  (abridged) We calculate near-infrared thermal emission spectra using a
doubling-adding radiative transfer code, which includes scattering by clouds
and haze. Initial temperature profiles and cloud optical depths are taken from
the drift-phoenix brown dwarf model. As is well known, cloud particles change
the spectrum compared to when clouds are ignored. The clouds reduce fluxes in
the near-infrared spectrum and make it redder than for the clear sky case. We
also confirm that not including scattering in the spectral calculations can
result in errors on the spectra of many tens of percent, both in magnitude and
in variations with wavelength. This is especially apparent for particles that
are larger than the wavelength and only have little iron in them. Scattering
particles will show deeper absorption features than absorbing (e.g. iron)
particles and particle size will also affect the calculated infrared colours.
Large particles also tend to be strongly forward-scattering, and we show that
assuming isotropic scattering in this case also leads to very large errors in
the spectrum. Thus, care must be taken in the choice of radiative transfer
method for heat balance or spectral calculations when clouds are present in the
atmosphere. Besides the choice of radiative transfer method, the type of
particles that are predicted by models will change conclusions about e.g.
infrared colours and trace gas abundances. As a result, knowledge of the
scattering properties of the clouds is essential when deriving temperature
profiles or gas abundances from direct infrared observations of exoplanets or
brown dwarfs and from secondary eclipse measurements of transiting exoplanets,
since scattering clouds will change the depth of gas absorption features, among
other things. Thus, ignoring the presence of clouds can yield retrieved
properties that differ significantly from the real atmospheric properties.
