The Sugar-to-Flower Shallow Cumulus Transition Under the Influences of
Diurnal Cycle and Free-Tropospheric Mineral Dust
Abstract
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.