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Abstract
Abstract. In situ observational data on the relative humidity (RH) in the upper troposphere and lowermost stratosphere (UT/LS), or tropopause region, collected aboard civil passenger aircraft in the MOZAIC (Measurements of OZone, water vapour, carbon monoxide and nitrogen oxides by in-service AIrbus airCraft) programme were reanalysed for the period 2000 to 2009. Previous analyses of probability distribution functions (PDFs) of upper troposphere humidity (UTH) data from MOZAIC observations from year 2000 and later indicated a bias of UTH data towards higher RH values compared to data of the period 1994 to 1999. As a result, the PDF of UTH data show a substantial fraction of observations above 100% relative humidity with respect to liquid water. Such supersaturations, however, do not occur in the atmosphere because there is always a sufficient number of condensation nuclei available, that trigger condensation as soon as liquid saturation is slightly exceeded. An in-depth reanalysis of the data set identified a coding error in the calibration procedure from year 2000 on. The error did not affect earlier data from 1994 to 1999. The full data set for 2000–2009 was reanalysed applying the corrected calibration procedure. Applied correction schemes and a revised error analysis are presented along with the reanalysed PDF of relative humidity with respect to liquid water (RHliquid) and ice (RHice).
Highlights
Upper troposphere humidity (UTH) is one of the still poorly understood climate variables, its role in the global climate system is considered essential (Solomon et al, 2010; Gettelman et al, 2011; Riese et al, 2012)
RHliquid values below 5 % prevail, the uncertainty of the MOZAIC Capacitive Hygrometer is insufficient for quantitative water vapour measurements, since sensor response time is too slow to equilibrate at the low relative humidity and low temperatures
The entire calibration data set since year 2000 was reanalysed and the MOZAIC data set was reprocessed using the corrected calibration functions
Summary
Upper troposphere humidity (UTH) is one of the still poorly understood climate variables, its role in the global climate system is considered essential (Solomon et al, 2010; Gettelman et al, 2011; Riese et al, 2012). Relative humidity data from the MOZAIC programme have been used for various scientific studies which include the distribution of RHice (Gierens et al, 1997, 1999, 2007; Stohl et al, 2001; Spichtinger et al, 2002; Kunz et al, 2008). The reanalysis period for atmospheric RH data presented here focuses on the first 15 years of MOZAIC observations. Gierens et al, 1999; Spichtinger et al., 2004) The reason for this bias towards higher humidity values is identified as an error in the pre- and post-flight calibration regularly conducted in the environmental simulation chamber at Jülich (Helten et al, 1998; Smit et al, 2000) from year 2000 onward.
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