The STAGGER-grid: A grid of 3D stellar atmosphere models II. Horizontal and temporal averaging and spectral line formation

Aims. We study the implications of averaging methods with different reference depth scales for 3D hydrodynamical model atmospheres computed with the Stagger-code. The temporally and spatially averaged (hereafter denoted as < 3D >) models are explored in the light of local thermodynamic equilibrium (LTE) spectral line formation by comparing spectrum calculations using full 3D atmosphere structures with those from < 3D > averages. Methods. We explored methods for computing mean < 3D > stratifications from the Stagger-grid time-dependent 3D radiative hydrodynamical atmosphere models by considering four different reference depth scales (geometrical depth, column-mass density, and two optical depth scales). Furthermore, we investigated the influence of alternative averages (logarithmic, enforced hydrostatic equilibrium, flux-weighted temperatures). For the line formation we computed curves of growth for Fe I and Fe II lines in LTE. Results. The resulting < 3D > stratifications for the four reference depth scales can be very different. We typically find that in the upper atmosphere and in the superadiabatic region just below the optical surface, where the temperature and density fluctuations are highest, the differences become considerable and increase for higher T-eff, lower log g, and lower [Fe/H]. The differential comparison of spectral line formation shows distinctive differences depending on which < 3D > model is applied. The averages over layers of constant column-mass density yield the best mean < 3D > representation of the full 3D models for LTE line formation, while the averages on layers at constant geometrical height are the least appropriate. Unexpectedly, the usually preferred averages over layers of constant optical depth are prone to increasing interference by reversed granulation towards higher effective temperature, in particular at low metallicity.