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

Climatological patterns over South America derived from COSMIC radio occultation data

Meteorological phenomena are closely linked to the presence of water vapor. They mainly originate and develop in the troposphere, where almost all the atmospheric water is concentrated. The Global Positioning System radio occultation (GPS RO) technique provides vertical profiles of refractivity from which other properties such as temperature and water vapor can be derived. The GPS RO capability to reproduce global, synoptic, and regional climatological patterns over South America, which is a mostly oceanic continent, is tested. From FORMOSAT-3/COSMIC mission data (2006-2010), our previous knowledge regarding global and synoptic/regional patterns of temperature, equivalent potential temperature, specific humidity, and pressure is verified. Special cases such as baroclinic disturbances arriving at South American midlatitudes and storm events over a mountain region near the Andes are analyzed. The temporal evolution and the latitude-longitude distribution in several layers of the variables listed above are well described with this technique. Copyright 2012 by the American Geophysical Union.Fil:Hierro, R. 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.Fil:De La Torre, A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Alexander, P. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Rolla, A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Gravity waves above the Andes detected from GPS radio occultation temperature profiles: Jet mechanism?

A significant wave activity (WA) in the upper troposphere and lower stratosphere, mainly during winter, was detected at midlatitudes in the southern hemisphere (30-40S) above the Andes Range, from an analysis of Global Positioning System Radio Occultation (GPS RO) temperature profiles retrieved by CHAMP (CHAllenging Mini-satellite Payload) and SAC-C (Satélite de Aplicaciones Científicas-C) Low Earth Orbit (LEO) satellites, between May 2001 and February 2006. The possible main gravity wave sources in this region are: i) orographic forcing, ii) geostrophic adjustment and iii) deep convection. The available vertical resolution of GPS RO soundings does not rule out any of these alternatives. Based on satellite imaginary, the WA enhancements cannot be attributed to deep convection events. Inertia-gravity waves (IGWs) could be generated after a geostrophic adjustment process, following a perturbation of the zonal jet situated above the Andes Mountains by mountain waves (MWs). The monthly WA intensity follows the zonal wind velocity strength according to its seasonal variability at jet altitudes. As the GPS-LEO lines of sight are roughly meridionally aligned and the morphology of the Andes at middle latitudes is predominantly north-south, it was possible to detect MWs as well as IGWs from GPS RO temperature profiles. This characteristic does not apply for other mountain range alignments. From the analysis of a numerical simulation at the time and location of a single RO event with very strong WA, two main modes of oscillation with horizontal wavelength around 40 and 200 km were identified. The first one is attributed to a MW and the second one to an IGW. Copyright 2006 by the American Geophysical Union.Fil:de la Torre, A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Alexander, P. 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.Fil:Menéndez, C. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

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

Heartbeat Classification in Wearables Using Multi-layer Perceptron and Time-Frequency Joint Distribution of ECG

Heartbeat classification using electrocardiogram (ECG) data is a vital assistive technology for wearable health solutions. We propose heartbeat feature classification based on a novel sparse representation using time-frequency joint distribution of ECG. Fundamental to this is a multi-layer perceptron, which incorporates these signatures to detect cardiac arrhythmia. This approach is validated with ECG data from MIT-BIH arrhythmia database. Results show that our approach has an average 95.7% accuracy, an improvement of 22% over state-of-the-art approaches. Additionally, ECG sparse distributed representations generates only 3.7% false negatives, reduction of 89% with respect to existing ECG signal classification techniques.Comment: 6 pages, 7 figures, published in IEEE/ACM International Conference on Connected Health: Applications, Systems and Engineering Technologies (CHASE

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

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