Characterizing 3-D structure of clouds is needed for a more complete understanding of the Earth’s radiative and latent heat fluxes. Here we develop and explore a ray casting algorithm applied to data ...from the Multi-angle Imaging SpectroRadiometer (MISR) onboard the Terra satellite, in order to reconstruct 3-D cloud volumes of observed clouds. The ray casting algorithm is first applied to geometrically simple synthetic clouds to show that, under the assumption of perfect, clear-conservative cloud masks, the reconstruction method yields overestimation in the volume whose magnitude depends on the cloud geometry and the resolution of the reconstruction grid relative to the image pixel resolution. The method is then applied to two hand-picked MISR scenes, fully accounting for MISR’s viewing geometry for reconstructions over the Earth’s ellipsoidal surface. The MISR Radiometric Camera-by-camera Cloud Mask (RCCM) at 1.1-km resolution and the custom cloud mask at 275-m resolution independently derived from MISR’s red, green, and blue channels are used as input cloud masks. A wind correction method, termed cloud spreading, is applied to the cloud masks to offset potential cloud movements over short time intervals between the camera views of a scene. The MISR cloud-top height product is used as a constraint to reduce the overestimation at the cloud top. The results for the two selected scenes show that the wind correction using the cloud spreading method increases the reconstructed volume up to 4.7 times greater than without the wind correction, and that the reconstructed volume generated from the RCCM is up to 3.5 times greater than that from the higher-resolution custom cloud mask. Recommendations for improving the presented cloud volume reconstructions, as well as possible future passive remote sensing satellite missions, are discussed.
The top-of-atmosphere (TOA) albedo is one of the key parameters in determining the Arctic radiation budget, with continued validation of its retrieval accuracy still required. Based on three years ...(2007, 2015, 2016) of summertime (May–September) observations from the Clouds and the Earth’s Radiant Energy System (CERES) and the Multi-angle Imaging SpectroRadiometer (MISR), collocated instantaneous albedos for overcast ocean and snow/ice scenes were compared within the Arctic. For samples where both instruments classified the scene as overcast, the relative root-mean-square (RMS) difference between the sample albedos grew as the solar zenith angle (SZA) increased. The RMS differences that were purely due to differential Bidirectional Reflectance Factor (BRF) anisotropic corrections ( σ A D M ) were estimated to be less than 4% for overcast ocean and overcast snow/ice when the SZA ≤ 70°. The significant agreement between the CERES and MISR strongly increased our confidence in using the instruments overcast cloud albedos in Arctic studies. Nevertheless, there was less agreement in the cloud albedos for larger solar zenith angles, where the RMS differences of σ A D M reached 13.5% for overcast ocean scenes when the SZA > 80°. Additionally, inconsistencies between the CERES and MISR scene identifications were examined, resulting in an overall recommendation for improvements to the MISR snow/ice mask and a rework of the MISR Albedo Cloud Designation (ACD) field by incorporating known strengths of the standard MISR cloud masks.
Soil aggregation plays a critical role in the maintenance of soil structure and crop productivity. Fertilization influences soil aggregation, especially by regulating soil organic carbon (SOC) and ...total nitrogen (TN) contents in aggregate fractions. Here, we conducted a fixed-site field experiment to quantify the effect of five N application rates: 0, 75, 150, 225, and 300 kg·N·ha−1, denoted as N0, N75, N150, N225, and N300, respectively, on soil aggregate stability, aggregate-associated SOC and TN sequestration and crop productivity. Soil aggregates were divided into >0.25 (>5, 5–2, 2–1, 1–0.5, 0.5–0.25) and <0.25 mm through wet and dry sieving methods. The results showed that long-term fertilization increased the proportion of macro-aggregates (>0.25 mm), decreased the proportion of micro-aggregates (<0.25 mm), and improved the aggregates stability. Compared with N0, the proportion of micro-aggregates in N225 was significantly decreased by 66.45% under wet sieving, while the proportion of >5 mm macro-aggregates in N225 was significantly increased by 19.24% under dry sieving (p < 0.05). With the increase in N application rate, the bulk SOC and TN contents first increased and then decreased, and the SOC and TN of N225 were significantly increased by 17.75% and 72.33% compared with N0 (p < 0.05). More specifically, fertilization promoted the distribution and enrichment of SOC and TN in macro-aggregates and reduced the C/N of the micro-aggregates and the contribution of SOC and TN in the micro-aggregates. Compared with N0, the contribution rate of macro-aggregates to SOC and TN of N225 under wet sieving was significantly increased by 84.13 and 17.18%, respectively, while the C/N of micro-aggregates of N225 under wet and dry sieving methods was significantly decreased by 45.95 and 31.74%, respectively (p < 0.05). Moreover, fertilization improved the yield, and N225 significantly increased the total yield by 80.68% compared with N0 (p < 0.05). In conclusion, N225 was the suitable N application for improving soil aggregate stability, carbon and nitrogen sequestration, and crop productivity on the Loess Plateau, China.
The predictability of the minimum sea ice extent (SIE) in the Arctic in September, especially for large anomaly years, is of strong current interest, given the rapid decline in sea ice amount. Our ...results show that June reflected solar radiation (RSR) is closely related to the underlying sea ice condition in that month and can be used to achieve September SIE predictions with good accuracy. The correlation coefficient between detrended June RSR and September SIE reaches 0.91 based on 16 year satellite observations, and the relatively high forecast skill using Modern‐Era Retrospective analysis for Research and Applications Version 2 reanalysis data is similar to or better than other complex prediction models. The results confirm the particular importance of the early summer sea ice state and help to explain the abrupt declines of September SIE in the 21st century (2007 and 2012).
Key Points
There is a strong lag relationship between June TOA RSR and September sea ice extent anomalies
June TOA RSR anomalies are mainly contributed from Pacific Sector's surface condition variations and less affected by clouds
TOA RSR is not properly represented by some of the reanalysis data sets, such as the ERA‐Interim
The operational cloud-motion tracking technique fails to retrieve atmospheric motion vectors （AMVs） in areas lacking cloud; and while water vapor shown in water vapor imagery can be used, the heights ...assigned to the retrieved AMVs are mostly in the upper troposphere. As the noise-equivalent temperature difference （NEdT） performance of FY-2E split win- dow （10.3-11.5 μm, 11.6-12.8 μm） channels has been improved, the weak signals representing the spatial texture of water vapor and aerosols in cloud-free areas can be strengthened with algorithms based on the difference principle, and applied in calculating AMVs in the lower troposphere. This paper is a preliminary summary for this purpose, in which the principles and algorithm schemes for the temporal difference, split window difference and second-order difference （SD） methods are introduced. Results from simulation and cases experiments are reported in order to verify and evaluate the methods, based on comparison among retrievals and the ＂truth＂. The results show that all three algorithms, though not perfect in some cases, generally work well. Moreover, the SD method appears to be the best in suppressing the surface temperature influence and clarifying the spatial texture of water vapor and aerosols. The accuracy with respect to NCEP 800 hPa reanalysis data was found to be acceptable, as compared with the accuracy of the cloud motion vectors.
Measurements on the Terra satellite by the Cloud and the Earth's Radiant Energy System (CERES), the Moderate Resolution Imaging Spectroradiometer (MODIS), and the Multiangle Imaging Spectroradiometer ...(MISR), between 2001 and 2015 over the polar regions, are analyzed in order to investigate the intercalibration differences between these instruments. Direct comparisons of colocated near‐nadir radiances from CERES, MODIS, and MISR show relative agreement within 2.4% decade−1. By comparison with the CERES shortwave broadband, MODIS Collection 6 is getting brighter, by 1.0 ± 0.7% decade−1 in the red band and 1.4 ± 0.7% decade−1 in the near infrared. MISR's red and near‐infrared bands, however, show darkening trends of −1.0 ± 0.6% decade−1 and −1.1 ± 0.6% decade−1, respectively. The CERES/MODIS or CERES/MISR visible and near IR radiance ratio is shown to have a significant negative correlation with precipitable water content over the Antarctic Plateau. The intercalibration results successfully correct the differential top‐of‐atmosphere trends in zonal albedos between CERES and MISR.
Key Points
Compared to CERES, MISR is darkening and MODIS is brightening, by ~1% per decade
The dependence of the spectral to broadband radiance ratio on precipitable water content is minimized by use of Dome C
The intercalibration of CERES and MISR successfully corrects the differential TOA albedo trends at low and middle latitudes
A solution to the common problem that sensors have in overestimating cloud fraction (CF) due to their finite resolution is implemented by the Multi‐angle Imaging SpectroRadiometer (MISR) to produce a ...resolution‐corrected CF product. Here we evaluate the efficacy of this product toward the development of an improved global cloud climatology. We found a large reduction in CF across various regions of the globe due to the correction, with reductions >0.4 in regions dominated by shallow cumulus clouds. The MISR resolution‐corrected CF lies within ±0.05–0.08 of 180 randomly selected scenes of 15‐m resolution Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data, where MISR uncorrected CF and Moderate Resolution Imaging Spectroradiometer (MODIS) CF overestimated this benchmark by 0.40–0.50. We conclude that the CF between 50°N and 50°S is much lower than that reported by the original MISR and MODIS and that the new MISR cloud product indeed provides improved estimates of CF.
Key Points
The 50°N to 50°S cloud fraction reduced from 0.65 to 0.47 based on new MISR cloud product that corrects for measurement resolution effects
Largest reductions (up to ~0.4) in cloud fraction occur over trade cumulus regions and smallest (<0.1) occur over stratocumulus regions
New MISR cloud fractions are within ±0.05–0.08 of regional ASTER benchmarks compared to overestimates of 0.4–0.5 from original MISR and MODIS
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