Effects of Improved Simulation of Precipitation on Evapotranspiration and Its Partitioning Over Land.

Evapotranspiration (ET) is a key component of the global hydrological cycle, which is strongly modulated by the occurrence of different rainfall intensities. Global climate models (GCMs) commonly suffer from "too much light rain" and a negative bias in the ratio of transpiration (T) to ET (T/ET). It is unclear whether these biases are related. Here we show that with the improved simulation of probability density functions of rainfall intensity by suppressing light‐rain occurrence using a stochastic convection parameterization in the NCAR CESM1.2, the canopy T increases in tropical forests while evaporation from canopy interception and bare soil decreases. The simulated T/ET is increased by 2.5% globally and up to 8% regionally, primarily attributable to reduced fraction of wet leaves due to less frequent light rain despite its weak intensity. These results imply that excessive light rain is an important cause of the negative T/ET bias in GCMs. Plain Language Summary: Water moves from the land surface to the atmosphere via evapotranspiration. Processes contributing to evapotranspiration (ET) include evaporation from soil and wet leaves, and transpiration through pores in plants (T). Rainfall intensity is known to be an important factor in regulating canopy interception and soil moisture, thus impacting ET. "Too much light rain and too little heavy rain" and underestimated ratio of T to ET (T/ET) are two common weaknesses of current global climate models (GCMs). Here we examine the impact of rainfall intensity on climatological ET and show that light rain has a major influence on ET and its components. By improving the representation of convection, the light rain (1–20 mm d−1) frequency is reduced. As a result, the fractional coverage of wet leaves of vegetation decreases, resulting in an increase in T and a decrease in evaporation from canopy interception and bare soil. Therefore, in GCMs, the issue of excessive light rain is a cause of the problem of underestimated T/ET. Key Points: The underestimated ratio of transpiration (T) to evapotranspiration (ET) can be largely attributed to too frequent light rainThe improved rainfall intensity spectrum greatly impacts evapotranspiration by directly reducing wet leaf fraction and canopy evaporationThe associated changes in solar radiation and other factors also affect evapotranspiration by increasing vegetation transpiration [ABSTRACT FROM AUTHOR]