Abstract
Abstract. We report a new shortwave infrared (SWIR) retrieval of the column-averaged HDO/H2O ratio from the Japanese Greenhouse Gases Observing Satellite (GOSAT). From synthetic simulation studies, we have estimated that the inferred δD values will typically have random errors between 20‰ (desert surface and 30° solar zenith angle) and 120‰ (conifer surface and 60° solar zenith angle). We find that the retrieval will have a small but significant sensitivity to the presence of cirrus clouds, the HDO a priori profile shape and atmospheric temperature, which has the potential of introducing some regional-scale biases in the retrieval. From comparisons to ground-based column observations from the Total Carbon Column Observing Network (TCCON), we find differences between δD from GOSAT and TCCON of around −30‰ for northern hemispheric sites which increase up to −70‰ for Australian sites. The bias for the Australian sites significantly reduces when decreasing the spatial co-location criteria, which shows that spatial averaging contributes to the observed differences over Australia. The GOSAT retrievals allow mapping the global distribution of δD and its variations with season, and we find in our global GOSAT retrievals the expected strong latitudinal gradients with significant enhancements over the tropics. The comparisons to the ground-based TCCON network and the results of the global retrieval are very encouraging, and they show that δD retrieved from GOSAT should be a useful product that can be used to complement datasets from thermal-infrared sounder and ground-based networks and to extend the δD dataset from SWIR retrievals established from the recently ended SCIAMACHY mission.
Highlights
Presence of cirrus clouds, the HDO a priori profile shape and it is important that climate models manage to atmospheric temperature, which has the potential of intro- reproduce the tropospheric water vapour concentrations but ducing some regional-scale biases in the retrieval
From comparisons to ground-based column observations from the Total Carbon Column Observing Network (TCCON), we find difa(Rlsiositehtaat lt.h, e20i1n2d)i.vTidhuealiHsportyoodcpeircsosceolsomagpreoyscitoaironrnedcotflywraetperrevsaepnoteudr between H216O and the Eheaavritehr HSDyOs(toermH218O) changes ferences between δD from Gases Observing Satellite (GOSAT) and TCCON of around during phase changes due to coSndceniesantiocneasnd evaporation
The GOSAT retrievals souurreumnednetrssotafntdhienHg DofOt/hOHe2cpOreorcaaetinsosecSsanicncvioeonlnvtreicdbueitne to improving the water cyallow mapping the global distribution of δD and its variations cle and allow critical testing of the water cycle representation with season, and we find in our global GOSAT retrievals in climate models
Summary
Bands 2 and 3 of GOSAT cover a useful spectral range from 5800–6400 cm−1 and 4800–5200 cm−1. The spectral window around 2.35 μm (4255 cm−1) used for the SCIAMACHY HDO/H2O retrieval as described in Frankenberg et al (2009) is not covered by GOSAT. We have selected a spectral window between 6439 cm−1 and 6464 cm−1 for the retrieval, which includes roughly 140 spectral points (Fig. 2) In this spectral range, HDO lines are relatively free from interference by H2O and strong solar lines. Extending the spectral range up to 1.568 μm to include additional HDO lines has shown to lead to H2O interferences in the HDO retrieval. Since the a priori profiles of H2O and HDO profiles already represent SMOW, we can directly use the ratio of the retrieved scaling factors instead of R/RSMOW
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