Making confident climate predictions requires accurately representing key features, like the Intertropical Convergence Zone (ITCZ), and processes in models. Most climate models erroneously simulate the ITCZ as two separate belts north and south of the equator. Researchers showed that inaccuracies in the simulated moisture convergence profile, which measures how much moist air is moving into a specific area, cause this error. Tropical precipitation preferentially falls in one of two states based on the corresponding moisture convergence profile shape: one that primarily follows the evaporation distribution and one that follows atmospheric moisture. Most models favor the former and produce excessive precipitation in regions of high surface evaporation near the equator.
The ITCZ has a substantial influence on the global energy and water cycles. The longstanding double ITCZ bias in climate models has important implications for projections of future changes in regional rainfall over land. This study highlights how accurately representing the depth of convective clouds in climate models allows them to correctly simulate the spatial distribution of precipitation over both tropical oceans and monsoon regions. By linking the representation of clouds with model rainfall climatology, this new diagnostic framework provides an important tool for understanding climate model biases and interpreting model projections.
This study uses moisture budget analysis to derive the relationship between precipitation and moisture. Researchers found that the parameters of the relationship depend on the vertical structure of moisture convergence. They determine whether the precipitation is balanced by evaporation or is independently controlled by moisture convergence. The parameters separate tropical precipitation into two regimes: a local evaporation-controlled regime with widespread drizzle, and a precipitable water-controlled regime with high-intensity precipitation. Most of the 17 examined historical simulations (from the Coupled Model Intercomparison Project Phase 6) favor the drizzle regime over the high-intensity precipitation regime, which is in disagreement with observations. The models thus overestimate precipitation over the high-evaporation, windy oceanic regions south and north of the equator, thereby producing a “double ITCZ” feature. This results in models underestimating precipitation over large tropical landmasses and the climatologically moist oceanic regions near the equator. In response to warming, the moisture convergence-related parameters have changed and led to a statistically significant increase in observed drizzle over the past few decades.