The impact of changes of atmospheric water mass ontropical cyclone intensification in ICON-A

For many modelling applications, the total mass of the atmosphere and hence the global mean surface pressure can be considered constant and invariant under precipitation and evaporation. This is also the case for the atmosphere model ICON-A: Changes to the atmospheric water mass are compensated by changes in dry air composition, hence the mass of an atmospheric layer is constant and only its physical properties change. However, there are limits to this simplification, especially when it comes to modelling very moist environments or atmospheres. In moist environments, water becomes a major contributor to atmospheric mass and surface pressure, thus changes in water mass from evaporation and precipitation can affect the surface pressure. 

By separating the atmospheric mass in ICON-A into contributions from dry air and from water constituents and by allowing the water component to vary with precipitation and evaporation, we are adding a simplified precipitation mass sink / evaporation mass source to the ICON-A model (for simplicity only referred to as ‘precipitation mass sink’).

The impact of this precipitation mass sink on atmospheric dynamics is investigated in a tropical cyclone test case: A vortex is initialised on a rotating aquaplanet and evolves into a tropical cyclone over the period of ten simulation days. Simulations with and without the precipitation mass sink are compared. The effect of the precipitation mass sink on cyclone development and, in particular, its strength are investigated.