Micro-spatial behavior of volcanic ashfall at low- and mid-rise building areas in a large city is investigated by numerical ashfall simulations with individual buildings approximately resolved. The effects of the inertial force acting on ash particles are included in the simulations to account for the deviation of ash flow from air flow caused by the large curvatures of air streams around buildings, whereas the inertia force can be neglected in the large-scale atmospheric ash-dispersion processes treated in operational ashfall prediction models. The ashfall rate and the spatial ash concentration in the urban boundary layer are calculated for six particle sizes (from -1 to 4 on the phi scale) under windy conditions. Dependence on the ash-particle size of the distribution of the ashfall rate at the ground, as well as the spatial concentration within the building canopy resulting from the disturbed wind caused by buildings, is shown to be apparent. The results also indicate that the spatial ash concentration can locally exceed 150% of the large-scale values that might be provided by ashfall predictions with a horizontal resolution on the order of kilometers. Understanding small-scale features of ashfall in cities is stressed to be important for developing measures against ashfall events.

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