Mathilde Ritman (she/her)

Abstract:

The radiative response of deep convective anvil clouds to anthropogenic aerosols is a major source of uncertainty. While aerosol-cloud interactions (ACI) in the convective core have been extensively studied, the microphysical mechanisms governing the full anvil lifecycle, from detrainment to dissipation, remain poorly constrained.This study examines the Cloud Radiative Effect (CRE) of deep convection through a microphysical process-rate lens. We perform three regional simulations with interactive aerosol using ICON-HAM-lite, comprising baseline, clean, and polluted runs. The simulations follow the TRACER-MIP protocol for a sea-breeze event over Houston, Texas. Using Lagrangian tracking with the tobac cloud tracking algorithm, we isolate individual convective cells and track their evolution from convective onset to the detrainment and dissipation of the resulting anvils. We then assess aerosol-cloud interactions over the lifecycle of the tracked cells by aligning their evolution with the onset of freezing, to ensure a consistent lifecycle comparison.Our results show that a 9-fold increase in aerosol concentration leads to a 2.5-fold increase in cloud droplet number concentration (CDNC). This suppresses warm-rain processes and enhances upward mass flux above the melting layer. As a result, it also lofts higher droplet concentrations, which can shape anvil characteristics by modulating the total ice surface area available for deposition and the net cross-section for riming. This creates a competition between enhanced riming, which promotes mass fallout, and increased vapour deposition, which sustains smaller ice crystals aloft. We conclude by investigating how these competing factors change the lifetime of the anvil and its net CRE.

Abstract:

The top-of-atmosphere net radiative effect of convective anvils is estimated to be close to zero and arises from a balance of significant short-wave cooling and long-wave warming over a complex diurnal cycle. When anvils are optically thick, the cooling due to daytime scattering of shortwave solar radiation dominates. In contrast, optically thin anvils have weaker scattering of solar radiation, so longwave warming becomes the dominant effect. Hence, it is essential to understand the controls of anvil radiative properties over the convective lifecycle, which arises from a complex interplay of convective cloud dynamics and microphysics. The convective mass flux modulates anvil extent, and changes in ice crystal size and morphology affect anvil lifetime and radiative properties. Convective anvils have been proposed to respond to global warming (cloud feedbacks) and anthropogenic aerosols (aerosol-cloud interactions). However, the associated uncertainties remain large and key relevant processes are not represented in the current generation of climate models. Emerging kilometre-scale climate models present new opportunities to examine these effects at the process level.In this work we bring together multiple research strands to quantify the controls of convective anvil clouds and associated radiative effects over the convective lifecycle towards understanding its sensitivity to climate and air pollution changes. We use the tobac cloud tracking framework to track convective cores and associated anvils in 4D across regional and global km-scale ICON model simulations which allows us to quantify the link between convective mass flux, anvil extent and anvil radiative properties. We apply this framework to regional high-resolution simulation of ICON coupled to HAM-lite, our reduced complexity aerosol model derived from the microphysical aerosol scheme HAM [Weiss et al., GMD, 2025], to explore the sensitivity of anvils and their radiative effects to aerosol perturbations in the context of the ORCHESTRA/EarthCARE Model Intercomparison Project (ECOMIP) as well as the TRACER campaign MIP. We find that an increase in aerosol increases cloud droplet numbers, suppresses warm rain formation, increases convective mass flux and thereby upper tropospheric ice water content and will discuss how these changes translate into anvil cloud radiative effects. Prototype next generation km-scale climate models are implicitly already including such anvil radiative effects; however, these currently remain unconstrained by observations. We develop novel observational constraints on the convective anvil cloud lifecycle through consistent tracking of convection using the tobac-flow cloud tracking framework [Jones et al., 2024] between MSG SEVIRI observations and forward simulated geostationary satellite radiances from ICON model output.  This reveals that deep convective systems in ICON grow too fast and show a faster dissipation of thick to thin anvils than observations, which affects their radiative effects. Our work provides novel approaches to improve our understanding of aerosol effects on convective clouds and climate. 

Abstract:

The top-of-atmosphere radiative effect of tropical anvil clouds varies with cloud opacity, and can range from substantially negative to largely positive. Recent climate model assessments have found a decrease in the proportion of thick, or opaque, anvil cloud with warming, resulting in a positive climate feedback. However, the mechanism for this change remains obscure.Lifecycle analysis of deep convective clouds tracked using tobac in the convection-permitting global ICOsahedral Non-hydrostatic model (ICON) shows how anvil area and opacity respond to convection. We find that both properties increase in response to increased convective intensity and convective area, but that their sensitivity to each is not equal. To interpret these results, we independently develop a simple analytical model that links anvil expansion and opacity to convective mass flux (CMF). The model predicts that higher CMF leads to greater anvil expansion, increasing the area of thick anvil cloud. But when anvil opacity also depends on convective intensity, we find a strong, non-linear increase in thick anvil amount in response to increasing CMF, consistent with the response observed in ICON. This implies a strong sensitivity of thick anvil amount to changes in the upper tail of the distribution of CMF and illustrates a possible mechanism by which changes in the distribution of cloud CMF could drive anvil thinning in a warming climate.

Abstract:

From 2017 to 2019, vast swathes of eastern Australia were affected by the severe and devastating Tinderbox Drought. Here, we present the first extended drought climatology for New England, spanning 1880 to 2022, and explore trends in drought characteristics over the past 142 years. We use newly recovered historical temperature and rainfall observations, the latest version of the Australian Bureau of Meteorology’s gridded rainfall dataset and a global gridded extreme dataset to assess changes in precipitation signatures and temperature events during droughts. Our analysis identifies 32 meteorological droughts from 1880 to 2022, lasting from 7 months to over 7 years. The climatology also reveals a change in the nature of drought, with a shift from events characterised by warm season rainfall deficiencies to events with greater rainfall reduction in the cool half of the year. Despite this shift, we also find a significant decrease in the number of cold extremes occurring during droughts, and an increase in hot extremes. Droughts in New England have been associated with a greater than average frequency of cold nights and frost days, but this relationship has weakened over recent decades. Conversely, they are generally associated with a greater than average frequency of hot days, a relationship that has increased over time. The Tinderbox Drought was the second-most extreme meteorological drought for New England in terms of rainfall deficit and drought severity, and was associated with the highest number of extreme warm temperature events. The new drought climatology for New England can now be used to provide regional drought information for decision makers and the community.