0 Assigned ID: 6189 ----- Attempting Submission 1 (Tue Apr 08 23:31:00 GMT 2025) ----- HST Phase I Proposal 6189 successfully submitted. Receipt: # 6189-1 ----- Attempting Submission 2 (Wed Apr 09 20:05:49 GMT 2025) ----- HST Phase I Proposal 6189 successfully submitted. Receipt: # 6189-2 ----- Attempting Submission 3 (Thu Apr 10 02:47:22 GMT 2025) ----- The Hubble Space Telescope has transformed exometeorology by enabling time-resolved spectroscopic monitoring of brown dwarfs. These groundbreaking observations revealed quasiperiodic variability and top-of-atmosphere inhomogeneities, fueling advances in light curve analyses, 2D surface mapping, and general circulation models. Yet, quantitative constraints on the physical mechanisms driving variability - particularly patchy clouds and hot spots - remain limited. We propose to fill this gap by developing the first framework for time-resolved atmospheric retrievals. Our work builds on a newly developed method - the first to characterize both patchy clouds and hot spots within a unified framework. This method has successfully fitted a single-epoch JWST spectrum of a late-L brown dwarf while reproducing its HST photometric variability, laying the groundwork for full time-series analysis. We will apply this framework to archival WFC3/G141 time-series spectra, paired with publicly available single-epoch spectra, of 13 variable brown dwarfs, deriving their atmospheric properties (e.g., cloud composition, sedimentation, vertical mixing, thermal anomalies), and tracking how they evolve over time. The sample spans a wide range of ages, inclinations, and spectral types, enabling population-level studies of brown dwarf weather. Our program provides a computationally tractable foundation for interpreting upcoming JWST spectroscopic monitoring, where time-resolved retrievals have not been developed. By unlocking the full potential of HST's archival data, we will deliver a scalable, open-source tool for atmospheric retrievals - fueling a new era of data-driven exometeorology. Dr. Zhoujian Zhang (PI) specializes in the atmospheric characterization of brown dwarfs and directly imaged exoplanets, with extensive experience in analyzing spectroscopic data using both atmospheric retrievals and forward modeling techniques (e.g., Zhang et al. 2025b, arXiv:2502.18559; 2025a, AJ, 169, 9; 2023, AJ, 166, 198; 2021, ApJ, 921, 5; 2021, ApJ, 916, 53). He led the development of the first atmospheric retrieval framework capable of simultaneously characterizing patchy clouds and hot spots (Zhang et al. 2025b, arXiv:2502.18559) - two key drivers of variability in brown dwarf atmospheres. This framework was successfully applied to fit both the JWST single-epoch spectrum of a variable L/T-transition brown dwarf and its photometric variability as observed by HST, demonstrating its power for interpreting heterogeneous atmospheres. This methodology provides the foundation for our proposed AR program. Dr. Paul Molliere is an expert in atmospheric modeling of brown dwarfs and exoplanets (Molliere et al. ApJ, 813, 47; Molliere et al. 2020, A\&A, 640, 131), and the connections between these objects' compositions and formation (Molliere et al. 2022, 934, 74). He is the lead developer of petitRADTRANS (Molliere et al. 2019, A\&A, 627, 67), an open-source software widely used for atmospheric retrievals of brown dwarfs and exoplanets. This software will also support the spectral retrieval analysis in our proposed AR program. Dr. Molliere is currently leading the retrieval study of JWST observations of the L8 variable brown dwarf, PSO318; their analysis has demonstrated that the retrieved models can also reproduce this brown dwarf's time-resolved HST WFC3/G141 spectra by allowing only patchy cloud fraction to evolve over time. This analysis serves as a pilot study for our proposed AR program, validating the feasibility of time-resolved atmospheric retrievals for the archival HST data. Stellar Physics and Stellar Types 0 false false false false false false false SUPPORTED