Linking the organization of precipitation extremes at sub-meso-alpha scales to surface wind fluctuations in a storm-resolving GCM

Convective systems exhibit a wide range of cloud and precipitation structures spanning spatial scales from a few kilometres to thousands of kilometres. While the organization of convection at the meso-alpha scale (200–2000 km) is relatively well-researched through observations and numerical modelling, much less is known about how convection organizes at smaller scales, down to a few kilometres, that are now accessible to kilometre-scale, storm-resolving models.To address this, we investigate the spatial organization of extreme precipitation in simulations of the storm-resolving model, ICON, coupled to the prognostic aerosol module, HAM-lite. Using month- long, kilometre-scale limited-area simulations over the Atlantic Intertropical Convergence Zone, conducted for the ORCESTRA measurement campaign period [1], we find that 99th-percentile precipitation extremes over the ocean exhibit robust scale-invariant organization across spatial scales from approximately 10 to 150 km, characterised by a fractal dimension of approximately 4/3.While individual convective updrafts are associated with strong surface convergence, their organisation at these scales is significantly influenced by cold pools which generate intense surface wind divergence. Consistent with this mechanism, grid points with large absolute values of surface wind divergence form spatial clusters that statistically resemble those of extreme precipitation. They tend to predominantly affect the intermittency of surface wind fluctuations, in a manner analogous to shocks in compressible turbulence. Building upon this analogy, we demonstrate that the surface wind fluctuations indeed exhibit a nearly-bifractal scaling — consistent with certain models of compressible turbulence [2] — and the scaling exponents of higher-order surface wind velocity structure functions appear to approach the co-dimension of the fractal set defined by the extreme precipitation events.This establishes a direct quantitative link between the spatial organization of precipitation extremes and surface wind fluctuations at sub–meso-alpha scales, highlighting implications for the development of simple yet physically grounded stochastic parameterizations of the latter in coarse- resolution GCMs. Furthermore, we assess the robustness of such organization to various climate- change and air pollution scenarios via perturbations to the prescribed sea-surface temperatures and aerosol emissions, respectively. References:[1] https://orcestra-campaign.org/intro.html[2] Mitra et al, Physical Review Letters 94, 194501 (2005).