Investigating the Role of Cloud‐Radiation Interactions in Subseasonal Tropical Disturbances

Abstract

Cloud locking, a method that prescribes cloud properties for radiative tendency calculations, is traditionally used to explore climate feedbacks, but here is applied novelly to investigate cloud-radiation interaction (CRI) impacts on subseasonal tropical variability. The approach minimizes mean state differences between control (CRI active) and experimental simulations (CRI disabled) of the Community Earth System Model. Disabling CRI weakens amplitudes of the Madden-Julian oscillation (MJO) by 10-35% and equatorial Rossby waves by 10-30% yet strengthens Kelvin waves by 10-40%. MJO weakening results from suppressed radiation-convection positive feedbacks and increased gross moist stability. Kelvin waves strengthen from reduced convective inhibition and reduced radiative damping on temperature variance. The results are compared to a recently proposed theory that describes a continuum of tropical disturbances. MJO survival, when its primary maintenance mechanism (CRI) is eliminated, stresses the importance of advection and surface flux processes. Plain Language SummaryTropical cloud systems exist on many scales, from squall lines to continent-sized cloud systems. Solar and thermal radiations interact with these cloudy "disturbances" in ways that can favor further development of the disturbance. The exact nature of these interactions-along with associated changes to other disturbance properties such as winds, temperature, and surface evaporation-are inadequately understood and therefore cause inaccuracies in weather and climate prediction models. We investigate the impact of cloud-radiation interactions by preventing clouds from interacting with radiative heating in our model. In response, certain types of tropical cloud disturbance become weaker while others strengthen. These results highlight the distinct roles of cloud feedbacks for different tropical phenomena and demonstrate how a poor representation of cloud processes might degrade the accuracy of weather and climate models.

Investigating the impact of cloud-radiative feedbacks on tropical precipitation extremes

“…Another approach is to lock the CREs by providing cloud fields to the radiative transfer calculation that are independent of the flow field. By using cloud fields from a separate simulation with a given model, this approach has been shown to maintain the mean climate very well 8,12,47,48 . The approach maintains the spatial, seasonal, and diurnal structure of CRE, but instantaneously removes the correlation between the atmospheric state and the clouds as seen by the radiative transfer.…”

“…The cloud-locking approach has been applied to CESM2 in both coupled and prescribed-SST configurations. The simulations have been described in detail in previous studies 12,48 ; we denote them using c (coupled) and f (fixed SST) modifying either "control" or "lock". Relevant to the present application, it has been shown that cloud-radiative feedbacks alter tropical variability 12 .…”

“…The simulations have been described in detail in previous studies 12,48 ; we denote them using c (coupled) and f (fixed SST) modifying either "control" or "lock". Relevant to the present application, it has been shown that cloud-radiative feedbacks alter tropical variability 12 . We extend that analysis by considering the impact of cloud-radiative feedbacks on extreme precipitation.…”

“…A spate of recent studies has shown that CREs also impact aspects of climate variability. Timescales from decadal 7,8 to interannual 9,10 to subseasonal 11,12 have all been studied. At the shorter timescales, emphasis has been given to the potential role of cloud-radiative feedbacks on the maintenance of the Madden-Julian oscillation (MJO) in the Indo-Pacific 13 .…”

“…Convective aggregation is thought to be closely related to observed forms of organized convection, such as moisture modes like the MJO and tropical depressions 26,27 . Recent work has also shown that in climate models, CREs impact aspects of organized convection 12 and that the representation of extreme precipitation depends on the degree of convective organization 34 . If CREs influence the convective organization, and if the convective organization is thought to be connected to extreme precipitation, it seems natural to ask how cloud-radiative feedbacks influence extreme precipitation.…”