Cirrus dominate the longwave radiative budget of the tropics. For the first time, we quantify the variability in cirrus properties and longwave cloud radiative effects (CREs) that arises from differences in microphysics within nudged global storm-resolving simulations from a single model. Nudging allows us to compute radiative biases precisely using coincident satellite measurements and to fix the large-scale dynamics across our set of simulations and isolate the influence of microphysics. We run five-day simulations with four commonly-used microphysics schemes of varying complexity (SAM1MOM, Thompson, M2005 and P3) and find that the tropical average longwave CRE varies over 20 W m$^{-2}$ between schemes. P3 best reproduces observed longwave CRE. M2005 and P3 simulate cirrus with realistic frozen water path but unrealistically high ice crystal number concentrations which commonly hit limiters and lack the variability and dependence on frozen water content seen in aircraft observations. Thompson and SAM1MOM have too little cirrus.

In clouds containing both liquid and ice that have temperatures between -3C and -8C, liquid droplets collide with large ice crystals, freeze, and shatter, producing a plethora of small ice splinters. This process, known as Hallett-Mossop rime splintering, can cause clouds to reflect less sunlight and to have shorter lifetimes. Here, we use a novel suite of five global cloud-resolving models, which break up the Earth’s atmosphere into columns with 2-4 km horizontal edges, to show that this microscale process has global implications. Simulations that include Hallett-Mossop rime splintering have reduced cumulus cloud cover over the Southern Ocean and reflect 12 Wm^(-2) less sunlight back to space over the same region, better matching satellite observed radiative fluxes. We evaluate simulated clouds using high-resolution visible images from the Himawari satellite, and radar reflectivities and two-dimensional images of cloud particles from the SOCRATES aircraft campaign. Cumulus clouds from simulations with Hallett-Mossop rime splintering included have more realistic cloud morphology, cloud vertical structure and ice crystal properties. We show that Hallett-Mossop rime splintering is an important control on cumulus cloud cover and cloud radiative effects over the Southern Ocean, and that including it in simulations improves model performance. We also demonstrate the key role that global cloud-resolving models can play in detangling the effects of clouds on Earth’s climate across scales, making it possible to translate the behavior of tiny cloud particles (10^(-8) m^2) to their impact on the radiative budget of the massive Southern Ocean basin (10^(14) m^2).

Climate models struggle to accurately represent the highly reflective boundary layer clouds overlying the remote and stormy Southern Ocean. We use in-situ aircraft observations from the Southern Ocean Clouds, Radiation and Aerosol Transport Experimental Study (SOCRATES) to evaluate Southern Ocean clouds in a cloud-resolving large-eddy simulation (LES) and two coarse resolution global atmospheric models, the CESM Community Atmosphere Model (CAM6) and the GFDL global atmosphere model (AM4), run in a nudged hindcast framework. We develop six case studies from SOCRATES data which span the range of observed cloud and boundary layer properties. For each case, the LES is run once forced purely using reanalysis data (‘ERA5-based’) and once strongly nudged to an aircraft profile (‘Obs-based’). The ERA5-based LES can be compared with the global models, which are also nudged to reanalysis data, and is better for simulating cumulus. The Obs-based LES closely matches an observed cloud profile and is useful for microphysical comparisons and sensitivity tests, and simulating multi-layer stratiform clouds. We use two-moment Morrison microphysics in the LES and find that it simulates too few frozen particles in clouds occurring within the Hallett-Mossop temperature range. We modify the Hallett-Mossop parameterization so that it activates within boundary layer clouds and we achieve better agreement between observed and simulated microphysics. The nudged GCMs achieve reasonable supercooled liquid water dominated clouds in most cases but struggle to represent multi-layer stratiform clouds and to maintain liquid water in cumulus clouds. CAM6 has low droplet concentrations in all cases and underestimates stratiform cloud-driven turbulence.

Cirrus control the longwave radiative budget of the tropics. For the first time, we quantify the variability in cirrus properties and longwave cloud radiative effects (CREs) that arises from using different bulk ice microphysical parameterizations within a single global storm-resolving model. We run five-day meteorologically-nudged simulations with four commonly-used microphysics schemes (M2005, Thompson, P3 and SAM1MOM) and evaluate them with satellite products and in situ observations. Tropical average longwave CRE varies over 20 W m$^{-2}$ between schemes. Within the Thompson scheme, rapid autoconversion of cloud ice to snow leads to deficient anvil cirrus even with radiatively active snow. SAM1MOM, which uses saturation adjustment for cloud ice, also has deficient anvil cirrus. M2005 and P3 simulate cirrus with realistic frozen water path, and P3 best reproduces observed longwave CRE. Even in those schemes, ice crystal number concentrations commonly hit limiters and lack the observed variability and dependence on frozen water content.