INVESTIGADORES
ALEXANDER Pedro Manfredo
artículos
Título:
Precision estimation in temperature and refractivity profiles retrieved by GPS radio occultations
Autor/es:
P. ALEXANDER; A. DE LA TORRE; P. LLAMEDO; R. HIERRO
Revista:
JOURNAL OF GEOPHYSICAL RESEARCH
Editorial:
AMER GEOPHYSICAL UNION
Referencias:
Año: 2014 vol. 119 p. 8624 - 8638
ISSN:
0148-0227
Resumen:
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.