Wenying Su

Advances in Understanding Top-of-Atmosphere Radiation Variability from Satellite Observations

This paper highlights how the emerging record of satellite observations from the Earth Observation System (EOS) and A-Train constellation are advancing our ability to more completely document and understand the underlying processes associated with variations in the Earth’s top-of-atmosphere (TOA) radiation budget. Large-scale TOA radiation changes during the past decade are observed to be within 0.5xa0Wm−2 per decade based upon comparisons between Clouds and the Earth’s Radiant Energy System (CERES) instruments aboard Terra and Aqua and other instruments. Tropical variations in emitted outgoing longwave (LW) radiation are found to closely track changes in the El Niño-Southern Oscillation (ENSO). During positive ENSO phase (El Niño), outgoing LW radiation increases, and decreases during the negative ENSO phase (La Niña). The coldest year during the last decade occurred in 2008, during which strong La Nina conditions persisted throughout most of the year. Atmospheric Infrared Sounder (AIRS) observations show that the lower temperatures extended throughout much of the troposphere for several months, resulting in a reduction in outgoing LW radiation and an increase in net incoming radiation. At the global scale, outgoing LW flux anomalies are partially compensated for by decreases in midlatitude cloud fraction and cloud height, as observed by Moderate Resolution Imaging Spectrometer and Multi-angle Imaging SpectroRadiometer, respectively. CERES data show that clouds have a net radiative warming influence during La Niña conditions and a net cooling influence during El Niño, but the magnitude of the anomalies varies greatly from one ENSO event to another. Regional cloud-radiation variations among several Terra and A-Train instruments show consistent patterns and exhibit marked fluctuations at monthly timescales in response to tropical atmosphere-ocean dynamical processes associated with ENSO and Madden–Julian Oscillation.

Multi-Instrument Comparison of Top-of-Atmosphere Reflected Solar Radiation

Abstract Observations from the Clouds and the Earth’s Radiant Energy System (CERES), Moderate Resolution Imaging Spectroradiometer (MODIS), Multiangle Imaging Spectroradiometer (MISR), and Sea-Viewing Wide-Field-of-View Sensor (SeaWiFS) between 2000 and 2005 are analyzed in order to determine if these data are meeting climate accuracy goals recently established by the climate community. The focus is primarily on top-of-atmosphere (TOA) reflected solar radiances and radiative fluxes. Direct comparisons of nadir radiances from CERES, MODIS, and MISR aboard the Terra satellite reveal that the measurements from these instruments exhibit a year-to-year relative stability of better than 1%, with no systematic change with time. By comparison, the climate requirement for the stability of visible radiometer measurements is 1% decade−1. When tropical ocean monthly anomalies in shortwave (SW) TOA radiative fluxes from CERES on Terra are compared with anomalies in Photosynthetically Active Radiation (PAR) from SeaWiF...

Aerosol and cloud interaction observed from high spectral resolution lidar data

[1]xa0Recent studies utilizing satellite retrievals have shown a strong correlation between aerosol optical depth (AOD) and cloud cover. However, these retrievals from passive sensors are subject to many limitations, including cloud adjacency (or three-dimensional) effects, possible cloud contamination, uncertainty in the AOD retrieval. Some of these limitations do not exist in High Spectral Resolution Lidar (HSRL) observations; for instance, HSRL observations are not affected by cloud adjacency effects, are less prone to cloud contamination, and offer accurate aerosol property measurements (backscatter coefficient, extinction coefficient, lidar ratio, backscatter Angstrom exponent, and aerosol optical depth) at a fine spatial resolution (<100 m) in the vicinity of clouds. Hence the HSRL provides an important data set for studying aerosol and cloud interaction. In this study, we statistically analyze aircraft-based HSRL profiles according to their distance from the nearest cloud, assuring that all profile comparisons are subject to the same large-scale meteorological conditions. Our results indicate that AODs from HSRL are about 8–17% higher in the proximity of clouds (∼100 m) than far away from clouds (4.5 km), which is much smaller than the reported cloud three-dimensional effect on AOD retrievals. The backscatter and extinction coefficients also systematically increase in the vicinity of clouds, which can be explained by aerosol swelling in the high relative humidity (RH) environment and/or aerosol growth through in-cloud processing (albeit not conclusively). On the other hand, we do not observe a systematic trend in lidar ratio; we hypothesize that this is caused by the opposite effects of aerosol swelling and aerosol in-cloud processing on the lidar ratio. Finally, the observed backscatter Angstrom exponent (BAE) does not show a consistent trend because of the complicated relationship between BAE and RH. We demonstrate that BAE should not be used as a surrogate for Angstrom exponent, especially at high RH.

Direct Aerosol Radiative Forcing Uncertainty Based on a Radiative Perturbation Analysis

To provide a lower bound for theuncertaintyin measurement-based clear- and all-skydirectaerosolradiative forcing (DARF), a radiative perturbation analysis is performed for the ideal case in which the perturbations in global mean aerosol properties are given by published values of systematic uncertainty in Aerosol Robotic Network (AERONET) aerosol measurements. DARF calculations for base-state climatological cloud and aerosol properties over ocean and land are performed, and then repeated after perturbing individual aerosol optical properties (aerosol optical depth, single-scattering albedo, asymmetry parameter, scale height, and anthropogenic fraction) from their base values, keeping all other parameters fixed. The total DARF uncertainty from all aerosol parameters combined is 0.5‐1.0 W m 22 , a factor of 2‐4 greater than the value cited in the Intergovernmental Panel on Climate Change’s (IPCC’s) Fourth Assessment Report. Most of the total DARF uncertaintyinthisanalysisisassociatedwithsingle-scatteringalbedouncertainty.Owingtothegreatersensitivity tosingle-scatteringalbedoincloudycolumns,DARFuncertaintyinall-skyconditionsisgreaterthaninclear-sky conditions, even though the global mean clear-sky DARF is more than twice as large as the all-sky DARF.

Effects of land use in Southwest Australia: 1. Observations of cumulus cloudiness and energy fluxes

The Southwest Australian region has large homogeneous tracts of differing vegetation types separated by a sharp transition called the vermin or bunny fence which runs for almost 750 km. Seasonal winter agriculture is found to the west of the fence, whereas to the east native perennial vegetation grows. Geostationary Meteorological Satellite-5 imagery are used to construct monthly cumulus cloud frequency of occurrence maps for the region 0800 to 1500 LT in hourly increments for 1999 and 2000. Moderate Resolution Imaging Spectroradiometer (MODIS) imagery are used to retrieve regional values of surface temperature, albedo, Normalized Difference Vegetation Index, fractional soil moisture availability, sensible and latent heat fluxes. High spatial resolution Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imagery are used to retrieve detailed values along the fence. MODIS imagery also is utilized to retrieve cloud optical thickness, droplet sizes, and liquid water paths. This study shows that higher soil moisture availability is found over agricultural areas during winter (September) and over native vegetation areas during summer (December). Latent heat fluxes are higher over native vegetation than over agricultural areas during summer, while sensible heat fluxes are lower. Cumulus clouds occur with higher frequency and have higher optical thicknesses, cloud liquid water contents, and effective radii over agricultural areas during the winter and over native perennial vegetation during the dry summer. This is due to higher latent heat fluxes and available energy over agriculture during winter and over native vegetation during summer. We conclude that land use differences result in differences in available soil moisture and surface energy fluxes, which in turn lead to the observed preferential enhancement of cumulus cloudiness and cumulus cloud properties.

An estimate of aerosol indirect effect from satellite measurements with concurrent meteorological analysis

[1]xa0Many studies have used satellite retrievals to investigate the effect of aerosols on cloud properties, but these retrievals are subject to artifacts that can confound interpretation. Additionally, large-scale meteorological differences over a study region dominate cloud dynamics and must be accounted for when studying aerosol and cloud interactions. We have developed an analysis method which minimizes the effect of retrieval artifacts and large-scale meteorology on the assessment of the aerosol indirect effect. The method divides an oceanic study region into 1° × 1° grid boxes and separates the grid boxes into two populations according to back trajectory analysis: one population contains aerosols of oceanic origin, and the other population contains aerosols of continental origin. We account for variability in the large-scale dynamical and thermodynamical conditions by stratifying these two populations according to vertical velocity (at 700 hPa) and estimated inversion strength and analyze differences in the aerosol optical depths, cloud properties, and top of atmosphere (TOA) albedos. We also stratify the differences by cloud liquid water path (LWP) in order to quantify the first aerosol indirect effect. We apply our method to a study region off the west coast of Africa and only consider single-layer low-level clouds. We find that grid boxes associated with aerosols of continental origin have higher cloud fraction than those associated with oceanic origin. Additionally, we limit our analysis to those grid boxes with cloud fractions larger than 80% to ensure that the two populations have similar retrieval biases. This is important for eliminating the retrieval biases in our difference analysis. We find a significant reduction in cloud droplet effective radius associated with continental aerosols relative to that associated with oceanic aerosols under all LWP ranges; the overall reduction is about 1.0 μm, when cloud fraction is not constrained, and is about 0.5 μm, when cloud fraction is constrained to be larger than 80%. We also find significant increases in cloud optical depth and TOA albedo associated with continental aerosols relative to those associated with oceanic aerosols under all LWP ranges. The overall increase in cloud optical depth is about 0.6, and the overall increase in TOA albedo is about 0.021, when we do not constrained cloud fraction. The overall increases in cloud optical depth and TOA albedo are 0.4 and 0.008, when we only use grid boxes with cloud fraction larger than 80%.

Deriving surface ultraviolet radiation from CERES surface and atmospheric radiation budget: Methodology

[1]xa0We describe an algorithm that retrieves the surface UVB (280–315 nm) and UVA (315–400 nm) irradiances from the Surface and Atmosphere Radiation Budget (SARB) product of Clouds and the Earths Radiant Energy System (CERES). The SARB product we use here routinely calculates the vertical profiles of shortwave, longwave, and window channel irradiances with inputs of retrievals from imagers collocated with CERES. The top of the atmosphere broadband irradiance from SARB is constrained by CERES broadband irradiance. The shortwave spectrum in the SARB calculation is divided into 15 bands, and the two ultraviolet spectral bands, band 5 (298.5–322.5 nm) and band 6 (322.5–357.5 nm), are used to generate surface UVB and UVA irradiances. In this study, we develop a set of ratio lookup tables to derive surface UVB and UVA irradiances from SARB band 5 and band 6 outputs. We show that the ratio of band 5 to UVB irradiance is sensitive to total column ozone, solar zenith angle, surface albedo, and the atmospheric profile in cloud-free conditions; in cloudy conditions, the ratio of band 5 to UVB irradiance is also sensitive to cloud optical depth and height. Additionally, we show that the ratio of band 6 to UVA irradiance is sensitive to solar zenith angle, surface albedo, and cloud optical depth. We also derive a UV index from the UVB irradiance. Our algorithm may be applied at any surface elevation or surface type, including snow and ice. Surface UV irradiances derived from the lookup table that we created agree well with those computed by the high-resolution, multistream radiative transfer code, with differences ranging from −10% to +4% for UVB and UVA irradiances. The relative differences for the UV index are higher, ranging from −26% to +16%.

Clouds and the Earth’s Radiant Energy System (CERES) Energy Balanced and Filled (EBAF) Top-of-Atmosphere (TOA) Edition-4.0 Data Product

The theoretical bases for the Release 1 algorithms that will be used to process satellite data for investigation of the Clouds and Earths Radiant Energy System (CERES) are described. The architecture for software implementation of the methodologies is outlined. Volume 2 details the techniques used to geolocate and calibrate the CERES scanning radiometer measurements of shortwave and longwave radiance to invert the radiances to top-of-the-atmosphere (TOA) and surface fluxes following the Earth Radiation Budget Experiment (ERBE) approach, and to average the fluxes over various time and spatial scales to produce an ERBE-like product. Spacecraft ephemeris and sensor telemetry are used with calibration coefficients to produce a chronologically ordered data product called bidirectional scan (BDS) radiances. A spatially organized instrument Earth scan product is developed for the cloud-processing subsystem. The ERBE-like inversion subsystem converts BDS radiances to unfiltered instantaneous TOA and surface fluxes. The TOA fluxes are determined by using established ERBE techniques. Hourly TOA fluxes are computed from the instantaneous values by using ERBE methods. Hourly surface fluxes are estimated from TOA fluxes by using simple parameterizations based on recent research. The averaging process produces daily, monthly-hourly, and monthly means of TOA and surface fluxes at various scales. This product provides a continuation of the ERBE record.

CERES Top-of-Atmosphere Earth Radiation Budget Climate Data Record: Accounting for in-Orbit Changes in Instrument Calibration

The Clouds and the Earth’s Radiant Energy System (CERES) project provides observations of Earth’s radiation budget using measurements from CERES instruments onboard the Terra, Aqua and Suomi National Polar-orbiting Partnership (S-NPP) satellites. As the objective is to create a long-term climate data record, it is necessary to periodically reprocess the data in order to incorporate the latest calibration changes and algorithm improvements. Here, we focus on the improvements and validation of CERES Terra and Aqua radiances in Edition 4, which are used to generate higher-level climate data products. Onboard sources indicate that the total (TOT) channel response to longwave (LW) radiation has increased relative to the start of the missions by 0.4% to 1%. In the shortwave (SW), the sensor response change ranges from −0.4% to 0.6%. To account for in-orbit changes in SW spectral response function (SRF), direct nadir radiance comparisons between instrument pairs on the same satellite are made and an improved wavelength dependent degradation model is used to adjust the SRF of the instrument operating in a rotating azimuth plane scan mode. After applying SRF corrections independently to CERES Terra and Aqua, monthly variations amongst these instruments are highly correlated and the standard deviation in the difference of monthly anomalies is 0.2 Wm−2 for ocean and 0.3 Wm−2 for land/desert. Additionally, trends in CERES Terra and Aqua monthly anomalies are consistent to 0.21 Wm−2 per decade for ocean and 0.31 Wm−2 per decade for land/desert. In the LW, adjustments to the TOT channel SRF are made to ensure that removal of the contribution from the SW portion of the TOT channel with SW channel radiance measurements during daytime is consistent throughout the mission. Accordingly, anomalies in day–night LW difference in Edition 4 are more consistent compared to Edition 3, particularly for the Aqua land/desert case.

Photosynthetically active radiation from Clouds and the Earth's Radiant Energy System (CERES) products

[1]xa0We describe a method that retrieves surface photosynthetically active radiation (PAR) and its direct and diffuse components from the Surface and Atmospheric Radiation Budget (SARB) product of Clouds and the Earths Radiant Energy System (CERES). The shortwave spectrum in the SARB Edition 2 is calculated in 15 bands, 4 of which are used to develop the PAR, in conjunction with the look-up tables described in this paper. We apply these look-up tables to existing CERES Terra Edition 2 products. The new retrieved surface PAR is validated with LI-COR PAR measurements at seven Surface Radiation Budget Network (SURFRAD) sites using data from March 2000 to June 2005. The relative bias of retrieved all-sky PAR at the SURFRAD sites is 4.6% (positive sign indicating retrieval exceeds measurement), and 54% of the all-sky samples are within the ±10% uncertainty of the LI-COR PAR measurements. The satellite field-of-view (FOV) is more representative of the ground instrument FOV under clear conditions, so 89% of clear-sky retrievals are within the uncertainty of the LI-COR PAR measurements at SURFRAD sites with positive biases at most sites. The retrieved PAR is also validated at the Atmospheric Radiation Measurement (ARM) Southern Great Plains Central Facility (CF) site using data from October 2003 to June 2004 for those FOVs having both LI-COR and Rotating Shadowband Spectroradiometer (RSS) ground measurements; for this small domain, all-sky relative biases are again positive (1.9%) for LI-COR but negative (−4.2%) for RSS. The direct-to-diffuse ratio derived from CERES is smaller than that from RSS for both clear and cloudy conditions. CERES also retrieves the broadband shortwave insolation, and the relative biases for the broadband retrievals are much less than those for PAR at the above sites. It appears that some of the ground-based measurements of PAR do not have the fidelity of those for broadband shortwave insolation.