First Odin sub-mm retrievals in the tropical upper troposphere: humidity and cloud ice signals M. Ekström, P. Eriksson, B. Rydberg, and D. P. Murtagh Department of Radio and Space Science, Chalmers University of Technology, Gothenburg, Sweden
Abstract. Odin-SMR is a limb-sounder operating in the 500 GHz region with the
capability of performing measurements down to altitudes of about 10 km with
relatively low influence of ice clouds. Until now spectra from tropospheric
tangent altitudes have been disregarded due to inadequate handling of
scattering. A first method to extract upper tropospheric quantities has now
been developed, yielding the humidity in two layers around 200 and 130 hPa
and information on cloud ice content above 200 hPa.
First results are compared with in situ MOZAIC measurements and presented to
give a global view of the horizontal distribution.
The seasonal structures are in agreement with other satellite measurements.
The main concern for these retrievals is the calibration performance. A
careful analysis indicates a systematic calibration error of about 1 K, but
also a random component that differs between the two bands. The random
calibration uncertainty results in retrieval errors of 10–60% depending on
humidity and band. Presently this prohibits use of single retrievals, but
averages can be presented with good accuracy. The fixed calibration error can
largely be removed, leaving the spectroscopic uncertainties to dominate the
humidity retrieval accuracy, with a worst case estimate of 30%.
However, the comparison of MOZAIC data and the measurements for the 200 hPa
layer shows a systematic difference of <10%.
This indicates that the actual systematic error is low and gives further
confidence in the capability of Odin-SMR to measure humidity in
the upper tropical troposphere.
Citation: Ekström, M., Eriksson, P., Rydberg, B., and Murtagh, D. P.: First Odin sub-mm retrievals in the tropical upper troposphere: humidity and cloud ice signals, Atmos. Chem. Phys., 7, 459-469, doi:10.5194/acp-7-459-2007, 2007.