PNNL

The Science

Evapotranspiration (ET) is the movement of water from land to the atmosphere by evaporation and transpiration. Moisture recycling describes the contribution from local ET to local precipitation. It has been quantified using models that often assume that local ET is well mixed with background moisture in the atmosphere. It is a strong assumption and may contradict reality. To test this well-mixed atmosphere assumption, scientists use water vapor tracers incorporated in a climate model to tag moisture from local ET and trace their evolution through different processes. Focusing on May 2015, a month with more than 20 mesoscale convective systems (MCSs) occurring in the southern Great Plains, scientists found that 76 percent of local ET is transported away from the region and the remaining 24 percent is transported upward, contributing to moisture recycling. MCSs play a critical role in boosting moisture recycling due to their strong upward motion that effectively pumps local ET to the upper atmosphere.

The Impact

Using the novel water vapor tracer tool allows researchers to examine detailed processes contributing to moisture recycling and test the well-mixed atmosphere assumption. It reveals how local ET is pumped upward by the strong updraft of MCSs, which in turn contributes to MCS precipitation, complementing our understanding of the interplay between local ET and MCS processes. It also allows scientists to understand the diurnal cycle of moisture recycling, which cannot be resolved by recycling models that assume a well-mixed atmosphere. More importantly, scientists found limited mixing of local ET with background moisture, challenging other methods that assume local ET is well-mixed vertically.

Summary

Moisture recycling, the contribution of local ET to local precipitation, has been studied using models assuming a well-mixed atmosphere. The latter may be a poor assumption that limits accurate estimation of moisture recycling in different regions. To test this assumption, scientists use a regional climate model enhanced with numerical water vapor tracers to tag moisture from local ET to physically trace ET’s moisture through various processes such as transport and mixing. Simulating May 2015, when more than 20 MCSs occurred over the southern Great Plains, scientists found 76 percent of local ET is transported away from the region, while the remaining 24 percent is pumped upward within this region, contributing to moisture recycling. Scientists also found that pumping local ET is closely associated with MCSs due to its strong upward motion that can effectively pump local ET to the upper atmosphere, particularly during early phases of their life cycle. In addition, the study using water vapor tracers indicates limited mixing of local ET with background moisture, challenging other methods that quantify moisture recycling based on the well-mixed atmosphere assumption.

PNNL Contact

L. Ruby Leung, Pacific Northwest National Laboratory, ruby.leung@pnnl.gov

Funding

The Department of Energy Office of Science, Biological and Environmental Research supported this research as part of the Regional and Global Model Analysis program area.

Related Links

Moisture Recycling through Pumping by Mesoscale Convective Systems, Journal of Hydrometeorology.