First Odin sub-mm retrievals in the tropical upper troposphere: humidity and cloud ice signals

[摘要] Odin-SMR is a limb-sounder operating in the 500 GHz region with thecapability of performing measurements down to altitudes of about 10 km withrelatively low influence of ice clouds. Until now spectra from tropospherictangent altitudes have been disregarded due to inadequate handling ofscattering. A first method to extract upper tropospheric quantities has nowbeen developed, yielding the humidity in two layers around 200 and 130 hPaand information on cloud ice content above 200 hPa.First results are compared with in situ MOZAIC measurements and presented togive 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. Acareful analysis indicates a systematic calibration error of about 1 K, butalso a random component that differs between the two bands. The randomcalibration uncertainty results in retrieval errors of 10–60% depending onhumidity and band. Presently this prohibits use of single retrievals, butaverages can be presented with good accuracy. The fixed calibration error canlargely be removed, leaving the spectroscopic uncertainties to dominate thehumidity retrieval accuracy, with a worst case estimate of 30%.However, the comparison of MOZAIC data and the measurements for the 200 hPalayer shows a systematic difference of <10%.This indicates that the actual systematic error is low and gives furtherconfidence in the capability of Odin-SMR to measure humidity inthe upper tropical troposphere.

[学科分类] 大气科学