Enhancing the Upper Boundary Layer Damping Scheme for km‐Scale ICON: A Question of Wavelength

Abstract The upper boundary condition treatment in atmospheric models has a pronounced impact on stability and performance. At the same time, the choice of damping settings can severely impact atmospheric states by not sufficiently preventing back‐reflections of vertically propagating waves from the model top or provoking back‐reflections from the damping layer. A particular challenge is the broad spectrum of upward propagating waves, with horizontal wavelengths between 10 and 10,000 km. In the Icosahedral Nonhydrostatic Weather and Climate Model (ICON), the numerical formulation includes a Klemp Rayleigh damping acting on the vertical velocity in higher atmospheric levels. However, Klemp Rayleigh damping does not yield a uniform damping efficiency over the horizontal wavelength spectrum. This necessitates overdamping of short wavelengths to sufficiently damp the long wavelengths, risking the introduction of artificial back‐reflection from the damping layer. In this work, an extended formulation of the upper‐level wave absorber is proposed that yields a more uniform attenuation of vertically propagating waves across the wave spectrum by introducing vertical diffusion damping on horizontal velocities. This damping scheme explicitly targets the challenges encountered in km‐scale simulations where an increasing number of wavelengths have to be accounted for. Global 10‐km resolution ICON simulations are conducted to investigate the suitability of the proposed upper‐level damping scheme. The results demonstrate improved simulation stability and more efficient damping of long waves. The representation of the mean atmospheric circulation and surface climate remains largely unchanged.