We present a first study showing that organization of trade cumulus (Tc) clouds can significantly enhance Tc response to climate change. Among four recently identified states of Tc organization, the “Flower” state has the highest and the “Sugar” state the lowest cloud fraction and cloud radiative effect. Using large-eddy simulations, we show that the organized “Flower” Tc state is strongly suppressed at the end of the 21st century, unlike the less organized “Sugar” Tc state and Tc studied previously. The primary cause of the suppression is down-welling long-wave radiation from increased greenhouse gas concentrations, which weakens the mesoscale circulation that organizes clouds into the “Flower” Tc state. The cumulus-valve mechanism, which is thought to limit Tc response to climate change, does not prevent this response. Our work unravels an unrecognized role of cloud organization in the cloud response to climate change.

The Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC) took place in January-February 2020. It was designed to understand the relationship between shallow convection and the large-scale environment in the trade-wind regime. Lagrangian large eddy simulations, following the trajectory of a boundary-layer airmass, can reproduce a transition of trade cumulus organization from “sugar” to “flower” clouds with cold pools, observed on February 2-3. The simulations were driven with reanalysis large-scale meteorology and ATOMIC in-situ aerosol data. During the transition, large-scale upward motion deepens the cloud layer. The total water path and optical depth increase, especially in the moist regions where flowers aggregate. This is due to mesoscale circulation that renders a net convergence of total water in the already moist and cloudy regions, strengthening the organization. An additional simulation shows that stronger large-scale upward motion reinforces the mesoscale circulation and accelerates the organization process by strengthening the cloud-layer mesoscale buoyant turbulence kinetic energy production.

A transition from sugar to flower shallow cumuli occurred under a layer of mineral dust on February 2, 2020, during the multinational ATOMIC and EUREC4A campaign. Lagrangian large eddy simulations following an airmass trajectory along the trade winds are used to explore radiative impacts of the diurnal cycle and mineral dust on the sugar-to-flower (S2F) cloud transition. The large-scale meteorological forcing is derived from the European Center for Medium-Range Weather Forecasts Reanalysis 5th Generation and based on in-situ measurements during the field campaign. A 12-hour delay in the diurnal cycle accelerates the S2F transition, leading to more cloud liquid water and precipitation at night. The aggregated clouds generate more, and stronger cold pools, which alter the original mechanism responsible for the organization. Although there is still mesoscale moisture convergence in the cloud layer, the near-surface divergence associated with cold pools transports the subcloud moisture to the drier surrounding regions. New convection forms along the cold pool edges, resulting in the next generation of flower clouds. The amount of cloud water, rain, and cold pools reduce after sunrise. The modulation of the surface radiative budget by free-tropospheric mineral dust poses a less dramatic effect on the S2F transition. Mineral dust absorbs shortwave radiation during the day, cooling the boundary layer temperature, enabling stronger turbulence, strengthening the mesoscale organization, and enlarging the aggregate areas. At night, the longwave heating effects of the mineral dust and more cloud liquid water warms the boundary layer, reducing the cloud amount and weakening the organization.

The low cloud bias in global climate models (GCMs) remains an unsolved problem. Coarse vertical resolution in GCMs has been suggested to be a significant cause of low cloud bias because planetary boundary layer parameterizations cannot resolve sharp temperature and moisture gradients often found at the top of subtropical stratocumulus layers. This work aims to lessen the low cloud problem by implementing a new computational method, the Framework for Improvement by Vertical Enhancement (FIVE), into the Energy Exascale Earth System Model (E3SM). Three physics schemes representing microphysics, radiation, and turbulence as well as vertical advection are interfaced to vertically enhanced physics (VEP), which allows for these processes to be computed on a higher vertical resolution grid compared to the rest of the E3SM model. We demonstrate the better representation of subtropical boundary layer clouds with FIVE while limiting additional computational cost from the increased number of levels. When the vertical resolution approaches the LES-like vertical resolution in VEP, the climatological low cloud amount shows a significant increase of more than 30% in the southeastern Pacific Ocean. Besides the improvement of low-level cloud amount, the skill scores of mid- and high-level cloud amounts are not negatively impacted partly because FIVE can avoid negative consequences of running deep convection parameterization at high vertical resolution.

An approach to improve the fidelity of Lagrangian large eddy simulation (LES) of boundary layer clouds is presented and evaluated with satellite retrievals and aircraft in-situ measurements. The Lagrangian LES are driven by reanalysis meteorology and follow trajectories of the boundary layer flow. They track the formation and evolution of a pocket of open cells (POC) underneath a biomass burning aerosol layer in the free troposphere. The simulations are evaluated with data from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) on board the Meteosat Second Generation (MSG) satellite, and in-situ aircraft measurements from the Cloud-Aerosol-Radiation Interactions and Forcing (CLARIFY) field campaign. The simulations reproduce the evolution of observed cloud morphology, cloud optical depth, and cloud effective radius, and capture the timing of the cloud state transition from closed to open cells seen in the satellite imagery on the three considered trajectories. They also reproduce a biomass burning aerosol layer identified by the in-situ aircraft measurements above the inversion of the POC. We find that entrainment of aerosol from the biomass burning layer into the POC is limited to the extent of having no impact on cloud- or boundary layer properties, in agreement with observations from the CLARIFY field campaign. The simulations reproduce in-situ cloud microphysical properties reasonably well. The role of the model and simulation setup and the resulting uncertainties and biases are presented and discussed, and research and development needs are identified.