Geoscience Reference
In-Depth Information
90
80
Dry
spell
Figure 13.14
The
probability that a spell of
weather will be extended by
a further day at Selangor,
Malaysia deduced by
counting the times this has
occurred in an observed
precipitation data series.
(Redrawn from Sumner,
1988, after Yap, 1973,
published with permission.)
70
Wet
spell
60
50
0
123
456
Length of spell (days)
7 8 9 101112131415
Important points in this chapter
Annual precipitation
: determines the nature of a region and its viability for
human habitation: long-term trends in its value may result from human
intervention in the global system, while fluctuations may be predictable (e.g.,
ENSO, NAO, etc.).
●
Intra-annual precipitation
: variations caused by seasonal changes in atmos-
pheric circulation, especially in the ITCZ, monsoons systems, and the
strength and location of westerly wind bands, give:
— at mid-latitudes, characteristic differences between winter and summer
climates depending on continental location (see text for details); and
— in the tropics, strong seasonality in precipitation, wind direction, and
tropical storms (see text for details).
These are documented visually using
isomers, pluviometric coefficients
, and
polar diagrams
or using numerate
seasonality indices
.
●
Daily precipitation
: generally has a strong diurnal variation linked to atmos-
pheric convection, and daily total values are very variable and have a highly
skewed probability distribution and so are problematic to analyze statisti-
cally, except perhaps as a median.
●
Precipitation trends
: are identified by methods that involve averaging/
summing precipitation, including: (a)
running means
; (b)
cumulative
deviations
; and (c)
mass curves
(see text for details).
●
Oscillations in precipitation
: are identified by methods that include:
(a)
autocorrelation
to define a
correlogram
; (b)
harmonic analysis
; and
(c)
numeric filters
(see text for details).
●
System signatures
: because the timing of in-storm precipitation is usually
different for frontal and convective storms, mass curves are used to investigate
the primary atmospheric mechanism giving rise to a storm.
●
