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Natalia Deligne

Pululagua diffusion coefficient

I noticed today that the diffusion coefficient provided for the Pululagua 2450 BP eruption is 92066, which is considerably more than the Cerro Negro (568) and Irazu (800) values, and also outside the recommended range of ~100 - 10,000 provided in the manual. Is this a typo? What should the value be?

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    Costanza Bonadonna

    Dear Natalia,

    The DC reported in Volentik et al. 2010 paper for Pululagua are correct. They are, however, very different from those of Cerro Negro and Irazu (and other eruptions) simply because the Pululagua eruption occurred with no wind.

    Consider that the DC in all analytical models (eg ASHFALL, HAZMAP, TEPHRA2) is simply an empirical parameters that accounts for many physical processes that are not fully described in these models. Consider that these analytical models are typically used for hazard assessment, and, therefore, they require fast computation times. This is why some physical processes are not fully described, but they are parameterized with simple empirical parameters, such as the DC. As a result, the DC is not only related to the actual atmospheric DC and can vary a lot from model to model and from eruption to eruption. This is also why it is a good practice to derive empirical parameters, as DC, from model inversions in order to find the best fit (e.g. Volentik et al. 2010; Connor and Connor 2006: http://scholarcommons.usf.edu/geo_facpub/1040/).

    In the particular case of Pululagua, the DC is very high because analytical models do not describe the horizontal cloud spreading due to gravity. As a result, the DC of large eruptions that occur in no wind will have to account also for the gravitational spreading. In contrast, Cerro Negro eruption is significantly smaller and was affected by higher wind than Pululagua. As a result, the associated DC will be smaller.

    Does that make sense?

    Best Regards

    Costanza Bonadonna

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