Understanding the Role of Radiative Forcing and Cloud-Circulation Feedbacks on Spatial Rainfall Shifts in CMIP6

A robust prediction of all climate models is for wet regions to become wetter and dry
regions to become drier in response to increased greenhouse gases. The physical
processes that drive this response arise from increased water vapor and are generally
considered to be well understood. In contrast, the processes that govern spatial shifts of
rain belts are less well understood, despite the fact that such changes also have profound
societal consequences. This is particularly relevant in the tropics and sub-tropics due to
their large spatial gradients between wet and dry regions.

Recent research has highlighted the importance of radiative forcing from both greenhouse
gases and aerosols in driving large-scale shifts in rainfall patterns through their influence
on the atmospheric circulation. The instantaneous radiative forcing from greenhouse
gases has been shown to drive large-scale changes in the monsoonal circulations.
Likewise, the strong hemispheric asymmetry in aerosol radiative forcing has been shown
to drive large-scale changes in the meridional circulation. Both of these radiatively-forced
circulation changes have direct impacts in modulating the regional distribution of rainfall.
Unfortunately, recent studies have highlighted significant biases in model calculations of
radiative forcing under identical emission scenarios. Such biases remain largely
undocumented since radiative forcing is rarely calculated or archived, despite its
fundamental role in determining the forced response to anthropogenic emissions.

Due to their strong influence on atmospheric heating rates, clouds play a key role in
regulating the large-scale circulation of the atmosphere and therefore the regional
distribution, frequency and intensity of rainfall. Recent studies suggest that regional
shifts in rainfall may also be amplified through circulation-driven cloud feedbacks that
respond to, and enhance, the radiatively-forced rainfall change. The selection of “Clouds,
Circulation, and Climate Sensitivity” as one of the WCRP Grand Challenges underscores
both the importance and current lack of understanding regarding these processes.

This proposal seeks to exploit model simulations from CMIP6 along with idealized
forcing scenarios from RFMIP and PDRMIP to better quantify and understand the role
of instantaneous radiative forcing and cloud-circulation feedbacks in modulating shifts in
the spatial distribution and intensity of rainfall.

The primary objectives of this proposal are to:
i) Develop and apply process-oriented metrics to quantify and evaluate model simulations
of instantaneous radiative forcing and cloud-circulation feedbacks;

ii) Quantify the impacts of radiatively-forced circulation changes on the regional
distribution of rainfall;

iii) Use historical observations in conjunction with spatial fingerprinting techniques to
better constrain the representation of these radiative and cloud processes in models.
In accomplishing these objectives, we will directly contribute to the NOAA MAPP goal
of developing and applying process-oriented metrics to better understand sources of
model bias involving “cloud and radiative processes” and their impact on “weather and
climate extremes”.

In accomplishing these objectives, we will directly contribute to the NOAA MAPP goal
of developing and applying process-oriented metrics to better understand sources of
model bias involving “cloud and radiative processes” and their impact on “weather and
climate extremes”.

Climate Risk Area: Water Resources