Comparison study of COSMIC RO dry air climatologies based on average profile inversion
Abstract. Global Navigation Satellite System (GNSS) radio occultation (RO) data enable
the retrieval of near-vertical profiles of atmospheric parameters like
bending angle, refractivity, pressure, and temperature. The retrieval step
from bending angle to refractivity, however, involves an Abel integral with
an upper limit of infinity. RO data are practically limited to altitudes below
about 80 km and the observed bending angle profiles show decreasing
signal-to-noise ratio with increasing altitude. Some kind of high-altitude
background data are therefore needed in order to perform this retrieval step
(this approach is known as high-altitude initialization). Any bias in the
background data will affect all RO data products beyond bending angle. A
reduction of the influence of the background is therefore desirable – in
particular for climate applications. Recently a new approach for the production of GNSS radio occultation
climatologies has been proposed. The idea is to perform the averaging of
individual profiles in bending angle space and then propagate the mean
bending angle profiles through the Abel transform. Climatological products of
refractivity, density, pressure, and temperature are directly retrieved from
the mean bending angles. The averaging of a large number of profiles suppresses noise in the data,
enabling observed bending angle data to be used up to 80 km without
the need for a priori information. Some background information for the Abel
integral is still necessary above 80 km. This work is a follow-up study, having the focus on the comparison of the
average profile inversion climatologies (API) from the two processing centers
WEGC and DMI, which study monthly COSMIC (Constellation Observing System for
Meteorology, Ionosphere, and Climate) data from January to March 2011. The
impact of different backgrounds above 80 km is tested, and different
implementations of the Abel integral are investigated. Results are compared
for the climatological products with ECMWF analyses, MIPAS, and SABER
data. It is shown that different implementations of the Abel integral have little
impact on the API climatologies. On the other hand, different extrapolations
of the bending angle profile above 80 km play a key role in the
resulting monthly mean refractivities above 35 km in altitude. Below that
respective altitude the API climatologies show a good agreement between the
two processing centers WEGC and DMI. Due to the downward propagation within
the retrieval, effects of the high-altitude initialization lead to
differences in dry-temperature climatologies down to 20 km in altitude. When applying an exponential extrapolation to the bending angles above
80 km at both centers, the dry-temperature climatologies agree among
WEGC, DMI, ECMWF analysis, and MIPAS up to 35 km in altitude within
 ±0.5  K and up to 40 km in altitude within ±1  K. We
conclude that the API retrieval is a valid approach up to the lower
stratosphere. It is a computationally efficient alternative method for
producing dry atmospheric RO climatologies.