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Thread: Atmospheric Aerosols and Climate

  1. #1 Atmospheric Aerosols and Climate 
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    New Publications:

    "Spatial and temporal variability in aerosol properties over the Mediterranean basin based on 6-year (2000-2006) MODIS data"
    Papadimas et al., 2008 JGR (JOURNAL OF GEOPHYSICAL RESEARCH-Atmospheres), VOL. 113, D11205, doi:10.1029/2007JD009189, 2008

    http://www.agu.org/pubs/crossref/200...JD009189.shtml

    Abstract

    The temporal variability of aerosol optical properties is investigated over the broader Mediterranean basin, with emphasis on aerosol optical depth (AOD) that is an effective measure of aerosol load. The study is performed using Collection 005 Level-3 mean daily spectral aerosol data from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on board the Terra and Aqua satellites, which cover the 6-year period from 2000 to 2006. The results of our analysis reveal a significant interannual variability of AOD in the study region. Specifically, the regional mean visible AOD over land and ocean has decreased over the period 2000–2006 by 20% in relative percentage terms (or by 0.04 in absolute terms). This tendency is statistically significant according to the Man-Kendall test. However, the decreasing tendency of AOD is not uniform over the whole basin. It appears mainly in the western parts of Iberian, Italian, and Balkan peninsulas (and coastal areas), as well as in the southern Anatolian peninsula. The analysis for summer (June to September) and winter (November to March) seasons revealed different tendencies in both AOD and precipitation. The summer-period AOD has decreased by 0.04 (or by 14%) probably due to decreased emission rates of anthropogenic pollution. In contrast, the winter AOD has increased by 0.03 (or 19%) mainly related to decreased precipitation (associated with an increasing tendency in the NAO index). The decreasing tendency in MODIS AOD is in good agreement with corresponding AOD tendencies based on data from Aerobot Robotic Network (AERONET) stations in the study region and ground based PM10 measurements at selected stations.


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  3. #2 MODIS Collections C005 and C004 
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    Assessment of the MODIS Collections C005 and C004 aerosol optical depth products over the Mediterranean basin
    Atmos. Chem. Phys., 9, 2987-2999, 2009

    http://www.atmos-chem-phys.net/9/298...2987-2009.html

    C. D. Papadimas, N. Hatzianastassiou, N. Mihalopoulos, M. Kanakidou, B. D. Katsoulis, and I. Vardavas

    Abstract
    The second generation Collection 005 (C005) MODIS operational algorithm for retrieval of aerosol properties was evaluated and validated for the greater Mediterranean basin (29.5 N46.5 N and 10.5 W38.5 E), a region with an atmosphere under siege by air pollution and diminishing water resources that are exacerbated by high aerosol loads and climatic change. The present study aims to quantify the differences between the C005 and the previous (C004) MODIS collections, and re-assess the results of previous studies that have been performed for the region using MODIS C004 aerosol optical depth (AOD) products. Daily data of AOD from EOS-Terra covering the 6-year period 20002006 were taken from both C005 and C004 Level-3 datasets, and were inter-compared and validated against ground-based measurements from 29 AERONET stations. The C005 data were found to significantly better agree with the AERONET data than those of C004. The correlation coefficient between MODIS and AERONET was found to increase from 0.66 to 0.76 and the slope of linear regression MODIS/AERONET from 0.79 to 0.85. The MODIS C005 data still overestimate/underestimate the AERONET AOD values smaller/larger than 0.25, but to a much smaller extent than C004 data. The better agreement of C005 with AERONET data arises from the generally lower C005 values, with regional mean AOD values equal to 0.27 and 0.22 for C004 and C005, respectively. This decrease, however, is not uniform over the region and involves a significant decrease over land and a small increase over the ocean for AOD values greater than 0.1 (opposite changes were found under aerosol-clean conditions). Both data sets indicate a decrease in the regional mean AOD over the period 20002006, equal to 20% based on C005 and 17% based on C004 datasets, though the intra-annual and inter-annual variation did not change significantly, thus indicating a systematic correction to C004 values.

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  4. #3  
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    Are "green" efforts - like field irrigation, fire suppression, cover-cropping - depleting the atmosphere of aerosols? If less aerosols, then less clouds, right? Then warming must follow.
    A pong by any other name is still a pong. -williampinn
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  5. #4  
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    Quote Originally Posted by Pong
    less aerosols, then less clouds, right? Then warming must follow.
    Not necessarily. Low level clouds have a cooling effect because they are highly reflective of incoming radiation. High level clouds are excellent absorbers of IR due to their cold temperature. The net effect of cloud cover is warming.

    If there's a reduction in cloud cover, the effect, either warming, cooling, or neutral, will depend on which clouds are reduced more - low level or high level. It's complex and by no means certain that warming would follow a reduction in cloud cover.
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    Quote Originally Posted by Pong
    If less aerosols, then less clouds, right? Then warming must follow.
    Not necessarily. If there remain sufficient cloud condensation nuclei, which is often the case over land, the amount of cloud cover will remain relatively unaffected. Another factor is the types of clouds; at low latitudes, decreasing particulates might block less solar radiation and result in more surface warming which in turn drives more convective clouds.

    Lastly if the lower number of aerosols also lowers the number of ice nuclei, it will makes rain production less effective therefore lowering precipitation and less scavenging of aerosols. Lack of sufficient ice nuclei is pretty common, which is why supercooling conditions are a common aviation hazard and precipitation processes often aren't optimum for low and mid level clouds.
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  7. #6  
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    Quote Originally Posted by Bunbury
    Not necessarily. Low level clouds have a cooling effect because they are highly reflective of incoming radiation. High level clouds are excellent absorbers of IR due to their cold temperature. The net effect of cloud cover is warming.
    I'm probably being dense here, but that seems wrong to me. My reasoning is that before we consider heat retention of the atmosphere we must consider if/how we got that heat. Heat from the sun, not heat presumed to start beneath high altitude clouds greenhouse gasses etc. You can't heat your home by insulating it.

    If you're telling me that high altitude cloud has lower albedo than (unclouded) Earth surface, my sources say otherwise.

    Also the common observation that cloudy weather precede a drop in temperature is real hard to dispel.

    Those are my thoughts and if I'm in error please help me out.

    Regarding "not necessarily". That casts general doubt over everything I've said, and isn't helpful. Could you have said "Yeh, mostly, except..."



    @ Lynx_Fox. I meant "aerosols" including "cloud condensation nuclei". I meant those aerosols like dust or salt that act as nuclei.

    Rephrase: If less CCN, then less clouds?

    What you said about unwashed air makes perfect sense but it's circular. You may as well point to any corrective feedback and say no end of it can starve. Yet you do say ice nuclei are scarce. Why? Why not others?
    A pong by any other name is still a pong. -williampinn
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  8. #7  
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    Quote Originally Posted by Pong
    Quote Originally Posted by Bunbury
    Not necessarily. Low level clouds have a cooling effect because they are highly reflective of incoming radiation. High level clouds are excellent absorbers of IR due to their cold temperature. The net effect of cloud cover is warming.
    I'm probably being dense here, but that seems wrong to me. My reasoning is that before we consider heat retention of the atmosphere we must consider if/how we got that heat. Heat from the sun, not heat presumed to start beneath high altitude clouds greenhouse gasses etc. You can't heat your home by insulating it.
    High clouds are thin and let in a far greater amount of Visible solar radiation than dense lower clouds. The combination of thin to Visible and effective blocking of IR usually result in them being a net increase in lower atmospheric temperature.


    @ Lynx_Fox. I meant "aerosols" including "cloud condensation nuclei". I meant those aerosols like dust or salt that act as nuclei.

    Rephrase: If less CCN, then less clouds?
    Not by much because most particles can act as CCN and there's usually plenty of them. Even changes by 4x only results in small percentage of cloud change (e.g., cloud density, albedo, lifting condensation level by less than 5%)

    What you said about unwashed air makes perfect sense but it's circular. You may as well point to any corrective feedback and say no end of it can starve. Yet you do say ice nuclei are scarce. Why? Why not others?
    Fewer natural particles have the crystallographic structure of ice that make them good Ice nuclei. Also ice nuclei concentration has a pretty dramatic effect on the altitude of the freezing layer, and altitude from where the precip formation starts, because it can the ice creation a temp from -40C to 0C.
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  9. #8  
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    Quote Originally Posted by Pong
    If you're telling me that high altitude cloud has lower albedo than (unclouded) Earth surface, my sources say otherwise.
    Huh? Where did I say that? I was comparing high and low level clouds with each other, not with the Earth's surface.

    If this helps, here's a quote from Gavin Schmidt in his book "Climate Change, Picturing the Science"

    The net effect of clouds depends on their altitude. Low, thick clouds reflect more sunlight back to space than they absorb, cooling the surface. For high, thin clouds the balance is the opposite - they also reflect sunlight, but due to the cold temperature aloft they are particularly effective at absorbing infrared.....In most models the high cloud positive feedback dominates, so that globally clouds amplify greenhouse-induced warming.
    Regarding "not necessarily". That casts general doubt over everything I've said, and isn't helpful. Could you have said "Yeh, mostly, except..."
    I thought "not necessarily" was a polite way of saying that a definitve statement like "warming must follow" is not justified by the current state of knowledge. I don't see why you would take offense at that.
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  10. #9  
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    Okay, so you both are saying that high altitude cloud even considered apart from low cloud or clear (not relatively) are net heaters, because they reflect outgoing radiation better than incoming. This "pointing down" happens because they admit much incoming VR, which then becomes IR they do reflect back. This works even not taking as granted an already warm Earth i.e. an Earth surface as cold as space would gain heat under these clouds.

    ***

    Plentiful supply of CCN I could accept on faith, but I see many examples of increased aerosol causing clouds. Large examples like the North Pacific cloud assembly line, where Central Asian dust combines with tropics humidity; or local ones like cloud forming in the wake of jets.

    I think that insufficient water or nuclei (or both) is pretty common. In context of global warming, I'm less concerned with vapour shortage since increased evaporation seems inevitable and besides it's beyond our control. Human history has not directly forced it either way. Aerosols though we certainly force significantly and have been for hundreds or thousands of years.
    A pong by any other name is still a pong. -williampinn
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  11. #10  
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    Thanks for paper, I want to know one thing , when we try to correlate the satellite data with our ground based observation ( which contains a large spans of values for a single day) one question always come in my mind that that satellite is taking observation only at its overpass time and involves a large region, but AOD (Aerosol Optical Depth) varies from morning to evening. So how to use satellite data effectively.
    And one more doubt, I have I have seen in papers that MODIS data at 1X1 KM resolution , how to get it.




    Quote Originally Posted by energyepsilon View Post
    New Publications:

    "Spatial and temporal variability in aerosol properties over the Mediterranean basin based on 6-year (2000-2006) MODIS data"
    Papadimas et al., 2008 JGR (JOURNAL OF GEOPHYSICAL RESEARCH-Atmospheres), VOL. 113, D11205, doi:10.1029/2007JD009189, 2008

    Spatial and temporal variability in aerosol properties over the Mediterranean basin based on 6-year (2000–2006) MODIS data

    Abstract

    The temporal variability of aerosol optical properties is investigated over the broader Mediterranean basin, with emphasis on aerosol optical depth (AOD) that is an effective measure of aerosol load. The study is performed using Collection 005 Level-3 mean daily spectral aerosol data from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on board the Terra and Aqua satellites, which cover the 6-year period from 2000 to 2006. The results of our analysis reveal a significant interannual variability of AOD in the study region. Specifically, the regional mean visible AOD over land and ocean has decreased over the period 2000–2006 by 20% in relative percentage terms (or by 0.04 in absolute terms). This tendency is statistically significant according to the Man-Kendall test. However, the decreasing tendency of AOD is not uniform over the whole basin. It appears mainly in the western parts of Iberian, Italian, and Balkan peninsulas (and coastal areas), as well as in the southern Anatolian peninsula. The analysis for summer (June to September) and winter (November to March) seasons revealed different tendencies in both AOD and precipitation. The summer-period AOD has decreased by 0.04 (or by 14%) probably due to decreased emission rates of anthropogenic pollution. In contrast, the winter AOD has increased by 0.03 (or 19%) mainly related to decreased precipitation (associated with an increasing tendency in the NAO index). The decreasing tendency in MODIS AOD is in good agreement with corresponding AOD tendencies based on data from Aerobot Robotic Network (AERONET) stations in the study region and ground based PM10 measurements at selected stations.
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