Moisture Sources and Sinks Derived from Water Isotopic Ratios from the Tropospheric Emission Spectrometer

Further understanding of the variations in the seasonal sources of mid-tropospheric moisture, including the contributions from vertical mixing of surface evaporation, moist convective detrainment, and the re-evaporation of falling rain, can provide refined knowledge of the global hydrological cycle. Stable water isotope measurements from the Tropospheric Emission Spectrometer (TES) are useful and unique in this regard since isotopic fractionations occurring during evaporation and condensation give rise to measurable variations in the isotopic composition of water vapor, which can be used to estimate the strength of regional hydrologic processes. The present study uses the isotopic ratios and moisture values provided from the TES to constrain a simple Lagrangian mass transport model. The model focuses on the rates of regional exchange of HDO and H2O during turbulent mixing and intense condensation, yet also provides estimates of the isotopic composition of the dominant moisture reservoirs. The final estimates of the isotopic composition and amount of atmospheric moisture at the source regions are analyzed to provide information on the dominant processes leading to the regional moisture reservoirs. The regional moisture exchange rates between the source regions and the low to mid-tropospheric air parcels are then compared with estimates from historical moisture budgets, and are further used to assess regional hydrologic strength and efficiency. The mass weighted estimates generally show vigorous moisture exchange occurring over land surfaces during the seasonal monsoons, where the timescale for complete replacement of the moisture in the parcels by the evaporative supply of boundary layer moisture is found to be on the order of two to four days. However, the results also implicate moist convective detrainment as a primary moisture source for the wettest monsoonal regions (e.g. the Amazon Basin), while moisture refreshment over the drier monsoonal regions (e.g., the Southwestern and N. Australian monsoon areas) is primarily associated with the evaporation of falling rain. Over the cool waters of the subtropical oceans, our estimates reveal a much slower timescale of approximately seven to ten days for moisture refreshment. Here, the dominant moisture exchange processes are found to be turbulent mixing and rainfall evaporation. Additionally, the mass transport model reveals the presence of the ‘amount effect’ in the HDO/H2O measurements of water vapor over several monsoonal regions, which is consistent with past work using the isotopic composition of rainfall. Using the isotopic measurements from TES, this study provides unique diagnostics of the processes that control the seasonal distribution of water vapor.