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On the retrieval of ice sheet temperature profile by means of low frequency wide band radiometry: sensitivity to the physical parameters
No full textIn a climate change era, improved information on ice sheets and ices shelves are required in order to monitor theirs changes
and the possible contribution to sea level rise over the 21st century and beyond ([1], WCRP). In recent years activities were
promoted by SCAR, CliC (WCRP), IASC, to improve models for predicting the future evolution of ice sheets and significant
progress has been reported [2]. Nevertheless, there are still large discrepancies between simulations [3, 4, 5] and large uncer-
tainties still remain related to our limited quantitative understanding of many of the processes that control the mass balance and
stability of ice sheets, and the inability to characterize such processes from space using existing or planned sensors. Foremost
among them is the vertical temperature profile within the ice, which is sparsely measured in situ, but is a primary control on the
rate at which ice flows by deformation. With the launch of L-band radiometers (SMOS, Aquarius, SMAP), which are capable
of observing ice sheet in depth, a new methodology was proposed to infer information by using satellite data and glaciological
models [6]. More recently, the use of microwave radiometry in a low frequencies range (i.e., a wide range 0.50–2.00 GHz)
was suggested to go deeper in the ice and retrieve the ice sheet and shelf temperature profile (i.e., from the top to the bottom)
through the use of techniques similar to those used by radiometric atmospheric sounders [7] and proposals for space mission
development are in preparation in Europe and U.S. The methodology was already successfully tested in airborne campaigns
in both Greenland and Antarctica [8, 9]. With the purpose of developing an operational ice sheet/shelf temperature retrieval
methodology appliable to spaceborne data, a model analysis started to better understand the contribution of observed brightness
temperature at the different frequencies as a function of depth.
The analysis was conducted by using a microwave emission model [10], able to simulate the brightness temperature, which
use as inputs the ice temperature and density profiles based on glaciological models [11, 12]. Ice sheet is discretized by 2-m
thick layers between surface and 50m in depth, then 50-m layers down to the bedrock. This allows to a better characterization
of the upper part of the ice sheet where the most rapid ice density changes happened. As the horizontal advection is neglected
in this model, the validity area is limited where the ice velocity [13] is lower than 10m yr-1. Four geophysical parameters
(i.e., ice thickness, surface temperature, snow accumulation, and geothermal heat flux) are needed to run models. Analysis
was performed over all Antarctica and information on these parameters are derived from literature [14, 15, 16, 17]. For each
considered 25×25km pixel, the nominal values for ice thickness (H), surface temperature (Tsurf), snow accumulation (Acc),
and geothermal heat flux (GHF) were used to compute the TB. Moreover, for the three last parameters, theirs uncertainty was
considered as follows: Tsurf±1.5K, and Acc±0.5 Acc. Regarding GHF, its value was transformed in a basal temperature (Tbasal)
by means of Robin model and the uncertainties were considered as Tbasal±4.5K.
In addition to TB, and to provide detailed information about the contribution of each ice sheet layer to the observed TB, the
weighting function has been computed [18, 19]. Considering that:
̃T ice
B =
∫ H
0
T (z) W (z) dz (1)
where T(z) is the temperature of the ice layer, the weighting function (W(z)) can be defined
W is the weight of the considered layer, ke is the extinction coefficient, and z is the depth.
Based on the weight computation, it is possible to identify for each frequency the ice sheet part proving the maximum
contribution to the signal. For that, the minimum thick of the normalized weight representing the 90% of the total weight
has been calculated. The portion of the function that subtend this percentage has been identified using a rolling integral. The
upper (ztop) and lower (zbottom) limits of these intervals have been calculated together with their differences (∆z). These values
were then normalized to the thickness in order to compare values within the continent and, finally, the uncertainty (σ∆z) was
calculated by considering the range of variability for all the parameters. Sensitivity to single parameters was also computed and
it was demonstrated that TB is function of H and Tsurf, but also Acc and GHF could influence TB under specific site conditions.
Obtained Results are summarized in Fig. 1 where ztop, zbottom, ∆z, and σ∆z maps of Antarctica are represented at 0.50,
0.95, 1.40, and 1.85 GHz respectively. At 0.50 GHz the lower 50% of the ice column contribute to the 90% of the simulated
TB with the zbottom always located at the ice/bedrock interface. Consequently, the first half of the ice column, including the
surface, shows a limited contribute to the total emission. The uncertainty to different parameters is then limited. Increasing the
frequency, ztop and zbottom rise in the ice column up to the surface, ∆z decreases and uncertainty increases at 0.95 and 1.40 GHz.
At 1.85 GHz uncertainty is low because it is less sensitive to GHF and accumulation variability.
These results confirmed that different frequencies are sensitive to different portion of the ice column and then can be used
to better estimate different geophysical parameters and constrain the ice temperature profile. Moreover, lowest and highest
frequencies could be used to constrain geothermal heat flux and surface temperature, respectively, with a relatively small
uncertainty whereas frequencies in the 0.95–1.40 GHz range could be potentially used to constrain snow accumulation even if
with a higher error. In addition, simulations show that in the inner part of Antarctica ice sheet TB decreases with frequency
while close to the coast the TB is almost frequency independent. Also, this information could be used to constraints the retrieval
depending on location. Detailed analysis of the obtained results and the advanced retrieval method will be presented at the
conference
Radiometric Microwave Measurements and Hydrological Modelling for Estimating Soil Moisture in the MASMEX Experiment
No full text
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Monitoring of Hydrological Parameters on Mountain Areas using Microwave Radiometric Measurements
No full textVariations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Ultrawideband propagation experiment trough the antarctica firn at the Concordia station in the 0.4 - 2 GHZ gfrequency range
No full textIn recent years, special attention has been paid, in microwave remote sensing from space, to extend observations to low frequencies (i.e lower than L band). Active mission at P-band (i.e. the ESA's Biomass mission based on SAR) will be launched in 2022 and missions based on low frequency microwave radiometer are proposed to both ESA and NASA. Because of the high penetration depth in ice, these sensors are particularly suitable for investigating parameters of the cryosphere. Nevertheless, in order to properly derive information on such geophysical parameters, there is the need to improve our knowledge of the interaction of electromagnetic waves and the different media. In this paper we present and discuss a first series of results obtained during a propagation measurement campaign carried out at the French-Italian Concordia research station during the summer campaign 2019-2020 in Antarctica in order to investigate on the electromagnetic properties of the Antarctic's firn in the frequency range 0.4-2 GHz. Specific instrumentation has been designed and assembled, essentially consisting of a transmitting unit and a receiving one, to be let down into two boreholes in the firn. Preliminary measurement results are presented and discussed
UHF Propagation Measurements through the Antarctic Firn at Concordia Station
No full textIn this paper we present and discuss the first series of results obtained during a propagation measurement campaign carried out at the French-Italian Concordia research station during last (2019/2020) summer campaign in Antarctica. Specific instrumentation has been designed and assembled, essentially consisting of a transmitting unit and a receiving one, to be let down into two boreholes in the firn. Preliminary measurement results are presented and discussed
Monitoring of melting refreezing cycles of snow with microwave radiometers: The Microwave Alpine Snow Melting Experiment (MASMEx 2002-2003)
No full textA study of the melting cycle of snow was carried out by using ground-based microwave radiometers, which operated con- tinuously 24 h/day from late March to mid-May in 2002 and from mid-February to early May in 2003. The experiment took place on the eastern Italian Alps and included micrometeorological and conventional snow measurements as well. The measurements con- firmed the high sensitivity of microwave emission at 19 and 37 GHz to the melting-refreezing cycles of snow. Moreover, micrometeoro- logical data made it possible to simulate snow density, tempera- ture, and liquid water content through a hydrological snowpack model and provided additional insight into these processes. Simu- lations obtained with a two-layer electromagnetic model based on the strong fluctuation theory and driven by the output of the hy- drological snowpack model were consistent with experimental data and allowed interpretation of both variation in microwave emission during the melting and refreezing phases and in discerning the con- tributions of the upper and lower layers of snow as well as of the underlying ground surface
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