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    The Estimation of Snowfall Rate Using Visibility

    Source: Journal of Applied Meteorology:;1999:;volume( 038 ):;issue: 010::page 1542
    Author:
    Rasmussen, Roy M.
    ,
    Vivekanandan, Jothiram
    ,
    Cole, Jeffrey
    ,
    Myers, Barry
    ,
    Masters, Charles
    DOI: 10.1175/1520-0450(1999)038<1542:TEOSRU>2.0.CO;2
    Publisher: American Meteorological Society
    Abstract: The relationship between liquid equivalent snowfall rate and visibility is investigated using data collected at the National Center for Atmospheric Research Marshall Snowfall Test Site during two winter field seasons and using theoretical relationships. The observational data include simultaneous liquid equivalent snowfall rate, crystal types, and both automated and manual visibility measurements. Theoretical relationships between liquid equivalent snowfall rate and visibility are derived for 27 crystal types, and for ?dry? and ?wet? aggregated snowflakes. Both the observations and theory show that the relationship between liquid equivalent snowfall rate and visibility depends on the crystal type, the degree of riming, the degree of aggregation, and the degree of wetness of the crystals, leading to a large variation in the relationship between visibility and snowfall rate. Typical variations in visibility for a given liquid equivalent snowfall rate ranged from a factor of 3 to a factor of 10, depending on the storm. This relationship is shown to have a wide degree of scatter from storm to storm and also during a given storm. The main cause for this scatter is the large variation in cross-sectional area to mass ratio and terminal velocity for natural snow particles. It also is shown that the visibility at night can be over a factor of 2 greater than the visibility during the day for the same atmospheric extinction coefficient. Since snowfall intensity is defined by the U.S. National Weather Service using visibility, this day/night difference in visibility results in a change in snowfall intensity category caused by only whether it is day or night. For instance, a moderate snowfall intensity during the day will change to a light snowfall intensity at night, and a heavy snowfall intensity during the day will change to a moderate snowfall intensity at night, for the same atmospheric extinction coefficient. Thus, the standard relationship between snowfall intensity and visibility used by many national weather services (1/4 mile or less visibility corresponds to heavy snowfall intensity, between 5/16 and 5/8 mile corresponds to moderate intensity, and greater than 5/8 mile corresponds to light intensity) does not always provide the correct indication of actual liquid equivalent snowfall rate because of the variations in snow type and the differences in the nature of visibility targets during day and night. This false indication may have been a factor in previous ground-deicing accidents in which light snow intensity was reported based on visibility, when in fact the actual measured liquid equivalent snowfall rate was moderate to heavy.
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      The Estimation of Snowfall Rate Using Visibility

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    http://yetl.yabesh.ir/yetl1/handle/yetl/4148158
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    contributor authorRasmussen, Roy M.
    contributor authorVivekanandan, Jothiram
    contributor authorCole, Jeffrey
    contributor authorMyers, Barry
    contributor authorMasters, Charles
    date accessioned2017-06-09T14:07:11Z
    date available2017-06-09T14:07:11Z
    date copyright1999/10/01
    date issued1999
    identifier issn0894-8763
    identifier otherams-12781.pdf
    identifier urihttp://onlinelibrary.yabesh.ir/handle/yetl/4148158
    description abstractThe relationship between liquid equivalent snowfall rate and visibility is investigated using data collected at the National Center for Atmospheric Research Marshall Snowfall Test Site during two winter field seasons and using theoretical relationships. The observational data include simultaneous liquid equivalent snowfall rate, crystal types, and both automated and manual visibility measurements. Theoretical relationships between liquid equivalent snowfall rate and visibility are derived for 27 crystal types, and for ?dry? and ?wet? aggregated snowflakes. Both the observations and theory show that the relationship between liquid equivalent snowfall rate and visibility depends on the crystal type, the degree of riming, the degree of aggregation, and the degree of wetness of the crystals, leading to a large variation in the relationship between visibility and snowfall rate. Typical variations in visibility for a given liquid equivalent snowfall rate ranged from a factor of 3 to a factor of 10, depending on the storm. This relationship is shown to have a wide degree of scatter from storm to storm and also during a given storm. The main cause for this scatter is the large variation in cross-sectional area to mass ratio and terminal velocity for natural snow particles. It also is shown that the visibility at night can be over a factor of 2 greater than the visibility during the day for the same atmospheric extinction coefficient. Since snowfall intensity is defined by the U.S. National Weather Service using visibility, this day/night difference in visibility results in a change in snowfall intensity category caused by only whether it is day or night. For instance, a moderate snowfall intensity during the day will change to a light snowfall intensity at night, and a heavy snowfall intensity during the day will change to a moderate snowfall intensity at night, for the same atmospheric extinction coefficient. Thus, the standard relationship between snowfall intensity and visibility used by many national weather services (1/4 mile or less visibility corresponds to heavy snowfall intensity, between 5/16 and 5/8 mile corresponds to moderate intensity, and greater than 5/8 mile corresponds to light intensity) does not always provide the correct indication of actual liquid equivalent snowfall rate because of the variations in snow type and the differences in the nature of visibility targets during day and night. This false indication may have been a factor in previous ground-deicing accidents in which light snow intensity was reported based on visibility, when in fact the actual measured liquid equivalent snowfall rate was moderate to heavy.
    publisherAmerican Meteorological Society
    titleThe Estimation of Snowfall Rate Using Visibility
    typeJournal Paper
    journal volume38
    journal issue10
    journal titleJournal of Applied Meteorology
    identifier doi10.1175/1520-0450(1999)038<1542:TEOSRU>2.0.CO;2
    journal fristpage1542
    journal lastpage1563
    treeJournal of Applied Meteorology:;1999:;volume( 038 ):;issue: 010
    contenttypeFulltext
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