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 June, 26th 2020

Title: « The Impact of Sea Surface Temperature on the Aggregation of Deep Convective Clouds »

Short abstract : This study investigates the impact of Sea Surface Temperature (SST) heterogeneities on the aggregation of convective clouds, using 3D cloud-resolving simulations of radiative-convective equilibrium.
The SST heterogeneities are either imposed or interactive. In imposed cases, a spatiotemporally fixed warm SST anomaly (Hot-spot) with radius R and temperature anomaly dT is introduced at the center of the domain. The hot-spot significantly accelerates aggregation and extends the range of SSTs for which aggregation occurs. A convective instability over the hot-spot leads to stronger convection and generates a large-scale circulation, forcing subsidence drying outside the hot-spot.  A large/warm hot-spot drives the aggregation even without radiative feedbacks. In cases where SST heterogeneities are interactive, the ocean is modeled as one layer slab ocean, with a constant mean but spatially varying temperature. The interactive SST decelerates the aggregation, especially with shallower slab. SST anomaly in dry regions is positive at first, thus opposing the diverging shallow circulation known to favor self-aggregation. With further drying, it becomes negative and favors the shallow circulation. The shallow circulation is found to be well correlated with the aggregation speed. It can be linked to a positive surface pressure anomaly, itself the consequence of SST anomalies and boundary layer radiative cooling. Including a diurnal cycle in simulations with interactive SST results in faster triggering of dry patches and accelerates the aggregation for shallow slabs, thus reducing the dependency of aggregation on slab depth.

Francoise GUICHARD (CNRM, Météo France, Toulouse)       Reviewer
Adrian TOMPKINS (ICTP, Trieste)                         Reviewer
Sandrine BONY (LMD, Sorbonne Université)                Examiner
Christopher HOLLOWAY (University of Reading, Reading)   Examiner
Jean-Pierre CHABOUREAU (Université de Toulouse)         Examiner
Caroline MULLER (LMD, ENS)                              Director 
Jean-Philippe DUVEL (LMD, ENS)                          Co-director
Fabio D’ANDREA (LMD, ENS)                               Co-director