229 research outputs found
Three examples where the specific surface area of snow increased over time
Snow on the ground impacts climate through its high albedo and affects atmospheric composition through its ability to adsorb chemical compounds. The quantification of these effects requires the knowledge of the specific surface area (SSA) of snow and its rate of change. All relevant studies indicate that snow SSA decreases over time. Here, we report for the first time three cases where the SSA of snow increased over time. These are (1) the transformation of a melt-freeze crust into depth hoar, producing an increase in SSA from 3.4 to 8.8m2 kgâ1. (2) The mobilization of surface snow by wind, which reduced the size of snow crystals by sublimation and fragmented them. This formed a surface snow layer with a SSA of 61m2 kgâ1 from layers whose SSAs were originally 42 and 50m2 kgâ1. (3) The sieving of blowing snow by a snow layer, which allowed the smallest crystals to penetrate into open spaces in the snow, leading to an SSA increase from 32 to 61m2 kgâ1. We discuss that other mechanisms for SSA increase are possible. Overall, SSA increases are probably not rare. They lead to enhanced uptake of chemical compounds and to increases in snow albedo, and their inclusion in relevant chemical and climate models deserves consideration
Integration of Argo trajectories in the Mediterranean Forecasting System and impact on the regional analysis of the western Mediterranean circulation
The impact of Argo float trajectory assimilation on the quality of ocean analyses is studied by means of an operational oceanographic model implemented in the Mediterranean Sea and a 3D-Var assimilation scheme. For the first time, both Argo trajectories and vertical profiles of temperature and salinity (TS) together with satellite altimeter data of sea level anomaly (SLA) are assimilated to produce analyses for short-term forecasts. The study period covers 3 months during winter 2005 when four Argo trajectories were present in the northwestern Mediterranean Sea. The scheme is first assessed computing the misfits between observations and model forecast and analysis. The misfit statistics appear improved for float trajectories, while they are not degraded for the other assimilated variables (TS profiles and SLA). This indicates that the trajectory integration is consistent with the other components of the assimilation system and provides new information on horizontal pressure gradients. Comparisons between analyses obtained with and without trajectory assimilation suggest that trajectory assimilation can have an impact on the description of boundary currents and their instabilities, as well as mesoscale activity at regional scales. Changes are depicted by intermediate water mass redistributions, mesoscale eddy relocations, and net transport modulations. These impacts are detailed and assessed considering historical and simultaneous in situ data sets. The results motivate the integration of Argo trajectories in the operational Mediterranean Forecasting System
Variational analysis of drifter positions and model outputs for the reconstruction of surface currents in the central Adriatic during fall 2002
Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 113 (2008): C04004, doi:10.1029/2007JC004148.In this paper we present an application of a variational method for the reconstruction of the velocity field in a coastal flow in the central Adriatic Sea, using in situ data from surface drifters and outputs from the ROMS circulation model. The variational approach, previously developed and tested for mesoscale open ocean flows, has been improved and adapted to account for inhomogeneities on boundary current dynamics over complex bathymetry and coastline and for weak Lagrangian persistence in coastal flows. The velocity reconstruction is performed using nine drifter trajectories over 45 d, and a hierarchy of indirect tests is introduced to evaluate the results as the real ocean state is not known. For internal consistency and impact of the analysis, three diagnostics characterizing the particle prediction and transport, in terms of residence times in various zones and export rates from the boundary current toward the interior, show that the reconstruction is quite effective. A qualitative comparison with sea color data from the MODIS satellite images shows that the reconstruction significantly improves the description of the boundary current with respect to the ROMS model first guess, capturing its main features and its exchanges with the interior when sampled by the drifters.Four of the authors are supported by the
Office of Naval Research, V.T. and A.G. under grants N00014-05-1-0094
and N00014-05-1-0095, P.M.P. under grant N00014-03-1-0291, and S.C.
under grant N00014-05-1-0730. CNR-ISMAR
activity was partially supported by P.O.R. ââCAINOââ (Regione Puglia),
VECTOR (Italian MIUR) project, and ECOOP (EU project)
Integration of ARGO trajectories in the Mediterranean Forecasting System and impact on the regional analysis of the Western Mediterranean circulation
The impact of ARGO float trajectory assimilation on the quality of ocean analyses is studied by
means of an operational oceanographic model implemented in the Mediterranean Sea and a 3D-var
assimilation scheme. For the first time, both ARGO trajectories and vertical profiles of temperature
and salinity (TS) together with satellite altimeter data of sea level anomaly (SLA) are assimilated to
produce analyses for short term forecasts. The study period covers three months during winter 2005
when four ARGO trajectories were present in the northwestern Mediterranean Sea. The scheme is
first assessed computing the misfits between observations and model forecast and analysis. The
misfit statistics appear improved for float trajectories, while they are not degraded for the other
assimilated variables (TS profiles and SLA). This indicates that the trajectory integration is
consistent with the other components of the assimilation system, and provides new information on
horizontal pressure gradients. Comparisons between analyses obtained with and without trajectory
assimilation suggest that trajectory assimilation can impact on the description of boundary currents
and their instabilities, as well as mesoscale activity at regional scales. Changes are depicted by
intermediate water mass redistributions, mesoscale eddy relocations and net transport modulations.
These impacts are detailed and assessed considering historical and simultaneous in-situ datasets.
The results motivate the integration of ARGO trajectories in the operational Mediterranean
Forecasting System
Integration of ARGO trajectories in the Mediterranean Forecasting System and impact on the regional analysis of the Western Mediterranean circulation
The impact of ARGO trajectory assimilation on the quality of ocean analyses is studied by means of
an operational oceanographic model implemented in the Mediterranean Sea and a 3D-var
assimilation scheme. For the first time, both ARGO trajectories and vertical profiles together with
satellite data are assimilated to produce analyses for short term forecasts. The study period covers
three months during winter 2005 when four ARGO trajectories were present in the northwestern
Mediterranean Sea. It is shown that their integration is consistent with the other components of the
assimilation system, and it contributes to refine the model error structure with new information on
horizontal pressure gradients. So the analysis benefits of a more accurate description of the
boundary currents and their instabilities that drive the mesoscale activity of regional circulations.
As a consequence, the trajectory assimilation remotely and significantly influences the basin scale
circulation. Changes can be depicted by intermediate water mass redistributions, mesoscale eddy
relocations or net transports modulations. These impacts are detailed and assessed considering
historical and contemporary datasets. The obtained qualitative and quantitative agreements motivate
the integration of ARGO trajectories in the operational Mediterranean Forecasting System
Using machine learning and Biogeochemical-Argo (BGC-Argo) floats to assess biogeochemical models and optimize observing system design
Numerical models of ocean biogeochemistry are becoming the major tools used to detect
and predict the impact of climate change on marine resources and to monitor
ocean health. However, with the continuous improvement of model structure
and spatial resolution, incorporation of these additional degrees of freedom
into fidelity assessment has become increasingly challenging. Here, we
propose a new method to provide information on the model predictive skill in a concise
way. The method is based on the conjoint use of a k-means clustering
technique, assessment metrics, and Biogeochemical-Argo (BGC-Argo) observations. The k-means
algorithm and the assessment metrics reduce the number of model data points
to be evaluated. The metrics evaluate either the model state accuracy or the
skill of the model with respect to capturing emergent properties, such as the deep
chlorophyll maximums and oxygen minimum zones. The use of BGC-Argo
observations as the sole evaluation data set ensures the accuracy of the
data, as it is a homogenous data set with strict sampling methodologies and
data quality control procedures. The method is applied to the Global Ocean Biogeochemistry Analysis and Forecast system of the Copernicus Marine
Service. The model performance is evaluated using the model efficiency
statistical score, which compares the modelâobservation misfit with the
variability in the observations and, thus, objectively quantifies whether the
model outperforms the BGC-Argo climatology. We show that, overall, the model
surpasses the BGC-Argo climatology in predicting pH, dissolved inorganic
carbon, alkalinity, oxygen, nitrate, and phosphate in the mesopelagic and
the mixed layers as well as silicate in the mesopelagic layer. However,
there are still areas for improvement with respect to reducing the modelâdata misfit for
certain variables such as silicate, pH, and the partial pressure of CO2
in the mixed layer as well as chlorophyll-a-related, oxygen-minimum-zone-related, and particulate-organic-carbon-related metrics. The method proposed
here can also aid in refining the design of the BGC-Argo network, in
particular regarding the regions in which BGC-Argo observations should be enhanced to
improve the model accuracy via the assimilation of BGC-Argo data or
process-oriented assessment studies. We strongly recommend increasing the
number of observations in the Arctic region while maintaining the existing
high-density of observations in the Southern Oceans. The model error in
these regions is only slightly less than the variability observed in
BGC-Argo measurements. Our study illustrates how the synergic use of
modeling and BGC-Argo data can both provide information about the performance of models
and improve the design of observing systems.</p
Evolution of the Snow Area Index of the Subarctic Snowpack in Central Alaska over a Whole Season. Consequences for the Air to Snow Transfer of Pollutants
Investigation of the secondary emission characteristics of CVD diamond films for electron amplification
a b s t r a c t Chemical vapour deposition (CVD) diamond offers great potential as a low-cost, high-yield, easily manufactured secondary electron emitter for electron multiplication in devices such as photomultiplier tubes. Its potential for high secondary electron yield offers several significant benefits for these devices including higher time resolution, faster signal rise time, reduced pulse height distribution, low noise, and chemical stability. We describe an experiment to characterize the secondary emission yield of CVD diamond manufactured using different processes and process parameters and discuss the degradation of secondary electron yield and experimental difficulties encountered due to unwanted electron beaminduced contamination. We describe techniques utilized to overcome these difficulties, and present measurements of secondary yield from CVD diamond dynodes in reflection mode. We discuss the application of CVD diamond dynode technology, both in reflection and transmission mode, to advanced high-speed imaging and photon-counting detectors and describe future plans in this area
Ion-Induced Dipole Interactions and Fragmentation Times : C -C Chromophore Bond Dissociation Channel
The fragmentation times corresponding to the loss of the chromophore
(C-- C bond dissociation channel) after photoexcitation at 263
nm have been investigated for several small peptides containing tryptophan or
tyrosine. For tryptophan-containing peptides, the aromatic chromophore is lost
as an ionic fragment (m/z 130), and the fragmentation time increases with the
mass of the neutral fragment. In contrast, for tyrosine-containing peptides the
aromatic chromophore is always lost as a neutral fragment (mass = 107 amu) and
the fragmentation time is found to be fast (\textless{}20 ns). These different
behaviors are explained by the role of the postfragmentation interaction in the
complex formed after the C--C bond cleavage
Physicochemical properties of bacterial cellulose obtained from different Kombucha fermentation conditions
The production of bacterial cellulose has been limited due to its high cost and low productivity. Alternative lowâcost sources of this biopolymer of high purity and biocompatibility are needed in order to benefit from its enormous potential. Kombucha tea is a trend functional beverage whose production is growing exponentially worldwide, and the bacteria present in this fermented beverage belonging to the genus Komagataeibacter are capable of producing a crystalline biofilm with interesting properties. Obtaining bacterial cellulose from Kombucha tea has already been studied, however several fermentation conditions are being optimized in order to scaleâup its production. In this study, we characterized the bacterial cellulose produced from three different Kombucha fermentation conditions. The scanning electron microscopy images revealed the crystalline structure of the biofilms. The energyâdispersive xâray analysis exhibited the chemical composition of the crystals. The thermogravimetric analysis showed a rate of degradation between 490 and 560°C and the differential scanning calorimetry confirmed the presence of crystalline and amorphous regions in the bacterial cellulose samples. The results suggested that crystalline cellulose could be obtained by varying the fermentation conditions of Kombucha tea
- âŠ