The influence of convective organization on tropical free-tropospheric temperature

Idealised simulations under the assumption of radiative-convective equilibrium (RCE) demonstrate that the spatial aggregation of convection can significantly influence the domain-mean climate. One notable implication is the warming of the free troposphere with increased convective organisation, resulting in greater atmospheric stability. However, atmospheric temperature is also closely tied to surface temperature in regions of deep convection. The interplay between convective organisation and surface temperature in modulating free-tropospheric temperature remains unclear.

To address this question, we conduct idealised cloud-resolving simulations incorporating a diurnal cycle and prescribed sea surface temperatures (SSTs). The SST is spatially fixed with temperature gradients: a warmer ocean hotspot surrounded by cooler ocean regions. We vary the temperature of the ocean hotspot to modify the temperature gradients between the hotspot and the surrounding oceans. Additionally, we introduce an island away from the hotspot by coupling the atmosphere to a 0.05-meter deep slab ocean model. The latent heat flux calculation in the slab ocean model is rescaled by a factor of 0.1 to represent the reduced latent heat fluxes typically observed over land. The presence of temperature gradients enables continuous convection over the hotspot, whereas convection over land occurs only in the afternoon, after being heated by incoming radiation. Consequently, the model successfully simulates a diurnal cycle, characterised by enhanced precipitation over land in the late afternoon and early evening, and increased precipitation over the ocean in the early morning.

We find that daily variations in atmospheric temperature are closely related to the daily evolution of convective organisation. Additionally, enhanced temperature gradients between the hotspot and the surrounding ocean further promote convective organisation. Consequently, convection is most organised, and the free troposphere is warmest, in the simulation with the highest hotspot temperature (and the largest temperature gradients). We test the modelling results with ERA5 reanalysis data and confirm that the degree of organisation plays a crucial role in modulating the tropical free-tropospheric temperature. However, organisation appears to be primarily important for daily variations in atmospheric temperature on timescales shorter than 20 days, while surface temperature in the deep convective region becomes more significant on longer timescales (greater than 20 days).