| Water vapor is by far the most important greenhouse gas in the atmosphere. An accurate knowledge of hydrological cycles and their feedback mechanisms regarding a changing climate is therefore indispensable for reliable climate predictions. The relative abundance of HDO provides a deeper insight into hydrological cycles as evaporation and condensation processes deplete heavy water in the gas phase. Only recently, global measurements of HDO depletions in the middle to lower troposphere were performed by the Tropospheric Emission Spectrometer aboard the Aura spacecraft, confirming the importance of rain evaporation and continental convection. However, these measurements exhibit little sensitivity toward the lowest layers of the atmosphere where the bulk of water is located and isotopic signals from evaporation processes largest. The SCIAMACHY instrument aboard ENVISAT allows for simultaneous retrievals of H2O and HDO in the short wave infrared spectral region, thus exhibiting high sensitivity toward the boundary layer. We will optimize retrievals of water and its isotopologue HDO with special focus on the impact of instrumental effects, such as an ice-layer build-up and increasing amount of malfunctioning detector pixels, as well as specific spectral fitting approaches. We will also reassess the spectroscopy for this specific wavelength range. Results will be validated with ground-based Fourier Transform measurements at tropical, mid and high-latitude sites. Some initial comparisons will be done of the SCIAMACHY HDO/H2O with the models containing the current knowledge and understanding of the hydrological cycle to identify important anomalies. |
