Environmental Engineering Reference
In-Depth Information
to climate studies may be less problematic. Many
questions remain, however, including:
.
Will the additional assimilation of water in
the form of clouds and rain into NWP models
improve their QPF accuracy to make them cred-
ible to drive hydrological models?
.
If so, what lead times are achievable as a function
of meteorological system type and scale of hydro-
logical problem?
.
What sort of hydrological problem lends
itself to combined nowcasting/NWP/hydrological
modelling?
effects). Most NWP models are best suited for
making forecasts in the first category, where the
model can take a large-scale view and effectively
use the large-scale fluid dynamical outcomes to
predict the likelymovement and development of a
front, for example. Thus the model is likely to get
the arrival time of the rain from a front with good
accuracy several hours, or even days ahead, but the
detailed patterns in the QPF are likely to be
quite wrong since there may be embedded con-
vection or local small-scale eddies not resolved
by the NWP model.
Orographic effects
Prospects/Conclusions
A particularly problematic area for QPFs occurs in
mountainous regions where the interaction be-
tween the large-scale wind patterns and the
high-resolution topography can result in a number
of significant highly non-linear behaviours, in-
cluding the triggering of small-scale effects such
as thunderstorms (Austin and Dirks 2005). In
many countries the main flood hazards are flash
floods in steep terrain. In these situations, the
meteorology is likely difficult to model and the
hydrology complex.
In New Zealand, for example, nearly all of the
heavy rainfalls come either from subtropical cy-
clones encountering mountainous terrain or from
strong onshore winds interacting with the South-
ern Alps. In both cases there is the need to allow
high-resolution terrain to interact with large-scale
flows. It is found that the NWP models give im-
proved results as the resolution of the terrain and
model are increased, leading to some hope for
better flash flood forecasts in mountainous areas.
There are currently great opportunities for real
improvements in the way meteorological fore-
casts are used for flood warning systems. Recent
developments include drastic improvements in
the resolution, physics and data assimilation of
quantitative rainfall amounts in NWP models.
Similarly the performance of distributed rainfall-
runoff models suitable for flood prediction has
notably improved. Moreover, large computing fa-
cilities have declined markedly in cost, including
inexpensive parallel processing clusters, making
multiple runs of the meteorological and hydrolog-
ical models operationally feasible. This in turn
opens up the possibility of working with ensem-
bles of model outputs, thus giving information
about the likely range and also perhaps extrema
of flooding events. Whilst the interpretation of
such results is not trivial, it is expected that they
will be of much greater value for those charged
with giving flood warnings.
References
Important Research Questions
Atlas, D.
(1990) Radar
in Meteorology. Chicago
Whilst the idea of driving hydrological flood pre-
diction models from the QPFs generated by atmo-
spheric NWP models is entirely obvious and
logical, there are serious questions about the abil-
ity of the NWP systems to represent and predict
rainfall amounts with sufficient accuracy, partic-
ularly for real-time flood forecasting. Applications
University Press.
Austin, G.L.and Austin, L.B. (1974) The use of radar in
urban hydrology. Journal of Hydrology, 22, 131-142.
Austin, G.L. and Dirks, K.N. (2005) Topographic Effects
on precipitationWylie Encyclopaedia of Hydrological
Sciences, Part 3,Meteorology and Climatology,.
M. Anderson (ed), (page numbers unknown as the
