Creating an accurate climate model powerful enough to encompass the entirety of Earth is a major goal that scientists have long pursued. Although current technology and knowledge still prevent even the best models from capturing the full complexity of the planet’s climate, that remains the ultimate goal of the Energy Exascale Earth System Model (E3SM).
Precipitation is one of the most challenging aspects of climate to model, so the accuracy with which it’s 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: Over land, rainfall in models tends to come too early in the day; over the ocean, peak rainfall occurs around 2:00 a.m. local time instead of between 4:00 a.m. and 6:00 a.m. In a new study, Xie et al. propose modifications to E3SM that appear to improve the 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 (dCAPE), which limits how easily and frequently precipitation occurs. In essence, dCAPE allows convection to build up potential energy over the course of the day but holds back the rain. 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. Much of the improvement behind the dCAPE constraint came from reducing how strongly convection was coupled to surface heating.
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. This layer is, indeed, where most atmospheric convection occurs, but the ULL change allows the model to capture atmospheric instability above the boundary layer, which may be key in predicting high-altitude, nocturnal convective systems. Nighttime thunderstorms are common in many areas, such as the Great Plains, that are downstream of large mountain ranges and appear to rely on convection occurring above the boundary layer.
With these two changes, E3SM did not improve in its predictions of the mean amount of precipitation across the entire globe, but it showed dramatic improvements in forecasting the timing of rainfall events, especially in its ability to accurately capture the diurnal cycle. The improvement represents an important advance and brings us one step closer to a milestone in climate modeling. (Journal of Advances in Modeling Earth Systems (JAMES), https://doi.org/10.1029/2019MS001702, 2019)
—David Shultz, Freelance Writer