Comparison of satellite limb-sounding humidity climatologies of the uppermost tropical troposphere

Abstract

Abstract. Humidity climatologies of the tropical uppermost troposphere from satellite limb emission measurements have been compared. Four instruments are considered; UARS-MLS, Odin-SMR, and Aura-MLS operating in the microwave region, and MIPAS in the infrared region. A reference for the comparison is obtained by MOZAIC in-situ measurements.The upper tropospheric humidity products were compared on basis of their empirical probability density functions and seasonally averaged horizontal fields at two altitude layers, 12 and 15 km. The probability density functions of the microwave datasets were found to be in very good agreement with each other, and were also consistent with MOZAIC. The average seasonal humidities differ with less than 10%RHi between the instruments, indicating that stated measurement accuracies of 20-30% are conservative estimates. The systematic uncertainty in Odin-SMR data due to cloud correction was also independently estimated to be 10%RHi. MI-PAS humidity profiles were found to suffer from cloud contamination, with only 30% of the measurements reaching into the upper troposphere, but under clear-sky conditions there is a good agreement between MIPAS, Odin-SMR and Aura-MLS.Odin-SMR and the two MLS datasets can be treated as independent, being based on different underlying spectroscopy and technology. The good agreement between the microwave limb-sounders, and MOZAIC, is therefore an important step towards understanding the upper tropospheric humidity. The found accuracy of 10%RHi is approaching the level required to validate climate modelling of the upper troposphere huCorrespondence to: M. Ekström (mattias.ekstrom@chalmers.se) midity. The comparison of microwave and infrared also stresses that microwave limb-sounding is necessary for a complete view of the upper troposphere.

Technical Note: Reanalysis of upper troposphere humidity data from the MOZAIC programme for the period 1994 to 2009

“…7 of Ekström et al (2007). In their consecutive study Ekström et al (2008) compared PDFs of RH ice measured by ODIN, AURA-MLS and UARS-MLS with MOZAIC UTH data optimised at 205 hPa; see Fig. 4 of their paper.…”

“…Most of the comparison has been performed on decadal averages of UTH data such that the impact of the wet bias is of minor influence on the results because the variability of UTH is very large in that region. Ekström et al (2007Ekström et al ( , 2008 compared RH ice values from ODIN (ODIN-SMR is a limb-sounder operating in the 500 GHz region) at 200 hPa with MOZAIC RH ice at 200 hPa for the period 2001-2004 over tropical regions. The agreement of the PDF for RH ice from ODIN and MOZAIC sensors is better than 5 % RH ice , which is within the retrieval error of ODIN.…”

“…Upper troposphere humidity (UTH) is one of the still poorly understood climate variables, although its role in the global climate system is considered essential (Solomon et al, 2010;Gettelman et al, 2011;Riese et al, 2012). The latest IPCC report (IPCC, 2013) states that the knowledge about potential trends and feedback mechanisms of upper tropospheric water vapour is low because of its large natural variability in the troposphere and relatively short records of observations.…”

“…MOZAIC RH data were also used for the validation of satellite instruments (e.g. Offermann et al, 2002;Ekström et al, 2007Ekström et al, , 2008Heise et al, 2008), global chemistry transport models (e.g. Law et al, 2000;Crowther et al, 2002) and ECMWF models (e.g.…”

On cloud ice induced absorption and polarisation effects in microwave limb sounding

“…In this respect, microwave techniques have an inherent advantage over infrared and optical techniques, because their sensitivity to clouds is significantly lower (e.g. Ekström et al, 2008).…”