Quantifying the contribution of different cloud types to the radiation budget in southern West Africa

The contribution of cloud to the radiation budget of southern West Africa (SWA) is poorly understood yet is important for understanding regional monsoon evolution and for evaluating and improving climate models, which have large biases in this region. Radiative transfer calculations applied to atmospheric profiles obtained from the CERES-CloudSat-CALIPSO-MODIS (CCCM) dataset are used to investigate the effects of 12 different cloud types (defined by their vertical structure) on the regional energy budget of SWA (5–10 °N, 8 °W-8 °E) during June-September. We show that the large regional mean cloud radiative effect in SWA is due to non-negligible contributions from many different cloud types; 8 cloud types have a cloud fraction larger than 5 % and contribute at least 5 % of the regional mean shortwave cloud radiative effect at the top of atmosphere. Low-clouds, which are poorly observed by passive satellite measurements, were found to cause net radiative cooling of the atmosphere, which reduces the heating from other cloud types by approximately 10 %. The sensitivity of the radiation budget to underestimating low-cloud cover is also investigated. The radiative effect of missing low-cloud is found to be up to approximately –25 W m-2 for upwelling shortwave irradiance at the top of atmosphere and 35 W m-2 for downwelling shortwave irradiance at the surface

Estudio de la relación entre los balances en superficie de onda larga y onda corta utilizando datos del proyecto SRB (Surface Radiation Budget)

Ponencia presentada en: III Congreso de la Asociación Española de Climatología “El agua y el clima”, celebrado en Palma de Mallorca del 16 al 19 de junio de 2002.[ES]En este trabajo se muestra la influencia de diferentes regímenes climáticos en el balance de radiación en superficie, a través de las comparaciones entre los balances de onda larga y onda corta en diferentes regiones del globo. Las regiones estudiadas son cuatro zonas desérticas o semidesérticas, seis oceánicas y dos localizadas en el Mediterráneo. Se observa una gran influencia de la cobertura nubosa en los balances de radiación de onda corta y larga, así como el efecto estacional en ambos hemisferios. Asimismo, aparecen diferencias claras si comparamos regiones del Mar Mediterráneo con regiones centrales del Océano Pacífico o el Atlántico.[EN]In this work we show the influence of different climatic regimes on the surface radiation balance through the comparisons between the long wave balance and the short wave balance over different regions in the globe. The studied regions are four deserted or semi-deserted areas, six ocean areas and two areas located in the Mediterranean basin. A large influence of cloud cover is observed in both short wave and long wave balances, as well as the seasonal effect in both hemispheres. Likewise, remarkable differences appear if we compare Mediterranean Sea regions with central regions of the Pacific or Atlantic Oceans

Evaluation of a General Circulation Model by the CERES Flux-By-Cloud Type Simulator

In this work, we use the Clouds and the Earths Radiant Energy System (CERES) FluxByCloudTyp data product, which calculates TOA shortwave and longwave fluxes for cloud categories defined by cloud optical depth () and cloud top pressure (), to evaluate the HadGEM2-A model with a simulator. The CERES Flux-by-cloud type simulator is comprised of a cloud generator that produces subcolumns with profiles of binary cloud fraction, a cloud property simulator that determines the (,) cloud type for each subcolumn, and a radiative transfer model that calculates TOA fluxes. The identification of duplicate atmospheric profiles reduces the number of radiative transfer calculations required by approximately 97.6%. In the Southern Great Plains region in JFD (January, February, and December) 2008, the simulator shows that simulated cloud tops are higher in altitude than observed, but also have higher values of OLR than observed, leading to a compensating error that results in an average value of OLR that is close to observed. When the simulator is applied to the Southeast Pacific stratocumulus region in JJA 2008, the simulated cloud tops are primarily low in altitude; however, the clouds tend to be less numerous, and have higher optical depths than are observed. In addition to the increase in albedo that comes from having too many clouds with higher optical depth, the HadGEM2-A albedo is higher than observed for those cloud types that occur most frequently. The simulator is also applied to the entire 60 N 60 S region, and it is found that there are fewer clouds than observed for most cloud types, but there are also higher albedos for most cloud types, which represents a compensating error in terms of the shortwave radiative budget

The impact of equilibrating hemispheric albedos on tropical performance in the HadGEM2-ES coupled climate model

AcceptedArticle in Press©2015. The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.©2015. The Authors. The Earth's hemispheric reflectances are equivalent to within±0.2Wm-2, even though the Northern Hemisphere contains a greater proportion of higher reflectance land areas, because of greater cloud cover in the Southern Hemisphere. This equivalence is unlikely to be by chance, but the reasons are open to debate. Here we show that equilibrating hemispheric albedos in the Hadley Centre Global Environment Model version 2-Earth System coupled climate model significantly improves what have been considered longstanding and apparently intractable model biases. Monsoon precipitation biases over all continental land areas, the penetration of monsoon rainfall across the Sahel, the West African monsoon "jump", and indicators of hurricane frequency are all significantly improved. Mechanistically, equilibrating hemispheric albedos improves the atmospheric cross-equatorial energy transport and increases the supply of tropical atmospheric moisture to the Hadley cell. We conclude that an accurate representation of the cross-equatorial energy transport appears to be critical if tropical performance is to be improved

The impact of equilibrating hemispheric albedos on tropical performance in the HadGEM2-ES coupled climate model

The Earth's hemispheric reflectances are equivalent to within ± 0.2 Wm-2, even though the Northern Hemisphere contains a greater proportion of higher reflectance land areas, because of greater cloud cover in the Southern Hemisphere. This equivalence is unlikely to be by chance, but the reasons are open to debate. Here we show that equilibrating hemispheric albedos in the Hadley Centre Global Environment Model version 2-Earth System coupled climate model significantly improves what have been considered longstanding and apparently intractable model biases. Monsoon precipitation biases over all continental land areas, the penetration of monsoon rainfall across the Sahel, the West African monsoon 'jump', and indicators of hurricane frequency are all significantly improved. Mechanistically, equilibrating hemispheric albedos improves the atmospheric cross-equatorial energy transport and increases the supply of tropical atmospheric moisture to the Hadley cell. We conclude that an accurate representation of the cross-equatorial energy transport appears to be critical if tropical performance is to be improved

The Cloud Feedback Model Intercomparison Project (CFMIP) contribution to CMIP6

The primary objective of CFMIP is to inform future assessments of cloud feedbacks through improved understanding of cloud-climate feedback mechanisms and better evaluation of cloud processes and cloud feedbacks in climate models. However, the CFMIP approach is also increasingly being used to understand other aspects of climate change, and so a second objective has now been introduced, to improve understanding of circulation, regional-scale precipitation, and non-linear changes. CFMIP is supporting ongoing model inter-comparison activities by coordinating a hierarchy of targeted experiments for CMIP6, along with a set of cloud-related output diagnostics. CFMIP contributes primarily to addressing the CMIP6 questions "How does the Earth system respond to forcing?" and "What are the origins and consequences of systematic model biases?" and supports the activities of the WCRP Grand Challenge on Clouds, Circulation and Climate Sensitivity. A compact set of Tier 1 experiments is proposed for CMIP6 to address this question: (1) what are the physical mechanisms underlying the range of cloud feedbacks and cloud adjustments predicted by climate models, and which models have the most credible cloud feedbacks? Additional Tier 2 experiments are proposed to address the following questions. (2) Are cloud feedbacks consistent for climate cooling and warming, and if not, why? (3) How do cloudradiative effects impact the structure, the strength and the variability of the general atmospheric circulation in present and future climates? (4) How do responses in the climate system due to changes in solar forcing differ from changes due to CO2, and is the response sensitive to the sign of the forcing? (5) To what extent is regional climate change per CO2 doubling state-dependent (non-linear), and why? (6) Are climate feedbacks during the 20th century different to those acting on long-term climate change and climate sensitivity? (7) How do regional climate responses (e.g. in precipitation) and their uncertainties in coupled models arise from the combination of different aspects of CO2 forcing and sea surface warming? CFMIP also proposes a number of additional model outputs in the CMIP DECK, CMIP6 Historical and CMIP6 CFMIP experiments, including COSP simulator outputs and process diagnostics to address the following questions. 1. How well do clouds and other relevant variables simulated by models agree with observations? 2. What physical processes and mechanisms are important for a credible simulation of clouds, cloud feedbacks and cloud adjustments in climate models? 3. Which models have the most credible representations of processes relevant to the simulation of clouds? 4. How do clouds and their changes interact with other elements of the climate system

Cloud feedbacks in extratopical cyclones: insight from long-term satellite data and high-resolution global simulations

A negative extratropical shortwave cloud feedback driven by changes in cloud optical depth is a feature of global climate models (GCMs). A robust positive trend in observed liquid water path (LWP) over the last two decades across the warming Southern Ocean supports the negative shortwave cloud feedback predicted by GCMs. This feature has been proposed to be due to transitions from ice to liquid with warming. To gain insight into the shortwave cloud feedback we examine extratropical cyclone variability and the response of extratropical cyclones to transient warming in GCM simulations. Multi-Sensor Advanced Climatology Liquid Water Path (MAC-LWP) microwave observations of cyclone properties from the period 1992–2015 are contrasted with GCM simulations, with horizontal resolutions ranging from 7 km to hundreds of kilometers. We find that inter-cyclone variability in LWP in both observations and models is strongly driven by the moisture flux along the cyclone's warm conveyor belt (WCB). Stronger WCB moisture flux enhances the LWP within cyclones. This relationship is replicated in GCMs, although its strength varies substantially across models. It is found that more than 80 % of the enhancement in Southern Hemisphere (SH) extratropical cyclone LWP in GCMs in response to a transient 4 K warming can be predicted based on the relationship between the WCB moisture flux and cyclone LWP in the historical climate and their change in moisture flux between the historical and warmed climates. Further, it is found that that the robust trend in cyclone LWP over the Southern Ocean in observations and GCMs is consistent with changes in the moisture flux. We propose two cloud feedbacks acting within extratropical cyclones: a negative feedback driven by Clausius–Clapeyron increasing water vapor path (WVP), which enhances the amount of water vapor available to be fluxed into the cyclone, and a feedback moderated by changes in the life cycle and vorticity of cyclones under warming, which changes the rate at which existing moisture is imported into the cyclone. Both terms contribute to increasing LWP within the cyclone. While changes in moisture flux predict cyclone LWP trends in the current climate and the majority of changes in LWP in transient warming simulations, a portion of the LWP increase in response to climate change that is unexplained by increasing moisture fluxes may be due to phase transitions. The variability in LWP within cyclone composites is examined to understand what cyclonic regimes the mixed-phase cloud feedback is relevant to. At a fixed WCB moisture flux cyclone LWP increases with increasing sea surface temperature (SST) in the half of the composite poleward of the low and decreases in the half equatorward of the low in both GCMs and observations. Cloud-top phase partitioning observed by the Atmospheric Infrared Sounder (AIRS) indicates that phase transitions may be driving increases in LWP in the poleward half of cyclones

Forcings, feedbacks and climate sensitivity in HadGEM3‐GC3.1 and UKESM1

Climate forcing, sensitivity and feedback metrics are evaluated in both the UK’s physical climate model HadGEM3-GC3.1at low (-LL) and medium(-MM) resolution and the UK’s Earth System Model UKESM1. The Effective Climate Sensitivity (EffCS)to a doubling of CO2 is 5.5K for HadGEM3.1-GC3.1-LL and 5.4 K for UKESM1. The transient climate response is 2.5K and 2.8K respectively. Whilst the EffCS is larger than that seen in the previous generation of models, none of the model’s forcing or feedback processes are found to be atypical of models, though the cloud feedback is at the high end. The relatively large EffCS results from an unusual combination of a typical CO2 forcing with a relatively small feedback parameter. Compared to the previous UK climate model, HadGEM3-GC2.0, the EffCS has increased from 3.2K to 5.5K due to an increase in CO2 forcing, surface albedo feedback and mid-latitude cloud feedback. All changes are well understood and due to physical improvements in the model.At higher atmospheric and ocean resolution(HadGEM3-GC3.1-MM), there is a compensation between increased marine stratocumulous cloud feedback and reduced Antarctic sea-ice feedback. In UKESM1 a CO2 fertilization effect induces a land surface vegetation change and albedo radiative effect. Historical aerosol forcing in HadGEM3-GC3.1-LL is -1.1 Wm-2. In HadGEM3-GC3.1-LL historical simulations cloud feedback is found to be less positive than in abrupt-4xCO2, in agreement with atmosphere-only experiments forced with observed historical sea-surface-temperature and sea-ice variations. However variability in the coupled model’s historical sea-ice trends hampers accurate diagnosis of the model’s total historical feedback

The Cloud Feedback Model Intercomparison Project Observational Simulator Package: Version 2

Abstract. The Cloud Feedback Model Intercomparison Project Observational Simulator Package (COSP) gathers together a collection of observation proxies or satellite simulators that translate model-simulated cloud properties to synthetic observations as would be obtained by a range of satellite observing systems. This paper introduces COSP 2, an evolution focusing on more explicit and consistent separation between host model, coupling infrastructure, and individual observing proxies. Revisions also enhance flexibility by allowing for model-specific representation of sub-grid scale cloudiness, provide greater clarity by clearly separating tasks, support greater use of shared code and data including shared inputs across simulators, and follow more uniform software standards to simplify implementation across a wide range of platforms. The complete package including a testing suite is freely available. </jats:p

Evaluation of ice cloud representation in the ECMWF and UK Met Office models using CloudSat and CALIPSO data

Ice cloud representation in general circulation models remains a challenging task, due to the lack of accurate observations and the complexity of microphysical processes. In this article, we evaluate the ice water content (IWC) and ice cloud fraction statistical distributions from the numerical weather prediction models of the European Centre for Medium-Range Weather Forecasts (ECMWF) and the UK Met Office, exploiting the synergy between the CloudSat radar and CALIPSO lidar. Using the last three weeks of July 2006, we analyse the global ice cloud occurrence as a function of temperature and latitude and show that the models capture the main geographical and temperature-dependent distributions, but overestimate the ice cloud occurrence in the Tropics in the temperature range from −60 °C to −20 °C and in the Antarctic for temperatures higher than −20 °C, but underestimate ice cloud occurrence at very low temperatures. A global statistical comparison of the occurrence of grid-box mean IWC at different temperatures shows that both the mean and range of IWC increases with increasing temperature. Globally, the models capture most of the IWC variability in the temperature range between −60 °C and −5 °C, and also reproduce the observed latitudinal dependencies in the IWC distribution due to different meteorological regimes. Two versions of the ECMWF model are assessed. The recent operational version with a diagnostic representation of precipitating snow and mixed-phase ice cloud fails to represent the IWC distribution in the −20 °C to 0 °C range, but a new version with prognostic variables for liquid water, ice and snow is much closer to the observed distribution. The comparison of models and observations provides a much-needed analysis of the vertical distribution of IWC across the globe, highlighting the ability of the models to reproduce much of the observed variability as well as the deficiencies where further improvements are required