Flower trade‐wind clouds are shallow mesoscale convective systems

Abstract

Flower clouds are trade‐wind shallow cumuli, with tops reaching 3 km altitude, organised into 100‐km wide clusters. They are widespread over the subtropics and associated with the strongest cloud radiative effect among trade‐wind cumuli mesoscale organisations. In the context of large uncertainty in climate projections due to the representation of shallow clouds, major knowledge gaps remain about the global impact of mesoscale organisations and the local processes driving them. Here, the processes governing the flower organisation are investigated based on the case study of February 2, 2020 from the Elucidate the Couplings Between Clouds, Convection, and Circulation (EUREC4$$ {}^4 $$A) campaign, east of Barbados. One flower cloud is simulated with a large‐eddy simulation (LES), using the Meso‐NH model at 100‐m horizontal grid spacing, and validated extensively with high‐resolution observations from the High Altitude and Long‐range Research Aircraft (HALO), dropsondes, and satellite measurements. The cloud‐top altitudes exhibit a trimodal distribution. The processes shaping flower clouds are wide cold pools and cloudy updrafts organised in one large arc at the western edge. These updrafts are responsible for the highest cloud tops and drive most of the vertical turbulent fluxes of sensible heat, humidity, and momentum. A mesoscale circulation takes place at the scale of the flower clouds and makes them very similar to deep mesoscale convective systems.

Process‐oriented evaluation of the oversea AROME configuration: Focus on the representation of cloud organisation

“…In agreement with Dauhut et al . (2022), the four case‐studies highlight that mesoscale vertical motions represent the imprint of the cloud systems, but the role of vertical velocities at larger scale (hundreds of km) on the cloud organisation remains an open question.…”

“…It is characterized in the vicinity of the HALO circle by the occurrence of Flowers‐structure (FL in the following) associated with marked cold pools. The physical mechanisms implied in such a cloud pattern are fully documented by recent studies based on LES (Narenpitak et al ., 2021; Dauhut et al ., 2022). For the environment E4, the 9th February at 1200 UTC (Figure 8, yellow dot) is selected.…”

“…LES would be a suitable tool to disentangle the mechanisms at play and their key spatio‐temporal scales. In particular, the question is open as to whether the positive anomalies in PW are due to enhanced boundary‐layer convective activity, to shallow convection outflow (Bretherton and Blossey, 2017; Narenpitak et al ., 2021; Dauhut et al ., 2022), or former cloud clusters which dissipate.…”

High-spatial-resolution retrieval of cloud droplet size distribution from polarized observations of the cloudbow

“…In the following, two case studies from 2 February 2020 are presented. On this day, the observed clouds had a clear flower organization (Stevens et al, 2020;Dauhut et al, 2022). Such cloud fields are characterized by shallow trade-wind cumuli organized into large stratiform clusters (20-200 km) with high rain rates and surrounded by a large clear-sky region (Stevens et al, 2020;.…”

Water isotopic characterisation of the cloud-circulation coupling in the North Atlantic trades. Part 1: A process-oriented evaluation of COSMOiso simulations with EUREC4A observations

“…Several studies have evaluated the performance of models in the lower troposphere of the trade-wind region during EUREC 4 A. Beucher et al (2022) tested the oversea configuration of the French regional model AROME, called AROME-OM (its output used by Dauhut et al, 2022, as initial and lateral boundary conditions for large eddy simulations). In their model setup, deep convection is treated explicitly, but shallow convection is parameterised.…”