  Isolated brown dwarfs provide remarkable laboratories for understanding
atmospheric physics in the low-irradiation regime, and can be observed more
precisely than exoplanets. As such, they provide a glimpse into the future of
high-SNR observations of exoplanets. In this work, we investigate several new
considerations that are important for atmospheric retrievals of high-quality
thermal emission spectra of sub-stellar objects. We pursue this using an
adaptation of the HyDRA atmospheric retrieval code. We propose a parametric
pressure-temperature (P-T) profile for brown dwarfs consisting of multiple
atmospheric layers, parameterised by the temperature change across each layer.
This model allows the steep temperature gradient of brown dwarf atmospheres to
be accurately retrieved while avoiding commonly-encountered numerical
artefacts. The P-T model is especially flexible in the photosphere, which can
reach a few tens of bar for T-dwarfs. We demonstrate an approach to include
model uncertainties in the retrieval, focusing on uncertainties introduced by
finite spectral and vertical resolution in the atmospheric model used for
retrieval (~8\% in the present case). We validate our retrieval framework by
applying it to a simulated data set and then apply it to the HST/WFC3 spectrum
of the T-dwarf 2MASS J2339+1352. We retrieve sub-solar abundances of H2O and
CH4 in the object at ~0.1 dex precision. Additionally, we constrain the
temperature structure to within ~100 K in the photosphere. Our results
demonstrate the promise of high-SNR spectra to provide high-precision abundance
estimates of sub-stellar objects.
