Dynamically Coupled Three-dimensional Cloud-Chemistry in Rapidly Rotating Irradiated Atmospheres: an Application to the Compact White Dwarf-Brown Dwarf system ZTFJ0038+2030

Abstract: Clouds and disequilibrium chemistry shape the observable properties of substellar and planetary atmospheres, and JWST time-resolved spectroscopy now enables direct tests of their three-dimensional structures. We present an update to the 3D atmospheric framework ADAM, coupling the dynamical core to a miniature kinetic chemistry scheme (mini-chem) and a saturation-relaxation tracer cloud module. Using this framework, we investigate the atmosphere of the brown dwarf orbiting the white dwarf ZTFJ0038+2030 in a 10-hour orbit. The brown dwarf receives stellar irradiation comparable to its internal heat flux, placing the atmosphere in a regime where the circulation is characterized by a standing Matsuno--Gill wave response and equatorial superrotation. Rapid rotation confines these structures closer to the equator, producing more symmetric broadband light curves with smaller hotspot offsets than canonical hot Jupiters, in agreement with recent JWST observations of this system. Although most species are globally homogenized, rapid rotation limits global-scale horizontal mixing and allows circulation-driven vertical transport to maintain strong CO and CO$_2$ inhomogeneities in the photosphere. These chemical inhomogeneities produce asymmetric phase curves in the CO and CO$_2$ absorption bands, which could qualitatively explain the observed CO$_2$ light-curve asymmetry. When clouds are included, Na$_2$S clouds dominate the photosphere and weaken the large-scale circulation, lowering photospheric CO and CO$_2$ abundances, while MgSiO$_3$ and Fe clouds rain out at deeper levels. Because the dominant Na$_2$S clouds reside mostly above the quench region, cloud-driven convection has limited impact on the quenched abundances, which are instead regulated mainly by large-scale circulation.