Water is arguably our most precious resource. Tropospheric water vapor comes from a combination of evaporation over the oceans and direct transpiration (plant respiration). Energy and heat within the atmosphere is efficiently and continuously in motion through the engine of condensation within the cloud and evaporation below the cloud.
Applying the principles of isotope chemistry to satellite data provides a powerful tool to improve scientific understanding of water vapor. Using TES measurements of water vapor (H2O) and the isotope HDO, or ‘heavy’ water (also known as deuterium), scientists have determined that a non-trivial fraction of Earth’s water gets into the free troposphere through terrestrial sources.
What are we learning about the global water cycle from TES?
TES measurements of water isotopes are helping track the origin and movement of water vapor throughout Earth’s atmosphere, and understand how the hydrological cycle behaves at different locations. Water isotopes are a good tracer for the origin, condensation, and evaporation history of an air parcel, since:
- lighter isotopes preferentially evaporate
- heavier isotopes preferentially condense
Enhanced condensation means more isotope depletion, and enhanced evaporation (water vapor source) means less isotope depletion. In general, global isotope distributions show increased depletion with latitude, and decreased depletion near regions of convection. (“Depletion” just means that the HDO/H2O ratio is smaller than some reference – usually the ocean.) TES uses a measure of HDO depletion compared to standard mean sea water concentrations of HDO to make the first ever global maps of the HDO/H2O ratio.
Underestimation of Continental Convection in Tropical Water Cycle
There is so much ocean water on Earth that it may come as a surprise to learn that the role of continental convection in the tropical water cycle has traditionally been underestimated. When condensation models are higher than observations, and evaporation models are lower than observations, additional water source terms or processes are needed to explain the discrepancies.
TES profiles also include information on the unique isotopic signals for each source, allowing scientists to differentiate and determine how much water evaporates into the atmosphere from the oceans verses terrestrial sources. Since TES is able to distinguish the water vapor that comes from evapotranspiration in the tropics, certain details concerning the role of plant transpiration as a water vapor source are being revealed.
This TES transect depicts isotope enrichment (red) in regions where there are water vapor sources (notably near the Amazon and the North Atlantic/Caribbean regions). Also, most distant from this (both higher altitude and higher latitude), the vapor is depleted (blue/less red) due to the preferential loss of heavy isotopes during precipitation.
Changes in the isotope ratio are expressed as delta-D (per million). Typical δD for tropospheric water vapor ranges from approximately -50 (near tropical land masses), to -79 (above tropical oceans), to -250 (at higher latitudes, dependant on season), to -800 in the upper troposphere. High H2O and HDO/H2O ratios over land indicate strong evapotranspiration as the water vapor source. For example, a relatively low HDO/H2O ratio with high H2O indicates re-evaporation of precipitation in tropical cloud systems. Outside the tropics there are seasonal effects which confound simple analysis.
Re-evaporation of Precipitation
TES measurements show that in the tropics, re-evaporation of precipitation is an important process controlling cloud formation. It turns out that evaporation of rain falling from the bottom of clouds is very important – typically between 20 and 50% is recycled during rainfall. Up to 70% of precipitation is reevaporated into the cloud. The identification of rainfall evaporation as an important rehydration mechanism in the tropics is helping to identify and quantify some of previously “hidden” sources of water.
These results are exciting, particularly with reference to the role that water plays in climate. Indeed, the starting and ending points for the arrows on water cycle process diagrams traditionally used in undergraduate level courses, may need to be revised to take into consideration these new findings. Cloud-water processes play directly into the radiative balance of the Earth, and are crucial to understanding climate change. As monitoring is continued we may even gain an opportunity to answer some outstanding questions regarding interdependencies between regional land cover and the global water cycle.
Further Interpretion of HDO/H2O Ratios
Sometimes, the water vapor is extra depleted, which happens when raindrops evaporate as they fall. This has implications for how water resides in the atmosphere, and ultimately climate. These differences, plus the fact that transport alone does not change the isotopes, allow us to identify specific processes.
Measurements of HDO also contain information about ice cloud formation and evaporation. The isotopic composition of water vapor over the ocean is a function of temperature. Air at high latitudes is HDO depleted due to the presence of stratospheric air. When cloud ice forms, HDO is concentrated in the ice and the air is HDO depleted. If the ice falls out then air has lower HDO. If the ice evaporates, then the amount of HDO increases.
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Back: Global Climate Change
Understanding how water travels between Earth's surface and atmosphere is crucial to being able to predict the climate and the availability of water in various parts of the world, and to formulating plans to help people adapt to climate changes brought about by global warming. TES helps to deepen our understanding of the water cycle by providing an important piece of the puzzle.