Isentropic primitive equations for the moist troposphere

Despite the knowledge that the potential temperature of an air parcel has a dependence on its water vapour content, potential temperature is often still calculated as if the parcel were dry, assuming that this moisture dependence is negligible. We show that such a dry potential temperature approximation is not suitable for tropical regions. Moisture gradient terms are seen in the isentropic primitive equations when Exner and Montgomery functions are generalised with moist specific heat capacities, forming a contribution to the horizontal momentum tendency comparable to that by the Montgomery function. This reflects how local horizontal gradients in potential temperature created by inhomogeneous water vapour distribution are relatively significant compared to gradients created by inhomogeneous temperature, in a large‐scale background of weak horizontal temperature gradient. In such an environment, water plays an active role in tropical atmospheric dynamics without the uptake or release of latent heat during phase changes. Hence, we suggest that the tropical troposphere is a place where the atmosphere can behave dynamically as a binary‐component fluid at local and regional scales.