Accurate Column Moist Static Energy Budget in Climate Models. Part 1:
Conservation Equation Formulation, Methodology, and Primary Results
Demonstrated Using GISS ModelE3
Abstract
This paper addresses the challenges in computing the column moist static
energy (MSE) budget in climate models. Residuals from such computations
often match other major budget terms in magnitude, obscuring their
contributions. This study introduces a methodology for accurately
computing the column MSE budget in climate models, demonstrated using
the GISS ModelE3. Multiple factors leading to significant residuals are
identified, with the failure of the continuous calculus’s chain rule
upon discretization being the most critical. This failure causes the
potential temperature equation to diverge from the enthalpy equation in
discretized models. Consequently, in models using potential temperature
as a prognostic variable, the MSE budget equation is fundamentally not
upheld, requiring a tailored strategy to close the budget. This study
introduces the “process increment method’ for accurately computing the
column MSE flux divergence. This method calculates the difference in the
sum of column internal energy, geopotential, and latent heats before and
after applying the dynamics scheme. Furthermore, the calculated column
flux divergence is decomposed into its advective components. These
computations enable precise MSE budget analysis. The most crucial
finding is that vertical interpolation into pressure coordinates can
introduce errors substantial enough to reverse the sign of vertical MSE
advection in the warm pool regions. In ModelE3, accurately computed
values show MSE import via vertical circulations, while values in
pressure coordinates indicate export. This discrepancy may prompt a
reevaluation of vertical advection as an exporting mechanism and
underscores the importance of precise MSE budget calculations.