The Relationship between Precipitation and Precipitable Water in CMIP6 Simulations and Implications for Tropical Climatology and Change
It is well documented that over the tropical oceans, column integrated precipitable water (PW) and precipitation (P) have a non-linear relationship. In this study moisture budget analysis is used to examine this P-PW relationship in a normalized precipitable water framework. It is shown that the parameters of the non-linear relationship depend on the vertical structure of moisture convergence. Specifically, the precipitable water values at which precipitation is balanced by evaporation vs. moisture convergence independently define a critical normalized precipitable water, PWnc, a measure of convective inhibition that separates tropical precipitation into two regimes: a local evaporation-controlled regime with wide-spread drizzle and a precipitable water-controlled regime. Most of the 17 CMIP6 historical simulations examined here have higher PWnc compared to ERA5, and more frequently they operate in the drizzle regime. When compared to observations they overestimate precipitation over the high-evaporation oceanic regions off the equator while underestimating precipitation over the large tropical land masses and over the climatologically moist oceanic regions near the equator. The responses to warming under the SSP585 scenario are also examined using the normalized precipitable water framework. It is shown that the critical normalized precipitable water value at which evaporation vs. moisture convergence balance precipitation decreases as a result of the competing dynamic and thermodynamic responses to warming, resulting in an increase in drizzle and total precipitation. Statistically significant historical trends corresponding to the thermodynamic and dynamic changes are detected in ERA5 and in low-intensity drizzle precipitation in the PERSIANN precipitation dataset.