Near‐cloud aerosol properties from the 1 km resolution MODIS ocean product

Abstract

This study examines aerosol properties in the vicinity of clouds by analyzing high-resolution atmospheric correction parameters provided in the MODIS (Moderate Resolution Imaging Spectroradiometer) ocean color product. The study analyzes data from a 2 week long period of September in 10 years, covering a large area in the northeast Atlantic Ocean. The results indicate that on the one hand, the Quality Assessment (QA) flags of the ocean color product successfully eliminate cloud-related uncertainties in ocean parameters such as chlorophyll content, but on the other hand, using the flags introduces a sampling bias in atmospheric products such as aerosol optical thickness (AOT) and Angstrom exponent. Therefore, researchers need to select QA flags by balancing the risks of increased retrieval uncertainties and sampling biases. Using an optimal set of QA flags, the results reveal substantial increases in optical thickness near clouds-on average the increase is 50% for the roughly half of pixels within 5 km from clouds and is accompanied by a roughly matching increase in particle size. Theoretical simulations show that the 50% increase in 550 nm AOT changes instantaneous direct aerosol radiative forcing by up to 8 W/m 2 and that the radiative impact is significantly larger if observed near-cloud changes are attributed to aerosol particles as opposed to undetected cloud particles. These results underline that accounting for near-cloud areas and understanding the causes of near-cloud particle changes are critical for accurate calculations of direct aerosol radiative forcing.

Effect of Cloud Fraction on Near-Cloud Aerosol Behavior in the MODIS Atmospheric Correction Ocean Color Product

“…The analysis used data collected over the northeast Atlantic Ocean during ten consecutive Septembers. Similar to the analysis of CALIPSO data [20], the results of MODIS data analysis showed that sampling of different cloud fractions near clouds and far from clouds can explain a significant but not dominant part (15%-30%) of the near-cloud AOT changes reported earlier [29]. For particle size, however, analyzing the Atlantic Ocean southwest of the United Kingdom yielded some opposite behaviors than the area near the Azores [20]: While sampling different cloud fractions near and far from clouds strengthened the near-cloud particle size changes in the composite statistics of the Azores, this effect weakened the near-cloud particle size changes in our composite statistics.…”

“…In contrast, the resampling made near-cloud particle size changes even stronger in our dataset-which implies that resampling would strengthen the particle size changes discussed in [29]. We also found that the exact method of resampling makes very little difference for optical thickness, and only a modest difference for particle size (characterized through the Angstrom exponent).…”

“…To help comparisons and putting earlier and current results in context, this paper analyzes a dataset that has been used earlier in [8,29]. (Previously, relationships between aerosol and cloud properties in the study region were also examined in [23].)…”

Response of Marine Boundary Layer Cloud Properties to Aerosol Perturbations Associated with Meteorological Conditions from the 19-Month AMF-Azores Campaign

“…There is also the limitation that cloud and aerosol properties cannot be obtained simultaneously over the same location. Recent studies have revealed that the aerosol optical depth (AOD) retrieved in the presence of nearby clouds can be significantly enhanced (Jeong and Li 2010;Várnai and Marshak 2014), which can lead to spurious correlations between aerosols and cloud properties (Costantino and Bréon 2013). These limitations can be ameliorated, or overcome, by using ground and in situ observations, which have already been used to investigate the influence of aerosols on cloud microphysical properties (Feingold et al 2001McComiskey et al 2009;Twohy et al 2013).…”

Near‐cloud aerosols in monsoon environment and its impact on radiative forcing

“…Under such scenarios, the situation is far more complicated due to changes in the physical and optical properties of aerosol in a humidified atmosphere near to the cloud zone [ Altaratz et al ., ], which has important consequences on the Earth's radiation balance [ Feingold and Morley , ]. Satellite and in situ observations have detected enhanced aerosol optical depth (AOD) near clouds, confirming radiative forcing anomalies when compared with cloud‐free zones [ Charlson et al ., ; Su et al ., ; Wen et al ., ; Várnai et al ., ; Várnai and Marshak , ]. On the other hand, new particles formed around growing convective clouds can also enhance the background aerosol number concentration and serve as cloud condensation nuclei that influence the radiative properties [ Hegg et al ., ; Perry and Hobbs , ; Clarke et al ., ; Heintzenberg and Charlson , ].…”