R. Hierro

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.

Wave activity at ionospheric heights above the Andes Mountains detected from FORMOSAT‐3/COSMIC GPS radio occultation data

An estimation of the ionospheric wave activity, derived from 4 years of FORMOSAT-3/ COSMIC GPS (Taiwans Formosa Satellite Mission 3/Constellation Observing System for Meteorology—Global Positioning System) radio occultation electron density data, is presented. A systematic enhancement at the eastern side of the Andes range with respect to the western side is observed. A fitting method to remove the wavelike component from each measured profile and estimate the wave activity is described. The differential effect introduced by the action of orography on the generation, to the eastern side of the Andes, of mountain waves, deep convection waves, or even secondary waves aloft after momentum deposition in the middle atmosphere, is suggested.

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.

REGIONAL POLLUTION DUE TO BIOMASS BURNING IN SOUTH AMERICA

The present study analyses a low level jet event that occurred inconjunction with biomass burning and focuses on the impact onsoutheastern South America. The aerosol transport is analyzed usingoutputs of the CATT-BRAMS modeling system. The relationships betweenthe low level jet and the smoke plume pattern and concentrations are shown.