  We present several new sets of grids of model stellar atmospheres computed
with modified versions of the ATLAS9 code. Each individual set consists of
several grids of models with different metallicities ranging from [M/H] = -2.0
to +1.0 dex. The grids range from 4000 to 10000 K in T_eff and from 2.0 to 5.0
dex in logg. The individual sets differ from each other and from previous ones
essentially in the physics used for the treatment of the convective energy
transport, in the higher vertical resolution of the atmospheres and in a finer
grid in the (T_eff, logg) plane. These improvements enable the computation of
derivatives of color indices accurate enough for pulsation mode identification.
In addition, we show that the chosen vertical resolution is necessary and
sufficient for the purpose of stellar interior modelling. To explain the
physical differences between the model grids we provide a description of the
currently available modifications of ATLAS9 according to their treatment of
convection. Our critical analysis of the dependence of the atmospheric
structure and observable quantities on convection treatment, vertical
resolution and metallicity reveals that spectroscopic and photometric
observations are best represented when using an inefficient convection
treatment. This conclusion holds whatever convection formulation investigated
here is used, i.e. MLT(alpha=0.5), CM and CGM are equivalent. We also find that
changing the convection treatment can lead to a change in the effective
temperature estimated from Stroemgren color indices from 200 to 400 K.
