1 Introduction
The tendency for convection to organize from isolated convective
updrafts into spatially coherent clusters has long been recognized in
numerical model simulations (Bretherton et al., 2005; Held et al., 1993;
Tompkins, 2001). This aggregation of convection occurs even in the
presence of uniform boundary conditions (self-organization) and not only
impacts the intensity of precipitation events (Bao & Sherwood, 2019),
but also modulates the larger-scale thermodynamic and radiative
properties of the tropics (e.g., Bony et al. (2020)). Increased
aggregation is associated with increased spatial variance of moisture;
dry regions become drier and moist regions become moister (Dai & Soden,
2020). The increased spatial variance of moisture directly impacts both
the intensity of precipitating systems and the larger-scale radiative
fluxes (Bony et al., 2020; Bony et al., 2016; Bretherton et al., 2005;
Wing et al., 2020).
The net effect of more aggregated convection is to dry the tropical free
troposphere, particularly in cloud-free regions, resulting in a net loss
of longwave radiation to space (Bretherton et al., 2005). This
large-scale influence of aggregation has been proposed as a potential
thermostat that may regulate the sensitivity of the tropics to radiative
forcing (Mauritsen & Stevens, 2015). However, the interaction with
radiation involves feedbacks. The amplification and expansion of dry
regions is believed to play a key role in triggering aggregation, and
radiative feedbacks involving both clouds and water vapor are essential
for maintaining the aggregation in idealized models (Wing et al., 2017).
Additionally, the relative importance of clear versus cloudy sky
radiative processes is not well understood, as is the contributions of
shortwave (SW) versus longwave (LW) radiation.
There is a long history of observational studies of convective
organization (see Holloway et al. (2017) for a recent review). Most
recently, Bony et al. (2020) found that increased aggregation in the
tropics is associated with a reduction in high cloud cover, a drier free
troposphere in the non-convective environment and increased emission of
infrared radiation to space. These features lead to a net radiative
cooling of the tropics.
Precipitation extremes are largely controlled by the amount of moisture
present in the atmosphere (Allen & Ingram, 2002; Pall et al., 2007;
Trenberth, 1999) and also atmospheric convergence (Liu et al., 2020),
whereas the global-mean precipitation is constrained by the global-mean
radiative cooling. From an energy balance perspective, latent heating
into the atmosphere must be balanced by atmospheric radiative cooling
given that heat capacity of the atmosphere is negligible (Allen &
Ingram, 2002). In model simulations, the change in extreme precipitation
in response to increases in greenhouse gases is found to depend on the
magnitude of warming (Pendergrass et al., 2015) and the change in
convective organization (Muller, 2013; Pendergrass et al., 2016). In
observations, the degree of aggregation at regional scales has also been
connected with extreme rainfall events (Dai & Soden, 2020).
Climate model projections indicate that the thermodynamic constraint
based on the Clausius-Clapeyron relation is a good predictor for extreme
precipitation changes in a warmer climate for regions where changes in
the circulation are small (Pall et al., 2007). However, this may not be
the case for regions with large changes in the atmospheric circulation,
such as the tropics (Emori & Brown, 2005; Vecchi & Soden, 2007) or for
tropical cyclones (Knutson et al., 2013; Liu et al., 2020). Indeed,
several studies suggest that the sensitivity of tropical precipitation
extremes is substantially larger than that predicted from the
Clausius-Clapeyron relationship (Allan & Soden, 2008; Norris et al.,
2019; O’gorman, 2015; Westra et al., 2013).
Among different factors contributing to radiative cooling of the
atmosphere, cloud radiative effects (CRE) have been received
considerable attentions. The Clouds On-Off Klimate Intercomparison
Experiment (COOKIE) is designed to investigate the role of CRE in the
climate system (Stevens et al., 2012). This project compares simulations
with clouds that are transparent to radiation (“clouds-off”) and those
including CRE (“clouds-on”). Using output from the COOKIE project,
Fermepin and Bony (2014) showed that low cloud radiative effects
increase tropical precipitation and strengthen winds near ocean surface.
However, Li et al. (2015) found that CRE can decrease precipitation in
the tropics but increase it at middle-to-high latitudes. Recently,
Medeiros et al. (2021) reported that extreme precipitation over tropical
ocean is strengthened by CRE. While mean precipitation and large-scale
circulation can be affected by removing CRE, it is unclear how
precipitation, especially extreme precipitation, will respond if
radiative-convective interactions are disabled.
In idealized models, studies found that interactive radiation is key to
convective aggregation over a limited domain (Muller & Bony, 2015;
Muller & Held, 2012; Wing & Emanuel, 2014; Yang, 2018). In simulations
of radiative convective equilibrium without rotation, Bao and Sherwood
(2019) found that extreme daily precipitation gets stronger when
convection is more aggregated with fully interactive radiation. Most
research on the coupling of clouds and radiation on smaller time/space
scales has focused on the impact of radiative feedbacks on convective
organization. These studies typically have been performed using high
resolution, radiative equilibrium simulations under very idealized
settings. This makes it difficult to ascertain the importance of
cloud-circulation feedbacks under more realistic situations (e.g., in
the presence of the SST gradients or wind shears that are present in our
planet). It also makes it more challenging to evaluate the model
simulations with observations.
In this study, we examine the impact of radiative interactions on the
spatial organization of convection and extreme precipitation events
under realistic boundary conditions. Instead of completely removing the
CRE, which dramatically changes the mean circulation and rainfall in a
model (e.g., as is done in COOKIE), synoptic-scale radiative
interactions are suppressed by prescribing radiative cooling rates using
their monthly climatological values. This enables us to examine the
behavior of two versions of a model with nearly identical large-scale
circulations but with differing degrees of convective organization. Our
simulations highlight the role of synoptic-scale radiative coupling in
enhancing convective aggregation and extreme precipitation by increasing
the horizontal gradient of radiative cooling which provides an
upgradient transport of energy from dry to moist regions.