Numerous studies using both global and regional models of the atmosphere have found daunting sensitivities of the structure and dynamics of the intertropical convergence zone (ITCZ) to the representations of unresolved processes, particularly the convective parameterization (CP). Evaluations of these results by comparison to high-resolution simulations with explicit convection have been rather limited, due to the large computational burden of using grid spacings less than 10 km over large domains representative of the Earth's tropics. This study introduces a framework that allows the use of cloud-resolving grid spacings over the tropics and larger spacings over remainder of the domain. The Weather Research and Forecasting (WRF) model is used in an ''aquachannel'' beta-plane configuration, zonally periodic with length equal to that of the real equator. This model reproduces the general circulation and eddy statistics of similarly configured aquaplanet models. A channel shortened to one third the length of the equator (the ''aquapatch'') also reproduces the zonal-mean circulations and eddies. Finally, nested grids embedded in the aquapatch are used to simulate tropical convection with 5.15 km resolution. The nested 5.15 km simulations produce broader and lighter rainfall distributions, making single ITCZs wider and smoothing out double ITCZ structures. They also show quite different rainfall production rates for atmospheric parameters such as convective available potential energy (CAPE) and column relative humidity (CRH). The apparent reason for these differences is that the higher resolution allows for the representation of squall lines and associated cold pools that propagate meridionally, redistributing rainfall away from the ITCZ.

This second part of a two‐part study uses Weather Research and Forecasting simulations with aquachannel and aquapatch domains to investigate the time evolution of convectively coupled Kelvin waves (CCKWs). Power spectra, filtering, and compositing are combined with object‐tracking methods to assess the structure and phase speed propagation of CCKWs during their strengthening, mature, and decaying phases. In this regard, we introduce an innovative approach to more closely investigate the wave (Kelvin) versus entity (super cloud cluster or “SCC”) dualism. In general, the composite CCKW structures represent a dynamical response to the organized convective activity. However, pressure and thermodynamic fields in the boundary layer behave differently. Further analysis of the time evolution of pressure and low‐level moist static energy finds that these fields propagate eastward as a “moist” Kelvin wave (MKW), faster than the envelope of organized convection or SCC. When the separation is sufficiently large the SCC dissipates, and a new SCC generates to the east, in the region of strongest negative pressure perturbations. We revisit the concept itself of the “coupling” between convection and dynamics, and we also propose a conceptual model for CCKWs, with a clear distinction between the SCC and the MKW components.

Clouds are one of the most influential components of Earth’s climate system. Specifically, the midlatitude clouds play a vital role in shaping Earth’s albedo. This study investigates the connection between baroclinic activity, which dominates the midlatitude climate, and cloud-albedo and how it relates to Earth’s existing hemispheric albedo symmetry. We show that baroclinic activity and cloud-albedo are highly correlated. By using Lagrangian tracking of cyclones and anticyclones and analyzing their individual cloud properties at different vertical levels, we explain why their cloud-albedo increases monotonically with intensity. We find that while for anticyclones, the relation between strength and cloudiness is mostly linear, for cyclones, in which clouds are more prevalent, the relation saturates with strength. Using the cloud-albedo strength relationships and the climatology of baroclinic activity, we demonstrate that the observed hemispheric difference in cloud-albedo is well explained by the difference in the population of cyclones and anticyclones, which counter-balances the difference in clear-sky albedo. Finally, we discuss the robustness of the hemispheric albedo symmetry in the future climate. Seemingly, the symmetry should break, as the northern hemisphere’s storm track response differs from that of the southern hemisphere due to Arctic amplification. However, we show that the saturation of the cloud response to storm intensity implies that the increase in the skewness of the southern hemisphere storm distribution toward strong storms will decrease future cloud-albedo in the southern hemisphere. This complex response explains how albedo symmetry might persist even with the predicted asymmetric hemispheric change in baroclinicity under climate change.

Convectively coupled Kelvin waves (CCKWs) represent a significant contribution to the total variability of the Intertropical Convergence Zone (ITCZ). This study analyzes the structure and propagation of CCKWs simulated by the Weather Research and Forecasting (WRF) model using two types of idealized domains. These are the “aquachannel,” a flat rectangle on a beta plane with zonally periodic boundary conditions and length equal to the Earth's circumference at the equator, and the “aquapatch,” a square domain with zonal extent equal to one third of the aquachannel's length. A series of simulations are performed, including a doubly nested aquapatch, in which convection is solved explicitly along the equator. The model intercomparison is carried out throughout the use of several techniques such as power spectra, filtering, wave tracking, and compositing, and it is extended to some simulations from the Aquaplanet Experiment (APE). Results show that despite the equatorial superrotation bias produced by the WRF simulations, the CCKWs simulated with this model propagate with similar phase speeds (relative to the low‐level mean flow) as the corresponding waves from the APE simulations. Horizontal and vertical structures of the CCKWs simulated with aquachannels are also in overall good agreement with those from aquaplanet simulations and observations, although there is a distortion of the zonal extent of anomalies when the shorter aquapatch is used.

Clouds are one of the most fascinating, important, and complex components of Earth's climate system (Siebesma et al., 2020). Despite their importance, theoretical understanding of what controls planetary-wide cloudiness is largely absent. For example, while we have a good understanding of how clouds form and interact with radiation (Cotton et al., 2014;Houze, 2014;Siebesma et al., 2020), it is difficult to use these theories to make claims about global cloudiness. Earth System Models (ESMs) and other bottom-up approaches do couple simple models of cloud formation to the global circulation. However, so far they have not been proven effective in constraining global cloudiness variability (Sherwood et al., 2020;Zelinka et al., 2020). This makes it difficult to transparently establish links between variability in global cloudiness and Earth's energy balance, or how such a link would change in a changing climate.Conceptual models could be useful in elucidating how clouds, circulation, and energy balance, are tied together. Existing theoretical work has linked cloudiness to circulation, and most examples of such work focus on particular circulation systems, like the tropical overturning circulation (Betts & Ridgway, 1989;Pierrehumbert & Swanson, 1995), or the Walker cell (Peters & Bretherton, 2005), or individual cyclones (Carlson, 1980). What is missing is a conceptual framework that both closes the top-of-atmosphere energy budget (and hence by necessity considers the planet as a whole), but also includes clouds. A suitable candidate for such a framework would be an energy balance model (Budyko, 1969;Ghil, 1981;North & Kim, 2017;Sellers, 1969) that explicitly represents dynamic cloudiness, likely as an implicit function of circulation measures or other state variables.

A wide range of the observed variability in the ITCZ is frequently explained in terms of equatorially trapped modes arising from Matsuno’s linear shallow-water model. Here, a series of zonally constant, meridionally symmetric aquachannel WRF simulations are used to study the propagation of tropical cloud clusters (CCs; patches of deep cloudiness and precipitation) in association with eastward-moving super cloud clusters (SCCs), also called convectively coupled Kelvin waves (CCKWs). Two independent but complementary methods are used: the first, from a local approach, involves a CC-tracking algorithm, while the second uses Lagrangian trajectories in a nonlocal framework. We show that the large-scale flow in low to midlevels advects the CCs either eastward or westward depending on model climatology, proximity to the CCKW axis, and latitude. Moreover, for most analyzed cases, sequences of CCs oscillate, describing qualitatively sinusoidal-like paths in longitude–time space, although with sharp transitions from westward to eastward motion due to westerly wind burst activity associated with the CCKWs. We also find that the discrete precipitation elements (CCs) are embedded in continuous tracks of positive moisture anomalies, which are parallel to the Lagrangian trajectories themselves. A conceptual model of the nonlinear SCC–CC interaction is presented.

The Northern and Southern Hemispheres reflect on average almost equal amounts of sunlight due to compensating hemispheric asymmetries in clear-sky and cloud albedo. Recent work indicates that the cloud albedo asymmetry is largely due to clouds in extratropical oceanic regions. Here, we investigate the proximate causes of this extratropical cloud albedo asymmetry using a cloud-controlling factor (CCF) approach. We develop a simple index that measures the skill of CCFs, either individually or in combination, in predicting the asymmetry. The index captures the contribution to the asymmetry due to interhemispheric differences in the probability distribution function of daily CCF values. Cloud albedo is quantified using daily MODIS satellite retrievals, and is related to range of CCFs derived from the ERA5 reanalysis product. We find that sea-surface temperature is the CCF that individually explains the largest fraction of the asymmetry, followed by surface wind. The asymmetry is predominantly due to low clouds, and our results are consistent with prior local-scale modelling work showing that marine boundary-layer clouds become thicker and more extensive as surface wind increases and surface temperature cools. The asymmetry is consistent with large-scale control of storm track intensity and surface winds by meridional temperature gradients: persistently cold and windy conditions in the Southern Hemisphere keep cloud albedo high year-round. Our results have important implications for global-scale cloud feedbacks and contribute to efforts to develop a theory for planetary albedo and its symmetry.

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