Spatiotemporal Continuity of Sequential Rain Suggested by 3-D Variogram |
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| Authors: | Tetsuya Shoji |
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| Institution: | (1) School of Frontier Sciences, The University of Tokyo, Kashiwa 277-8583, Japan |
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| Abstract: | A series of rainfalls observed in central Japan from noon on the 13th to midnight on the 14th, August 1999 (36 h), has been
analyzed by spatiotemporal variograms in order to reveal the continuity of rain precipitation in a 3-D space defined by geographic
coordinates and time. All instances of zero precipitation are considered, but have been treated as four different cases: case
0 excludes all zero data, case 1 includes a zero datum neighboring to each finite value, case 2 includes a zero neighboring
to each finite value and the next neighboring zero, and a fourth case (termed case A) includes all zeros. Hourly precipitation
has a statistical distribution best approximated by a Weibull model, and somewhat less well by a normal distribution, in all
four cases. A rectangular variogram of measured values of total precipitation shows that the best continuity appears approximately
along the N-S direction (the ranges given by directional variograms are 500 and 80 km in the N-S and W-E directions, respectively).
In contrast, temporally stacked rectangular variograms of hourly precipitation shows that the best continuity direction is
W-E in all cases (the ranges in case A are 50 and 100 km along the N-S and W-E directions, respectively). A spatial variogram
gives a spatial range independently of time, whereas a temporal variogram gives a temporal range. When geographic coordinates
are normalized by the spatial range (here 80 km given by the temporally stacked omnidirectional variogram in case A), and
time is normalized by the temporal range (here 7 h given by the spatially stacked temporal variogram), geographic coordinates
and time can be treated as equivalent variables. Consequently, a spatiotemporal variogram can be calculated along a given
direction in 3-D space using the normalized coordinates. The continuity direction of a series of rainfalls can be best understood
by display on a Wulff net, where each range value is written at a point corresponding to the direction. The direction of the
best continuity is N0°W+20° in the normalized space. A rectangular variogram in the normalized space, in which the horizontal
and vertical axes represent N-S direction and time, respectively, suggests that the series of heavy rainfalls examined here
had a continuity pattern that was elongated from west to east (the range values are 20–30 km and 100 km along N-S and W-E,
respectively), and that migrated from south to north with a speed of 30 km/h. |
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| Keywords: | Rain hazard spatiotemporal variogram Wullf net stacking Japan |
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