More accurate modeling of precipitation’s diurnal cycle

Precipitation is one of the most challenging aspects of climate to model, so the accuracy with which it is represented is therefore frequently cited as a barometer for the quality of climate forecasting. A shortcoming in current climate models is their inability to accurately predict diurnal precipitation patterns. Over land, the daily precipitation cycle is strong: As the Sun heats the ground, near-surface air temperatures increase rapidly, creating columns of rising hot air that form convection cells in the atmosphere. This process results in the formation of large cumulus clouds that can create strong thunderstorms and heavy precipitation. These storms tend to occur late in the afternoon, after convection has occurred through the warmest part of the day. The ocean, meanwhile, heats up much more slowly than land, so daily precipitation cycles over the water are much weaker; most precipitation falls just before dawn and is driven by a combination of factors.

Most current climate models do not capture these diurnal patterns very well. In a new study, Shaocheng Xie (AEED) and colleagues propose modifications to the Energy Exascale Earth System Model (E3SM) that improve this model’s ability to capture diurnal precipitation cycling. The researchers’ two-part revision focuses on the processes that trigger convection in the model.

First, they addressed the issue of daytime precipitation occurring too early by adding a new constraint, called dynamic convective available potential energy, which limits how easily and frequently precipitation occurs. This helps eliminate cases in which the model often produced too many low-intensity rainfall events instead of fewer, larger events later in the day. The second modification to E3SM also focused on convection. Dubbed the unrestricted air parcel launch level (ULL), the adjustment relaxes a restriction in the model that previously mandated that convection always occur close to Earth—in the boundary layer. The ULL change allows the model to capture atmospheric instability above the boundary layer, which may be key in predicting high-altitude nighttime thunderstorms.

With these two changes, the E3SM model shows dramatic improvements in capturing the timing of rainfall events—especially the diurnal cycle—although the model only marginally improves modeling of the mean amount of precipitation across the entire globe. The paper describing this work has been selected by the AGU journal editors as a “research spotlight” for their publication Eos.

[S. Xie, Y.‐C. Wang, W. Lin, H.‐Y. Ma, Q. Tang, S. Tang, X. Zheng, J.‐C. Golaz, G.J. Zhang, and M. Zhang, Improved Diurnal Cycle of Precipitation in E3SM With a Revised Convective Triggering Function, Journal of Advances in Modeling Earth Systems, available online on June 22, 2019, doi: 10.1029/2019MS001702.]