Retrieval of the ultraviolet aerosol optical depth during a spring campaign in the Bavarian Alps

A measurement campaign was organized in March 1999 in the Bavarian Alps as part of the European project, Characteristics of the UV Radiation Field in the Alps (CUVRA), to analyze the effect of altitude, aerosols, and snow cover on ground-level UV spectral irradiance. We present the results of simultaneous measurements of aerosol optical depth (AOD) made at various sites on two cloudless days in March 1999. The two days exhibited different aerosol conditions. Results derived from spectral measurements of UV irradiance are compared with data from filter radiometer measurements made at discrete wavelengths extending from the UV to the near IR. The different methods generated values for the AOD that were in good agreement. This result confirms that one can use either method to retrieve the AOD with an uncertainty of approximately 0.03-0.05. On 18 March, high turbidity was observed at low altitude (400-nm AOD approximately 0.5 at 700 m above sea level), and the AOD decreased regularly with altitude; on 24 March, the turbidity was much less (0.11 at 700 m above sea level). On both days very low AODs (0.05-0.09) were measured at 3000 m above sea level. The spectral dependence of the AOD is often parameterized by the angstrom relationship; the alpha parameter is generally difficult or impossible to retrieve from spectral measurements because of the relatively narrow wavelength range (320-400 nm), and only one of the spectro-radiometers used during the campaign permits this retrieval. In most cases, during this field campaign, alpha was found by filter sunphotometers to be 1.1-1.5.     

Method to determine snow albedo values in the ultraviolet for radiative transfer modeling

For many cases modeled and measured UV global irradiances agree to within +/-5% for cloudless conditions, provided that all relevant parameters for describing the atmosphere and the surface are well known. However, for conditions with snow-covered surfaces this agreement is usually not achievable, because on the one hand the regional albedo, which has to be used in a model, is only rarely available and on the other hand UV irradiance alters with different snow cover of the surface by as much as 50%. Therefore a method is given to determine the regional albedo values for conditions with snow cover by use of a parameterization on the basis of snow depth and snow age, routinely monitored by the weather services. An algorithm is evolved by multiple linear regression between the snow data and snow-albedo values in the UV, which are determined from a best fit of modeled and measured UV irradiances for an alpine site in Europe. The resulting regional albedo values in the case of snow are in the 0.18-0.5 range. Since the constants of the regression depend on the area conditions, they have to be adapted if the method is applied for other sites. Using the algorithm for actual cases with different snow conditions improves the accuracy of modeled UV irradiances considerably. Compared with the use of an average, constant snow albedo, the use of actual albedo values, provided by the algorithm, halves the average deviations between measured and modeled UV global irradiances.     

Analysis of direct solar ultraviolet irradiance measurements in the French Alps. Retrieval of turbidity and ozone column amount

Direct ultraviolet spectral solar irradiance is regularly obtained by the difference between global and diffuse irradiances at the French Alpine station of Brian?on; the data of years 2001 and 2002 are analyzed in this paper. Comparison with modeled values is used for cloud screening, and an average UV-A aerosol optical depth is used as an index of turbidity; it is found to be around 0.05 for the clear winter days and around 0.2 in summer. Langley plots are used to verify the instrument calibration; they confirm the expected uncertainty smaller than 5%. The ozone total column amount is estimated with an uncertainty between -3 and Dobson units; comparisons with TOMS (Total Ozone Mapping Spectrometer) overpass values shows agreement within the expected uncertainties of both instruments.     

Recent snow cover variability in the Italian Alps

The historical record of snow duration (from 1950 to 2009) and of snowfall (from 1960 to 2009) collected in the Italian Alps are presented and analysed. A reduction of the snow cover duration and of the snowfall stronger in springtime was detected during the last 40 years with the greatest decreasing rate during the 1990s. The last decade is characterised by a recovery from the documented decreasing trend mainly evident between 800 m and 1500 m. Principal component trend analysis of the snow duration and of the snowfall showed a long term decreasing trend. The change point test showed the existence of breakpoints between 1984 and 1994 that characterise the snow duration and snowfall time series analysed by elevation range and by seasons. These breakpoints mark a drastic trend variability in the time series: a positive trend characterises the time series before the breakpoint and a decreasing trend characterises the historical record after the breakpoint. The described negative trends result from the documented decrease in winter and spring precipitation. This in turn may either relate to a change in fraction of liquid to solid precipitation, and/or be associated to an increase of the temperatures. Northern Hemisphere and Italian Alps snow cover trends strongly correlate in the frequency domain. Among the dominant frequencies the 11.2 period was detected that may relate to the 11-year solar activity cycle.     

Study of snow climate in the Japanese Alps: Comparison to snow climate in North America

In the case of the Japanese Alps, it is experientially known that there is a notable snow climate difference between the Japan Sea side mountains and the Pacific Ocean side mountains. For the purpose of improving avalanche safety, we studied the snow climate characteristics using meteorological and snow pit data collected from two study plots in the mountain regions. Ten years of meteorological data and 4–10 years of snow pit data were employed in the study. A snow climate classification scheme proposed in North America was used to determine the snow climate of these study plots. The general snowpack characteristics for each snow climate presented in previous studies were used in the present study to determine the snowpack characteristics of the study plots. Both meteorological and snow pit data suggested that the Japan Sea side mountains have the same characteristics as the maritime snow climate in North America. On the other hand, the Pacific Ocean side mountains have unique characteristics caused by a combination of continental and maritime climate influences. The Pacific Ocean side mountains have similar characteristics to the continental snow climate of North America, however, that climate is different in that it is characterized by a large amount of rainfall and a high predominance of faceted crystals and wet grains. We identified a new snow climate for the Pacific Ocean side mountains of the Japanese Alps, a “rainy continental snow climate.”     

Study of snow climate in the Japanese Alps: Comparison to snow climate in North America

In the case of the Japanese Alps, it is experientially known that there is a notable snow climate difference between the Japan Sea side mountains and the Pacific Ocean side mountains. For the purpose of improving avalanche safety, we studied the snow climate characteristics using meteorological and snow pit data collected from two study plots in the mountain regions. Ten years of meteorological data and 4–10 years of snow pit data were employed in the study. A snow climate classification scheme proposed in North America was used to determine the snow climate of these study plots. The general snowpack characteristics for each snow climate presented in previous studies were used in the present study to determine the snowpack characteristics of the study plots. Both meteorological and snow pit data suggested that the Japan Sea side mountains have the same characteristics as the maritime snow climate in North America. On the other hand, the Pacific Ocean side mountains have unique characteristics caused by a combination of continental and maritime climate influences. The Pacific Ocean side mountains have similar characteristics to the continental snow climate of North America, however, that climate is different in that it is characterized by a large amount of rainfall and a high predominance of faceted crystals and wet grains. We identified a new snow climate for the Pacific Ocean side mountains of the Japanese Alps, a “rainy continental snow climate.”     

A note on the solar elevation dependence of clear-sky snow albedo

Recent attempts to match the short-wave albedo of snow for clear skies using approximate spectral solar fluxes and solutions of the radiative transfer equation for snow were unsuccessful until a separate surface reflection term was introduced. A separate consideration of specular reflection from surface snow grains has been objected to as being ad hoc. Results based on a new parametrization of short-wave radiation are discussed. Compared with previous radiation models, the new model gives higher diffuse insolation and predicts higher albedos. The difference between observed and predicted albedos is substantially reduced without invoking surface reflection.     

Effect of topography on average surface albedo in the ultraviolet wavelength range

The reflectivity of the 22 km x 24 km region surrounding Sonnblick Observatory near Salzburg, Austria (3104-m altitude, 47.05 degrees N, 12.95 degrees E), was calculated with a three-dimensional albedo model. The average albedo of the region was calculated at 305 and 380 nm for different solar zenith angles, ground reflectances, and solar azimuth angles. To determine geometrical effects, we first carried out the simulations without taking account of the effects of the atmosphere. The ratio to the reflectivity of a corresponding flat surface area (area with the same ground characteristics) was always less than 1 and showed a decrease with increasing solar zenith angle and with diminishing ground reflectance. Even when the ground reflectance was 100%, the average albedo was less than 1. The effect of the atmosphere was then taken into consideration in these calculations and was found to diminish the reflected components. This diminishing effect was compensated for, however, by the scattered irradiance. Finally, simulations of real conditions (nonhomogeneous ground reflectivities) were performed for different snow lines in the Sonnblick region. The average albedos obtained when all the surroundings were covered with snow were 0.32-0.63 with low solar zenith angles and 0.38-0.77 with a 40 degrees solar zenith angle.     

Retrieval of the albedo and phase function from exiting radiances with radiative perturbation theory

We use radiative perturbation theory to develop a retrieval technique for determining the radiative properties of a scattering medium, such as the Earth's atmosphere, based on measurements of the radiation emerging at either the top or bottom of the medium. In a previous paper [J. Quant. Spectrosc. Radiat. Transfer 54, 695 (1995)] we have shown the capacity of radiative perturbation theory to describe variations in exiting intensity as a linear combination of the parameters that characterize the scattering medium. Here we show that it is possible to set up a matrix relation such that the matrix inversion solves the inverse scattering problem. Using simulated data, we observe that the quality of the solution can be controlled by studying the singular values associated with the kernel matrix, obtaining in this way a stable solution, even in the presence of noise.     

Modelling soil removal from snow avalanches: A case study in the North-Western Italian Alps

Snow avalanches can exert considerable erosive forces on soils. If a snow avalanche flows directly over bare ground, basal shear forces may scrape away and entrain soil. Sediments mix up with the avalanche body and may be found within the run-out snow deposit. Based on a previous field campaign aiming to quantify the amount of sediments trapped within avalanche bodies for the study site of Lavancher, in the Aosta Valley, NW-Italy, we developed here a soil erosion model, which we preliminarily applied to that site. An already developed and tested 1-D avalanche dynamics model was modified to include soil erosion. Soil removal was triggered according to two different mechanisms, namely excess of shear and critical velocity. We used equations from the available literature to model the shear stress exerted by the avalanche flow upon the ground underneath. Critical threshold for soil removal of either shear or velocity were also retrieved from the available literature, possibly depending upon soil texture and geotechnical properties. The model performs well in reproducing soil removal for three wet-dense avalanches that occurred in the study site.Use of excess of shear mechanism to evaluate erosion seems more robust, as less dependent upon flow velocity, utmost uncertain. Albeit more accurately measured events of soil eroding avalanches seem necessary to test its performance, the model can be used henceforth as a basis for further refinement concerning geomorphologic contribution of avalanches.     

Modeling of the influence of snow reflectance on ultraviolet irradiance for cloudless sky

The amplification of UV irradiance at the Earth's surface that is due to successive reflections between the snow-covered ground and the scattering atmosphere is analyzed by a method based on decoupling the atmosphere and the surface functions. For a uniform Lambertian surface the amplification factor for the global irradiance depends only on the product of the surface reflectance and the atmospheric backscatter. It varies with wavelength, reaching a maximum near 320 nm; this maximum is close to 50% for clean snow. In UV-B the amplification depends strongly on tropospheric ozone. For non-Lambertian, nonuniform surfaces it is possible, by the same method, to define effective or average reflectances.     

In-situ monitoring of the time evolution of the effective thermal conductivity of snow

We report on a 3-month long time series of in-situ measurements of the effective thermal conductivity (k_eff) of snow at 6 heights in an Alpine snowpack in the Mont-Blanc mountain range, France, at an altitude of 2400 m. Measurements were carried out automatically every 2 days using heated-needle probes embedded in the snowpack. The experimental procedure used is presented in detail and demonstrates the applicability of single heated-needle probes for the evaluation of k_eff in snow, both for long-term measurements within the snowpack and occasional use in the field. Results based on 139 automatically collected data show k_eff values ranging between 0.04 and 0.35 W m^− 1 K^− 1, and a consistent pattern of effective thermal conductivity increase throughout the measurements campaign. The temporal rate of change of k_eff varies up to 0.01 W m⁻¹ K^− 1day^− 1, with maximum values just after snowfall.     

Retrieval of total ozone abundances from the ultraviolet region of spectra recorded with an ultraviolet-visible spectrometer

A method is described for deriving total ozone abundances from zenith-sky intensities measured by an UV-visible spectrometer, known as the System of Analysis of Observations at Zenith (SAOZ). Total ozone abundances are determined by comparison of intensity ratios measured at two wavelengths in the UV region with ratios computed with a radiative transfer model. The wavelength pair 320-307 nm was used in this study. Spectra recorded by the SAOZ spectrometer in Oslo (60 degrees N) from the beginning of May 1995 to the end of August 1995 were analyzed, and the results were compared with total ozone measured with a Brewer instrument. The relative difference in derived ozone abundances for the whole period, including days with thin and medium-thick cloud covers, is -0.18 +/- 1.46%. We study the effects of clouds and varying ground albedo on the derived total ozone. Clouds result in an overestimation of the derived total ozone. The error increases with the optical depth tau of the cloud from approximately 2% for an optically thin cloud (tau = 0.5) to approximately 10% for a thick cloud (tau = 50). The ratio between measured intensities at 550 and 350 nm, the so-called color index, can be used as a measure of the cloud optical depth for thin and medium-thick clouds. The effects of thin and medium-thick clouds on the derived ozone abundances can be compensated for by use of an empirical relationship found between the measured color index and the error in the inferred ozone abundances caused by cloud scattering. We also study the influence of changes in the ground albedo and in the ozone profiles on derived total ozone values.     

Case study of modeled aerosol optical properties during the SAFARI 2000 campaign

We present modeled aerosol optical properties (single scattering albedo, asymmetry parameter, and lidar ratio) in two layers with different aerosol loadings and particle sizes, observed during the Southern African Regional Science Initiative 2,000 (SAFARI 2,000) campaign. The optical properties were calculated from aerosol size distributions retrieved from aerosol layer optical thickness spectra, measured using the NASA Ames airborne tracking 14-channel sunphotometer (AATS-14) and the refractive index based on the available information on aerosol chemical composition. The study focuses on sensitivity of modeled optical properties in the 0.3-1.5 microm wavelength range to assumptions regarding the mixing scenario. We considered two models for the mixture of absorbing and nonabsorbing aerosol components commonly used to model optical properties of biomass burning aerosol: a layered sphere with absorbing core and nonabsorbing shell and the Maxwell-Garnett effective medium model. In addition, comparisons of modeled optical properties with the measurements are discussed. We also estimated the radiative effect of the difference in aerosol absorption implied by the large difference between the single scattering albedo values (approximately 0.1 at midvisible wavelengths) obtained from different measurement methods for the case with a high amount of biomass burning particles. For that purpose, the volume fraction of black carbon was varied to obtain a range of single scattering albedo values (0.81-0.91 at lambda=0.50 microm). The difference in absorption resulted in a significant difference in the instantaneous radiative forcing at the surface and the top of the atmosphere (TOA) and can result in a change of the sign of the aerosol forcing at TOA from negative to positive.     

Assessment of the accuracy of snow surface direct beam spectral albedo under a variety of overcast skies derived by a reciprocal approach through radiative transfer simulation

With radiative transfer simulations it is suggested that stable estimates of the highly anisotropic direct beam spectral albedo of snow surface can be derived reciprocally under a variety of overcast skies. An accuracy of +/- 0.008 is achieved over a solar zenith angle range of theta0 < or = 74 degrees for visible wavelengths and up to theta0 < or = 63 degrees at the near-infrared wavelength lambda = 862 nm. This new method helps expand the database of snow surface albedo for the polar regions where direct measurement of clear-sky surface albedo is limited to large theta0's only. The enhancement will assist in the validation of snow surface albedo models and improve the representation of polar surface albedo in global circulation models.     

Cross-comparison of meteorological and avalanche data for characterising avalanche cycles: The example of December 2008 in the eastern part of the French Alps

In December 2008, an intense avalanche cycle occurred in the eastern part of the southern French Alps. Southerly atmospheric fluxes that progressively evolved into an easterly return caused important snowfalls with return periods up to 10 years. Cold temperatures and drifting snow had important aggravating effects. The return period for the number of avalanches was above 50 years in two massifs and some of the avalanche had very long runouts that exceeded historical limits recorded in the French avalanche atlas. Using this case study, this paper illustrates and discusses how avalanche reports, snow and weather data and results from numerical modelling of the snow cover can be combined to analyse abnormal temporal clusters of snow avalanches. For instance, it is shown how statistical techniques developed in other fields can be used to test the significance of different explanatory factors, extract spatio-temporal patterns, compare them with previous cycles and quantify the magnitude/frequency relationship at different scales.     

Retrieval of the effective complex refractive index of Bruggeman nanoliquids from surface plasmon resonance reflectance using the maximum entropy method

We show that surface plasmon resonance reflectance measurement can be used to extract the effective complex refractive index of a Bruggeman nanoliquid, i.e. a solution consisting of spherical nanoparticles embedded into a host liquid such as water, using the maximum entropy method. This retrieval can be immediately performed once the optical properties of the host liquid are known. In this paper we investigate the upper limit of the volume fraction of spherical nanoparticles in a solution that still yields a reliable result in the retrieval based on the maximum entropy method. Finally, we show that by squeezing the spectrum we can significantly reduce the errors in the analysis and retrieve the effective complex refractive index for a high volume fraction of nanoparticles.     

Scientific Production in Puerto Rico in Science and Technology during the Period 1990 to 1998

This paper analyses the research activity conducted by Puerto Rican scientists in science and technology in the period 1990 to 1998. The Science Citation Index (SCI) database was used to analyse scientific production by geographic area, type of insittution, document typology, language coverage, visibility of publications, subjects addressed and collaboration between local and international authors and institutions. Scientific production was observed to nearly double over the period studied and found to be concentrated in the academic sector, primarily in the city of San Juan, specifically in the University of Puerto Rico's Rio Piedras, Medical Sciences and Mayagüez campuses. Puerto Rican scientific production in the period studied was greater than in any other Caribbean country and the sixth largest in all of Latin America. Papers are mainly published in highly visible journals and scientific articles are the vehicle most commonly used to reach the scientific community. Co-operation indices between authors and institutions are high and the principal areas in which research is published are Medicine, Chemistry, Life Sciences and Physics.     

The results of cosmic radiation in-flight TEPC measurements during the CAATER flight campaign and comparison with simulation

The European-Commission-supported project DOSMAX (Dosimetry of Aircrew Exposure to Radiation During Solar Maximum) was aimed at measuring aircrew exposure to cosmic radiation on-board the aircraft during solar maximum. During a dedicated international comparison mission (Co-ordinated Access to Aircraft for Transnational Environmental Research; CAATER) different measurement techniques have been compared by six European institutes (Results of the CAATER Mission, DOSMAX Meeting, Dublin, June 2004). In this paper, we present the tissue-equivalent proportional counter (TEPC) measurements carried out by ARC Seibersdorf research (ARCS), Austria, and Institut de Radioprotection et de S?reté Nucléaire (IRSN), France, together with a comparison with simulation results under the same conditions. The whole flight campaign consists of four different in-flight investigations performed at two different geographical positions at 12.2 km (FL 400) and 9.8 km (FL 320). One location was chosen above Rome (42 degrees North, 12 degrees East), Italy, for high cut-off rigidity (6.4 GV) and the second above Aalborg (57 degrees North, 10 degrees East), Denmark, for low cut-off rigidity (1.8 GV). The TEPC measurements are presented in terms of absorbed dose and ambient dose equivalent as well as microdosimetric spectra as a function of lineal energy. For the same conditions of the CAATER flights the response of the TEPC has also been simulated by using the Monte Carlo Transport Code FLUKA (version 2003). The results from simulations are compared with measurements and they show a reasonable agreement.     

Some physical drivers of changes in the winter storm tracks over the North Atlantic and Mediterranean during the Holocene

The winter climate of Europe and the Mediterranean is dominated by the weather systems of the mid-latitude storm tracks. The behaviour of the storm tracks is highly variable, particularly in the eastern North Atlantic, and has a profound impact on the hydroclimate of the Mediterranean region. A deeper understanding of the storm tracks and the factors that drive them is therefore crucial for interpreting past changes in Mediterranean climate and the civilizations it has supported over the last 12?000 years (broadly the Holocene period). This paper presents a discussion of how changes in climate forcing (e.g. orbital variations, greenhouse gases, ice sheet cover) may have impacted on the 'basic ingredients' controlling the mid-latitude storm tracks over the North Atlantic and the Mediterranean on intermillennial time scales. Idealized simulations using the HadAM3 atmospheric general circulation model (GCM) are used to explore the basic processes, while a series of timeslice simulations from a similar atmospheric GCM coupled to a thermodynamic slab ocean (HadSM3) are examined to identify the impact these drivers have on the storm track during the Holocene. The results suggest that the North Atlantic storm track has moved northward and strengthened with time since the Early to Mid-Holocene. In contrast, the Mediterranean storm track may have weakened over the same period. It is, however, emphasized that much remains still to be understood about the evolution of the North Atlantic and Mediterranean storm tracks during the Holocene period.