P. Alexander

Precision estimation in temperature and refractivity profiles retrieved by GPS radio occultations

The Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) is a six-satellite Global Positioning System (GPS) radio occultation (RO) mission that started in April 2006. The close proximity of these satellites during some months after launch provided a unique opportunity to evaluate the precision of GPS RO temperature and refractivity profile retrievals in the neutral atmosphere from nearly collocated and simultaneous observations. In order to work with nearly homogeneous sets, data are divided into five groups according to latitude bands during 20 days of July. For all latitude bands and variables, the best precision values (about 0.1%) are found somewhere between 8 and 25 km height. In general, we find that precision degrades significantly with height above 30 km and its performance becomes there worse than 1%. Temperature precision assessment has been generally excluded in previous studies. Refractivity has here, in general, a precision similar to dry temperature but worse than wet temperature in the lower atmosphere and above 30 km. However, it has been shown that the better performance of wet temperature is an artificial effect produced by the use of the same background information in nearly collocated wet retrievals. Performance in refractivity around 1% is found in the Northern Hemisphere at the lowest heights and significantly worse in the southern polar zone above 30 km. There is no strong dependence of the estimated precision in terms of height on day and night, on latitude, on season, or on the homogeneity degree of each group of profiles. This reinforces the usual claim that GPS RO precision is independent of the atmospheric conditions. The roughly 0.1% precision in the 8–25 km height interval should suffice to distinguish between day and night average values, but no significant differences are found through a Student t test for both populations at all heights in each latitude band. It was then shown that the present spatial density of GPS RO does not allow to analyze smaller latitudinal bands, which could lead to smaller dispersions associated with the day and night means, where it would then be potentially possible to detect significant statistical differences among both categories. We studied the uncertainties associated with the background conditions used in the retrievals and found that their contribution is negligible at all latitudes and heights. However, they force an artificial improvement of wet temperature precision as compared to the dry counterpart at the lowest and highest altitudes studied. In addition, we showed that there is no detectable dubious behavior of COSMIC data prior to day 194 of year 2006 as warned by the data providers, but our result applies only to the precision issue and cannot be extended to other features of data quality. Regarding accuracy, we estimated an average bias of 0.1 K for GPS RO temperature between about 10 and 30 km height and somewhat larger at lower altitudes. We expect a roughly −0.5 K bias above 35 km altitude. Regarding refractivity, a −0.2% bias of the measurements was estimated below about 8 km height.

Limb sounders tracking topographic gravity wave activity from the stratosphere to the ionosphere around midlatitude Andes

Several studies have shown that the surroundings of the highest Andes mountains at mid-latitudes in the Southern Hemisphere exhibit gravity waves (GW) generated by diverse sources which may traverse the troposphere and then penetrate the upper layers if conditions are favorable. There is a specific latitude band where that mountain range is nearly perfectly aligned with the north-south direction, which favors the generation of wavefronts parallel to this orientation. This fact may allow an optimization of procedures to identify topographic GW in some of the observations. We analyze data per season to the east and west of these Andes latitudes to find possible significant differences in GW activity between both sectors. GW effects generated by topography and convection are expected essentially on the eastern side. We use satellite data from two different limb sounding methods: the Global Positioning System (GPS) radio occultation (RO) technique and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument, which are complementary with respect to the height intervals, in order to study the effects of GW from the stratosphere to the ionosphere. Activity becomes quantified by the GW average potential energy in the stratosphere and mesosphere and by the electron density variance content in the ionosphere. Consistent larger GW activity on the eastern sector is observed from the stratosphere to the ionosphere (night values). However, this fact remains statistically significant at the 90% significance level only during winter, when GW generated by topography dominate the eastern sector. On the contrary, it is usually assumed that orographic GW have nearly zero horizontal phase speed and will therefore probably be filtered at some height in the neutral atmosphere. However, this scheme relies on the assumption that the wind is uniform and constant. Our results also suggest that it is advisable to separate night and day cases to study GW in the ionosphere, as it is more difficult to find significant statistical differences during daytime. This may happen because perturbations induced by GW during daytime are more likely to occur in a disturbed environment that may hinder the identification of the waves.

Oscillation modes of humidity over the Amazon basin derived from GPS RO profiles

[1]xa0The Global Positioning System radio occultation (GPS RO) technique provides vertical profiles of refractivity from which water vapor can be derived. It is possible to reproduce global, synoptic, and regional climatological patterns. From Formosa Satellite 3/Constellation Observing System for Meteorology Ionosphere and Climate mission data (2006–2013), the variability of the moistest region of Southern Hemisphere as the Amazon basin is analyzed. Applying different spatial and temporal filters, oscillation modes of the integrated specific humidity (Q) are found. A slight decreasing trend in Q is found during the studied period. Zonal variability of this variable averaged in time between Amazon basin latitudes presents a main mode of oscillation of a wavelength of one quarter of the Earth (T4). A secondary mode of wavelength at around T6 wavelength is also found after high-pass filtering the original signal. In turn, temporal variability averaged over Amazon basin latitudes shows a wavelength at around 12 months, while secondary modes of 6 months are found.

Numerical simulations of windblown dust over complex terrain: the Fiambalá Basin episode in June 2015

On the 13 June 2015, the London Volcanic Ash Advisory Centre (VAAC) warned the Buenos Aires VAAC about a possible volcanic eruption from the Nevados Ojos del Salado volcano (6,879 m), located in the Andes mountain range on the border between Chile and Argentina. A volcanic ash cloud was detected by the SEVIRI instrument on board the Meteosat Second Generation (MSG) satellites from 14:00 UTC on 13 June. Further studies concluded that the phenomenon was caused by remobilization of ancient pyroclastic deposits (circa 4.5 Ka Cerro Blanco eruption) from the Bolsón de Fiambalá (Fiambalá 5 Basin) in northwestern Argentina. In this paper, we provide the first comprehensive description of the dust episode through observations and numerical simulations. We have investigated the spatio-temporal distribution of aerosols and the emission process over complex terrain to gain insight into the key role played by the orography and the condition that triggered the long-range transport episode. Numerical simulations of windblown dust were performed using the WRF-ARW/FALL3D modeling system with meteo10 rological fields downscaled to a spatial resolution of 2 km in order to resolve the complex orography of the area. Results indicated that favourable conditions to generate dust uplifting occurred in northern Fiambalá Basin, where orographic effects caused strong surface winds. According to short-range numerical simulations, dust particles were confined to near-ground layers around the emission areas. On the other hand, dust aerosols were injected up to 5-6 km high in central and southern regions of the Fiambalá Basin, where intense ascending airflows are driven by horizontal convergence. 15 Long-range transport numerical simulations were also performed to model dust cloud spreading over northern Argentina. Results of simulated vertical particle column mass were compared with the MSG-SEVIRI retrieval product. We tested two numerical schemes: with the default configuration of the FALL3D model, we found difficulties to simulate transport through orographic barriers, whereas an alternative configuration, using a numerical scheme to more accurately compute the horizontal advection in abrupt terrains, substantially improved the model performance. 20 1 Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-851, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: 19 December 2016 c

A Method to Determine Gravity Wave Net Momentum Flux, Propagation Direction, and “Real” Wavelengths: A GPS Radio Occultations Soundings Case Study

Fil: Alexander, Pedro Manfredo. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Instituto de Fisica de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Fisica de Buenos Aires; Argentina