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Cold, low-mass, field brown dwarfs are important for constraining theterminus of the stellar mass function, and also for optimizing atmosphericstudies of exoplanets. In 2020 new model grids for such objects were madeavailable: Sonora-Bobcat and ATMO 2020. Also, new candidate cold brown dwarfswere announced, and new spectroscopic observations at lambda ~4.8 um werepublished. In this paper we present new infrared photometry for some of thecoldest brown dwarfs, and put the new data and models together to explore theproperties of these objects. We reconfirm the importance of mixing in theseatmospheres, which leads to CO and NH_3 abundances that differ by orders ofmagnitude from chemical equilibrium values. We also demonstrate that the newmodels retain the known factor >~3 discrepancy with observations at 2 <~ lambdaum <~ 4, for brown dwarfs cooler than 600 K. We show that the entire 1 <~lambda um <~ 20 energy distribution of six brown dwarfs with 260 <= T_eff K <=475 can be well reproduced, for the first time, by model atmospheres whichinclude dis-equilibrium chemistry as well as a photospheric temperaturegradient which deviates from the standard radiative/convective equilibriumvalue. This change to the pressure-temperature profile is not unexpected forrotating and turbulent atmospheres which are subject to diabatic processes. Alimited grid of modified-adiabat model colors is generated, and used toestimate temperatures and metallicities for the currently known Y dwarfs. Acompilation of the photometric data used here is given in the Appendix.

Measuring and Replicating the 1-20 um Energy Distributions of the Coldest Brown Dwarfs: Rotating, Turbulent and Non-Adiabatic Atmospheres