Geoscience Reference
In-Depth Information
A further characteristic of seasonal climate, of value for agricultural planning, is
the
time of onset
of seasonal rainfall, but there is no universally accepted way of
defining this. Some popular choices include:
specifying the date on which the precipitation exceeds an arbitrary amount
(e.g., 1 inch or 25 mm of rain);
●
specifying the date on which the precipitation exceeds a selected fraction of
the estimated potential evapotranspiration for the remainder of the growing
season;
●
fitting a Markov chain model (see later in this chapter) to the precipitation
pattern of early rains to define a date when there is a change in the probabil-
ity of rainfall after antecedent rain.
●
Daily variations
Although it is comparatively simple to demonstrate the overall intra-annual
variation in precipitation for most places, daily precipitation is far more
variable. The probability distribution for daily precipitation is always positively
skewed and often quite strongly so. Zero precipitation is quite common - there
is no guarantee of rain every day even during a wet season in the humid
tropics. Anomalously high daily rainfall can also occur, occasionally reaching
values as high as 1 m of rain per day at some locations. The derivation of a
robust mean daily rainfall is therefore statistically futile. The median daily
precipitation, i.e., the value for which occurrence of greater of less precipitation is
equally probable, is arguably a more stable measure of daily precipitation
climate. It may also be advantageous to specify daily rainfall in terms of days
with greater than a set amount of precipitation, e.g., defining days with precipi-
tation greater than 0.25 mm per day as 'rain days,' and days with more than 1 mm
as 'wet days'.
Because the convection process can be important over continents, a distinct
diurnal cycle in precipitation is commonly observed throughout the year in
tropical regions and during the summer months in temperate regions. Figure 13.2
shows an example of this for Manaus, Brazil. In some situations, such as that
shown in Fig. 13.2, it is the time of first occurrence of rainfall that is most obviously
linked to peak convective activity in the middle of the day. This is because severe
storms, once started, tend to be self-supporting and can last into the evening.
The daily pattern of variation in precipitation can also be complicated by the
presence of mountains. Moreover, near coasts or the edges of large lakes, differential
surface heating gives rise to diurnal changes in local air flows which translate into
ascent, and the timing of these flows can impact the diurnal pattern of precipitation.
Interestingly, over oceans, peak convective activity and associated precipitation is
often at night, a feature which has been ascribed to the cooling of cloud tops by
outward radiation at night.
