Dry mass versus total mass conservation in the IFS

The continuity equation of the IFS dynamical core assumes that the total atmospheric air mass in
the model is conserved. However, in the real atmosphere, there are physical processes that act as sources/sinks of total water. Therefore, while the dry air mass remains constant, the total air mass may change slightly in time. This note aims at clarifying the current state of the IFS with respect to dry mass versus total mass conservation. It also describes an alternative system of equations which conserves dry mass instead of total mass, following an approach that is similar to the one implemented in the Meteo-France model ARPEGE. This alternative form of the continuity equation is available in IFS from cycle 46R1. Preliminary testing shows that the impact on IFS forecast skill is small but slightly positive. These changes also imply a modification in tracer advection equation. Tracers in IFS are defined as specific ratios (ratios with respect to moist air density) but ”behave” as dry mixing ratios considering that the standard IFS continuity equation assumes that moist rather than dry air is conserved. If an IFS specific ratio is converted to a mixing ratio, its evolution becomes sensitive to the variation of dry air mass which is anti-correlated with the variation of total water. This is very noticeable for carbon tracers such as CO2, a gas that is very well mixed with its background air. An outcome of the new dry mass conserving continuity formulation is that dry air is no longer anticorrelated with its water content. However, in order to be used by atmospheric composition forecasts modifications will be required to the tracer mass fixers used there. Until this issue is addressed, the recommended option for IFS atmospheric composition forecasts is to keep using the current options for continuity and mass fixers which have been proven to produce good quality results.