From Sugar to Flowers: A Transition of Shallow Cumulus Organization During ATOMIC

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. A Lagrangian large eddy simulation, 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 simulation is driven with reanalysis large‐scale meteorology, and in‐situ aerosol data from ATOMIC and its joint field study EUREC4A. 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 U.S. counterpart of the European EUREC 4 A campaign. • Took place in January -February 2020 in the Atlantic Ocean east of Barbados. • To understand the relationship between shallow convection and large-scale conditions in the trade wind regime.

Shallow Cumulus Organization
• Different states of shallow cumulus organization often correlate with different meteorological states. • They can be categorized into four states: sugar, gravel, fish, and flowers (i.e., Bony et al., 2020 1 ).

Objectives
• To reproduce the transition of shallow cumulus organization -from sugar to flowersobserved on February 2-3, 2020.
• To determine primary mechanisms responsible for the transition. • To understand the relationship between the large-scale vertical velocity and the transition of the mesoscale organization.
-Aside from the control simulation (CTL), an additional simulation called WeakW is run with weaker large-scale vertical velocity when the cloud layer was deepening ( Fig. 3a-b). • Lagrangian LES can reproduce the transition of shallow cumulus organization from sugar to flowers observed on February 2-3, 2020 during ATOMIC.
• While the large-scale upward vertical wind deepens the cloud layer, the mesoscale wind leads to net moisture convergence in the moist and cloudy areas. This renders moist areas moister, assisting cloud organization.
• Stronger large-scale upward motion strengthens the mesoscale circulation and accelerates the transition process.

Sugar to Flowers Transition
• Positive large-scale vertical wind (W) deepens the cloud layer; total water path (TWP) increases during the deepening (Fig. 3a-b).
• TWP binned into quartiles has a wider and asymmetric distribution as the organization becomes stronger (Fig. 3c).
• The TWP variance is also a proxy for organization (Fig. 3d).
• Total (cloud+rain) optical depth (OPD) binned by TWP is largest in the moistest quartile (Q4) in which flowers aggregate. (Fig. 3e) • During the time with stronger upward vertical motion, CTL has higher optical depth than WeakW in all quartiles -with the greatest difference in Q4 (Fig. 3f).

Mechanisms of Transition
• The mesoscale perturbation of W from domain mean (W'') is positive in the cloud plumes and negative in the inversion aloft (Q4): ➡ Ascending air in the cloud plumes supports shallow convection. ➡ Net convergence of moisture in the moist patches: -Moisture convergence in and below the cloud plumes.
-Moisture divergence in the stratiform clouds and inversion.
➡ Descending air in drier patches suppresses cloud formation.
• Consistent with a previous study of other shallow cumulus cases. 5 • Stronger W'' in CTL drives stronger net moisture convergence w′ ′ (Top) Fig. 2: Snapshots of total optical depth from CTL and WeakW. (Right) Fig. 3: Time series of large-scale vertical velocity, total water path (TWP) binned into quartiles, normalized TWP variances, total optical depth (OPD) binned by TWP, and the difference in binned OPD between CTL and WeakW. (Bottom) Fig. 4: Vertical profiles of the domain-mean vertical velocity and the mesoscale perturbation coarse-grained to 16-km tiles at 16 UTC on Feb 2.