Investigation of aerosol-cloud interactions during Hurricane Humberto: a case study with EarthCARE ATLID data and MesoNH simulations.

Understanding Aerosol-Cloud Interactions (ACI) is today at the heart of our ability to accurately model the current and future states of Earth’s climate. High levels of aerosols lead to bright clouds with high albedo, effectively enhancing the overall cooling radiative forcing of aerosols up to –1.3 W.m⁻², rivalling the warming radiative forcing from greenhouse gases (+3.2 W.m⁻²) according to IPCC (2023). These interactions could be altered in unforeseen ways by changes in the global aerosol mix, either human-made or natural, with unforeseen consequences for climate. Improving our understanding of ACI is also the key to more accurate short-term weather predictions and severe weather alerts, such as hurricane events. Today, there is still no consensus on how aerosols radiative and microphysical effects impact the variations in precipitations, intensity, and structure of tropical cyclones. A good understanding of the properties and concentrations of aerosols that act as cloud condensation nuclei (CCN) and ice nucleating particles (INP) is required to better model and predict those extreme events. 

We will present our investigation of ACIs in a case study: the intense tropical cyclone Humberto (category 5 at maximum intensity) that occurred in September 2025 in the Atlantic Ocean. During its cyclogenesis and intensification period, a layer of Saharan dust aerosols was transported to the cyclone, representing a meteorological system where multiple and specific ACIs occur. The study is based on vertically-resolved optical measurements from the ATLID space lidar aboard the ESA-JAXA satellite EarthCARE, from which the nature and properties of cloud-relevant particles are retrieved. The POLIPHON method is applied to estimate CCN and INP concentrations from level 2 lidar products as a function of aerosol subtypes and relative humidity of the atmosphere. In parallel, we will show results from the simulation of the Humberto cyclone using the Meso-NH mesoscale atmospheric model, which numerically integrates the chemical and microphysical processes of aerosols. With a horizontal resolution of the simulation below 5 km, the model explicitly resolves atmospheric dynamics such as convection. By combining observations with modeling, we will describe how interactions between clouds and aerosols affect the lifetime and properties of the Humberto cyclone, which interacts mainly with Saharan dust and marine aerosols.