airglow imager observations of atmospheric gravity waves

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. ???, XXXX, DOI:10.1029/,

Airglow Imager Observations of Atmospheric Gravity

Waves at Alice Springs and Adelaide Australia

During the Darwin Area Wave Experiment

(DAWEX)

J. H. Hecht,1

S. Kovalam,3

P. T. May,2

G. Mills,2

R. A. Vincent,3

R. L.

Walterscheid,1

and J. Woithe,4

J. H. Hecht and R. L. Walterscheid, Space Science Applications Laboratory, The Aerospace

Corporation, M2-259,P. O. Box 92957, Los Angeles CA 90009. ([email protected];

[email protected])

S. Kovalam,R. A. Vincent, and J. Woithe, Department of Physics and Mathematical Physics,

University of Adelaide, Adelaide 5005 SA Australia. ([email protected]; rvin-

[email protected];[email protected])

P. T. May and G. Mills, Bureau of Meteorology Research Centre Melbourne, 3001 Vic Australia.

([email protected])

1Space Science Applications Laboratory ,

The Aerospace Corporation, Los Angeles,

CA.

2Bureau of Meteorology Research Centre

Melbourne, 3001 Vic Australia

D R A F T March 3, 2004, 6:53pm D R A F T

X - 2 HECHT ET AL.: OBSERVATIONS OF GRAVITY WAVES AT TWO SITES DURING DAWEX

Abstract. The Darwin Area Wave Experiment occurred in Australia from

October to December 2001. An objective was to characterize the atmospheric

gravity wave field produced from intense convective activity that is routinely

observed around Darwin during November and December. Two airglow im-

agers were sited at Adelaide and at Alice Springs, each located over 1000 km

south of Darwin. Waves were observed propagating predominantly towards

the southeast, with some going to the northwest, but with none observed go-

ing from east to west. The lack of waves propagating towards the west sug-

gests some wind filtering mechanism below 80 km altitude. Waves observed

over Alice Springs were analyzed in detail on three nights. On 11/16 they

were seen propagating towards the northwest. It is proposed that they were

generated by dynamical events associated with a cutoff low pressure system

present over southwest Australia. On 11/17 and 11/19 the observations are

consistent with wave generation by convective activity present in the Dar-

win area. Thus, as proposed in Walterscheid et al. [1999] and Hecht et al.

[2001a], the ducting of waves from distant sources is shown to be a viable

explanation for the quasi-monochromatic waves frequently observed in air-

3Department of Physics and

Mathematical Physics, University of

Adelaide, Adelaide 5005 SA Australia

4ATRAD, Adelaide SA Australia


HECHT ET AL.: OBSERVATIONS OF GRAVITY WAVES AT TWO SITES DURING DAWEX X - 3

glow observations. Walterscheid et al. [1999] suggested that ducting of waves

from the extensive region of deep cumulous convection over northern Aus-

tralia explained the strong poleward directionality seen in the summer months.

The present study suggests that propagation from northern Australia is se-

lective and ducted waves from this region may not be the primary source of

waves over Adelaide when convection is occurring over central Australia.

(c)2004TheAerospaceCorporation.



1. Introduction

Since the first mesopause region airglow imaging observations about 30 years [Peterson

and Kieffaber, 1973] many of the observed structures have been attributed to the passage

of atmospheric gravity waves (AGWs) that are generated in the troposphere and propagate

upwards to the upper mesosphere and lower thermosphere [Moreels and Herse, 1977; Hecht

et al., 1995; Swenson et al., 1995; Taylor et al., 1995; Nakamura et al., 1999; Smith et

al., 2000]. Passage of these waves through the airglow emission layer in the mesopause

perturbs the neutral density and temperature which subsequently perturbs the airglow

emission. Images of the airglow when AGWS are present show bright and dark regions

that, when seen in a time sequence, appears as a series of wave-like structures moving

across the sky.

It is not clear exactly what are the dominant source of the AGWs observed in the air-

glow images. A general picture of AGWs generated in the troposphere include primarily

orographic production (flow of air over large mountains) or dynamical production asso-

ciated with intense convective activity [Wang and Geller, 2003]. Many of the airglow

observations to date are at sites that could be seeing AGWs generated from either source.

However, many of the observed AGWs have short horizontal wavelengths suggesting they

should originate only a few hundred kilometers from the observing site. Thus, in many

cases there is no obvious source for the observed AGWs.

Several recent studies have examined this second point and concluded that many of

the AGWs observed by airglow imagers are ducted over distances of one to two thousand

kilometers [Walterscheid et al., 1999; Hecht et al., 2001a]. The Walterscheid et al. [1999]



study is particularly interesting as they reported on nine months of airglow observations at

Adelaide Australia from April 1995 to January 1996, a period encompassing the summer

monsoon period in Northern Australia. AGWs were observed to come mainly from the

north to north west, a direction devoid of any large mountains and a region that is often

quite dry in the spring and summer. They hypothesized that the AGWs were being

generated by the intense convective activity that occurs near Darwin in November and

December every year. This, however, means that these AGWs travelled nearly 3000 km,

an event only likely if they were ducted. Walterscheid et al. [2001] showed theoretically

that deep tropical convection, such as is found in the Darwin area, can indeed populate

the lower thermosphere with fast short wavelength AGWs.

A second study further investigated the ducting hypothesis using data from a site in Illi-

nois also far distant from large mountains [Hecht et al., 2001a]. The primary directionality

around (Northern Hemisphere) summer solstice was of AGWs being generated from the

south and southwest away from larger mountain ranges but consistent with generation by

convective sources. These waves were hypothesized to have been ducted although simple

modelling suggested that ducting may only be effective over somewhat shorter distances

of 1000 to 2000 km. If accurate this would cast some doubt that the waves seen Adelaide

routinely originated near Darwin.

The Darwin Area Wave Experiment (DAWEX) was conceived as an effort to study

AGW production in the troposphere and stratosphere by (a) the nearly daily convective

event known as ”Hector” that occurs over the Tiwi islands just north of Darwin during

the November pre monsoon buildup, and (b) the regular monsoon activity that occurs

over Darwin area in December [Hamilton et al., 2004]. As part of this experiment it



was desired to also understand whether the AGWs produced by such events might be

the source of the AGWs seen in airglow imagers. Airglow imagers were located close to

Darwin, at Katherine, and at Wyndham [Hamilton et al., 2004] in order to observe AGWs

locally generated by these events.

Two other observation sites were located at Adelaide and at Alice Springs at distances

of approximately 2600 and 1300 km from Darwin, respectively. Based on the Hecht et

al. [2001a] study AGWs could be ducted to the Alice Springs site. The results from

those observations sites were analyzed in order to determine if any of the AGWs seen in

those instruments can be attributed to any specific weather activity in general, and to the

convective activity which was the focus of DAWEX in particular.)

2. Experimental Instrumentation And Technique

The unique observational data reported on in this work are the combined airglow imager

data obtained at Alice Springs and Adelaide by the instruments described below. Ground-

based weather data obtained by the Australian Bureau of Meteorology Research Center

(BMRC) instruments and remote sensing weather data from the Japanese meteorological

satellite are used in the interpretation of the imager data. While the sources for these

latter data are more fully described in Hamilton et al. [2004] a brief summary of the

instrumentation is given below. Wind and temperature data are obtained from a variety

of sources including the two Medium Frequency (MF) radars deployed for DAWEX by

the University of Adelaide. The details of the wind and temperature databases used in

this paper are also described more fully in the overview paper of Hamilton et al. [2004]

but will also be summarized below. The location of the important sites are shown in

Hamilton et al. [2004] but are called out with respect to distance below and on the figures



in this paper where appropriate. A knowledge of the distances involved are important for

tracing the path from AGW generation in the troposphere to an observation in the lower

tthermosphere.

The data described in this paper originate from two nearly identical airglow imagers.

One was deployed at Buckland Park which is about 40 km north of Adelaide (3455S,

13836E), and a second at Alice Springs (2342S, 13353E) Australia. Buckland Park, also

the site of an MF radar, is approximately 1270 km due south from Alice Springs, and 2580

km southsoutheast from Darwin (1228S 13051E). Alice Springs is approximately 1290 km

southsoutheast of Darwin km. Katherine (1427S,13216E) the site of a second MF radar

and an airglow imager which is discussed in Hamilton et al. [2004] is 270 km southeast of

Darwin, but is almost 1043 km due north of Alice Springs. The Tiwi Islands (Bathurst and

Melville islands at approximately 1127S and 13020E) are just offshore Darwin at a distance

of approximately 126 km. Thus, several of these locations (Tiwi Islands, Katherine, Alice

Springs, and Adelaide) form a chain that runs almost north to south across the central

portion of Australia. As discussed in Hamilton et al. [2004] another imager was deployed

at Wyndham (1547S, 12810E) which is 460 km west of Katherine. However, in this paper

only the results from the Alice Springs and Buckland Park imagers will be discussed.

2.1. Airglow Imagers

The airglow instruments at Buckland Park (BP) and at Alice Springs (AS) are modified

versions of the Aerospace CCD nightglow camera which was originally described by Hecht

et al. [1994]. The modified version was described in Hecht et al. [2001b]. This instrument

obtains images of the OH Meinel (hereinafter OHM) and O2 Atmospheric (hereinafter

O2A) band emissions. A sequence of five images are obtained, each at 1 min integration,



through separate narrow passband filters. Two of the filters cover two different rotational

lines of OHM (6,2) band, two filters cover different portions of the O2A (0,1) band, and

one filter covers the background and has almost no airglow emission in its passband. The

latter is used to correct the airglow images for background skylight. Thus one can obtain,

besides images of the airglow, the intensity and temperature of the OH Meinel and O2

Atmospheric emissions. Examples of this technique are found in previous studies [Hecht

et al., 1997a, b, 2000]. The imager now uses a 1536 by 1024 Kodak CCD chip. The pixels

are binned 8 x 8 resulting in images that have 192 x 128 pixels. The resultant field of view

is now 46◦ by 69◦ giving a field of view at 90 km altitude of approximately 75 x 122 km.

For those most part in this paper the main data discussed will be the AGW wavelengths

and vector velocity. In past studies it has been found that the O2A band AGWs greatly

resemble those seen in the OHM images.

The DAWEX campaign consisted of three separate intensive observation periods (IOPs).

IOP1 occurred during the new Moon period in October and was supposed to measure

pre-Hector and pre-monsoon convective activity. IOP2 occurred the new Moon period in

November and was designed to measure the HECTOR convective activity, while IOP3,

which occurred during the December new Moon period, was designed to measure monsoon

convective activity when the convection is widespread but relatively weaker in terms of

vertical motion.. Unfortunately data from the BP and AS imagers were only available

during IOP2. During IOP3 the AS imager was not operating and due to weather data

from BP were obtained on only a few nights. However, the previous study of Walterscheid

et al. [1999] provides some information on the transition from pre-monsoon to monsoon

activity.



2.2. Wind and Temperature Data Sets

Hamilton et al. [2004] describe how the datasets were combined to form average wind and

temperature data sets appropriate for IOP2. Briefly they determined, from a comparison

with in situ measurements during DAWEX, that the United Kingdom Meteorological

Office (UKMO) analysis was sufficient to provide wind and temperature data up to the

stratopause. These data were extended to 100 km by using the wind climatology from

the Upper Atmosphere Research Satellite (UARS) atmospheric reference project (URAP)

[Swinbank and Ortland, 2003]. Atmospheric tides which are important above 70 km were

incorporated by using the Global Scale Wave Model (GSWM00) [Hagen et al., 1999].

This model was tuned so as to match the measured MF radar winds, during DAWEX, at

Katherine and at BP. The resultant average wind and temperature data sets for IOP2 are

shown in Hamilton et al. [2004] for two times; at 1800 LT (830 UT) which is near sunset,

and at 0600 LT (2030 UT) which is near sunrise. For the purpose of the discussion in

this paper Figures 1 and 2 show the IOP2 zonal and meridional wind component at 1800

LT, 0000 LT (1430 UT), and at 0600 LT, the middle time being close to the observations

of AGW activity on several nights. For the purposes of the ray tracing discussion the

wind and temperature fields were linearly interpolated to provide climatologies for times

between the three listed above.

2.3. Other Meteorological Data Sets

Hamilton et al. [2004] discuss some of the data sets available for this study. In addition,

BMRC produces daily maps of ground level rainfall over the Australian landmass obtained

from rain gauges. Data used to produce those maps were overlaid on some of the AGW



activity maps produced below in order to show potential regions of convective activity.

Rainfall is shown when it exceeds an arbitrary level here taken as 2.5 mm over 24 hours.

BMRC performs an operational analysis of the winds across Australia at different pres-

sure levels. These can be used to determine the presence of supergeostrophic winds, a

possible source of AGWs in the troposphere.

A third data source are the IR brightness temperatures measured from the Japanese

Geostationary meteorological satellite (GMS). These are calculated with a pixel size of

4km by 4km from the two IR channels on the satellite and three hourly imagery has been

used here. The brightness temperature gives a good measure of the height of the cloud

tops for optically thick clouds.

2.4. Model Analysis

Since this work is concerned with possible sources of the AGWS seen in airglow images it

is necessary to incorporate ray tracing techniques into the analysis. Ray-tracing techniques

are used to computationally investigate the effects of background wind and temperature

variation on gravity wave propagation. These techniques as applied to AGW propagation

are well summarized in Jones [1969] Lighthill [2001], Marks and Eckermann [1995], and

Eckermann and Marks [1999].

For waves with a dispersion relationship G(w, k,x,t) where w,x, k, and t are the fre-

quency, position vector, time and wave number vector, respectively then the following

equations describe the ray path and the refraction of the wave vector along the ray.

dx/dt = ∂G/∂k (1)



dk/dt = −∂G/∂x (2)

dω/dt = ∂G/∂t (3)

Equations 1-3 show how the ground-based group velocity, the wave vector, and the

ground-based wave frequency are modified in the presence of winds and wind and tem-

perature gradients.

Following Jones [1969] and Marks and Eckermann [1995], the non-hydrostatic dispersion

relation appropriate for gravity waves on a slowly varying background flow is expressed

as

ω2

i= (ω − uk − vl)2 =

N2(k2 + l2) + f 2(m2 + 1/4h2)

k2 + l2 + m2 + 1/H2(4)

where,ω is the ground based wave frequency, ωi is the intrinsic wave frequency, k,l,m are

the wave number vectors in the x, y, and z directions, f is the inertial frequency, N the

Brunt Vaisala frequency, and H is the density scale height. From Equation 4 an expression

for m, the vertical wave number, follows as

m2 =(k2 + l2)(N2

− ω2)

ω2− f 2

− 1/4h2 (5)

Equations 4 and 5 neglects a term ω2/c2, where c is the speed of sound, which is found

in the more complete dispersion relation given in Gossard and Hooke [1975]. This is

done for computational purposes but for the wave frequencies considered here this term

is negligible. Furthermore, terms including f, the inertial frequency, are also negligible

for the wave frequencies considered in this work. Equations 4 and 5 can then be used to



derive via Equations 1-3, the motion of the wave packet through the atmosphere. For this

work ray tracing was performed in two different modes.

First, the raytracing equations appropriate to non-hydrostatic gravity waves, as given in

Appendix A of Marks and Eckermann [1995], were used to trace wave packets from their

source taken near 10 km to the airglow emission layer near 85 km or vica versa. In this case

equation 3 above was not used as the time variations of the atmosphere were small. For

each ray there was specified an initial longitude, latitude, altitude, a wave azimuth and a

ground based horizontal phase speed. Group velocities were evaluated and were then used

to calculate the rays paths. The vertical wavenumber, m, was chosen to be appropriate

to wi so that the vertical group velocity is positive ensuring upward wave propagation

of the wave energy. In order to cover the variety of typically observed wave parameters

1000 such rays were simulated with phase speeds of 30-70 m/s, horizontal wavelengths of

30-60 km, propagation azimuths between 135-165 degrees and wave periods in the range

8-30 minutes. These rays were launched at 0000 LT, 0600 LT and 1800 LT from a fixed

point. This was done both in the forward direction (rays launched from Katherine at 10

km altitude and followed to 85 km altitude) and backwards (rays followed backwards in

time from an observation at Alice Springs at 85 km altitude to a source region at 10 km

altitude).

Second a simplified case also considered where once the ray reached 85 km the ray

path was followed assuming the wave was trapped between layers of evanescence and only

propagated horizontally. In the trapped region the wave packets are assumed to be freely

propagating, bouncing back and forth between layers of evanescence. This simplified the

analysis, following Hecht et al. [2001a], and allowed a consideration of whether the winds



significantly rotated the wave vector or significantly moved the wave packet closer to or

further away from Alice Springs. While rigorously, this ignores the effects of winds at the

boundaries where the waves are evanescent, these effects should be small since the packet

spends most of the time in the free propagation region.

3. Results and Discussion

3.1. Airglow Observations and Background Meteorology

Table 1 lists the convective events that occurred during IOP2 in the Katherine/Darwin

area. These could potentially lead to AGW production in the upper troposphere which

then propagate to the 85 km airglow observation region. However, these events are only

part of the extensive tropospheric disturbances that occurred during IOP2 and IOP3.

Figures 3 through 8 place these into perspective by showing rainfall maps over the main

Australian landmass during the 6 nights of airglow observations discussed in this paper.

11/16/01

This day was characterized by a strong Hector over the Tiwi Islands from 0400 to 0900

UT. The rainfall map shows extensive rainfall in the Katherine/Darwin area. However,

there was also major rainfall on the northeastern coast as well as a storm in the waters off

the southwest coast. This latter storm will be discussed below. The airglow observations

were only from AS on this night. AGWs came over AS from the direction of Hector to

the NNW, and from the SW. AGWs from the NNW only appeared for a brief period at

the end of the observing period while those from the SW were present from 1200 to 1700

UT.

11/17/01



Rainfall surrounded BP but AGWs were only observed originating from the NNW

during 1130 to 1300 UT. Another major Hector occurred from 0240 to 0710 UT and

squall line rainfall occurred over Darwin from 0640 to 0900 UT. At AS AGWs were

observed originating from the NNW, the direction towards the Hector event, during the

period of 1100 to 1400 UT. Even though rainfall occurred south of the city AGWs were

not observed from that direction. Rain was also reported along the east and southwest

coasts of Australia.

11/18/01

Rain occurred over three distinct regions of Australia; in the Katherine/Darwin area,

and in the SW and SE corners of the continent. The rainfall over Katherine/Darwin

consisted of convective activity from 0815-1500 UT. AGWs were only reported from AS

and they came from the N between 1040 and 1100 UT and from the NNW from 1100 to

1700 UT.

11/19/01

The rainfall activity was similar to the previous night. Over Katherine there was squall

line activity from 0800 to 1100 UT. At AS, AGWs were observed originating from the

NNW from approximately 1600 to 1900 UT. Prior to that AGWs originated from the SE

from 1345 to 1500 UT. At BP, AGWS came from the W from 1630 to 1800 UT and from

the SW from 1800 to 1820 UT. Rain also occured on the southwest coast.

12/15/01

This was during IOP3 which was generally characterized by more monsoon like rainfall

[Hamilton et al., 2004] than the isolated convective and squall lines seen during IOP2.

On this day there was extensive rainfall over almost the entire northeast and northcentral



(Katherine/Darwin) portion of Australia. Yet at BP waves were still seen to come from

the NNW at 1630 to 1730 UT.

12/16/01

While not as extensive as on the previous day rainfall occurred over the northeast and

northcentral part of Australia. AGWs were seen at BP between 1600 and 1800 from the

NNW and between 1130 and 1420 from the SW.

The wave directionality is quite striking although consistent with previous studies [Wal-

terscheid et al., 1999; Hecht et al., 2001a]. AGWs are not seen to come from the east

despite there being places in that direction where there was storm activity. AGWs are

seen to originate predominantly from the NNW although occasionally they are seen from

the SW. One explanation, following Hecht et al. [2001a], for the predominant NNW origi-

nation is that the AGWs are trapped in a lossy duct. The process of getting into the lossy

duct with enough energy so that they can propagate a significant distance greatly restricts

the range of available directions. This explanation however, depends on the the presence

of strong westward winds below 80 km. Such winds would also explain, through critical

level absorption, the absence of westward propagating waves. From Figure 1, at sunrise

and sunset such strong westward winds do seem to be present. Alexander et al. [2004]

however, specifically modelled AGW production on 11/17/01 over the Darwin area. They

found that there were predominant NE and SE propagation directions which depended

both on the tropospheric winds at the altitude of the wave forcing and the filtering by

the tropospheric winds above that altitude. Thus, part of the directionality observed may

also be influenced by the winds present in the troposphere.



Typically airglow imager observations occurred between about 1030 and 1900 UT. How-

ever, AGWs originating from the NNW appeared generally towards the end of that period

consistent possibly, as is shown below, with generation in the troposphere in the late af-

ternoon over Katherine/Darwin. AGWs from the SW generally appeared earlier in the

observation period. These presumably could originate from sources that occurred at more

random periods throughout the day.

To investigate this further we provide a more detailed look at three events; a source for

the observation of AGWs seen originating to the SW of AS on 11/16/01, and the source

of AGWS originating from the NNW of AS on 11/17/01 and 11/19/01. These events were

chosen not only because they represent the two predominant directionalities observed but

because there are known isolated sources which may be the cause of the observations.

3.2. Wave Observations 11/16/01 UT and the Presence of SuperGeostrophic

Winds

On 11/16/01 AGWs at AS were seen from 1230 to 1700 UT coming from the SW. This

was the most extensive set of observations of AGWs coming from this direction during

DAWEX and is even more interesting in that strong tropospheric weather occurred in

that direction during this period.

Figure 9 shows the BMRC operational surface analysis on the morning before the wave

event. A complex cut-off low system is dominating southwestern Australia, just west

of the the direction from which the AGWs were observed. These type of systems often

produce significant rainfall and have vigorous rainbands. The rainfall map (Figure 3)

does indicate that a significant amount of rainfall occurred in the direction from which

the AGWs are seen to originate. Over the next 24 hours the whole system has moved



eastward over the region from which the AGWs were observed suggesting that this system

was the origin of the observed AGWs. It is interesting to note that while rainfall occurred

over the southwest portion of the Australian landmass on all four observation nights of

IOP2 only on two nights were AGWs observed from this direction at Alice Springs and

only on this night did observations occur for more than one hour.

Figure 10 shows a satellite picture of the clouds associated with this low at 0800 UT

shortly before the observed AGWs. The coldest cloud tops in the low pressure region

are only about -40 to-50C and the tropopause temperature from the Eucla soundings,

located near the coast on the border between the states of South and Western Australian

(SA/WA) are about -65 to 70 C. These temperatures are therefore inconsistent with

vigorous convective activity. The radar data confirm this as there appears to be relatively

little intense rainfall associated with this low pressure system in the period around 0900

UT. This is significant since the phase velocity of the AGWs are about 200 km/hr it would

take approximately 6 to 8 hours to travel from this system to Alice Springs neglecting

the effects of background winds. Thus, AGWs launched at 0900 UT might reach Alice

Springs during the 1230 to 1700 UT period where waves were observed that night.

The cloud complex near 35S 130E is associated with the cut-off low. There is a strong

upper level jet overlaying this system as is shown in the Bureau of Meteorology analyses.

Figure 11 shows an isotach analysis of the winds at 250 hPa. This shows the presence

of a wind jet above the cutoff low which exits near the SA/WA border. Figure 12 shows

the ageostrophic component of the wind with the arrow pointing to the large amplitude

ageostrophic motions associated with the jet near the SA/WA border. The magnitude

of these motions are almost one third of the total wind speed. This supergeostrophic



flow had components of approximately equal magnitude both along the height contours,

through the ridge, and normal to the height contour associated with the exit from the

jet. These large amplitude ageostrophic motions means the flow is a long way from being

balanced and thus may be conducive to the generation of gravity waves.

There have been a number of studies that reported on AGWs generated by ageostrophic

motions [Plougonven et al., 2003; O’Sullivan and Dunkerton, 1995; Fritts and Luo, 1992].

Typically, the periods for these AGWs were much longer than are reported in this study.

Data however, suggest that dynamical events associated with fronts and/or the jet stream

are the source of short horizontal wavelength AGWs at least in the upper troposphere and

lower stratosphere [Fritts and Nastrom, 1992]. A study of AGWs generated by large wind

shears did show that the AGWs generated by such events would only reach the mesophere

if their intrinsic frequency was high [Buehler et al., 1999; Buehler and McIntyre, 1999].

While the data strongly suggest that these observed AGWs are indeed associated with

this cutoff low system it is not clear as to their origin. The rainfall did not appear to

be associated with any significant convective activity and other nights where there was

significant rainfall (e.g. 11/18) did not produce observed AGWs at AS for any extended

temporal period. However, the observed directionality of the AGWs on 11/16/01 is di-

rectly to the center of the rainfall maximum. While ageostrophic motions are also a

candidate source the modeling to date indicates this would produce larger scale waves

than observed here. Whether the current models could resolve such small-scale waves is

an open question.



3.3. Wave Observations at AS on 11/17/01 and 11/19/01 UT and their

Relation to Convective Activity Near Katherine

Most of the AGW activity seen at AS consisted of waves propagating from the NNW

over the imager site. A straight line extrapolation intersects the Katherine/Darwin area

suggesting that convective activity there could be responsible for the AGWs seen at AS.

The night of 11/19/01 UT is especially noteworthy as there appears to be a relatively

isolated (in time) convective event that occurs during the late afternoon and early evening

near Katherine. and subsequent post midnight observations of AGW activity over AS.

Figures 13 and 14 shows the development of the convective activity from satellite infrared

images of cloud cover centered over the Tiwi islands. The images are shown every 3 hours

from 0200 to 1100 UT. Figure 13 shows relatively clear skies over the Katherine area.

However as shown in Figure 14 cold cloudtops suggestive of convective activity appears

at 0800 and 1100 UT in the Katherine area. Figure 15 shows a radar reflectivity map

indicating that vigorous rainfall is associated with these clouds around Katherine. To

investigate further whether AGWs launched from this event could be responsible for the

observations of AGWs over AS beginning near 1600 UT ray traces were performed to

trace back the origin in the troposphere, assuming no ducting, of AGWs seen over AS at

85 km altitude. Figure 16 shows the background wind and temperature field that a ray

launched at 85 km passes through to reach 10 km altitude, i.e. a ray traced backwards

in time. The other panels shows the AGW intrinsic periods and the x and y horizontal

distances for the wave packet. Figure 17 shows a map of the ray paths. Clearly, consistent

with Walterscheid et al. [1999] AGWs that are not ducted with relatively horizontal

wavelengths only travel a few hundred km from their source in the troposphere to reach



85 km altitude. It takes about 2 hours to reach 85 km from a 10 km origin. None of the

rays reach Katherine and as their is little weather or orographic features in this region it

is difficult to understand what would launch these AGWs.

Figure 18 shows two maps of rays which are launched at 10 km from Katherine and

followed to 85 km altitude. The top panel is for AGWs launched at 1430 UT the same

time as the backwards ray trace shown in Figure 17. The results are the same; AGW wave

packets travel only a few hundred kms away from Katherine not reaching AS. Interestingly,

although the wave vector is taken as 135 to 160 degree azimuth and thus the packets are

initially launched in those directions, the background winds blow the packets so that by

the time they reach 85 km they are almost due south of Katherine. The bottom panel

shows another set of ray traces this time launched at 0830 UT close to the time that

the convective activity actually occurred near Katherine. Again, most of the rays only

travel a few hundred kms from Katherine. However, there are some rays that actually

reach AS. The reason for this can be seen in Figure 19 which shows the background winds

and temperatures and intrinsic period for the AGW waves. Some of the AGWs suffer

critical level reflection as there is a strong meridional wind towards the south at this time.

However, calculations show that even those wave packets reach 85 km in under 3 hours

so again these would not account for the AS observations where AGWs are not seen until

about 1600 UT. Note that the strong southward wind decreases with time so that waves

launched later would not suffer this reflection.

However, earlier studies such as Walterscheid et al. [1999] and Hecht et al. [2001a]

suggested that once AGWs reached 85 km they were trapped in a leaky duct that allowed

them to travel longer distances than implied by the ray traces shown in Figure 18. In Hecht



et al. [2001a] the wave packets were considered to be freely propagating throughout most

of the trapped region and the packets were assumed to bounce off the upper and lower

boundaries where the AGWs become evanescent. So a simple ray trace was performed

using Equations 1 to 5 but only looking at the horizontal propagation, assuming there

was no net vertical propagation for a packet that is bouncing between trapping layers

above and below. This allowed a look at how the wave packet would be blown by the

meridional and zonal winds and to investigate whether the horizontal wave vector would

change in direction and magnitude. For this analysis the winds were taken as constant

and either those at 85 or 90 km altitude representative of the altitudes of the OHM and

the lower portion of the O2A emission. Horizontal wavelengths of 40 and 50 kms and

observed periods of 15 and 20 minutes were used in the calculation. Note that on this

night while the median AGW horizontal wavelength was approximately 50 km a range

of wavelengths were observed. For these calculations it was found that the wind and

temperature gradients were small enough so that the wave vector remained essentially

unchanged during the ray trace. AGWs of 40 km horizontal wavelength, a 15 minute

period, background winds representative of 85 km, and launched at 1000 UT from 1600S

latitude would reach 2400S latitude at 1600 UT. This is consistent with the observations

at AS at that time. Such a packet would essentially travel due south. However, if the

AGW saw background winds representative at 90 km altitude it would be blown a few

hundred kms to the east. This is not surprising as the at 85 km are rather weak and those

at 90 km (and above) are stronger.

Since the wave packet would spend part of the time experiencing rather weak winds

blowing the packet somewhat towards the west and some time above 90 km where it



would see stronger winds blowing the packet a few hundred kms to the east the combined

ray trace analysis suggests that the AGWs seen at AS in the post midnight to sunrise

timeframe would have been launched a few hundred kms west of Katherine in the period

after 0800 UT. This certainly seems consistent with the convective activity shown in

Figure 14.

There is some evidence for winds that can trap these waves as seen in Figure 2. In the

bottom panel, at 2030 UT (0600 LT) the winds just below 80 km and apparently just

above 100 km are strongly northward which would essentially trap southward propagating

waves. This trap had slowly moved down during the night and the bottom portion is

present between 80 and 85 km at 1430 UT. At 0830 UT (1800 LT) the bottom is centered

around 90 km. In Hecht et al. [2001a] it was found that for certain propagation directions

the wave packets could tunnel through the bottom of the trap retaining enough energy to

propagate considerable distance. Interestingly for IOP2 because this trap is a function of

longitude the wave packets can enter the trapped region from below closer to the equator

where relatively less energy would be lost in penetrating the evanescent region than at

more southerly longitudes. As the wave packets move south and the evanescent region

becomes thicker the trapped region would become relatively less lossy than in the Hecht

et al. [2001a] simulation.

Finally, we note that Alexander et al. [2004] specifically modelled AGW generation

for the Hector and squall activity events on 11/17. They found that for both Hector

and squall line events the wave field had mainly northeast and southeast directionality

consistent with the directionality observed at AS on both 11/17 and 11/19. For Hector

type storms the horizontal wavelengths are mostly below 50 km and the phase speeds are



less than 40 m/s. For squall like events the horizontal wavelengths and phase velocities

can be somewhat larger. On 11/17 AGWs were observed at AS from 1100-1300 UT

suggestive, based on the ray trace for 11/19, on generation in the Darwin area from 0300

to 0500 UT. This is the period of the intense Hector event. The AS observations showed

horizontal wavelengths around 35 km, somewhat shorter than those observed on 11/19

consistent with Alexander et al. [2004]. The observed phase speeds on 11/17 are close to

60 m/s, somewhat above their model results.

3.4. Observations at BP and Monsoon Activity

As at AS there were many observations of AGWs originating from the NNW. The

rainfall map on 12/15 and 12/16 for example, shows some rainfall occurring close to AS

and between AS and BP, and these tropospheric weather systems may be responsible for

the BP observations on those nights. However, on 11/19 there is no rainfall close to BP

or even south of Katherine that could plausibly be the origin of the AGWs observed over

BP.

These data do not resolve the question as to the origin of the AGWs seen over BP.

While it cannot be ruled out that these are generated by ageostrophic winds their hori-

zontal wavelengths, as discussed previously, may be too short based on current models.

Previously, Walterscheid et al. [1999] suggested that ducting of waves from the extensive

region of deep cumulous convection over northern Australia explained the strong poleward

directionality seen in the summer months at BP. The analysis of Hecht et al. [2001a] as

well as the ray tracing in the previous section suggest that wave propagation over such

large distances is a selective process (for example,strongly trapped waves generated at

certain azimuth angles during favorable tidal phases) and sufficiently prolific wave sources



closer to BP would be favored . In general, attributing the BP observations to long-range

ducting of AGWs generated by convective events over northern Australia in the late af-

ternoon in accordance with the usual diurnal cycle of convection over land seems unlikely

since the wave group velocities are too small to allow them to reach BP during the evening

observation period. However, in the far north, AGWs may originate over water where the

diurnal cycle can be reversed (e.g., nocturnal convection over coastal waters) or fairly

weak (especially closer to the equator). Future coordinated observations are probably

required to determine their origin.

It is also worth noting that even though there was extensive monsoon activity to the E

and NE of BP on several nights AGWs were not seen from these directions,. The westward

winds present at or below 80 m at sunrise and sunset may filter out the westward traveling

waves.

4. Conclusions

The main results of this paper are the following.

1. Observations of airglow emissions at two sites in central Australia, Alice Springs

(AS) and Buckland Park (BP), revealed the presence of wavelike perturbations in the

airglow intensity attributed to atmospheric gravity waves passing through the emission

layer. The waves were found to originate mainly from the NNW of the site propagating

towards the SSE. Some waves were seen to originate to the SW of the sites propagating

towards the NE. No waves were seen propagating from the east to the west. Waves seen

coming from the NNW were mainly seen close to or after local midnight while the waves

seen coming from the SW were seen earlier.



2. An analysis of the waves seen over AS on 11/19/01 was consistent with those waves

being generated by convective activity in the Katherine area around sunset. However, if

that was the origin of the waves seen at AS then those waves must have been trapped or

ducted.

3. An analysis of the waves seen over AS on 11/16/01 coming from the SW revealed

no obvious strong convective source for those waves, although the AGWs clearly are

associated with a cutoff low pressure system present over southwest Australia. While it

is possible that they were dynamically generated by ageostropic winds associated with

this low modeling studies to date suggest that such waves would have long horizontal

wavelengths and periods inconsistent with our observations.

4. On several nights there was evidence of tropospheric weather between considerably

south of Katherine, and sometimes between AS and BP, which may have been the source

of the wave observations at BP. However, waves were seen at BP coming from the NNW

on 11/19 even though was no obvious origin such as convective activity south of Katherine

that could account for the observations. While it cannot be ruled out that these waves

were (a) generated dynamically by ageostrophic winds or (b) convectively generated by

activity over northern Australia or even closer to the equator, their origin is still unknown.

5. The lack of waves propagating westward suggests some wind filtering mechanism

either in the troposphere or the mesosphere.



Acknowledgments.

Thanks to Peter Strickland for the considerable help at Alice Springs. We also thank

Dr. Elizabeth E. Ebert of BMRC for providing us with the rainfall maps. The Aerospace

results could not have been obtained without the invaluable help given by Kirk Crawford

in all aspects of this project. JHH and RLW were supported by NSF grant ATM-0122772

and by The Aerospace Corporation MOIE program.

References

Alexander, M. J., P. T. May, and J. H. Beres, Gravity waves generated by convection in

the Darwin area during DAWEX, J. Geophys. Res., this issue, 2004.

Beer, T., Atmospheric Waves, 300pp., John Wiley & Sons, New York, 1974.

Buehler, O., M. E. McIntyre, and J. F. Scinocca, On shear-generated gravity waves that

reach the mesosphere. part I: Wave generation, J. Atmos. Sci., 56, 37493763, 1999.

Bhler, O., and M. E. McIntyre, On shear-generated gravity waves that reach the meso-

sphere. part I: Wave propagation, J. Atmos. Sci., 56, 37643773, 1999.

Eckermann, S. D. and C. J. Marks, An idealized ray model of gravity wave-tidal interac-

tions, J. Geophys. Res., 101, 21195-21212, 1996.

Fritts, D. C., and Z. Luo, Gravity wave excitation by geostrophic adjustment of the jet

streams, part I: Two-dimensional forcing, J. Atmos. Sci., 49, 681697, 1992.

Fritts, D. C., and G. D. Nastrom, Sources of mesoscale variability of gravity waves. part

II: Frontal, convective, and jet stream excitation, J. Atmos. Sci., 49, 111127, 1992.

Gossard, E. E. and W. H. Hooke, Waves in the atmosphere, atmospheric infrasound and

gravity waves - their generation and propagation, 456pp., Elsevier, Amsterdam, 1975.



Hagen, M. E., M. D. Burrage, J. M. Forbes, J. Hackney, W. J. Randel, and X. Zhang,

GSWM-98: Results for migrating solar tides, J. Geophys. Res., 104, 6813-6828, 1999.

Hamilton, K., R. A. Vincent, and P. T. May, The DAWEX field campaign to study gravity

wave generation and propagation J. Geophys. Res., this issue, 2004.

Hecht, J. H., R. L. Walterscheid, and M. N. Ross, First measurements of the two-

dimensional horizontal wavenumber spectrum from CCD images of the nightglow, J.

Geophys. Res., 99, 11,449-11,460, 1994.

Hecht, J. H., S. K. Ramsay Howat, R. L. Walterscheid, and J. R. Isler, Observations of

Spectra of Brightness Fluctuations of the OH Meinel Nightglow During ALOHA 93,

Geophys. Res. Lett., 22, 2873-2876, 1995a.

Hecht, J. H., R. L. Walterscheid, D. C. Fritts, J. R. Isler, D. C. Senft, C. S. Gardner,

and S. J. Franke, Wave breaking signatures in OH airglow and sodium densities and

temperatures 1. Airglow imaging, Na lidar, and MF radar observations, J. Geophys.

Res., 102, 6655-6668, 1997a.

Hecht, J. H., R. L. Walterscheid, J. Woithe, L. Campbell, R. A. Vincent, and I. M. Reid,

Trends of airglow imager observations near Adelaide, Australia, Geophys. Res. Lett, 24,

587-590, 1997b.

Hecht, J. H., C. Fricke-Begemann, R. L. Walterscheid, R. L. and J. Hoffner, Observations

of the breakdown of an atmospheric gravity wave near the cold summer mesopause at

54N, Geophys. Res. Lett., 27, 879-882, 2000.

Hecht, J, R. Walterscheid, M. Hickey, S. Franke, Climatology and modeling of quasi-

monochromatic atmospheric gravity waves observed over Urbana Illinois, J. Geophys.

Res., 106(D6), 5181-5196, 2001a.



Hecht, J. H., R. L. Walterscheid, and R. A., Vincent, Airglow Observations of Dynamical

(Wind Shear-Induced) Instabilities over Adelaide, Australia Associated with Atmo-

spheric Gravity Waves, J. Geophys. Res., 106, 28189-28197, 2001b.

Jones, W. L., Ray tracing for internal gravity waves, J. Geophys. Res., 74, 2028-2033,

1969

Lighthill, J. Waves in fluids, 504pp., Cambridge University Press, Cambridge, 2001.

Marks, C. J. and S. D. Eckermann, A three-dimensional nonhydrodynamic ray-tracing

model for gravity waves: Formulation and preliminary results for the middle atmo-

sphere, J. Atmos Sci., 52, 1959-1984, 1995.

Moreels, G., Herse, M. Photographic evidence of waves around the 85 km level, Planet.

Space Sci., 25, 265-273, 1977.

Nakamura, T., A. Higashikawa, T. Tsuda, and Y. Matsushita, Seasonal variations of

gravity wave structures in OH airglow with a CCD imager at Shigaraki, Earth, Planets,

and Space, 51, 897-906, 1999.

O’Sullivan, D., and T. J. Dunkerton, Generation of inertia-gravity waves in a simulated

life cycle of baroclinic instability, J. Atmos. Sci., 52, 36953716, 1995.

Peterson, A. W., and L. M. Kieffaber, Infrared photography of OH airglow structures,

Nature, 242, 321-322, 1973.

Plougonven R., H. Teitelbaum, V. Zeitlin, Inertia gravity wave generation by the tropo-

spheric midlatitude jet as given by the Fronts and Atlantic Storm-Track Experiment

radio soundings, J. Geophys. Res., 108 (D21), 4686, doi:10.1029/2003JD003535, 2003.

Smith, S. M., M. Mendillo, J. Baumgardner, and R. R. Clark, Mesospheric gravity wave

imaging at a subauroral site: First Results from Millstone Hill, J. Geophys. Res., 105,



27119-27130, 2000.

Swenson, G. R., M. J. Taylor, P. J. , Espy, C. S. Gardner, and X. Tao, ALOHA-93

measurements of intrinsic gravity wave characteristics using the airborne airglow imager

and ground-based Na wind/temperature lidar, Geophys. Res. Lett., 22, 2841-2844, 1995.

Swinbank, R. and D. A. Ortland, Compilation of wind data for the UARS reference

atmosphere project, J. Geophys. Res., 108, 4615, doi:10.1029/2002JD003135, 2003.

Taylor, M. J., M. B. Bishop, and V. Taylor, All-sky measurements of short period gravity

waves imaged in the OI(557.7 nm), Na (589,2 nm) and near infrared OH and O2 (0,1)

nightglow emissions during the ALOHA-93 campaign, Geophys. Res. Lett., 22, 2833-

2836, 1995.

Walterscheid, R. L., J. H. Hecht, R. A. Vincent, I. M. Reid, J. Woithe, and M. P.

Hickey, Analysis and Interpretation of Airglow and Radar Observations of Quasi-

Monochromatic Gravity Waves in the Upper Mesosphere and Lower Thermosphere over

Adelaide, Australia (35 S, 138 E), J. Atmos. Solar-Terr. Phys. , 61, 461-468, 1999.

Walterscheid, R. L., G. Schubert, D. G. Brinkman, Small-scale gravity waves in the upper

mesosphere and lower thermosphere generated by deep tropical convection,J. Geophys.

Res., 106(D23), 31825-31832, 10.1029/2000JD000131, 2001.

Wang, L., and M. A. Geller, Morphology of gravity-wave energy as observed from 4 years

(19982001) of high vertical resolution U.S. radiosonde data, J. Geophys. Res., 108(D16),

4489, doi:10.1029/2002JD002786, 2003.



Table 1. Convective Events in the Katherine/Darwin During IOP2

Date Time Comments

11/16 0400-0900 UT Major Hector (Tiwi)11/17 0240-0710 UT Major Hector (Tiwi)11/17 0640-0900 UT Squall Line (Darwin)11/18 0815-1500 UT Convection (Katherine/Darwin)11/19 0800-1100 UT Squall Line (Katherine/Wyndham)



Figure 1. Plots of the Climatology of the Zonal Wind during IOP2 at three times



Figure 2. Plots of the Climatology of the Meridional Wind during IOP2 at three times



Figure 3. Map of Australia showing regions of significant rainfall as colored contour regions

for the 24 hour period on 11/16/01 UT. The location of Buckland Park, Alice Springs, Katherine

and Bathhurst Island are marked. Arrows at those sites show the direction of propagation for

AGWs. The period when those AGWs were seen the horizontal wavelength are shown. The

length of the arrow is proportional to the horizontal wavelength.A colorbar is shown to indicate

the color assigned to the rainfall in mm over the 24 hours.



Figure 4. Same as Figure 3 but for 11/17/01 UT.



Figure 9. Operational surface analysis of pressure prepared by BMRC at 0000 UT on 11/16/01

(Top) and 11/17/01 (Bottom).



Figure 10. Japanese Meteorological Satellite Infrared Color Temperature Image over Australia

at 0800 UT on 11/16/01. The grey scale to the right shows the temperatures associated with

each color. The coldest cloudtops appear as white.



Figure 11. BMRC analysis of wind speeds at 250 hPa over Australia at 0500 UT on 11/16/01.



Figure 12. BMRC analysis of ageostrophic wind speeds at 250 hPa over Australia at 0500 UT

on 11/16/01.



Figure 13. Japanese Meteorological Satellite Infrared Color Temperature Image over Australia

on 11/19/01 with the image centered over the Tiwi Islands. The location of Katherine, Wyndham,

and Alice Springs are marked with red letters K, W, and A respectively. The scales are are

latitude and longitude and the color bar indicates the cloud top temperature in K. (Top) 0200

UT. (Bottom) 0500 UT.



Figure 14. Same as Figure 13 except for time (Top) 0800 UT. (Bottom) 1100 UT.



Figure 15. This map shows the radar reflectivity at 0808 UT on 11/19/01. The color scale is

in decibels with red indicating the most intense reflection and indicative of intense rainfall. This

map shows only a portion of the northcentral region Australia imaged in Figure 14. The location

of Katherine is marked with a K. Wyndham is located just off the southwest corner. The scales

are the distance in km from the center of the map.



Figure 16. Four plots appropriate to a ray trace of AGWs seen at 85 km and followed

backwards in time to their origin near 10 km. The background atmosphere is taken at 1430 UT

(0000 LT) (Top left) Solid line-zonal winds in m/s, Dotted line-meridional winds in m/s, Dashed

Line-Temperature in K. (Top Right) The AGW intrinsic period. (Bottom left) Distance travelled

in the x direction. (Bottom right) Distance travelled in the y direction.



Figure 17. A map of Australia showing some results for ray trace paths of AGWs seen at 85

km over Alice Springs and followed backwards in time to their origin at 10 km altitude. The ray

trace background conditions are taken to be those appropriate for 1430 UT (0000 LT).



Figure 18. Maps of Australia showing some results for ray trace paths of AGWs launched

from Katherine at 10 km altitude and followed to 85 km altitude. (Top) Rays launched at 1430

UT (0000 LT). (Bottom) Rays launched at 0830 UT (1800 LT).



Figure 19. A forward ray trace with AGWs launched at 10 km altitude and followed forward

in time until they reach 85 km. (Bottom) 1430 UT (0000 LT). (Top) 0830 UT (1800 LT).


airglow imager observations of atmospheric gravity waves

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