Observations of foehn interaction with lake breeze and valley wind ...

Observations of foehn interaction with lake breeze and valley
wind circulations, Lake Tekapo, New Zealand.
Hamish A. McGowan
School of Geography, Planning and Architecture, The University of Queensland,
St. Lucia, Brisbane 4072, Australia.
1. Introduction
Foehn winds in the eastern Southern Alps, New Zealand, known locally as the nor’wester, have been the focus
of research since the early 1990s, in relation to their role in dust transport and dispersion (McGowan 1997),
interaction with local thermally driven circulations (McGowan and Sturman 1996) and significance in alpine
hydro-meteorology (Sinclair et al. 1996; Katzfey 1995a,b; Chater and Sturman 1998). The rationale for this
research has been to answer significant knowledge gaps in the scientific understanding of foehn conditions in
this region and to improve foehn forecasting in light of the rapid development of the alpine zone for tourism and
associated recreational activities, viticulture and silviculture. Foehn wind events also present a significant hazard
to property and associated infrastructure and utilities, as well as to local aviation operations in support of
tourism.
This paper presents observations of foehn interaction with thermally generated circulations including a lake
breeze circulation. Measurements were made during the austral summer in February 1999 as part of the Lake
Tekapo Experiment (LTEX) (Sturman et al. 2003).
2. Physical Setting and Instrumentation
The Lake Tekapo hydro-catchment is situated approximately 180 km southwest of Christchurch in the central
Southern Alps of New Zealand. Lake Tekapo dominates the southern portion of the catchment with a surface
area of approximately 87 km 2 and is bordered to the east and north by mountain ranges with peaks exceeding
1800 m. The Godley and Macaulay River valleys located to the north of the lake are incised into these mountain
ranges and airflow within these valleys is primarily channelled up (southerly wind) or down valley (northerly
wind). The surrounding mountain ranges effectively shelter the lake basin from gradient winds allowing light
diurnally reversing thermally driven circulations to dominate, while foehn winds are most common in the
headwaters of the catchment (McGowan et al. 2002). Vegetation cover consists of a mixture of exotic grasses
and degraded tussock grassland at lower altitudes, while at higher altitudes, taller snow tussocks predominate
with some scrub. Vegetation cover above the snowline is scarce and snow, ice and rock surfaces prevail.
Meteorological observations presented in this paper were collected by a network of automatic weather stations,
pilot balloon stations and an instrumented light aircraft (Sturman et al. 2003), while synoptic analyses were
provided by MetService, New Zealand.
3. Observations on14 February 1999
The mean sea level analysis for the 14 February 1999 was
characterised by a large anticyclone centred to the east of New
Zealand (Figure 1). This produced favourable conditions for
thermally generated wind systems to develop, while at higher levels
(>2500 m) a northwesterly airflow of 5 to 10 ms -1 prevailed.
Nocturnal drainage flows were monitored in the Lake Tekapo
catchment until approximately 0700 NZST, when the transition to
the daytime lake breeze – valley wind system was recorded. These
thermally generated circulations were well established in the lake
basin by 1000 NZST, while the onset of foehn conditions was
monitored in the headwaters of the Godley Valley by a station
located 600 m above the valley floor.
1008
1000
L
1004
5kts
160E
1008
1016
1025
50S
1008
180
1024
H
10kts
40S
30S
Figure 1. Mean sea level analysis 14 February 1999
at 1200 NZST.

Over the following four hours, the foehn front advanced down valley
eroding the local thermally generated flows from above, as seen in
Figure 2. The foehn then decoupled from the surface approximately 5
km north of the lake shoreline as it encountered the lake breeze front,
which was associated with whirlwinds and associated dust
entrainment. The foehn then remained decoupled from the surface
before grounding midway down the lake between 1400 and 1500
NZST. Coupling of the foehn to the cold lake surface is believed to
have coincided with the descending limb of the lake breeze circulation
cell which prevailed throughout the day over the northern lake
shoreline. The northerly foehn then decoupled from the surface again
near the southern lake shoreline as it encountered the valley wind
originating from southeast of Tekapo Village (Figure 3). Pilot balloon
flights conducted adjacent to Tekapo Village identified this airflow to
be approximately 900 m deep beneath the prevailing gradient westerly
airstream.
Vertical mixing ratio profiles measured by an instrumented light
aircraft between 1400 and 1500 NZST are presented in Figure 4. They
show that below approximately 500 m agl. the mixing ratio profiles
over the northern shoreline and south of Tekapo Village were similar,
decreasing from 7 g kg -1 to 5.6 g kg -1 as the aircraft climbed through the
local lake breeze and valley wind system respectively. However, over the
middle of the lake the drier foehn airmass displayed mixing ratios of 4.5 g kg -1
to 5.5 g kg -1 . These drier conditions were also monitored by weather stations
located on the lakeshore adjacent to the position of the aircraft sounding and
were associated with light northerly winds of 3 to 4 ms -1 as shown in Figure 3.
The airmass south of Lake Tekapo became drier above 500 m until about 900 m
agl. Above this height, mixing ratio values remained nearly constant at
approximately 4 g kg -1 reflecting larger scale airmass characteristics associated
with the prevailing gradient westerly airflow. Over the northern shoreline,
mixing ratios remained nearly constant at 5.5 g kg -1 between 500 m to 1100 m
agl. before decreasing rapidly as the influence of surface moisture from
surrounding mountain ranges became less. The mid-lake profile indicates a
more layered structure above 500 m than either of the other two profiles, but
the general trend shown is a decrease in mixing ratio values with increasing
height as the influence of local surface properties on the foehn airflow became
less.
Cessation of this event began at approximately 1530 NZST with the foehn front
retreating up valley. This often coincides with the intrusion of the regional scale
plain-mountain circulation into the Lake Tekapo Basin, as discussed by
Kossmann et al. (2002). However, on this occasion the cessation of foehn
conditions appeared to be a function of reduced sensible heat flux causing a
decoupling of the wind aloft from the surface in the study area. By 2000 NZST
foehn conditions were confined to the headwaters of the Godley River valley
with local thermally generated winds monitored at other sites in the lake basin
including the onset of cold air drainage. Two hours latter the nocturnal drainage
wind regime was fully established throughout the catchment.
4. Summary
This paper has presented observations of the complex interaction between the
topographically modified foehn nor’wester and local thermally generated winds
Height agl.(m)
2000
1800
1600
1400
1200
1000
800
600
400
Anti-wind?
Topographically channelled
foehn airflow
200
0
0400 0600 0800 1000 1200 1400 1600
Wind speed Time (NZST)
Long barbs 2.5ms-1, Short barbs 1.25ms-1 Valley wind
Mountain wind
Figure 2. Vertical profiles of wind speed and
direction calculated from pilot balloon flights
conducted in the Godley Valley, 14 February
1999.
Southern Alps
Godley River
Tekapo
Village
S
Legend
Wind speed Scale
5 ms
N
10 km
-1
S
Lake
Tekapo
Foehn
front
Macaulay River
Aircraft
sounding
S
Figure 3. Surface wind field
characteristics 14 February 1999 at
1420 NZST.
S

monitored in the Lake Tekapo hydro-catchment.
Observations were made during weak to
moderate trans-barrier gradient airflow, as
indicated in Figures 1 and 2, suggesting that such
conditions may occur more frequently than
previously thought. Under these conditions, as
monitored on the 14 February 1999, foehn onset
occurred following sunrise and the onset of
surface heating, while cessation coincided with
sunset, or soon after, indicating a strong
association between foehn occurrence and the
surface energy balance – specifically sensible
heat flux. During early February 1999 surface
daytime temperatures over the dry alluvium in the
Godley River valley typically exceeded 40°C, with
Height (m)
Mixing Ratio (g kg -1 2200
2000
1800
1600
1400
1200
1000
800
600
400
200
0
0 1 2 3 4 5 6 7 8
Legend
)
Mid-lake Northern shoreline South of Tekapo Village
Figure 4. Vertical mixing ratio profiles obtained by a light aircraft
between 1400 – 1500 NZST, 14 February 1999.
corresponding sensible heat flux density values of 130 to 200 Wm 2 . The resulting convection combined with
topographic channelling of airflow is believed to trigger the observed daytime grounding of the foehn, which is
typically confined to the Godley and Macaulay River valleys. Local and regional scale thermotopographic
circulations dominate elsewhere in the catchment under such conditions, including a lake breeze circulation
which displays a complex interaction with the foehn. A climatology of foehn events is now planned to improve
knowledge of foehn occurrence in the headwaters of alpine catchments such as Lake Tekapo.
Acknowledgements:
The LTEX programme was funded by Marsden Fund Grant UOC602, awarded by the Royal Society of New
Zealand. The author acknowledges the assistance of all members of the LTEX programme and residents of Lake
Tekapo who allowed the installation of monitoring equipment on their property. This study was completed with
funding from The University of Queensland and the Ian Potter Foundation, Melbourne, Australia.
References:
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rainfall into the Waimakariri Catchment, Southern Alps, New Zealand. International Journal of
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