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An improved approximation for the moist-air entropy potential temperature $θ_s$

The moist-air entropy is defined in Marquet (QJRMS 2011, arXiv:1401.1097) by $\boxed{s = s_{ref} + c_{pd} \: \ln(θ_{s})}$ in terms of two constant values ($s_{ref}$, $c_{pd}$) and a potential entropy temperature denoted by $θ_s$. It is shown in Marquet (2011) that a quantity denoted by $(θ_{s})_1$ plays the role of a leading order approximation of $θ_{s}$. The aim of this note is to demonstrate in a more rigorous way that $(θ_{s})_1$ is indeed the leading order approximation of $θ_{s}$, and to derive a second order approximation which may be used in computations of values, gradients or turbulent fluxes of moist-air entropy. Some impacts of this second order approximation are described in this brief version of a note to be submitted to the QJRMS.