Download Nowcasting orographic rainfall : the effects of microphysical processes

NOWCASTING OROGRAPHIC RAINFALL
4.04
Geoff Austin*, Andrew Peace and Joanne Purdy
Atmospheric Physics Group
University of Auckland
New Zealand
1. INTRODUCTION
The Southern Alps, for example, result in an annual
rainfall as high as 14 meters per year in some
Over the years, a number of experimental
locations near the divide compared with 5 at the coast
programmes have been undertaken in various parts of
and 2 on the lee side. From a cloud microphysical
the world investigating the effects of mountainous
point of view there is a problem with this in that the
topography on rainfall patterns, following the
strong north westerly winds which produce this rain
pioneering work of Browning et al. in the South of
takes only 15 minutes or so to travel from the coast to
Wales (Browning, 1980). Of particular note are the
the top of the mountain range (height about 2,000m)
investigations in the Northwest United States, the
which is insufficient time for the clouds produced by
European Alps and the South Island of New Zealand
the air being forced up over the mountains to produce
(Wratt et al., 1996). These studies have revealed
precipitation sized particles. Nevertheless, these
many similarities in the observed mechanisms and
events usually but not always produce heavy rain
atmospheric conditions. Perhaps more significant are
giving a difficult nowcasting problem. Extensive
the differences in the stability and humidity of the air
studies of these events has lead to the hypothesis that
masses involved in these environments, the forcing
the predominant rainfall mechanism in the region
synoptic conditions, and the vertical and horizontal
involves acceleration of rainfall generation from low
scales of the topographic features concerned which
level condensation by the presence of large, pre-
can give rise to differing dominant microphysical
existing, synoptically generated cloud droplets within
processes.
the ascending air mass.( Purdy and Austin (2003)). A
.
nowcasting scheme that estimated rainfall rates by
assuming stable advection of moist air over the
2. NOWCASTING OROGRAPHIC RAIN IN NEW
topography, but excluded intervals when IR satellite
ZEALAND
data showed no pre-existing cloud was developed
and tested. In general the new scheme worked well in
Nearly all of the heavy rainfall events in New Zealand
that it showed significant improvement over the same
are strongly affected by topography and include strong
scheme but without the pre-existing cloud criterion
westerly air flows over Southern Alps, Sub Tropical
particularly in predicting low rain periods, but proved
Cyclones that produce strong on shore winds
to be prone to false alarms from the difficulty of
predominantly onto the east coast of the North Island
assessing the existence of low cloud in the presence
and winds derived from strong synoptic depressions
of cirrus overcast. The rerunning of the scheme with
which can occur almost anywhere.
the cirrus overcast periods being catgorised as low
cloud free improved the forecasts as did a scheme
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involving the use of visible satellite data to distinguish
Corresponding author address: Geoff Austin, Physics
the cirrus only cases. It seems essential therefore in
Department, University of Auckland, Auckland, New
nowcasting rainfall in this region that the pre-existing
Zealand. :email; [email protected]
cloud be included alongside the mesoscale model to
obtain reasonable results.
It should be noted that the seeder- feeder mechanism,
so important in South Wales, normally plays little or no
role in the Southern Alps situation, a conclusion
supported by data from two small radars located in the
mountain range, Figure 1.
Figure 1 Mobile autonomous weather radar which can
be deployed in mountainous terrain
However, on one occasion at least, with unusually
light north westerly winds, such evidence was
obtained (Purdy et al (2005)). The event of interest
occurred between 1740hrs and 1815hrs NZDT on the
21st October 1996. Over the preceding four days
there was a moist flow from the north-west and a total
of 392mm rain fell at the Vertically Pointing Radar
(VPR) site, which was located in a valley running into
the western slopes of the Alps.
The scanning radar, located 30km upwind, showed a
sudden rapid increase in raining area over the space
of only 15min (see figure 2a – 2d) with much of this
increase occurring in only four minutes.
orographic component. Strictly speaking the resulting
rain rate should not only be enhanced by the snowfall
from aloft but should be greater than the sum of the
rainfall from both systems to indicate scavenging of
small low level cloud droplets by precipitation falling
from the synoptic system is occurring. Finally the
snowfall must be a result of synoptic rather than
orographic lifting. This was determined from the
satellite data.
Sub Tropical Cyclones on the other hand, mainly
affect the east coast of the North Island and push very
moist air over relatively modest topography. Small
Figure 2 Scanning radar data showing a rapid
mobile radar deployments to these events have
increase in raining area in the 4 minutes between
shown remarkable degrees of modulation of the
figures 2b and 2c.
rainfall patterns by the topography down to kilometre
scale. Our current mesocale model cannot match this
An explanation for the sudden widespread increase in
resolution nor does it reproduce the complex
reflectivity is provided by the VPR data (Figure 3),
microphysical processes that are clearly going on..
which clearly shows a separate system dropping snow
Investigations are underway to develop a simple
into the shallow orographic rainfall which combines
empirical model which includes very high resolution
with cloud associated with the shallow rainfall to result
topography and is driven by the larger scale flows
in a rapid increase in reflectivity over a wide area.
obtained from the mesoscale model as well as insitu
radar observations of processes. It is too early to say
how effective this scheme will be but certainly the
flood producing rainfall patterns are complex, with
important catchment to catchment variability of great
practical importance, currently not reproduced by the
model.
CONCLUSIONS
The effects of topography on precipitation depend on
Figure 3 Vertically pointing radar image showing
the effects of the topography on the wind flows. These
snow falling into low level orographic rainfall. A factor
in turn can have significant effects on the predominant
of two rainfall enhancement was observed by
microphysical processes which a mesoscale model
raingauges at the ground..
may have trouble resolving. Our experiments suggest
that rainfall prediction procedures in such cases can
Relatively curved fall-streaks in the upper system
be greatly enhanced by careful use of satellite cloud
indicate that it is snowfall as does the defined bright-
data and by placing small radars in the mountainous
band visible at around 2300m. While these VPR data
regions concerned to allow direct observation of the
show an interaction characteristic of the seeder-feeder
dominant rainfall producing processes.
mechanism they can only be viewed as evidence of
this specific orographic enhancement process if the
shallow rainfall, which is analogous to the orographic
cloud in the classical seeder feeder model, has an
REFERENCES
Browning KA (1980) Structure, mechanism and
prediction of orographically enhanced rain in Britain.
Orographic effects in Planetary Flows, R. Hide and
PW West, Eds. GARP Publication Series No. 23, 85114.
Purdy, J, Austin, G.L (2003) The Role of Synoptic
Cloud Orographic Rainfall in the Southern Alps of New
Zealand. Meterol Appl 10, 355-365
Purdy, J, Austin, G.L., Seed, A.W., Cluckie, I.D.
(2005) Radar Evidence of Orographic Enhancement
due to the Seeder Feeder Mechanism. Met apps
(accepted)
Wratt, DS, Ridley, RN, Sinclair, MR, Larsen, HR,
Thompson, SM, Henderson, R, Austin, GL, Bradley,
SG, Auer, A, Sturman, AP, Owens, IF, Fitzharris, BB,
Ryan, BF, and Gayet, J-F. (1996). The New Zealand
Southern Alps Experiment. Bull. Am. Met. Soc. 77,
683-692.