The spectral signature of recent climate change

Spectrally resolved measurements of the Earth’s reflected shortwave (RSW) and outgoing longwave radiation (OLR) at the top of the atmosphere intrinsically contain the imprints of a multitude of climate relevant parameters. Here, we review the progress made in directly using such observations to diagnose and attribute change within the Earth system over the past four decades. We show how changes associated with perturbations such as increasing greenhouse gases are expected to be manifested across the spectrum and illustrate the enhanced discriminatory power that spectral resolution provides over broadband radiation measurements. Advances in formal detection and attribution techniques and in the design of climate model evaluation exercises employing spectrally resolved data are highlighted. We illustrate how spectral observations have been used to provide insight into key climate feedback processes and quantify multi-year variability but also indicate potential barriers to further progress. Suggestions for future research priorities in this area are provided

Evaluation of Cirrus Parameterizations for Radiative Flux Computations in Climate Models Using TOVS-ScaRaB Satellite Observations

Combined simultaneous satellite observations are used to evaluate the performance of parameterizations of the microphysical and optical properties of cirrus clouds used for radiative flux computations in climate models. Atmospheric and cirrus properties retrieved from Television and Infrared Observation Satellite (TIROS-N) Operational Vertical Sounder (TOVS) observations are given as input to the radiative transfer model developed for the Met Office climate model to simulate radiative fluxes at the top of the atmosphere (TOA). Simulated cirrus shortwave (SW) albedos are then compared to those retrieved from collocated Scanner for Radiation Budget (ScaRaB) observations. For the retrieval, special care has been given to angular direction models. Three parameterizations of cirrus ice crystal optical properties are represented in the Met Office radiative transfer model. These parameterizations are based on different physical approximations and different hypotheses on crystal habit. One parameterization assumes pristine ice crystals and two ice crystal aggregates. By relating the cirrus ice water path (IWP) retrieved from the effective infrared emissivity to the cirrus SW albedo, differences between the parameterizations are amplified. This study shows that pristine crystals seem to be plausible only for cirrus with IWP less than 30 g m−2. For larger IWP, ice crystal aggregates lead to cirrus SW albedos in better agreement with the observations. The data also indicate that climate models should allow the cirrus effective ice crystal diameter (De) to increase with IWP, especially in the range up to 30 g m−2. For cirrus with IWP less than 20 g m−2, this would lead to SW albedos that are about 0.02 higher than the ones of a constant De of 55 μm

Evaluation of five satellite top-of-atmosphere albedo products over land

Five satellite top-of-atmosphere (TOA) albedo products over land were evaluated in this study including global products from the Advanced Very High Resolution Radiometer (AVHRR) (TAL-AVHRR), Moderate Resolution Imaging Spectroradiometer (MODIS) (TAL-MODIS), and Clouds and the Earth’s Radiant Energy System (CERES); one regional product from the Climate Monitoring Satellite Application Facility (CM SAF); and one harmonized product termed Diagnosing Earth’s Energy Pathways in the Climate system (DEEP-C). Results showed that overall, there is good consistency among these five products, particularly after the year 2000. The differences among these products in the high-latitude regions were relatively larger. The percentage differences among TAL-AVHRR, TAL-MODIS, and CERES were generally less than 20%, while the differences between TAL-AVHRR and DEEP-C before 2000 were much larger. Except for the obvious decrease in the differences after 2000, the differences did not show significant changes over time, but varied among different regions. The differences between TAL-AVHRR and the other products were relatively large in the high-latitude regions of North America, Asia, and the Maritime Continent, while the differences between DEEP-C and CM SAF in Europe and Africa were smaller. Interannual variability was consistent between products after 2000, before which the differences among the three products were much larger

Comparison of radiative energy flows in observational datasets and climate modeling

This study examines radiative flux distributions and local spread of values from three major observational datasets (CERES, ISCCP, and SRB) and compares them with results from climate modeling (CMIP3). Examinations of the spread and differences also differentiate among contributions from cloudy and clear-sky conditions. The spread among observational datasets is in large part caused by noncloud ancillary data. Average differences of at least 10 W m-2 each for clear-sky downward solar, upward solar, and upward infrared fluxes at the surface demonstrate via spatial difference patterns major differences in assumptions for atmospheric aerosol, solar surface albedo and surface temperature, and/or emittance in observational datasets. At the top of the atmosphere (TOA), observational datasets are less influenced by the ancillary data errors than at the surface. Comparisons of spatial radiative flux distributions at the TOA between observations and climate modeling indicate large deficiencies in the strength and distribution of model-simulated cloud radiative effects. Differences are largest for lower-altitude clouds over low-latitude oceans. Global modeling simulates stronger cloud radiative effects (CRE) by +30 W m-2 over trade wind cumulus regions, yet smaller CRE by about -30 W m-2 over (smaller in area) stratocumulus regions. At the surface, climate modeling simulates on average about 15 W m-2 smaller radiative net flux imbalances, as if climate modeling underestimates latent heat release (and precipitation). Relative to observational datasets, simulated surface net fluxes are particularly lower over oceanic trade wind regions (where global modeling tends to overestimate the radiative impact of clouds). Still, with the uncertainty in noncloud ancillary data, observational data do not establish a reliable reference. © 2016 American Meteorological Society

New look at the Earth's radiation balance from an A-train observational perspective, A

2010 Fall.Includes bibliographical references.The weather and climate of the Earth are driven by interactions of the longwave and shortwave radiation between the Earth's atmosphere and surface. Past studies have tried to derive the Earth radiative budget through the use of models and passive satellite sensors. These past efforts did not have information about the vertical distribution of cloud or aerosols within the atmosphere that significantly influence radiative transfer within the atmosphere. This problem was improved upon with the launch of CloudSat and CALIPSO in 2006. These satellites provide the information on the vertical distribution of clouds. From CloudSat, a fluxes and heating rates product was produced to study the radiative budget, but this was limited to some degree because of undetected clouds and aerosol that have non-negligible effects on the radiative balance. This study addresses these issues by combining CALIPSO and MODIS data with CloudSat to detect and obtain the properties of cloud and aerosol undetected by the CloudSat CPR. The combined data were used to create a cloud and aerosol mask that identified distributions of undetected cloud and aerosol globally and quantified their radiative effects both seasonally and annually. Low clouds were found to have the highest impacts of nearly -6 Wm-2. High clouds globally have little effect, trapping 1 Wm-2, with the majority of the impact in the tropics. Four case studies are presented to show how heating rates change in the vertical due low cloud, cirrus, precipitation, and aerosol. The cloud and aerosol mask was used to create seasonal global distributions of cloud radiative effect using all clouds detected by CloudSat and CALIPSO, and the direct effect of aerosols estimated at the TOA. Using fluxes at the top and bottom of the atmosphere global distributions of outgoing and incoming radiation are shown, and an annual radiation budget of the Earth is derived. Clouds globally are found to have a radiative forcing of -20 Wm-2 at the TOA. The radiative budget of the Earth is calculated in two ways; using normalized shortwave fluxes by the average solar daily insolation, and by changing the solar zenith angle to simulate the diurnal cycle. Finally, the product is validated by comparing the outgoing and surface fluxes with CERES and ISCPP flux products

Long-term global distribution of earth's shortwave radiation budget at the top of atmosphere

The mean monthly shortwave (SW) radiation budget at the top of atmosphere (TOA) was computed on 2.5° longitude-latitude resolution for the 14-year period from 1984 to 1997, using a radiative transfer model with long-term climatological data from the International Satellite Cloud Climatology Project (ISCCP-D2) supplemented by data from the National Centers for Environmental Prediction – National Center for Atmospheric Research (NCEP-NCAR) Global Reanalysis project, and other global data bases such as TIROS Operational Vertical Sounder (TOVS) and Global Aerosol Data Set (GADS). The model radiative fluxes at TOA were validated against Earth Radiation Budget Experiment (ERBE) S4 scanner satellite data (1985–1989). The model is able to predict the seasonal and geographical variation of SW TOA fluxes. On a mean annual and global basis, the model is in very good agreement with ERBE, overestimating the outgoing SW radiation at TOA (OSR) by 0.93 Wm^-2 (or by 0.92%), within the ERBE uncertainties. At pixel level, the OSR differences between model and ERBE are mostly within ±10 Wm^-2, with ±5 Wm^-2 over extended regions, while there exist some geographic areas with differences of up to 40 Wm^-2, associated with uncertainties in cloud properties and surface albedo. The 14-year average model results give a planetary albedo equal to 29.6% and a TOA OSR flux of 101.2 Wm^-2. A significant linearly decreasing trend in OSR and planetary albedo was found, equal to 2.3 Wm^-2 and 0.6% (in absolute values), respectively, over the 14-year period (from January 1984 to December 1997), indicating an increasing solar planetary warming. This planetary SW radiative heating occurs in the tropical and sub-tropical areas (20° S–20° N), with clouds being the most likely cause. The computed global mean OSR anomaly ranges within ±4 Wm^-2, with signals from El Niño and La Niña events or Pinatubo eruption, whereas significant negative OSR anomalies, starting from year 1992, are also detected

Insights into the diurnal cycle of global Earth outgoing radiation using a numerical weather prediction model

A globally complete, high temporal resolution and multiple-variable approach is employed to analyse the diurnal cycle of Earth’s outgoing energy flows. This is made possible via the use of Met Office model output for September 2010 that is assessed alongside regional satellite observations throughout. Principal component analysis applied to the longwave component of modelled outgoing radiation reveals dominant diurnal patterns related to land surface heating and convective cloud development, respectively explaining 68.5 and 16.0% of the variance at the global scale. The total variance explained by these first two patterns is markedly less than previous regional estimates from observations, and this analysis suggests that around half of the difference relates to the lack of global coverage in the observations. The first pattern is strongly and simultaneously coupled to the land surface temperature diurnal variations. The second pattern is strongly coupled to the cloud water content and height diurnal variations, but lags the cloud variations by several hours. We suggest that the mechanism con- trolling the delay is a moistening of the upper troposphere due to the evaporation of anvil cloud. The shortwave component of modelled outgoing radiation, analysed in terms of albedo, exhibits a very dominant pattern explaining 88.4 % of the variance that is related to the angle of incoming solar radiation, and a second pattern explaining 6.7 % of the variance that is related to compensating effects from convective cloud development and marine stratocumulus cloud dissipation. Similar patterns are found in regional satellite observations, but with slightly different timings due to known model biases. The first pattern is controlled by changes in surface and cloud albedo, and Rayleigh and aerosol scattering. The second pattern is strongly coupled to the diurnal variations in both cloud water content and height in convective regions but only cloud water content in marine stratocumulus regions, with substantially shorter lag times compared with the longwave counterpart. This indicates that the shortwave radiation response to diurnal cloud development and dissipation is more rapid, which is found to be robust in the regional satellite observations. These global, diurnal radiation patterns and their coupling with other geophysical variables demonstrate the process-level understanding that can be gained using this approach and highlight a need for global, diurnal observing systems for Earth outgoing radiation in the future

On the meridional heat transport and its partition between the atmosphere and oceans

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2009.Includes bibliographical references (p. 185-196).In this thesis I study the meridional heat transport of the climate system and its partition between the atmosphere and oceans using models and data. I focus on three primary questions: (1) What is the total heat transport and its partition between the two fluids? (2) What sets the magnitude of the total heat transport and to what extent is it sensitive to the details of atmospheric and oceanic circulation? (3) How robust is the partition of heat transport between the two fluids and how sensitive is it to bathymetric constraints on ocean circulation? For these studies I employ a series of aqua-planet calculations using a coupled atmosphere-ocean-ice model in which idealized ocean basins impose various geometrical constraints on ocean circulation. A wide range of ocean heat transports and heat transport partitions are found, but with more modest variations in the total heat transport. Differences in the total heat transport are associated with the presence or absence of polar ice which is found to be sensitive to the ability of the ocean to carry heat to high latitudes. These model results, as well as data, are analyzed in the context of earlier work suggesting that the total meridional heat transport should be insensitive to the details of the atmospheric and oceanic circulation. The heat transport partitions in these aqua-planet calculations are also analyzed. The calculations all feature the same gross partition as the present climate with the ocean dominating near the equator and the atmosphere dominating at middle and high latitudes. While this suggests that this overall partition may be a robust feature of the climate system, there are important differences associated with the presence or absence of a meridional barrier to zonal flow in the ocean.(cont.) These results are diagnosed in the context of simple models and scalings which compare the strength of the atmospheric and oceanic circulations and the energy contrasts across the flows. Parallels are drawn with present and paleo climate. Finally, I produce a new estimate of the total meridional heat transport employing the method of minimum variance estimation, data from the Clouds and the Earth's Radiant Energy System instruments, and a prior estimate. This new estimate yields a peak poleward heat transport of 5.6 ± 0.8 PW at 35°N and 35°S with a northward transport of 0.1 ± 0.9 PW at the equator. This represents a 27% reduction in the standard error relative to the prior estimate. An estimate of the partition is made using direct ocean heat transport estimates with the atmospheric component computed as a residual.by Daniel Enderton.Ph.D