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Tropical upper-level clouds (TUCs) control the radiation budget in a climate system and strongly influence surface temperatures. This study examines global mean surface temperature changes due to the percent change in TUC cover, which is referred to as the tropical upper-level cloud radiative effect (TUCRE, in units of Kelvin per %). We use a radiative-convective equilibrium model that can control both upper- and lower-level cloud layers separately in three idealized regions (extratropics, tropical moist, and tropical dry regions) and two sub-regions (clear-moist and cloudy-moist regions) within the tropical moist regions. In the simulation, tropical reflectivity based on the TUC fraction assumes a primary role in determining the TUCRE. Accurate estimate of the TUCRE requires careful prescriptions according to actual satellite observations. We use the extent of TUC fraction and reflectivity obtained from 18 years (2003–2020) of satellite data on daily MODIS cloud properties. Our results show that the estimated net TUCRE ranges from 0.19 to 0.33 K/%, with a higher TUC fraction leading to higher temperatures (a warming effect) in the climate system. This means that a longwave TUCRE dominates over a shortwave TUCRE. When upper- and lower-level clouds interplay in the model, the range of the TUCRE was greater with a combination of two cloud layers, although all values were positive. The TUCRE is greater by 0.22 to 0.40 K/% when upper- and lower-level clouds are negatively coupled, because the Earth warms due to a decline in the reflectance of solar radiation. When upper- and lower-level clouds are positively coupled, the TUCRE is lower by 0.14 to 0.30 K/%, as less radiation reaches the Earth through combined cloud layers. Finally, we test the sensitivity of the TUCRE with five TUC fractions and 15 combinations of tropical reflectivity. Comparing our results with the TUCREs estimated from climate models will help us understand how TUC cover affects climate, and should greatly reduce uncertainty associated with cloud feedback. [ABSTRACT FROM AUTHOR]

The Southern Ocean is covered by a large amount of clouds with high cloud albedo. However, as reported by previous climate model intercomparison projects, underestimated cloudiness and overestimated absorption of solar radiation (ASR) over the Southern Ocean lead to substantial biases in climate sensitivity. The present study revisits this long-standing issue and explores the uncertainty sources in the latest CMIP6 models. We employ 10-year satellite observations to evaluate cloud radiative effect (CRE) and cloud physical properties in five CMIP6 models that provide comprehensive output of cloud, radiation, and aerosol. The simulated longwave, shortwave, and net CRE at the top of atmosphere in CMIP6 are comparable with the CERES satellite observations. Total cloud fraction (CF) is also reasonably simulated in CMIP6, but the comparison of liquid cloud fraction (LCF) reveals marked biases in spatial pattern and seasonal variations. The discrepancies between the CMIP6 models and the MODIS satellite observations become even larger in other cloud macro- and micro-physical properties, including liquid water path (LWP), cloud optical depth (COD), and cloud effective radius, as well as aerosol optical depth (AOD). However, the large underestimation of both LWP and cloud effective radius (regional means ∼20% and 11%, respectively) results in relatively smaller bias in COD, and the impacts of the biases in COD and LCF also cancel out with each other, leaving CRE and ASR reasonably predicted in CMIP6. An error estimation framework is employed, and the different signs of the sensitivity errors and biases from CF and LWP corroborate the notions that there are compensating errors in the modeled shortwave CRE. Further correlation analyses of the geospatial patterns reveal that CF is the most relevant factor in determining CRE in observations, while the modeled CRE is too sensitive to LWP and COD. The relationships between cloud effective radius, LWP, and COD are also analyzed to explore the possible uncertainty sources in different models. Our study calls for more rigorous calibration of detailed cloud physical properties for future climate model development and climate projection. [ABSTRACT FROM AUTHOR]

Clouds are a dominant modulator of the energy budget. The cloud shortwave radiative effect at the surface (CRE) is closely related to the cloud macro- and micro-physical properties. Systematic observation of surface irradiance and cloud properties are needed to narrow uncertainties in CRE. In this study, 1-min irradiance and Total Sky Imager measurements from 2005 to 2009 at Xianghe in North China Plain are used to estimate cloud types, evaluate cloud fraction (CF), and quantify the sensitivities of surface irradiance with respect to changes in CF whether clouds obscure the sun or not. The annual mean CF is 0.50, further noting that CF exhibits a distinct seasonal variation, with a minimum in winter (0.37) and maximum in summer (0.68). Cumulus occurs more frequently in summer (32%), which is close to the sum of the occurrence of stratus and cirrus. The annual CRE is −54.4 W m−2, with seasonal values ranging from −29.5 W m−2 in winter and −78.2 W m−2 in summer. When clouds do not obscure the sun, CF is a dominant factor affecting diffuse irradiance, which in turn affects global irradiance. There is a positive linear relationship between CF and CRE under sun-unobscured conditions, the mean sensitivity of CRE for each CF 0.1 increase is about 1.2 W m−2 [79.5° < SZA (Solar Zenith Angle) < 80.5°] to 7.0 W m−2 (29.5° < SZA < 30.5°). When clouds obscure the sun, CF affects both direct and diffuse irradiance, resulting in a non-linear relationship between CF and CRE, and the slope decreases with increasing CF. It should be noted that, although only data at Xianghe is used in this study, our results are representative of neighboring areas, including most parts of the North China Plain. [ABSTRACT FROM AUTHOR]

Based on 12 years (March 2000–February 2012) of monthly data from Clouds and the Earth's Radiant Energy System energy balanced and filled (CERES-EBAF), this study systematically evaluates the applicability of Advanced Very High Resolution Radiometer (AVHRR) and second Along-Track Scanning Radiometer and advanced ATSR (AATSR) flux products at the top-of-the-atmosphere (TOA), and the ability of atmosphere-only simulations of the Coupled Model Intercomparison Project Phase6 (CMIP6/AMIP) model in reproducing the observed spatial–temporal patterns of TOA cloud radiative effect (CRE) over the Tibetan Plateau (TP). Results show that TOA radiative fluxes from AVHRR and AATSR can be used to analyze their spatial/temporal characteristics over TP region, especially for AVHRR, but none of them can capture the observed CRE trend since 2000. In particular, when using AATSR TOA radiative flux in clear-sky of TP, the large bias of SW flux (regional mean about 30.48 Wm−2) compared with CERES-EBAF must be taken seriously. The multimodel ensemble mean (MEM) can sufficiently reproduce the temporal changes of CREs, particularly the shortwave CRE. Regarding the geographical pattern of CREs of MEM, the annual mean deviations of longwave CRE are very small, while obvious underestimations can be found in the southeastern TP for shortwave CRE. Additionally, the spatial distribution of CREs is difficult to reproduce for many individual models due to albedo and temperature biases of surface. Our results also demonstrated that MEM still has evident difficulties to capture realistic CRE trends in TP due to poor simulations in surface and cloud properties (particularly cloud fraction). [ABSTRACT FROM AUTHOR]

This study uses 2001-2014 satellite observations and reanalyses to investigate the seasonal characteristics of Cloud Radiative Effects (CREs) and their associations with cloud fraction (CF) and precipitation over the Asian monsoon region (AMR) covering Eastern China (EC) and South Asia (SA). The CREs exhibit strong seasonal variations but show distinctly different relationships with CFs and precipitation over the two regions. For EC, the CREs is dominated by shortwave (SW) cooling, with an annual mean value of − 40 W m for net CRE, and peak in summer while the presence of extensive and opaque low-level clouds contributes to large Top-Of-Atmosphere (TOA) albedo (>0.5) in winter. For SA, a weak net CRE exists throughout the year due to in-phase compensation of SWCRE by longwave (LW) CRE associated with the frequent occurrence of high clouds. For the entire AMR, SWCRE strongly correlates with the dominant types of CFs, although the cloud vertical structure plays important role particularly in summer. The relationships between CREs and precipitation are stronger in SA than in EC, indicating the dominant effect of monsoon circulation in the former region. SWCRE over EC is only partly related to precipitation and shows distinctive regional variations. Further studies need to pay more attention to vertical distributions of cloud micro- and macro-physical properties, and associated precipitation systems over the AMR. [ABSTRACT FROM AUTHOR]

A new global mean energy budget diagram is offered for discussion and further examination. The main motivation for creating this figure was the observation that a quasi-discrete flux quantity structure seems to appear behind the best published energy budget data. This structure underneath the observed global energy flow system might represent an idealized, hypothetic normal (steady) state onto which the actual climatic regimes and their changes can be projected. The unit of the all-sky structure is the value of the flux element called longwave cloud radiative effect (LWCRE), termed also the greenhouse effect of clouds; under prevailing average conditions, it turns out to be numerically equal to the all-sky surface transmitted irradiance, S(all). There is also a clear-sky structure, as reported in earlier studies, where the unit of measure is one S(clear). Three important features are independent of the discrete units: (a) the energies at the surface are equal to the total energy at top-of-atmosphere plus one LWCRE; (b) the energies in the atmosphere are equal to the energy at the surface plus two LWCRE; (c) the shortwave (SW) radiation absorbed by the surface is equal to the longwave (LW) energy in the all-sky greenhouse effect. The aim of our study is to present the system as it reveals itself in the data; theoretical explanation is out of our recent scope. Graphical Abstract: [ABSTRACT FROM AUTHOR]

Future responses of cloud regimes are analyzed for five CMIP5 models forced with observed SSTs and subject to a patterned SST perturbation. Correlations between cloud properties in the control climate and changes in the warmer climate are investigated for each of a set of cloud regimes defined using a clustering methodology. The only significant (negative) correlation found is in the in-regime net cloud radiative effect for the stratocumulus regime. All models overestimate the in-regime albedo of the stratocumulus regime. Reasons for this bias and its relevance to the future response are investigated. A detailed evaluation of the models' daily-mean contributions to the albedo from stratocumulus clouds with different cloud cover fractions reveals that all models systematically underestimate the relative occurrence of overcast cases but overestimate those of broken clouds. In the warmer climate the relative occurrence of overcast cases tends to decrease while that of broken clouds increases. This suggests a decrease in the climatological in-regime albedo with increasing temperature (a positive feedback); this is opposite to the feedback suggested by the analysis of the bulk in-regime albedo. Furthermore we find that the inter-model difference in the sign of the in-cloud albedo feedback is consistent with the difference in sign of the in-cloud liquid water path response, and there is a strong positive correlation between the in-regime liquid water path in the control climate and its response to warming. We therefore conclude that further breakdown of the in-regime properties into cloud cover and in-cloud properties is necessary to better understand the behavior of the stratocumulus regime. Since cloud water is a physical property and is independent of a model's radiative assumptions, it could potentially provide a useful emergent constraint on cloud feedback. [ABSTRACT FROM AUTHOR]

Idealized climate change experiments using fixed sea-surface temperature are investigated to determine whether zonally symmetric aquaplanet configurations are useful for understanding climate feedbacks in more realistic configurations. The aquaplanets capture many of the robust responses of the large-scale circulation and hydrologic cycle to both warming the sea-surface temperature and quadrupling atmospheric CO. The cloud response to both perturbations varies across models in both Earth-like and aquaplanet configurations, and this spread arises primarily from regions of large-scale subsidence. Most models produce a consistent cloud change across the subsidence regimes, and the feedback in trade-wind cumulus regions dominates the tropical response. It is shown that these trade-wind regions have similar cloud feedback in Earth-like and aquaplanet warming experiments. The tropical average cloud feedback of the Earth-like experiment is captured by five of eight aquaplanets, and the three outliers are investigated to understand the discrepancy. In two models, the discrepancy is due to warming induced dissipation of stratocumulus decks in the Earth-like configuration which are not represented in the aquaplanet. One model shows a circulation response in the aquaplanet experiment accompanied by a cloud response that differs from the Earth-like configuration. Quadrupling atmospheric CO in aquaplanets produces slightly greater adjusted forcing than in Earth-like configurations, showing that land-surface effects dampen the adjusted forcing. The analysis demonstrates how aquaplanets, as part of a model hierarchy, help elucidate robust aspects of climate change and develop understanding of the processes underlying them. [ABSTRACT FROM AUTHOR]

Physical processes responsible for tropospheric adjustment to increasing carbon dioxide concentration are investigated using abrupt CO 2 quadrupling experiments of a general circulation model (GCM) called the model for interdisciplinary research on climate version 5 with several configurations including a coupled atmosphere–ocean GCM, atmospheric GCM, and aqua-planet model. A similar experiment was performed in weather forecast mode to explore timescales of the tropospheric adjustment. We found that the shortwave component of the cloud radiative effect (SWcld) reaches its equilibrium within 2 days of the abrupt CO 2 increase. The change in SWcld is positive, associated with reduced clouds in the lower troposphere due to warming and drying by instantaneous radiative forcing. A reduction in surface turbulent heat fluxes and increase of the near-surface stability result in shoaling of the marine boundary layer, which shifts the cloud layer downward. These changes are common to all experiments regardless of model configuration, indicating that the cloud adjustment is primarily independent of air–sea coupling and land–sea thermal contrast. The role of land in cloud adjustment is further examined by a series of idealized aqua-planet experiments, with a rectangular continent of varying width. Land surface warming from quadrupled CO 2 induces anomalous upward motion, which increases high cloud and associated negative SWcld over land. The geographic distribution of continents regulates the spatial pattern of the cloud adjustment. A larger continent produces more negative SWcld, which partly compensates for a positive SWcld over the ocean. The land-induced negative adjustment is a factor but not necessary requirement for the tropospheric adjustment. [ABSTRACT FROM AUTHOR]

An extended cloud-clustering method to assess the seasonal variation of clouds is applied to five CMIP5 models. The seasonal variation of the total cloud radiative effect (CRE) is dominated by variations in the relative frequency of occurrence of the different cloud regimes. Seasonal variations of the CRE within the individual regimes contribute much less. This is the case for both observations, models and model errors. The error in the seasonal variation of cloud regimes, and its breakdown into mean amplitude and time varying components, are quantified with a new metric. The seasonal variation of the CRE of the cloud regimes is relatively well simulated by the models in the tropics, but less well in the extra-tropics. The stratocumulus regime has the largest seasonal variation of shortwave CRE in the tropics, despite having a small magnitude in the climatological mean. Most of the models capture the temporal variation of the CRE reasonably well, with the main differences between models coming from the variation in amplitude. In the extra-tropics, most models fail to correctly represent both the amplitude and time variation of the CRE of congestus, frontal and stratocumulus regimes. The annual mean climatology of the CRE and its amplitude in the seasonal variation are both underestimated for the anvil regime in the tropics, the cirrus regime and the congestus regime in the extra-tropics. The models in this study that best capture the seasonal variation of the cloud regimes tend to have higher climate sensitivities. [ABSTRACT FROM AUTHOR]

We analyse the dependence of the cloud radiative effect (CRE) and cloud amount on mid-tropospheric pressure velocity ( ω) and sea surface temperature (SST) and point out the shortcomings of using these two proxies separately as means to separate cloud regimes. A bivariate approach is proposed to overcome these shortcomings and it is used to systematically investigate marine cloud properties at different spatial and time scales in the present-day (1985-2001) tropical climate. During the 1997-1998 El Niño, the greatest regional change in CRE and cloud cover coincides with the greatest local change in circulation and SST. In addition, we find that the cooling effect of the stratiform low clouds reduces at the rate of approximately 1 W/m per percent of cloudiness reduction in the subsident cold pools of the Pacific ocean. During El Niño, the transition between different cloud regimes gives rise to opposing cloud feedbacks. The sign of the total feedback is controlled by the cloud optical thickness. More generally, we find that the largest part of the cloud response to El Niño, when averaged over the tropical Pacific, is not directly associated with ω and SST changes, so other factors must play a role as well. [ABSTRACT FROM AUTHOR]

The cloud phase composition of cold clouds in the Antarctic atmosphere is explored using data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instruments for the period 2000-2006. We used the averaged fraction of liquid-phase clouds out of the total cloud amount at the cloud tops since the value is comparable in the two measurements. MODIS data for the winter months (June, July, and August) reveal liquid cloud fraction out of the total cloud amount significantly decreases with decreasing cloud-top temperature below 0°C. In addition, the CALIOP vertical profiles show that below the ice clouds, low-lying liquid clouds are distributed over ∼20% of the area. With increasing latitude, the liquid cloud fraction decreases as a function of the local temperature. The MODIS-observed relation between the cloud-top liquid fraction and cloud-top temperature is then applied to evaluate the cloud phase parameterization in climate models, in which condensed cloud water is repartitioned between liquid water and ice on the basis of the grid point temperature. It is found that models assuming overly high cut-offs (≫ −40°C) for the separation of ice clouds from mixed-phase clouds may significantly underestimate the liquid cloud fraction in the winter Antarctic atmosphere. Correction of the bias in the liquid cloud fraction would serve to reduce the large uncertainty in cloud radiative effects. [ABSTRACT FROM AUTHOR]