On the Accuracy of the Moist Static Energy Budget when Applied to Large-Scale Tropical Motions

The moist static energy (MSE) budget is widely used to understand moist atmospheric thermodynamics. However, the budget is not exact, and the accuracy of the approximations that yield it has not been examined rigorously in the context of large-scale tropical motions (horizontal scales ≥ 1000 km). A scale analysis shows that these approximations are most accurate in systems whose latent energy anomalies are considerably larger than the geopotential and kinetic energy anomalies. This condition is satisfied in systems that exhibit phase speeds and horizontal winds on the order of 10 m s−1 or less. Results from a power spectral analysis of data from the DYNAMO field campaign and ERA5 qualitatively agree with the scaling, although they indicate that the neglected terms are smaller than what the scaling suggests. A linear regression analysis of the MJO events that occurred during DYNAMO yields results that support these findings. It is suggested that the MSE budget is accurate in the tropics because motions within these latitudes are constrained to exhibit small fluctuations in geopotential and kinetic energy as a result of Weak Temperature Gradient (WTG) balance.