The lower boundary layer is probably the least observed part of
tropical cyclones. Good quality measurements of conditions above the surface, but below
several hundred metres, are exceedingly rare - in contrast to the situation in much of the
rest of the storm. Yet it is here, in the "habitation layer", that the storm
impacts most directly on humanity, both through direct wind damage, and through the
generation of ocean waves and storm surge.
The approach to understanding the tropical cyclone boundary layer
has been to extrapolate theories, developed and verified in more benign conditions, and
apply them in the severe conditions of the cyclone core.
Yet there is some evidence to suggest that air-sea interaction at
high wind speeds cannot be extrapolated from low wind speed and land-based behaviour.
Frank (1977) found that standard bulk formulae considerably overestimated the surface
drag. Pudov (1993) observed the air-sea temperature difference increasing to over 5 K as
wind speeds rose in two South China Sea typhoons. Analyses of Atlantic buoy data by Black et
al(1993) and Cione and Black (1998) produced similar results, while similar
observations were reported in Hurricane Andrew by Breaker et al (1994, their Fig
2). Fairall et al (1994) have explained these observations in terms of the
evaporation of sea spray, a process which appears to only become active at very high wind
speeds. However, the impact of these large coolings on the boundary layer structure and
dynamics is still an open question.
Similarly, observations of boundary layer wind profiles often show
wind maxima in the lower boundary layer. Examples include early
measurements from the towers at North West Cape (Wilson 1979) , aircraft measurements
(Moss and Merceret 1975), balloon measurements (Korolev et al 1990) and recent
measurements by the latest dropsonde equipment (Black and Shay 1998).
North West Cape is the tip of the long
peninsula which encloses the Exmouth Gulf. It is influenced by tropical cyclones on
average about once every two to three years and is within the core region at least once
every ten years. A low eroded ridge, Cape Range, runs along the peninsula and peters out
near the Cape. The Australian Bureau of Meteorology has a conventional surface and upper
air observational site at Learmonth, with an associated weather watch radar nearby on top
of Cape Range. The Harold E. Holt naval communications base is located on the flat
low-lying area towards the tip of the peninsula, and is jointly owned and operated by the
United States and Australian Navies.
The main VLF antenna for the naval base is located right at the tip
of the peninsula, and is supported by a central tower (Tower Zero) surrounded by two
concentric circles each of six smaller towers. The towers range from 304 to 392 m in
height and are the tallest located on a tropical cyclone prone coast in the world. When
constructed, they were the tallest man-made structures in the southern hemisphere.
Tower Zero has carried anemometers at several levels since the early
1970's, although these have been in a poor state of repair for some years. Unfortunately,
the anemometers were too close to the tower to be free of flow distortion effects and were
insufficiently maintained and calibrated, and so it was not possible to be absolutely
certain of their reliability and scientific utility. They did, however, produce the
interesting observations of low level jets.
View
from the southwest of the antenna array.
Towers 0, 1 and 5 are marked. The transmitter building at the base of Tower 0 is
approximately 4 storeys tall. The Exmouth Gulf is a few hundred metres to the right of the
road, over a low line of dunes.
The site is influenced by tropical cyclones on average about once
every year or two and is within the core region at least once every ten years. It is under
direct surveillance by an Australian Bureau of Meteorology 5 cm radar near Learmonth
(location shown above, which is routinely archived and can provide detailed rainfall and
convective structure observations. The Bureau also maintains a conventional surface and
upper air observational program at Learmonth.
We began by mounting prototype sets of instruments and loggers on
Tower 5 in 1995. After a couple of rounds of testing to solve problems with interference
from the Navy's transmitter, a set of instruments comprising a McVann synchrotach
anemometer and Rotronics humidity and temperature probe, connected to a Unidata data
logger. We decided to log each level of instruments in situ, as the resulting short cable
lengths reduced both the risk of damage in a cyclone, and also the likelihood of RF
interference. At this time, we also tested an ATI sonic anemometer, which turned out to be
such a marvelous osprey perch that a pair of them built a nest on the adjacent platform .
In January 1996, a 920 MHz profiler with RASS was installed adjacent
to the towers arrays in collaboration with NOAA ETL. An ATI sonic anemometer was installed
at 42 m on Tower 5 at the same time, although this unfortunately failed almost
immediately. The following month, Tropical Cyclone Jacob passed by the Cape, with the
profiler capturing measurements through 3 rainbands. A discussion of these observations
will be included here soon!! The profiler was removed and returned to the U.S.A. in May
1997. We plan to replace it before the 1998/99 cyclone season.
Installing the profiler and RASS system at Point Murat.
In September and October 1996, Towers 1 and 5 were instrumented.
These were chosen as they were the two towers closest to the beaches and offering the best
exposure to tropical cyclone winds with a marine fetch. Two towers were instrumented to
enable coverage of wind from all directions, as the Navy's operations precluded running
booms out from more than one face of any tower. Doing two towers also allowed us to get a
short land fetch for more wind directions, and we may even be able to observe internal
boundary layer development.
Jeff
Kepert adjusting the anemometer at 252m on Tower 5.
The instrumentation on each tower consists of five levels of wind,
temperature and humidity instruments, ranging from 42to 252m. The anemometers are located
on sleds which slide along tracks on top of rigid booms extending 7.2m out from the
seaward faces of the towers. The long booms were installed so that flow distortion effects
will be minimised. Sleds on top of rigid booms, rather than pivoting or telescoping booms,
were used to maximise strength and stability in high winds.
Boom,
temperature enclosure and electronics boxes at 252 m on Tower 5.
Initially, the towers had to be climbed to download the loggers -
1040 rungs to the top! In October 1997 we installed a radio modem system to enable remote
downloads and reprograms of the loggers. If this is unsuccessful (it is not yet 100%
reliable) the loggers still have a capacity of several months of data before any data is
lost.
Finally, in December 1997 Campbell CSAT-3 sonic anemometers were
installed at four levels on tower 5. These take 3-dimensional wind and speed of sound
measurements at 10 Hz, which are radioed down to a base station PC which calculates the
full set of means, variances, covariances, spectra, cospectra and quadrature spectra. As
well as providing a useful backup for the existing "slow" instruments, these
detailed direct measurements of the turbulence will allow us to not only know what the
conditions in the lower boundary layer are, but also understand the physical processes
that produce them.
Tower 1 with projecting booms.Tower 1
after installation of meteorological equipment. Note booms projecting towards the left.
Our operational plans include a pre-season maintenance program as
well as a calibration check immediately after any events.
A future surface site will measure wind, temperature, humidity,
rainfall, and sea surface temperature.
This project would be impossible without the cooperation and support
of the Australian and U.S. Navies, and the staff of the Learmonth observing station.
Thanks are due particularly to Gordon MacDermott, Russ Levien, Jeff Callon, Steve Summers,
Bruce Alden, Dave Edwards and John Hallworth. Many people have shared freely of their
expertise, with particular thanks to Chris Fairall and Frank Bradley. The project forms
part of the Australian Tropical Cyclone Coastal Impacts Project, and is partly supported
by the US Office of Naval Research under grant N-00024-94-1-0493.
Black, P.G., G.J. Holland and V. Pudov, 1992: Observations of
air-sea temperature difference in tropical cyclones as a function of wind speed. In BMRC
Research Report 46: Parameterisation of Physical Processes: Papers Presented at the Fifth
BMRC Modelling Workshop, Melbourne, Australia. Available from BMRC, GPO Box 1289,
Melbourne Victoria 3001, Australia.
Black, P.G. and L.K. Shay, 1998: Air-Sea interaction processes
relevant to tropical cyclone intensity change. In preprints volume, Symposium on
Tropical Cyclone Intensity Change, 78th AMS Annual Meeting, Phoenix, U.S.A.,
11-16 January.
Breaker, L.C., L.D. Burroughs, Y.Y. Chao, J.F. Gulp, N.L. Guinano
Jnr, Rl. Teboulle and C.R. Wong, 1994: The impact of Hurricane Andrew on the near-srface
marine environment in the Bahamas and the Gulf of Mexico. Wea. and Forc., 9,
542-556.
Cione, J.J. and P.G. Black, 1998: Surface thermodynamic observations
within the tropical cyclone inner core.. In preprints volume, Symposium on Tropical
Cyclone Intensity Change, 78th AMS Annual Meeting, Phoenix, U.S.A., 11-16
January.
Fairall, C.W., J.D. Kepert and G.J. Holland, 1994: The effect of sea
spray on surface energy transports over the ocean. The Global Atmosphere and Ocean
System, 2, 121-142.
Frank, W.M., 1977: The structure and energetics of the tropical
cyclone: I. Storm structure. Mon. Wea. Rev., 105, 1119-1135.
Korolev, V.S., S.A. Petrichenko and V.D. Pudov, 1990: Heat and
moisture exchange between the ocean and atmosphere in tropical storms Tess and Skip. Meteorologiya
i Gidrologiya, 2, 108-111. (English translation in Soviet Meteorology and
Hydrology, 2, 92-94).
Moss, M.S. and F.J. Merceret, 1976: A note on several low-level
features of Hurricane Eloise (1975). Mon. Wea. Rev., 104, 967-971.
Pudov, V.D., 1993: The ocean response to the cyclones influence and
its possible role in their tracks formation. In Tropical Cyclone Disasters, edited
by J. Lighthill, Z. Zhemin, G.J. Holland and K.A. Emanuel, Peking University Press,
Beijing, China, 1993.
Wilson, K.J., 1979: Characteristics of the subcloud layer wind
structure in tropical cyclones. Paper presented at International Conference on Tropical
Cyclones, Perth, Australia.
