The Alpha--Omega Problem in the Assessments of Atmospheric Power

Abstract

Atmospheric power is commonly assessed with two Lorenz-cycle formulations treated as equivalent: the $αω$ formulation, yielding gas expansion power $W_ω$, and the $\mathbf{v}\cdot\nabla z$ formulation, yielding kinetic-energy generation by horizontal pressure gradients, $W_K$. We show that, for a statistically stationary global moist atmosphere, $W_ω=W_K+W_P+W_{\rm nh}$, where $W_P$ is the power of lifting precipitating water and $W_{\rm nh}$ is power associated with departures from gas-phase hydrostatic balance. Since $W_P\simeq1$ W m$^{-2}$ globally, equality would require $W_{\rm nh}\simeq-W_P<0$, so the associated vertical force imbalance would have to do negative work globally. Using six-hourly ERA5, JRA-3Q, and MERRA-2 fields for 2025, we find that resolved $W_ω$ varies by a factor of 1.7 (1.9--3.2 W m$^{-2}$), versus 1.2 for $W_K$ (2.4--2.8 W m$^{-2}$). Zonal-mean $αω$ profiles have area-weighted contributions near $\pm50$ W m$^{-2}$, making global $W_ω$ a cancellation residual, whereas $\mathbf{v}\cdot\nabla z$ profiles vary much less with latitude. The difference $W_ω-W_K$ ranges from $-0.61$ to $+0.87$ W m$^{-2}$. Thus reanalysis-derived $W_ω$ cannot provide an independent constraint on atmospheric power. Two treatments of the omitted surface layer give cross-reanalysis mean $W_K$ of 2.8 and 3.1 W m$^{-2}$, with corrected values spanning 2.6--3.5 W m$^{-2}$. Both exceed the observation-based Lorenz available-potential-energy generation estimate of 1.2 W m$^{-2}$. Assuming $W_{\rm nh}\geq0$, adding $W_P\simeq1$ W m$^{-2}$ gives total atmospheric power of about 3.8--4.1 W m$^{-2}$, motivating investigation of additional power-generation mechanisms.