Ground-based remote sensing of HDO/H<sub>2</sub>O ratio profiles: introduction and validation of an innovative retrieval approach

Abstract

Abstract.We propose an innovative approach for analysing ground-based FTIR spectra which allows us to detect variabilities of lower and middle/upper tropospheric HDO/H 2 O ratios. We show that the proposed method is superior to common approaches. We estimate that lower tropospheric HDO/H 2 O ratios can be detected with a noise to signal ratio of 15% and middle/upper tropospheric ratios with a noise to signal ratio of 50%. The method requires the inversion to be performed on a logarithmic scale and to introduce an inter-species constraint. While common methods calculate the isotope ratio posterior to an independent, optimal estimation of the HDO and H 2 O profile, the proposed approach is an optimal estimator for the ratio itself. We apply the innovative approach to spectra measured continuously during 15 months and present, for the first time, an annual cycle of tropospheric HDO/H 2 O ratio profiles as detected by groundbased measurements. Outliers in the detected middle/upper tropospheric ratios are interpreted by backward trajectories.

Airborne in situ vertical profiling of HDO / H<sub>2</sub><sup>16</sup>O in the subtropical troposphere during the MUSICA remote sensing validation campaign

“…Quality assessment of remotely sensed δD has been performed theoretically (Worden et al, 2006;Schneider et al, 2006;Schneider and Hase, 2011;Lacour et al, 2012;Schneider et al, 2012;Boesch et al, 2013). So far empirical validation studies of tropospheric δD remote sensing products have been made with very few indirect references or have used a δD reference that itself was not comprehensively validated (Schneider and Hase, 2011;Boesch et al, 2013).…”

The MUSICA MetOp/IASI H<sub>2</sub>O and δD products: characterisation and long-term comparison to NDACC/FTIR data

“…In recent years there has been great progress in remote-sensing observations of water vapour isotopologues. In the meanwhile ground-based FTIR (Fourier Transform InfraRed) instruments (Schneider et al, 2006 have been used for measuring δD in the lower and middle troposphere. Furthermore, there are space-borne scientific sensors that measure middle tropospheric δD (Worden et al, 2007) and δD at and above the upper troposphere and lower stratosphere (e.g.…”

Observation of tropospheric δD by IASI over western Siberia: comparison with a general circulation model

“…The methodology used to retrieve HDO / H 2 O ratios from IASI radiances is overall similar to that presented in Lacour et al (2012), which itself was stimulated by the earlier developments on the retrieval of δD presented by Schneider et al (2006) and Worden et al (2006). As compared to Lacour et al (2012), the retrieval settings have been optimized for the specificity of the high-latitude region analysed here.…”

“…This method also has the large advantage of minimizing the error on the δD profile (e.g. Schneider et al, 2006) as demonstrated for example by Worden et al (2006Worden et al ( , 2007Worden et al ( , 2012 on TES measurements, on IASI measurements by Schneider and Hase (2011) and Lacour et al (2012), or for ground-based measurements by Schneider et al (2006Schneider et al ( , 2010Schneider et al ( , 2012.…”

“…The HDO / H 2 O correlated approach makes it possible to constrain the retrieval taking into account the probability density function of the ln(HDO)-ln(H 2 O) distribution which is more stable than H 2 O variability (Schneider et al, 2006). Performing the retrieval on a natural logarithm scale and including the correlation between ln(HDO) and ln(H 2 O) helps to constrain the joint HDO / H 2 O retrieval to a physically meaningful solution.…”