BlackHoleWeather – Chaotic cold accretion across the meso-scale: Variability and kinematics

Accretion onto supermassive black holes (SMBHs) in realistic halos is inherently time-variable, governed by the interplay of turbulence, radiative cooling, and multi-phase condensation. In chaotic cold accretion (CCA), multi-phase clouds and filaments condense out of the hot atmosphere and feed the central SMBH stochastically. We investigate how varying turbulence regulates the variability, radial transport, and kinematics of CCA, focusing on the meso-scale that connects halo rain to the inner inflow. We analysed 3D hydrodynamic simulations with a GPU-accelerated code, including radiative cooling and driven subsonic turbulence in a stratified galaxy group, resolving scales from tens of kiloparsecs down to sub-parsecs, probing two turbulent weather regimes. In both regimes, SMBH accretion proceeds through CCA, remains strongly super-Bondi, and then varies by up to ∼,2 dex. The runs diverge mainly at meso scales: strong stirring sustains an extended, fragmented multi-phase feeding and a clear inflow enhancement at 0.1--1 kpc, whereas weaker turbulence yields a smoother, more centrally concentrated cascade. Yet the innermost feeding rates remain similar, implying that SMBH accretion is not directly supply-limited by the macro-scale weather. The accretion rate distributions peak at low Eddington ratios, indicating a predominantly maintenance-mode state. The accretion rate power spectra follow a broken power law, with pink noise on long and medium timescales and a steeper red-noise tail at high frequencies, consistent with a parsec-scale collisional damping transition. The CCA modes are aptly captured by two complementary diagnostics: the (≡ t_ ̊m cool /t_ ̊m eddy ) ≈ 1 identifies the soft X-ray gas as the gateway of the condensation cascade, while the k-plot (line broadening vs shift) shows that the weather distinction is strongest on meso scales, where the stormy regime produces broader, more overlapping multi-phase kinematics than the rainy regime. The meso-scale is the critical bridge between halo rain and micro-scale CCA feeding, which regulates the spatial transport, kinematic imprint, and temporal coherence of SMBH growth.