FY 2023

The sugar-to-flowers cumulus transition under the influences of diurnal cycle and free-tropospheric mineral dust

Narenpitak, P., J. Kazil, T. Yamaguchi, P.K. Quinn, and G. Feingold

J. Adv. Model. Earth Syst., 15(1), e2022MS003228, doi: 10.1029/2022MS003228, View open access article online at AGU/Wiley (external link) (2023)

A shallow cumulus cloud transition from a sugar to flower type of organization occurred under a layer of mineral dust on 2 February 2020, during the multinational Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC) and the Elucidating the Role of Clouds-Circulation Coupling in Climate (EUREC4A) campaigns. Lagrangian large eddy simulations following an airmass trajectory along the tradewinds are used to explore radiative impacts of the diel 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 Fifth Generation and based on aerosol measurements from the U.S. Ronald H. Brown Research Vessel and the French ATR-42 Research Aircraft during the field campaigns. A 12-hr delay in the diel cycle accelerates the S2F transition at night, leading to more cloud liquid water and precipitation. 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, generating new flower clouds. The modulation of the surface radiative budget by free-tropospheric mineral dust poses a less dramatic effect on the S2F transition. Mineral dust releases longwave radiation, reducing the cloud amount at night, and absorbs shortwave radiation during the day, cooling the boundary-layer temperature and increasing the overall cloud amount. Cloud-top radiative heating because of more clouds strengthens the mesoscale organization, enlarging the aggregate areas, and increasing the cloud amount further.

Plain Language Summary. During a joint field study called ATOMIC and EUREC4A, a transition between two cloud systems took place during the day on 2 February 2020. Very small and shallow clouds called “sugar” transitioned into deeper and wider cloud aggregates called “flowers.” A dense mineral-dust layer was also observed above the tradewind cumulus cloud field, likely modulating the radiation interacting with the clouds. High-resolution simulations are applied to help understand the same cloud transition as if it had taken place at night, and to explore the impacts of mineral dust on the transition. A 12-hr delay in the daily cycle such that the transition occurs at night affects the cloud transition more significantly than when the transition occurs during the day under a layer of mineral dust. The cloud transition that occurs at night produces more clouds and rain. The mineral dust blocks the solar radiation and cools the air beneath during the day, but does not change the cloud and rain amount as much.