53 research outputs found
Selective beat averaging to evaluate ventricular repolarization adaptation to deconditioning after 5 days of head-down bed rest
The study of QT/RR relationship is important for the clinical evaluation of possible risk of ventricular tachyarrhythmia. Our aim was to assess the effects of 5-days of head-down (-6 degrees) bed-rest (HDBR) on ventricular repolarization. High fidelity 12-leads Holter ECG was acquired before (PRE), the last day of HDBR (HDT5), and five days after its conclusion (POST). X, Y, Z leads were derived (inverse Dower matrix) and vectorcardiogram computed. Selective beat averaging applied to the night period resulted in averages preceded by the same stable heart rate (for each 10 msec bin amplitude, in the range 900-1200 msec). For each template (i.e., one for each bin), T-wave maximum amplitude (Tmax), T wave area, R-Tapex and R-Tend were computed. Results (in 8 male volunteers) showed that, compared to PRE, at HDT5 both R-Tapex and R-Tend resulted significantly shortened (-5% and -3%, respectively), together with a decrease in T-wave area (-7%), while Tmax was unchanged. At POST, duration parameters showed a trend towards their control values (-1.5% and -3%, respectively) while amplitude parameters resulted restored. Despite the short-term BR, cardiac adaptation to deconditioning affected ventricular repolarization during the night period. © 2012 CCAL
Orographic and convective gravity waves above the Alps and Andes Mountains during GPS radio occultation events – a case study
Gravity waves (GWs) and convective systems play a fundamental role in
atmospheric circulation, weather, and climate. Two usual main sources of GWs
are orographic effects triggering mountain waves and convective activity. In
addition, GW generation by fronts and geostrophic adjustment must also be
considered. The utility of Global Positioning System (GPS) radio occultation
(RO) observations for the detection of convective systems is tested. A
collocation database between RO events and convective systems over
subtropical to midlatitude mountain regions close to the Alps and Andes is
built. From the observation of large-amplitude GW structures in the
absence of jets and fronts, subsets of RO profiles are sampled. A
representative case study among those considered at each region is selected
and analyzed. The case studies are investigated using mesoscale Weather Research and Forecasting (WRF)
simulations, ERA-Interim reanalysis data, and measured RO temperature
profiles. The absence of fronts or jets during both case studies reveals
similar relevant GW features (main parameters, generation, and propagation).
Orographic and convective activity generates the observed GWs. Mountain waves
above the Alps reach higher altitudes than close to the Andes. In the Andes
case, a critical layer prevents the propagation of GW packets up to
stratospheric heights. The case studies are selected also because they
illustrate how the observational window for GW observations through RO
profiles admits a misleading interpretation of structures at different
altitude ranges. From recent results, the distortion introduced in the
measured atmospheric vertical wavelengths by one of the RO events is
discussed as an illustration. In the analysis, both the elevation angle of
the sounding path (line of tangent points) and the gravity wave aspect ratio
estimated from the simulations and the line of sight are taken into account.
In both case studies, a considerable distortion, over- and underestimation of
the vertical wavelengths measured by RO, may be expected
A method to improve the determination of wave perturbations close to the tropopause by using a digital filter
GPS radio occultation satellite data allowed to analyze in the last decade for the first time a large amount of atmospheric temperature profiles including both the troposphere and the stratosphere all over the globe. Wave amplitude enhancements have been systematically observed around tropopause levels, which are apparently due to artifacts generated by any digital filter used to isolate the wave components from these data. We present a new filtering method which can be equally applied to temperature or refractivity profiles. It was tested with synthetic temperature data based on NCEP reanalyes and observed wave climatologies and it was also statistically validated with GPS radio occultation profiles from the COSMIC mission. The suggested technique significantly reduces artificial enhancements around the tropopause, mainly at low latitudes, where a sharp lapse rate change usually exists. This represents an improvement in comparison to previous applications of standard filters. In addition it would allow the study of longer vertical wavelengths than previously done with other filtering procedures
Temperature Profiles From Two Close Lidars and a Satellite to Infer the Structure of a Dominant Gravity Wave
Gravity waves (GW) are a crucial coupling mechanism for the exchange of energy and momentum flux (MF) between the lower, middle, and upper layers of the atmosphere. Among the remote instruments used to study them, there has been a continuous increment in the last years in the installation and use of lidars (light detection and ranging) all over the globe. Two of them, which are only night operating, are located in Río Gallegos (−69.3◦ W, −51.6◦ S) and Río Grande (−67.8◦ W, −53.8◦ S), in the neighborhood of the austral tip of South America. This is a well-known GWhot spot from late autumn to early spring. Neither the source for this intense activity nor the extent of its effects have been yet fully elucidated. In the last years, different methods that combine diverse retrieval techniques have been presented in order to describe the three-dimensional (3-D) structure of observed GW, their propagation direction, their energy, and the MF that they carry. Assuming the presence of a dominant GWin the covered region, we develop here a technique that uses the temperature profiles from two simultaneously working close lidars to infer the vertical wavelength, ground-based frequency, and horizontal wavelength along the direction joining both instruments. If in addition within the time and spatial frame of both lidars there is also a retrieval from a satellite like SABER (Sounding of the Atmosphere using Broadband Emission Radiometry), then we show that it is possible to infer also the second horizontal wavelength and therefore reproduce the full 3-D GWstructure. Our method becomes verified with an example that includes tests that corroborate that both lidars and the satellite are sampling the same GW. The improvement of the Río Gallegos lidar performance could lead in the future to the observation of a wealth of cases during the GWhigh season. Between 8 and 14 hr (depending on the month) of continuous nighttime data could be obtained in the stratosphere and mesosphere in simultaneous soundings from both ground-based lidars.Facultad de Ciencias Astronómicas y GeofísicasConsejo Nacional de Investigaciones Científicas y Técnica
Tropospheric Products from High-Level GNSS Processing in Latin America
ARTÍCULO PUBLICADO EN REVISTA EXTERNA. The present geodetic reference frame in Latin America and the Caribbean is given by a
network of about 400 continuously operating GNSS stations. These stations are routinely
processed by ten Analysis Centres following the guidelines and standards set up by the
International Earth Rotation and Reference Systems Service (IERS) and International
GNSS Service (IGS). The Analysis Centres estimate daily and weekly station positions
and station zenith tropospheric path delays (ZTD) with an hourly sampling rate. This
contribution presents some attempts aiming at combining the individual ZTD estimations
to generate consistent troposphere solutions over the entire region and to provide reliable
time series of troposphere parameters, to be used as a reference. The study covers ZTD
and IWV series for a time-span of 5 years (2014–2018). In addition to the combination
of the individual solutions, some advances based on the precise point positioning technique
using BNC software (BKG NTRIP Client) and Bernese GNSS Software V.5.2 are presented.
Results are validated using the IGS ZTD products and radiosonde IWV data. The agreement
was evaluated in terms of mean bias and rms of the ZTD differences w.r.t IGS products
(mean bias 1.5 mm and mean rms 6.8 mm) and w.r.t ZTD from radiosonde data (mean
bias 2 mm and mean rms 7.5 mm). IWV differences w.r.t radiosonde IWV data (mean
bias 0.41 kg/m2 and mean rms 3.5 kg/m2).Sitio de la revista: https://link.springer.com/chapter/10.1007/1345_2020_12
Interpretation of gravity wave signatures in GPS radio occultations
The horizontal averaging of global positioning system radio occultation retrievals produces an amplitude attenuation and phase shift in any plane gravity wave, which may lead to significant discrepancies with respect to the original values. In addition, wavelengths cannot be straightforwardly inferred due to the observational characteristics. If the waves produce small departures from spherical symmetry in the background atmosphere and under the assumption that the refractivity kernel may be represented by a delta function, an analytical expression may be derived in order to find how the retrieved amplitudes become weakened (against the original ones). In Particular, we study the range of waves that may be detected and the consequent reduction in variance calculation, which is found to be around 19%. A larger discrepancy was obtained when comparing an occultation variance with the one computed from a numerical simulation of that case. Wave amplitudes can be better resolved when the fronts are nearly horizontal or when the angle between the occultation line of sight and the horizontal component of the wave vector approaches π/2. Short horizontal scale waves have a high probability of becoming attenuated or of not being detected at all. We then find geometrical relations in terms of the relative orientation between waves and sounding, so, as to appropriately interpret wavelengths extracted from the acquired data. Only inertio-gravity waves, which exhibit nearly horizontal fronts, will show small differences between detected and original vertical wavelengths. Last, we analyze the retrieval effect on wave phase and find a shift between original and detected wave that generally is nonzero and approaches π /4 for the largest horizontal wavelengths. Copyright 2008 by the American Geophysical Union.Fil:Alexander, P. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:de la Torre, A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Llamedo, P. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina
A gravity waves study close to the Andes mountains in Patagonia and Antarctica with GPS radio occultation observations
We first study the seasonal and geographical behavior of gravity
wave activity in the lower stratosphere over the southernmost Andes
mountains and their prolongation in the Antarctic Peninsula by
global positioning system (GPS) radio occultation (RO) temperature
profiles, obtained between years 2002 and 2005 by the CHAllenging
Minisatellite Payload (CHAMP) mission. The observed features
complement observations in the same zone by other satellite passive
remote sensing instruments, which are able to detect different
height regions and other spectral intervals of the wave spectrum.
Comparisons with previous GPS RO studies in smaller areas than the
one covered in our analysis are also established. Significant
seasonal variation of wave activity is observed in our work, in
agreement with results from other instruments. The locations of
significant cases indicate that topography is an important source.
Some strong wave activity is also found over open ocean. Critical
level filtering is shown to have an attenuation effect, implying
that a large fraction of the observed activity can be considered to
be an outcome of mountain waves. The studied region has a
significant advantage as compared to other regions of our planet: it
generates wavefronts nearly aligned with the North-South direction
(almost parallel to the mountains), whereby this geometry favors the
wave detection by the nearly meridional line of sight characterizing
most of the GPS RO observations used. A distribution of the observed
gravity waves in terms of amplitudes and wavelengths is also
presented
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