers of a little known and, to our knowledge, recently discovered phenomenon associated with the passage of tsunamis near a coast. It provides further insight into this phenomenon. Observations of long‐period effects of the passage of tsunamis on the horizontal components of nearshore seismometers were initially made by Yuan et al. (2005). A quantitative assessment was first made by Okal (2007). These observations were made on the Indian Ocean tsunami of December 2004. We observe similar long‐period oscillations for the tsunamigenic Maule, Chile, earthquake of 27 February 2010 at two broadband seismometers used as hydroacoustic T‐station of the International Monitoring System (IMS) network and located on Socorro Island, Mexico. Polarization and amplitude analysis of the long‐period arrivals on the two closely spaced seismometers allows us to propose an explanation for the polarization of observations that were not explained in Okal (2007). Our preferred interpretation of the phenomenon is that the surface of the island close to the shore is tilted by the loading from the local sea elevation change due to the passage of the long‐wavelength tsunami wave. Whereas Okal ignores the presence of the island in his physical analysis, this tilting of horizontal seismic sensors induced by this slow dynamic process explains observations from several tsunamis and we would expect similar observations on near‐coastal continental stations. This hypothesis is confirmed by the quantitative assessment and the analysis of the data at the …

Are deep‐ocean‐generated surface‐wave microseisms observed on land?

AbstractRecent studies attribute land double‐frequency (DF) microseism observations to deep water generation. Here we show that near‐coastal generation is generally the dominant source region. This determination is based on observations at land and ocean seismic stations, buoys, gravity‐wave hindcasts, and on beamforming results from continental seismic arrays. Interactions between opposing ocean wave components generate a pressure excitation pulse at twice the ocean wave frequency that excites pseudo‐Rayleigh (pRg) wave DF microseisms. pRg generated in shallow coastal waters have most of their energy in the solid Earth (“elastic” pRg) and are observed by land‐based and seafloor seismometers as DF microseisms. pRg generated in the deep ocean have most of their energy in the ocean (“acoustic” pRg) and are continuously observed on the ocean bottom, but acoustic pRg does not efficiently transition onto continents. High‐amplitude DF signals over the [0.2, 0.3] Hz band observed on the deep seafloor are uncorrelated with continental observations and are not clearly detectable at individual continental stations or by land seismic‐array beamforming. Below 0.2 Hz, modeling and some observations suggest that some deep water‐generated elastic pRg energy can reach continental stations, providing that losses from scattering and transition across the continental‐shelf boundary to the shore are not substantial. However, most observations indicate that generally little deep‐ocean‐generated DF microseism energy reaches continental stations. Effectively, DF land observations are dominated by near‐coastal wave activity.

Ten years of measured UV Index from the Spanish UVB Radiometric Network

An analysis is made of the UV Index (UVI) obtained from the ultraviolet erythemal solar radiation (UVER) data measured by the Spanish UVB Radiometric Network between the years 2000 and 2009. Previously, the daily UVI has been evaluated using two different criteria: (a) the value corresponding to solar noon; and (b) the daily maximum value. The mean percentage of agreement is 92% if we consider the cases for which the difference is zero or one UVI unit. These results are similar to those obtained in a previous work where only 2years were analyzed.In all the stations the UVI reaches very high values (8–10) in spring–summer, and the very high and extreme (⩾11) UVI values are more dependent on the continental effect than on the latitude effect. From the UVI values it is possible to classify the stations into four groups: Coastal stations, Continental stations (more than 200km from the coast), Southern stations (Coastal stations but with similar values of UVI as the Continental ones due to their low latitude) and Canary Islands stations (1400km southwest from the Iberian Peninsula thus lower latitude).The monthly mean maximum of UVI is reached in July due to the annual evolution of the total ozone column. This value corresponds, for a skin phototype II, to three times the minimal erythemal dose (MED) in an hour in a Coastal station, 3.5 MEDs in an hour measured in a Continental or Southern station and up five MEDs in an hour in the Izaña station (Canary Islands).The cumulative dose on a horizontal plane over an average year has been calculated for each station. More than 40% of the annual dose is received in summer, about 35% in spring, more than 11% in autumn and less than 10% in winter except for the stations in the Canary Islands where the difference between seasons is less significant.

Observation of the variations of the atmospheric electric field at YBJ, Tibet

The variability noticed in fair weather atmospheric electric field (AEF) recorded at Yangbajin (YBJ, 90°31′50″E, 30°06′38″N), a continental station located at Tibet Plateau, China, during the period 2006–2011, has been examined firstly in relation to the variations of three selected local meteorological parameters. Secondly, a test has been carried out for a hypothesis that numerical reduction on the variations of fair weather AEF data according to the meteorological parameters like air pressure, temperature and relative humidity, could weaken its meteorological effect and thus help to study the variations of AEF. The reduction effectively suppresses the annual variation, but the shape of the diurnal variation remains the same, and is widely different from the Carnegie Curve. Finally, the seasonal effects of diurnal variations are analyzed. The most distinctive characteristics of the fair weather AEF here is that the pattern of diurnal variations exhibits a double fluctuation, which is modulated by the sunrise and sunset effect. The phase of the diurnal curve has a little shift with seasons.

Major element concentrations in Mangrove Pore Water, Sepetiba Bay, Brazil

Concentrations of cations and anions of major elements (Na+, Ca2+, Mg2+, K+, Cl-, SO4 2-) were analyzed in the pore water of a mangrove habitat. Site specific major element concentrations were identified along a four piezometric well transect, which were placed in distinct geobotanic facies. Evapotranspiration was evident in the apicum station, given the high salinity and major element concentrations. The station landward of an apicum was where major element/Cl- ratios standard deviations are greatest, suggesting intense in situ diagenesis. Molar ratios in the most continental station (4) are significantly lower than the nearby freshwater source, indicating a strong influence of sea water flux into the outer reaches of the mangrove ecosystem and encroaching on the Atlantic rain forest. Indeed, the SO4 2-/Cl- and Ca2+/Cl- ratios suggest limited SO4 2- reduction and relatively high Ca2+/Cl- ratios indicate a region of recent saltwater contact.

TransCom continuous experiment: comparison of <sup>222</sup>Rn transport at hourly time scales at three stations in Germany

Abstract. Fourteen global atmospheric transport models were evaluated by comparing the simulation of 222Rn against measurements at three continental stations in Germany: Heidelberg, Freiburg and Schauinsland. Hourly concentrations simulated by the models using a common 222Rn-flux without temporal variations were investigated for 2002 and 2003. We found that the mean simulated concentrations in Heidelberg are related to the diurnal amplitude of boundary layer height in each model. Summer mean concentrations simulated by individual models were negatively correlated with the seasonal mean of diurnal amplitude of boundary layer height, while in winter the correlation was positive. We also found that the correlations between simulated and measured concentrations at Schauinsland were higher when the simulated concentrations were interpolated to the station altitude in most models. Temporal variations of the mismatch between simulated and measured concentrations suggest that there are significant interannual variations in the 222Rn exhalation rate in this region. We found that the local inversion layer during daytime in summer in Freiburg has a significant effect on 222Rn concentrations. We recommend Freiburg concentrations for validation of models that resolve local stable layers and those at Heidelberg for models without this capability.

Dynamical Downscaling of Wind Speed in Complex Terrain Prone To Bora-Type Flows

AbstractThe results of numerically modeled wind speed climate, a primary component of wind energy resource assessment in the complex terrain of Croatia, are given. For that purpose, dynamical downscaling of 10 yr (1992–2001) of the 40-yr ECMWF Re-Analysis (ERA-40) was performed to 8-km horizontal grid spacing with the use of a spectral, prognostic full-physics model Aire Limitée Adaptation Dynamique Développement International (ALADIN; the “ALHR” version). Then modeled data with a 60-min frequency were refined to 2-km horizontal grid spacing with a simplified and cost-effective model version, the so-called dynamical adaptation (DADA). The statistical verification of ERA-40-, ALHR-, and DADA-modeled wind speed on the basis of data from measurement stations representing different regions of Croatia suggests that downscaling was successful and that model accuracy generally improves as horizontal resolution is increased. The areas of the highest mean wind speeds correspond well to locations of frequent and strong bora flow as well as to the prominent mountain peaks. The best results are achieved with DADA and contain bias of 1% of the mean wind speed for eastern Croatia while reaching 10% for complex coastal terrain, mainly because of underestimation of the strongest winds. Root-mean-square errors for DADA are significantly smaller for flat terrain than for complex terrain, with relative values close to 12% of the mean wind speed regardless of the station location. Spectral analyses suggest that the shape of the kinetic energy spectra generally relaxes from k−3 at the upper troposphere to the shape of orographic spectra near the surface and shows no seasonal variability. Apart from the buildup of energy on smaller scales of motions, it is shown that mesoscale simulations contain a considerable amount of energy related to near-surface and mostly divergent meso-β-scale (20–200 km) motions. Spectral decomposition of measured and modeled data in temporal space indicates a reasonable performance of all model datasets in simulating the primary maximum of spectral power related to synoptic and larger-than-diurnal mesoscale motions, with somewhat increased accuracy of mesoscale model data. The primary improvement of dynamical adaptation was achieved for cross-mountain winds, whereas mixed results were found for along-mountain wind directions. Secondary diurnal and tertiary semidiurnal maxima are significantly better simulated with the mesoscale model for coastal stations but are somewhat more erroneous for the continental station. The mesoscale model data underestimate the spectral power of motions with less-than-semidiurnal periods.

Temporal and spatial variability of the stable isotopic composition of atmospheric molecular hydrogen: observations at six EUROHYDROS stations

Abstract. Despite the potential of isotope measurements to improve our understanding of the global atmospheric molecular hydrogen (H2) cycle, few H2 isotope data have been published so far. Now, within the EUROpean network for atmospheric HYDRogen Observations and Studies project (EUROHYDROS), weekly to monthly air samples from six locations in a global sampling network have been analysed for H2 mixing ratio (m(H2)) and the stable isotopic composition of the H2 (δ(D,H2), hereafter referred to as δD). The time series thus obtained now cover one to five years for all stations. This is the largest set of ground station observations of δD so far. Annual average δD values are higher at the Southern Hemisphere (SH) than at the Northern Hemisphere (NH) stations; the maximum is observed at Neumayer (Antarctica), and the minimum at the non-arctic NH stations. The maximum seasonal differences in δD range from ≈18 ‰ at Neumayer to ≈45 ‰ at Schauinsland (Southern Germany); in general, seasonal variability is largest at the NH stations. The timing of minima and maxima differs per station as well. In Alert (Arctic Canada), the variations in δD and m(H2) can be approximated as simple harmonic functions with a ≈5-month relative phase shift. This out-of-phase seasonal behaviour of δD and m(H2) can also be detected, but delayed and with a ≈6-month relative phase shift, at Mace Head and Cape Verde. However, no seasonal δD cycle could be observed at Schauinsland, which likely reflects the larger influence of local sources and sinks at this continental station. At the two SH stations, no seasonal cycle could be detected in the δD data. If it is assumed that the sink processes are the main drivers of the observed seasonality in m(H2) and δD on the NH, the relative seasonal variations can be used to estimate the relative sink strength of the two major sinks, deposition to soils and atmospheric oxidation by the hydroxyl (OH) radical. For the NH coastal and marine stations this analysis suggests that the relative contribution of soil uptake to the total annual H2 removal increases with latitude.

Variation of aerosol optical thickness with atmospheric water vapour : A case study over a continental station Mysore, India

Atmospheric measurements in a continental, low latitude station Mysore (12.3° N) has been carried out, for the period December 2003 to June 2006. Measurements were made using a sunphotometer with five bands in the visible and near-infrared range of the solar spectrum. To bring out the wavelength dependence of Aerosol Optical Thickness (AOT) on atmospheric water vapour, typically two wavelength channels are being used, one at 500 nm and the other at 1020 nm. A linear dependence between AOT and water vapour on meteorologically calm days is the important observation made. Growth rate of AOT is found to be larger at shorter wavelength (500 nm) than that of the longer wavelength (1020 nm). A mass-plot representation is followed on monthly basis, which is nothing but the graphical plot of spectral AOT versus water vapour of the scans for all the clear sky days of a particular month. Further investigations reveal that some months exhibit a single trend of growth of AOT with water vapour whereas double trend is the scenario for other months. These results provide insight into the changes in the atmospheric aerosol characteristics with precipitable water vapour, which is the subject matter of this paper.