Geoscience Reference
In-Depth Information
14 Precipitation
Analysis in Space
Introduction
There is substantial spatial variation in precipitation fields due to differences in
the type and scale of atmospheric processes that cause precipitation, and to local
or regional influences such as topography and the wind direction at the time the
precipitation was produced. For this reason the temporal analyses of precipitation
described in Chapter 13 are site or at least regionally specific because the intensity
and probability analyses described are for point precipitation data. However, as
mentioned in Chapter 12, gauge data are a poor representation of area-average
precipitation, and assuming they are representative can be dangerous, particularly
in the case of short duration samples and in climates prone to convective storms.
Adjacent gauges are not always consistent, even monthly and annual average
precipitation data may vary significantly, and this is true even in areas with
comparatively low topography.
The movement of storms relative to the ground and the fact that they develop
and decay influences the precipitation pattern on the ground. Some gauges may
experience very heavy rainfall while others see no rain for a particular storm.
Ideally, estimates of the precipitation falling in a particular area would track and
accurately model the passage of a rain storm over the area and calculate the
distribution of the precipitation in space and time. By convoluting this with
the shape and topography in a drainage basin, the area-average precipitation could
be computed. Doing this is at least difficult, and perhaps impossible given the
inherently chaotic nature of the precipitation-producing processes in the
atmosphere. An approximate alternative is to derive empirical models based on
observations that relate intensity to storm area and to use these to estimate mean
areal precipitation.
Notwithstanding the problems involved with analyzing the spatial organization
and distribution of precipitation, the subject has demanded and continues to demand
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