Geoscience Reference
In-Depth Information
a physical estimate of what might be the greatest possible precipitation given a
certain set of extreme atmospheric conditions. PMP is a hypothetical concept
which is defined as '
the analytically estimated greatest possible depth of precipi-
tation that is physically possible and reasonably characteristic over a geographi-
cal region at a certain time of year
'. PMP is usually defined with respect to a
given area, often a drainage basin, and includes estimates of the inflow of mois-
ture over the basin and the maximum likely amount of that moisture which
could be precipitated.
One atmospheric variable likely to exert control on the PMP is
W
, the
total
precipitable water
in the atmosphere overlying the region.
W
can be calculated (in
mm) from data measured during a radiosonde ascent through the atmosphere
(see http://amsglossary.allenpress.com/glossary/search?id=precipitable-water1)
by taking the integral:
P
1
top
e
∫
W
=
.
dp
(14.2)
g
P
−
e
P
ground
where
g
is the acceleration due to gravity,
e
and
P
are respectively the vapor pres-
sure and atmospheric pressure (in kPa) measured as a function of height by the
radiosonde, and
P
ground
and
P
top
are the atmospheric pressure at ground level and
the top of ascent (when contact is lost or the balloon bursts). Total precipitable
water can also be estimated from surface dew point assuming the moist adiabatic
lapse rate prevails throughout the atmosphere.
The name
total precipitable water
is inaccurate because not all of the water in the
atmosphere can be precipitated by any known mechanism. Consequently, in addi-
tion to depending on
W
, the calculation of PMP needs to recognize and make
allowance for realistic restrictions on the rate of convergence of water vapor
toward a storm and the maximum effect of vertical motion within a storm. One
approach used to estimate the PMP is to adopt (and if necessary transpose from
elsewhere) models of real extreme storms to estimate these additional restrictions,
but then to index these to local extreme values of
W
. However, the assumptions
and generalizations made when adopting the storm model approach are such that
a sometimes preferred technique involves the use of actual storm occurrences,
which are then 'maximized' to become an extreme storm for the area using the
highest observed surface dew points and most extreme morphological conditions.
Available regional depth-area-duration and maximum intensity information as
well as local isohyetal maps may used, or these may adopted from areas that are
similar. Time factors such as season, time of day and storm duration may also be
taken into account. In regions with topography the estimation of PMP is much
more difficult. Several different methods have been attempted but as yet none has
universal acceptance. Figure 14.10 shows an example map of all season average
probable maximum precipitation for the eastern USA.
The performance of PMP estimates can be evaluated against maximum
observed storm totals. Studies of this type in the USA (e.g., Reidel and Schreiner,
1980) suggest that observed maximum precipitation is approximately 60% of PMP.
