A long-standing and pervasive problem within the modelling community is the proper representation of cloud albedo over the Southern Hemisphere (SH) oceanic region. Errors persist despite the extensive evidence that these are related to the unique microphysical characteristics of the austral clouds. In this study we investigate additional causes of cloud albedo biases over the 50˚–65˚ oceanic band using CMIP6 simulations and a cloud-controlling factor (CCF) approach on daily timescales. We gain further insight by replicating our method over the equivalent oceanic region in the Northern Hemisphere (NH).Cloud albedo, computed from upwelling and downwelling shortwave radiation at surface and top of the atmosphere, is averaged into bins of vertical velocity, surface wind, and sea-surface temperature. The performance of fifteen models in both atmospheric-only and ocean-coupled configurations is evaluated against CERES satellite retrievals in combination with ERA5 reanalysis for the 2000–2014 period.When averaging cloud albedo by vertical velocity bins, we find that shallow boundary-layer (deep convective) clouds are consistently underpredicted (overpredicted) over the high-latitude oceans of the SH. We repeat the method for the 50˚–65˚ band in the North Atlantic and Pacific oceans and find that similar compensating errors exist.Another important result is that the SH cloud biases occur for sea-surface temperatures below 4°C. We show that a connection exists between this empirical finding and the biases as determined from microphysical effects, i.e.: a deficit of cloud albedo is due to models producing glaciated rather than supercooled liquid water clouds. Our CCF method allow us to see that in such cases, models tend to simulate NH clouds for the SH.We also find that the positive sign of the cloud albedo hemispheric asymmetry (SH-NH difference over the 50°–65° band) is consistently predicted by nearly all models, many of which also predict a similar magnitude to observations. However, this is a consequence of compensating errors as individually most models tend to either overpredict or underpredict cloud albedo in both hemispheres.

The 50˚–65˚ latitude band exhibits the largest hemispheric asymmetry of cloud albedo over the oceans as well as the largest negative Southern Ocean (SO) cloud albedo biases in CMIP models. In this study, we show that cloud albedo regressed against sea-surface temperatures (SSTs) highlights essential differences between the observed Northern and Southern hemisphere climatologies, and between the SO’s simulated and observed albedos. The threshold 4˚–5˚C stands out as a regime separator in both comparisons.By linking our empirical findings with the extensive evidence that model errors are related to the unique microphysical characteristics of the SO environment, we hypothesize that cloud albedo as a function of SST may act as a predictor of the presence/absence of supercooled liquid water cloud content.Using satellite-retrieved cloud optical thickness (COT) and cloud top temperature (CTT), we verify that a regime separation of COT as a function of CTT exists between the Northern and Southern hemispheres (for CTT< -12˚C), which becomes more noticeable under midlevel subsidence conditions (i.e., low, boundary layer clouds).Our simple and straightforward method using macrophysical variables can be easily applied in model evaluation with an insight in microphysics performance, especially given the scarcity of archived cloud-specific variables by the participating CMIP models. For example, it is well known that models tend to produce glaciated rather than supercooled liquid water clouds, and we show that in many cases models are simulating Northern Hemisphere clouds for the SO. We also detect that some of the CMIP models produce the right climatological cloud albedo over the SO but for the wrong reasons.

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 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 modeling 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.

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.

&lt;p&gt;Recent work has shown that the hemispheric asymmetry in cloud albedo is maximized over extratropical oceans: the Southern Ocean exhibits greater climatological cloud albedo than its northern counterpart. We investigate the dynamical causes of such asymmetry by evaluating how albedo responds to a series of cloud controlling factors, namely: sea surface temperature (SST), pressure velocity at 500mb (&amp;#969;&lt;sub&gt;500&lt;/sub&gt;), Estimated Inversion Strength (EIS), Marine Cold Air Outbreak (MCAO) index, SST-T&lt;sub&gt;2m&lt;/sub&gt; (&amp;#916;T&lt;sub&gt;sfc&lt;/sub&gt;), and surface wind (V&lt;sub&gt;sfc&lt;/sub&gt;). A cloud albedo parameterization applied to MODIS optical thickness and fractional cloud cover is used in conjunction with ERA-Interim reanalysis products over oceanic points in the 50&amp;#176;&amp;#8211;65&amp;#176; bands and for a 15-year period. Cloud properties are bin-averaged according to the range of variability of each predictor, using a 1-day timescale. We find that although &amp;#969;&lt;sub&gt;500&lt;/sub&gt; strongly controls cloud albedo, it cannot explain the observed hemispheric asymmetry. Instead, we find that surface wind most skillfully explains the hemispheric albedo difference, due to the much greater winds in the Southern Ocean. We further show that V&lt;sub&gt;sfc &lt;/sub&gt;is not only a predictor of cloud albedo but it also controls physical processes in the boundary layer such that stronger winds ultimately lead to thicker and more horizontally extended cloud decks. The interhemispheric albedo asymmetry is significantly reduced in winter, responding to a strengthening of winds in the North Atlantic and Pacific Oceans during this season. Our findings have significant implications regarding GCM cloud biases over the Southern Ocean for the current climate, as well as for cloud feedback in a warming planet.&lt;/p&gt;

Clouds are primary modulators of Earth's energy balance. It is thus important to understand the links connecting variabilities in cloudiness to variabilities in other state variables of the climate system, and also describe how these links would change in a changing climate. A conceptual model of global cloudiness can help elucidate these points. In this work we derive simple representations of cloudiness, that can be useful in creating a theory of global cloudiness. These representations illustrate how both spatial and temporal variability of cloudiness can be expressed in terms of basic state variables. Specifically, cloud albedo is captured by a nonlinear combination of pressure velocity and a measure of the low‐level stability, and cloud longwave effect is captured by surface temperature, pressure velocity, and standard deviation of pressure velocity. We conclude with a short discussion on the usefulness of this work in the context of global warming response studies.

Clouds are primary modulators of Earth’s energy balance. It is thus important to understand the links connecting variabilities in cloudiness to variabilities in other state variables of the climate system, and also describe how these links would change in a changing climate. A conceptual model of global cloudiness can help elucidate these points. In this work we derive simple representations of cloudiness, that can be useful in creating a theory of global cloudiness. These representations illustrate how both spatial and temporal variability of cloudiness can be expressed in terms of basic state variables. Specifically, cloud albedo is captured by a nonlinear combination of pressure velocity and a measure of the low-level stability, and cloud longwave effect is captured by surface temperature, pressure velocity, and standard deviation of pressure velocity. We conclude with a short discussion on the usefulness of this work in the context of global warming response studies.

Understanding planetary cloudiness is of major importance for Earth's energy balance and potential for warming, but so far we lack pathways to approach planetary cloudiness theoretically. On the one hand, it is difficult to connect the microphysics of cloud formation to planetary wide cloudiness. On the other hand, a representation of cloudiness in energy balance models simply does no exist yet. In this work we want to provide simple means to treat planetary cloudiness in an energy balance model. We utilize a top-down approach and directly decompose the energetic signature of planetary cloudiness into a simple model composed of simple components. Vertical wind speed and estimated inversion strength are enough to capture all major characteristics of cloudiness in both shortwave and longwave spectral signatures. Other variables provide only minor improvements to the fits, while surface horizontal wind speed seems to be important for capturing hemispheric asymmetries in cloudiness. We use our results to argue that cloudiness can be incorporated into conceptual models based on mean temperature and equator-to-pole temperature difference.

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.

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.

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.

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.

The Intertropical Convergence Zone is characterized by a hierarchy of transients within a large range of spatial and temporal scales that propagate both eastward and westward with varying speeds in synoptic-scale-like waves. Among these tropical structures, the Convectively Coupled Kelvin Waves constitute one of the most significant in terms of contribution to the total ITCZ variability. The first part of this work analyzes the senstivitity of simulated CCKWs to domain size, horizontal resolution and forcing, using the WRF model. The simulations are performed using two types of idealized domains: the “aquachannel” (an oceanic surface with the Earth dimensions, but extending to the latitude of 60 degrees in both hemispheres on the beta plane), and the “aquapatch” (similar configuration as the aquachannel except for its longitudinal extent, which is 1/3 of the former, or approximately 13000 km), with both cases using periodic boundary conditions. The aquapatch is integrated in low and high resolution, and also in a doubly-nested configuration, in which convection is solved explicitly in the innermost grid. 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”. The second part of this work addresses a topic that has not previously been studied: the life cycle of the CCKWs. A subjective technique is applied to isolate early, mature and decay stages, and then spatial structures as well as propagation speeds of each phase are compared, resulting in some distinctive differences. Moreover, this analysis reveals a “de-coupling” between super cloud clusters and the pressure wave, which is found to be connected with the dissipation of the clusters.