Environmental Engineering Reference
In-Depth Information
Figure 5.11
Correlation patterns associated with (a) air mass
storms and (b) low-pressure centres, during May-September
in Illinois. Source: After Huff and Shipp (1969).
5.11a), it is apparent that the gauges close to the central station are quite strongly
correlated. Nevertheless, by the distance of about 20 km the degree of correlation is low.
Again, this is probably due to the effect of local variability caused by the passage of
small summer convection storms. If, instead, we look at frontal storms or storms
associated with low-pressure systems, we get a different picture (Figure 5.11b). Now
most of the area shows a close correlation with the central gauge - indeed, almost a
perfect correlation - indicating that these more general storms affected the whole area
equally, despite occasions of variability referred to earlier.
These results indicate some of the atmospheric factors controlling rainfall variability.
Convectional storms give high levels of spatial variation, while cyclonic rainfall is
spatially much more uniform. In the tropics, where a great proportion of the rainfall
comes from convectional storms, the spatial variation is particularly marked. The storms
often build up without any significant movement, so areas just beyond the limits of the
cloud may receive no rainfall at all. Sometimes the storms develop over a wider area,
perhaps 500 km
2
, but even so they do not give rain everywhere. Using the correlation
method, it has been found that in some areas the relationships between raingauge totals
fall to zero within 100 km and are negative beyond. In other words, if rainfall were high
for a particular period in one area, it would tend to be low beyond 100 km. Over the short
term these differences may be considerable, but in the long term we would expect them to
balance out.
