The NonLinear Morning Glory Wave of Southern California Victor C' Tsai, Hiroo Kanamori, and Juliette

1
The Non-Linear Morning Glory Wave of Southern
CaliforniaVictor C. Tsai, Hiroo Kanamori, and
Juliette ArtruSeismological Laboratory,
California Institute of Technology, Pasadena,
CA(tsai_at_its.caltech.edu, hiroo_at_gps.caltech.edu,
juliette_at_gps.caltech.edu)
1. Abstract A pulse-like disturbance traveling
across the Los Angeles basin was observed on Oct
12, 2001 with seismographs of the TriNET network.
This wave had a period of about 1000 s, and a
propagation speed of about 10 m/s, much slower
than seismic waves. The seismograph data was
compared with barograph data and a good
correlation was found so the wave was determined
to be atmospheric in origin. It had an amplitude
of about 1 mbar, but it was not known what
process could produce such a wave. Since the
initial finding, we inspected all the TriNET
barograph and seismograph data for a period of
two and a half years (from Jan 2000 to July
2002), and found 5 more similar events. Another
event occurred in 1988. Each of the events has
an amplitude between 0.8 and 1.3 mbar, a period
between 700 and 1400 s, and a propagation speed
between 5 and 25 m/s. Analysis of these data has
led us to the conclusion that the wave is a
solitary wave (a non-linear internal gravity
wave) similar to the spectacular morning glory
wave observed in Australia. We present data here
that supports the hypothesis that this
morning-glory wave of Southern California is
caused by an excitation of the stable inversion
layer by some atmospheric condition or seismic
disturbance as it enters the LA basin. In
particular, we believe it may be associated with
stormy weather, winds such as the Santa Ana
Winds, and large teleseismic events.
Furthermore, the morning-glory wave could
contribute to the recently reported excitation of
the background free oscillations of the Earth.
Additionally, because of its large amplitude, it
could have important implications for aviation
safety as was suggested earlier for the
morning-glory waves in Australia.
Figure 7 In a plot of period as a function of a
(a/h), lines of constant speed are as displayed
below. Since period and wave speed are
measurable quantities, we can determine a from
the graph. a does not satisfy altlt1, thus the
wave is highly nonlinear.
4. Discovery and Data Collection On October 12,
2001 we observed a pulse-like disturbance of 1000
s with a speed of 10 m/s traveling through the
Los Angeles Basin. Although the signal was
detected using seismographs, the wave speed was
much slower than seismic waves suggesting an
alternative source. A good correlation was found
between seismograph data and barograph data so
the wave was determined to be atmospheric in
origin. Our initial analysis led us to examine
all the relevant TriNET seismograph and barograph
data available between January 2000 and August
2002. We found 5 additional morning-glory events
giving us a total of seven events with
characteristics as displayed in Table 1. Figure
3 Time series data for the July 2002 event.
The signals are well correlated in the 1000 s
period range allowing us to use seismograph
response as a proxy for barometric response.
(Note both graphs unfiltered)
5. Temperature Inversion Layer In most places,
temperature drops with increasing height,
following an almost adiabatic profile. The LA
Basin is atypical in that it is surrounded by
mountains on one side and ocean on the other (see
Figure 6). Therefore, the temperature of the air
first increases before finally decreasing as
normal. The typical height of this inversion is
approximately 1 km. The importance of the
thermal inversion layer for the Los Angeles
morning-glory wave is that temperature
differences cause a difference in air density.
To a first approximation, (4)   where r
air density, and T temperature. Since the
amount of temperature inversion is on the order
of 5 to 10 degrees C, the fractional change in
density should be around a few percent. Figure
6 Warm air comes from the desert over the
mountains and subside as cool ocean air arrives
underneath thus forming an inversion layer with
cool air underneath warm air.
7. Mechanisms for Excitation Although we have no
conclusive evidence, three possibilities seem
connected with morning-glory excitation.
2. Introduction A spectacular set of roll clouds
has been observed in Australia for many years.
Due to their frequent occurrence in the morning
hours, the atmospheric condition has been dubbed
the morning glory. However, the morning glory
phenomenon has never been observed anywhere else
in the world. Figure 1 Typical morning glory
photographed near the Gulf of Carpentaria in
northern Australia
Taken from http//www.dropbears.com/brough/
Figure 8 This record section shows signs of a
high-frequency atmospheric event that begins
outside the LA Basin. It is most likely a storm
system or Santa Ana winds. The morning glory
event (not pictured) occurs when the disturbance
reaches the LA Basin.
Figure 9 Four of the seven events found occur
within 10 hours of a large (6.1-6.3M)
teleseismic event. Although we do not know of a
mechanism that would relate the two events, the
overall probability of random concurrence is
about 1.2. Some type of resonance or feedback
mechanism may provide the necessary link between
them.
3. Solitary Waves In 1978, Christie et al. gave
an in depth study of the morning glory
phenomenon, classifying it as a solitary wavean
internal, non-linear gravity wave. Solitary
waves have been observed and studied for a long
time. The wave profile and wave speed satisfy
the following relations
(1) (2) , a maximum
amplitude, g gravitational acceleration h
undisturbed fluid height This solution arises
from the solution of the KdV (Korteweg-deVries)
Equation (3) where
and Figure 2 Schematic of a solitary
wave wave profilesech(x-ct)2
Figure 4 Stations from which seismograph and
barograph data were obtained.
6. Analysis and Wave Parameters Based on previous
work done by Christie et al. and data collected
for the seven events in the Los Angeles Basin, we
believe that the Los Angeles morning-glory wave
is a solitary wave that forms from the excitation
of the inversion layer. Following Christies
two-layer model (a uniform layer with a thickness
h overlain by a half space) for atmospheric
solitary waves gives the following
relations   (5) (6) (
7)   where the additional parameters are t, the
pulse width (or for multiple oscillations, the
time between successive wave troughs), and DP,
the difference in ground pressure. The measured
quantities are c, DP, and t. g and r are taken
to be known.
8. Consequences Perhaps the most important
consequences of the LA morning-glory wave are for
aviation safety and the excitation of the Earths
normal modes. Crashes of commercial airplanes
have been linked to transient and highly
energetic atmospheric events. The morning-glory
wave is one of these transient, high-energy
atmospheric events and thus a potential aviation
hazard as has been pointed out by Christie et
al. Recently Suda et al. have shown that the
Earth continually oscillates for unknown reasons.
The fact that the oscillations occur in the same
frequency band as the morning-glory events leads
us to conjecture that morning-glory waves are an
important contributor to the background
oscillations of the Earth. There are many areas
in the world that have inversion layers and thus
can develop morning glories. Enough
morning-glory events could potentially transfer
enough energy to the ground to excite the amount
of normal mode oscillations detected.
Figure 5 Though difficult to pick out from the
time series, the morning-glory has a very
distinct spectral signal. On this spectrogram
for the July 2002 event, it appears as a red dot
at a frequency of 1510-4 Hz.
9. Acknowledgements The SURF program and the
Bonsalls for financial support