PhD Thesis Presentation - Cloud Radiative Effects on Tropical Dynamics: Convection, Intraseasonal Variability, and Emergent Constraints on Extreme Precipitation

The cloud-radiative effect (CRE) plays a fundamental role in modulating the climate mean state, large-scale circulation, precipitation, and variability, particularly over the tropics. Its representation and mechanisms are worth further investigation. This thesis summarizes three completed studies related to cloud-radiative processes: (i) how CRE influences the climate mean state and the Madden-Julian Oscillation (MJO); (ii) how CRE shapes tropical precipitation and circulation; and (iii) how the tropical radiation-precipitation (R-P) relation, a CRE-relevant concept, constrains the projections of tropical extreme precipitation.

First, CESM2 aquaplanet experiments show that intensified CRE changes the tropical mean state via strengthened cloud-radiation-circulation feedback, leading to an environment favorable for convection but unfavorable for its eastward propagation. The altered mean state indirectly affects the MJO, while longwave cloud-radiative interactions directly influence its maintenance and propagation, strengthening the MJO and slowing its eastward movement.

Second, controlling CRE intensity in CESM2 experiments reveals opposite responses of tropical mean precipitation and the Hadley cell between aquaplanet and zonally asymmetric simulations. Planetary-scale zonal convective aggregation and the coupling between the Hadley cell and Walker circulation are highlighted as key mechanisms driving these opposite responses.

Third, the nonlinear tropical R-P relation was examined using observations and 21 CMIP6 models, and was quantified by a CRE parameter. The R-P relation, reflecting the CRE, can be used to constrain projections of tropical extreme precipitation, indicating that most climate models underestimate future extreme precipitation while overestimating its fractional change.

Overall, these studies provide a systematic understanding of CRE's role in the tropical climate system, deepen our knowledge of tropical atmospheric dynamics, highlight important model biases associated with CRE representation, and offer valuable guidance for improving climate model development and supporting climate adaptation strategies.