Convective Self-Aggregation over an Interactive Sea Surface with Diurnal Radiative Forcing

Abstract:

The spontaneous organization of mesoscale convection over the seas has been recognized as having a profound impact on the atmosphere's radiative budget in idealized simulations. As clusters of deep convection form, the domain-mean atmosphere dries while moist regions experience increased extreme precipitation events. The forces and feedback driving aggregation have been scrutinized over the past decade in an effort to predict its occurrence. Particularly, surface fluxes have been identified as a key mechanism in two ways.

First, when the sea surface temperature is allowed to react to radiative imbalances (as caused by inhomogeneous moisture distributions), shaded areas cool down which reduces the evaporation and convective rates. This results in a negative feedback which directly depends on the surface's heat capacity. Further, mimicking a radiative diurnal cycle was shown in recent studies to affect the organization of convection as increased temperature fluctuations caused spontaneous cloud clustering. Second, surface heat fluxes being directly proportional to the surface tangential velocity, areas of large sweeping circulation impact ground temperature. Notably, cold pool's gust fronts are known to redistribute moisture away from precipitating clouds, acting as a negative feedback on aggregation.

A key aspect linking the aforementioned two mechanisms has received little attention to this day. The diurnal cycle forms a thick bulk layer of oscillating sea temperature but only its top skin temperature determines the outgoing radiative cooling. Owing to its thickness, it exhibits a strong response to the diurnal cycle and is particularly sensitive to wind speeds. Indeed, strong winds can suppress the skin diurnal oscillation altogether, while no winds could result in large temperature fluctuations similar to that over land. This enhanced surface feedback is often discarded in slab ocean models which consider a single thick bulk layer. Therefore, we propose in this work to investigate by means of idealized cloud resolving simulations the combined effects of diurnal cycle and fully-coupled surface fluxes with skin/bulk layers energy exchanges on the process of convective aggregation.