Lecture 9 of the
ICTP/UniTN/UniAQ Joint International Seminar Series
on Weather and Climate: From Fundamentals to Applications
Register in advance for this meeting:
Climate model simulations are known to be sensitive to parameter choices in the sub grid-scale representation of deep convection, as deep convection plays a critical role in the transport of heat and momentum globally. Over the years, it has also become evident that the intermodel spread in the warming response to anthropogenic forcing is largely driven by uncertainties in the magnitude of the cloud feedback in the tropics, specifically the low cloud feedback. In this talk, I will discuss how parameterization differences among models and changes to deep convection in response to anthropogenic warming are likely contributing significantly to the intermodel spread in the tropical cloud feedback. I will present evidence of two physical pathways linking deep convection to low clouds and their response to anthropogenic forcing: the "Radiation-Subsidence" Pathway and the "Stability" Pathway. In a warmer world, the tropical overturning circulation is projected to weaken. We find that the overturning circulation does not weaken as much in climate models with more stable tropospheres, which ultimately leads to a more positive low cloud feedback (Stability Pathway). Differences in deep convective parameterization modifying deep convection onset thresholds – such as the fractional rate of entrainment into convective updrafts – can contribute significantly to this intermodel spread in static stability. Additionally, changes to the total area occupied by deep convection in the tropics modify the high cloud fraction, which is linked to subsidence changes and the low cloud feedback (Radiation-Subsidence Pathway). Results from both the Coupled Model Intercomparison Project (CMIP6) and a perturbation physics ensemble in the Community Earth System model (NCAR CESM) will be presented and discussed.