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Gravity wave observation from the Cloud Imaging and Particle Size (CIPS) Experiment on the Aeronomy of Ice in the Mesosphere (AIM) Spacecraft
Amal Chandran 1, 2,* , David Rusch 2 , S. E. Palo 1 , G. E. Thomas 2 , M.J. Taylor 3
1 Department of Aerospace Engineering, University of Colorado, Boulder
2 Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder
3 Center for Atmospheric and Space Sciences and Physics department, Utah
State University, Logan
* Corresponding Author. Tel: +17202407727
Email address: [email protected]
Abstract 1
In this paper we present first results of gravity wave observations on polar mesospheric clouds during the 2
summer of 2007, in the northern hemisphere. The Cloud Imaging and Particle Size (CIPS) experiment is one of 3
the three instruments on board the Aeronomy of Ice in the Mesosphere (AIM) spacecraft that was launched into 4
a sunsynchronous orbit on April 25, 2007. CIPS is a 4 camera widefield (120° x 80°) imager designed to 5
measure PMC morphology and particle properties and has a spatial resolution of 1 x 2 km in the nadir. One of 6
the objectives of AIM is to investigate gravity wave effects on PMC formation and evolution. CIPS images 7
show distinct wave patterns and structures in PMC’s. These structures range from a few kilometers to a few 8
hundred kilometers, similar to ground based photographs of Noctilucent Clouds (NLC’s). The horizontal 9
wavelengths of the observed waves range between 15 and 300 km, with smaller wavelength structures of less 10
than 50 km being most common. We present examples of individual wave events observed by CIPS and 11
statistics on wave structures observed in the northern hemisphere during the summer months of 2007. We also 12
present a global map of gravity wave events observed from CIPS. The spectrum of the PMC structures for the 13
three summer months show a clear peak at wavelengths less than 50 km. 14
Keywords: polar mesospheric clouds, gravity waves, mesosphere 15
1. Introduction 16
Photographs of Noctilucent Clouds (NLC’s) and Polar Mesospheric Clouds (PMCs) often exhibit a 17
distinct wave structure with spacing of ~ 10 to 100 km or more. These structures are described as “bands” while 18
the smaller scale features, with a spacing of ~ 3 to 10 km have been termed “billows” or “whirls” depending on 19
their form [Witt, 1962; Haurwitz and Fogle, 1969; Fritts and Rastogi, 1985; Gadsden and Parvianien, 1995; 20
Thomas, 1991]. 21
The structures seen in the clouds can potentially yield information about the dynamics of wave motion 22
in the upper mesosphere where they are believed to control the mesospheric circulation [Lindzen, 1982; Holton, 23
1983]. Indeed they are known to be directly responsible for altering the circulation which leads to the very low 24
polar temperatures responsible for the conditions necessary for ice formation. Remote sensing of the mesopause 25
region using airglow imaging techniques have shown evidence of gravity waves with length scales similar to 26
those seen in NLC near the mesopause [Swenson and Espy, 1995; Taylor and Garcia, 1995; Hines, 1968; Fritts, 27
1984]. Henceforth in this paper, we will refer to PMC linear structures having three or more spatiallycoherent 28
peaks and troughs in the scattered radiance as 'gravity waves (GW)'. In actuality they are proxy indicators of 29
waves, through the combined effects of the periodic changes in temperature, water vapor and vertical motion. 30
GWS propagate upward from lower altitudes where they grow in amplitude and become unstable 31
('break') in the upper mesosphere and lower thermosphere. They deposit substantial momentum and energy in 32
this region could play an important, if not a crucial, role in PMC formation and destruction [Turco et al., 1981; 33
Jensen and Thomas, 1993; Rapp et al., 2002]. Groundbased views of NLC are possible during summer in a 34
limited latitude zone (~5060 o ) where the lighting conditions allow scattering of sunlight to be visible against a 35
relatively dark sky. These views reveal the nearly ubiquitous presence of waves, at least at these latitudes during 36
the NLC season (approximately two months from midMay to midAugust in the NH [Gadsden, 1998]). The 37
smallerscale NLC 'billows' which often accompany larger band structures may be manifestations of the wave 38
breaking process itself, wherein waves become convectively unstable and create secondary waves normal to the 39
original wave front [Fritts et al., 1993]. The scales of internal atmospheric gravity waves typically encompass 40
horizontal wavelengths of a few tens of km to several thousand km, and ~1 to several tens of km in the vertical 41
[Manson, 1990, Fritts, 1984]. Typical horizontal phase speeds of1060 m/s have been reported by Haurwitz and 42
Fogle [1969], but values exceeding 100 m/s are known to occur, often opposite the direction of the bulk flow. 43
The bands seen in ground based NLC photography typically exhibit periods of less than an hour and horizontal 44
scales of up to a few hundred km and represent only a fraction of the total wave spectrum [Fritts, 2003]. The 45
longer period (with larger horizontal scales) gravity waves are expected to play an important role for PMC 46
formation than the short period waves as their time scales are similar to the expected PMC growth/decay time. 47
Microphysical modeling predicts that the dividing period is about seven hours, below which ice particles are 48
destroyed by the wave, and above which the ice particles can be at least temporarily enhanced in size, and thus 49
brightness [Rapp et al., 2002]. Experimental evidence that shortperiod gravity wave activity is inversely 50
proportional to PMC backscattering was provided by Gerrard et al. [1998; see also Thayer et al., 2003]. Lidar 51
backscattering of PMC at Söndrestrom, Norway indicate persistent gravity wave influences with periods of 2 to 52
3 hours [Thayer et al., 2003]. There is an abundance of temperature measurements in rocket flights that show 53
that highlatitude, summertime GW cause significant fluctuations in temperature, exceeding 5K. For example, 54
Rapp et al., [2002] showed that NLC occurred in the immediate vicinity of negative fluctuations in temperature 55
in three of seven rocketborne highresolution (200 m) ionization gauge measurements. Their detailed model 56
simulation showed that this correlation resulted from a complex interplay between growth, sedimentation and 57
vertical velocity fluctuations. 58
Ground based observations being local and almost all of them being south of 70 o latitude 59
(predominantly in the northern hemisphere) are limited in their use to study the largescale distribution of 60
gravity waves. PMC mapping from space can yield this type of information over the entire summertime polar 61
cap. Carbary et al. [2003], using data from the ultraviolet and visible imaging and spectrographic imaging 62
instrument (UVVISI) on the Midcourse Space Experiment (MSX) satellite, observed horizontal structures in 63
PMC’s of typical size 100 km with some structures >100 km. Nadir angles of 84 o limited the spatial resolution 64
along the line of sight, but structures of a few km were resolved normal to the viewing direction. Hundreds of 65
images were processed, from a total of ~25,000 separate images taken over 22 separate orbits in the southern 66
hemisphere (SH) of 1997/98 and in the northern hemisphere (NH) 1999. However, the reported images covered 67
only narrow strips, about 100 km wide, and were not sufficiently numerous to define latitudinal or seasonal 68
characteristics. The Cloud Imaging and Particle Size Experiment (CIPS) represents a significant advance over 69
MSX for several reasons: (1) The viewing geometry is more favorable, ranging from nadir viewing to a 70
maximum of 60 o offnadir; (2) the field of view is an order of magnitude greater (~1000 km x 2000 km) 71
permitting overlapping coverage of the polar region up to 82 o ; and (3) the CIPS coverage of the polar region has 72
100% duty cyclewith 15 orbits per day over the full northern2007 PMC season. The CIPS experiment was 73
carried on board the Aeronomy of Ice in the Mesosphere (AIM) satellite, which was launched on April 23, 2007 74
into a nearpolar sunsynchronous orbit of ~ 600 km height with an inclination of 97.8 o . Mounted on the 75
earthward side of the spacecraft, the CIPS cameras possess an unparalleled view of the PMCs over the polar 76
region at a uniform (~ 5 km) spatial resolution. It should be noted that the noonmidnight sunsynchronous 77
geometry of the AIM orbit means that only two bands of local solar time are sampled, centered around 2200 hrs 78
(the orbital upleg for the NH at 6474 o N) and 1400 hrs(the downleg). It is possible that GW characteristics may 79
depend upon local time, for example, if they are affected by tidal winds (for example, see Liu and Hagan, 1998 80
for numerical simulations of tidal interactions with gravity waves). This limitation should be kept in mind in 81
comparison with other data. 82
2. The CIPS Instrument 83
The CIPS experiment is a panoramic UV nadir imager with a spectral triangular bandpass, centered at 84
265 nm extending from 258nm to 274 nm (halfpower points). This region in the UV is chosen to maximize 85
cloud contrast, due to the relative weakness of the Rayleighscattered sky background from absorption of solar 86
radiation in the ozone Hartley bands. CIPS consist of four cameras with each camera taking 34 images per orbit. 87
Each camera uses a 2048 x 2048 pixel detector binned to provide a 1x 2 km resolution in the nadir at 83 km, the 88
nominal PMC altitude. Onboard binning results in a 360 (along track) x 180 (cross track) array of science 89
pixels [McClintock et al., 2008 (this issue)]. The combined camera array has a 120 o x 80 o field of view. 90
Projected to cloud altitude, the total field of view of ~ 2000 x 1000 km centered at nadir. CIPS takes multiple 91
exposures of the polar atmosphere, permitting a variety of scattering angles to be measured of the same volume 92
of space. Since PMC scatter light more efficiently in the forwardscattering direction, the sevenimage 93
combination helps determine cloud presence and thus allows a background separation and mapping of the much 94
weaker PMC. The details of this separation procedure is described in more detail in Bailey et al. [2008, this 95
issue] and Rusch et al. [2008, this issue]. The 5km spatial resolution enables the mapping of PMC structures at 96
scales which allows full resolution of the NLC bands. The images are marginal for viewing the smallscale 97
(wavelengths <10 km) billow structures, and will not be discussed in this paper. 98
3. Analysis 99
The first step in the analysis of waves is to manually identify clear wave events in the data. A wave 100
event is defined as a regular set of three or more spatiallycoherent linear features in the CIPS albedo maps. 101
Shown in Figure 2a is a four camera CIPS ‘bowtie’ image. There is an overall increase of PMC albedo, from 102
left to right, as the scattering angle increases. The fore and aft cameras have an integration time of 0.73 seconds 103
and the nadir cameras have an integration time of 0.75 seconds. (The UV albedo (sr 1 ) is defined as the ratio of 104
the scattered irradiance divided by the incoming solar irradiance, averaged over the spectral band pass.) Distinct 105
wave patterns can be seen near the top left of the image. The next step is to trace a series of pixelwide sections 106
normal to the wave fronts (as indicated by the line across the CIPS images in Figure 2a). A loworder 107
polynomial is fitted to each trace essentially performing a highpass filter. This polynomial fit is designed to 108
remove most of the underlying smoothlyvarying PMC signature. The difference of the albedo values from the 109
fit is calculated and the variations are assumed to be albedo perturbations purely due to the wave dynamics. 110
Next, a wavelet analysis is performed on these difference values of albedo, a method which is well suited to a 111
finite 'wave packet'. The wavelet transform technique is used to analyze time series that contain many different 112
frequencies, or in this case of a spatial scan, with many composite wavelengths. This technique has been found 113
suitable for analysis of CIPS images which often show multiple wave structures. Since here we are primarily 114
interested in the wavelet power criteria, the choice of wavelet function is not really critical, however the Morlet 115
wavelet was chosen since it can give information about both amplitude and phase and is better suited for data 116
with oscillatory behavior [Torrence et al., 1998]. The Morlet wavelet consists of a plane wave modified by a 117
Gaussian. The Morlet wavelet function is given by the equation: 118
2 / 4 / 1 0
2 0 ) ( η η ω π η ψ − − = e e i (1) 119
where 0 ω is the non dimensional “wavenumber” and η is a non dimensional “distance” parameter. The 120
wavelet used has zero mean and is localized in distance [Farge, 1992]. Figure 1 shows the shape of the Morlet 121
wavelet used for the analysis. In the wavelet analysis, the mean power spectrum for each series analyzed is 122
computed first, and if a peak in the wavelet power spectrum is significantly above this background power, then 123
it is be assumed to be a true feature. We have used a significance level of 99 % for our analysis which implies 124
that the peaks have 1 % probability of being caused by noise in the data. An alternative method is to use a fast 125
Fourier transform (FFT) analysis. The wavelet results were compared with an FFT analysis on the same 126
difference values. The FFT analysis and wavelet analysis both clearly identify the dominant wavelength in the 127
series. However, the secondary waves with lesser power are more easily brought out in wavelet analysis as 128
compared to FFT analysis. 129
4. Results. 130
We present wave structures seen in PMC’s in the northern hemisphere during the summer months of 131
June, July and August. A visual analysis was first performed on the data and from this we were able to identify 132
a large number of wave events. All these wave events were analyzed and here we present statistics on these 133
preliminary results. The spectra of PMC wavelengths range from 13 km to approximately 400 km. In this 134
section we present two specific examples of gravity waves in PMC’s. Figure 2 is an example of a short 135
wavelength gravity wave and figure 3 is an example of a long wavelength gravity wave. Figure 2 a. is a CIPS 136
'bowtie' image clouds taken at the peak of the PMC season in July. This image is a composite of the four fields 137
of view, all taken simultaneously which shows distinct wave patterns in PMC’s. In Figure 2 a., a wave field can 138
be seen extending ~ 1000 km between 110 o W and 120 O W and 6567 O N. A wavelet analysis for two sections 139
shown in figure 2 a., indicates a dominant wavelength of 45 km as shown in figure 2 b. Figure 2 c. shows the 140
geographical region over which the coherent wave pattern is seen. The dashed box in fig 2a. corresponds to the 141
boxed region in figure 2c. over which the coherent wave pattern is seen. 142
Figure 3 a. is a CIPS image taken at the start of the PMC season on June 5 over Northern Greenland in 143
orbit 611. It indicates the presence of a long wavelength gravity wave. This wave field can be seen in multiple 144
orbits, separated by about 90 minutes. Another interesting feature of this wavefield is that it seems to aligned 145
along latitudinal lines. Figure 3 b. shows how the wave fields line up in consecutive orbits in images taken 96 146
minutes apart. The wave structure analyzed at three sections indicated by 1, 2 and 3 in figure 5 a shown in the 147
wavelet plot in figure 6, indicate the presence of a wave with a horizontal wavelength close to 250 km and a 148
secondary wave with a horizontal wavelength of 100 km. At section (1.) the 250 km wave is stronger and 149
dominates the wavelet plot. At section (3.) the 100 km wave dominates and at section (2.) the wavelet plot 150
shows a mixture of the two waves as an equal distribution of power between the two waves at 100 km and 200 151
km. This change in the dominant wave structure seen in the clouds occurs over a distance of approximately 600 152
km in the alongtrack direction. The top most plot in figure 6 is a wavelet analysis of the same section as section 153
i. in orbit 611, from an overlapping image from orbit 610 taken 96 minutes earlier. This plot also shows the 154
presence of the same kind of wave structures as that seen in the analysis of the three sections in orbit 611 with a 155
secondary peak at 100 km and a strong peak between 210 – 250 km. An analysis of the geolocation of the wave 156
structure indicates that the wave has moved in the northwest by 85 km in 96 minutes yielding a speed of ~ 55 157
km/hr or ~ 15 m/s. This speed is of course the component sum of the bulk speed of the wind and the phase 158
a
speed. Since Period is given by V P / λ = , whereλ is the wavelength and V is the wave velocity, a horizontal 159
wave speed of 55 km/hr for a 250 km horizontal wavelength wave indicates a period of 4.5 Hours. 160
Taking the horizontal wavelength and the period of the gravity wave, the vertical wavenumber can be 161
estimated from the dispersion relation for internal gravity waves [e.g., Gill, 1982], which is given by: 162
2 2
2 2 2 2 2
z x
z x
k k k F k N
+ +
= ω , (2) 163
Here x x k λ π / 2 = is the horizontal, and z z k λ π / 2 = is the vertical wave number of the wave; N is the Brunt 164
Vailsala frequency; and T / 2π ω = is the frequency. F is the coriolis parameter and for the timescales of the 165
waves considered here, it can be neglected. The vertical wavelength calculated from the dispersion relation for 166
the wave structure in figure 3 is 8.5 km. 167
4.1. Statistical distribution of wave structures. 168
Table 1 summarizes the preliminary wave structures detected throughout the northern hemisphere 169
summer season. In June 41.18 % of all observed waves have horizontal wavelengths greater than 100 km. In 170
July it drops to 32.35 % and in August it again rises to 42.86 %. In July during the peak of the cloud season we 171
observe a greater number of smaller wavelength structures. Figure 5 shows the same data as in Table 1 in a 172
histogram plot. The wavelengths have been put in 15 km bins. Although the spectrum looks rather ragged 173
because of the small number of events, there is a definite peak near 3540 km in horizontal wavelength during 174
all three months during the summer. The histogram plot for the whole season in figure 5 clearly illustrates this. 175
During the months of June and August the distribution is more evenly distributed between wavelengths even 176
though there is a small number of events at wavelengths <50 km. In the month of July there is a bigger peak at 177
about 35 km. Figure 6. plots the geographical location of the observed wave events color coded for different 178
months. There is a concentration of wave events over NW Greenland, Eastern Canada and over the Arctic 179
Ocean just north of central Siberia. 180
5. Discussion & Conclusion 181
Large scale structures with wavelengths more than 200 km as well as small scale structures of very few 182
km’s in wavelength can be seen in PMCs. The clouds generally seem to favor structures between less than 50 183
km. However the extent of observed wave structures in clouds lies between 15 km and 400 km in wavelength. 184
The structures seen in CIPS images are in accordance with previous investigations which have noted 185
wavelengths of 20 100 km in both PMCs as well as their presumed drivers, gravity waves [e.g., Witt, 1962, 186
Fritts, 1984; Espy et al., 1995]. 62 % of all wave events observed during the season are < 100 kms in 187
wavelength as opposed to only 12.5 % with wavelength > 200 kms. This is quite different from results of 188
Carbary et al. who had noted that PMC’s tend to favor structures at scales of 500 – 1000 km and saw very few 189
structures less than 100 km. During our preliminary investigations, we detected gravity waves in all four CIPS 190
cameras, though a wave event was rarely observed extending across cameras and therefore the images were 191
generally not analyzed across cameras. This limited our analysis to structures less than 500 km. Analysis of the 192
CIPS images for wavelengths greater than 500 km is part of our immediate future research. Based on our initial 193
results, an interesting observation is that, in July we see 67 % of smaller wavelength waves, while in June and 194
August we see 58 and 57 % respectively, There is a significant rise in the number of smaller scale gravity waves 195
observed during the peak of the cloud season. While we generally do not see a preferential east west alignment 196
of gravity waves along latitudinal lines, there are a lot of wave events which are aligned east west along 197
latitudinal lines similar to observations by Carbary et al. We were limited to a lower latitude of 65 degrees 198
below which the clouds were not bright enough for our preliminary visual analysis to detect gravity waves. 199
Most of the wave events observed have very small timescales and are rarely seen in consecutive orbits making it 200
difficult to get relative phase speeds and parameterization of the wave. Simultaneous AIM measurements along 201
with ground based measurements using lidar and photography of wave events will yield information about the 202
vertical profiles of the waves and help in gravity wave parameterization and help in better understanding 203
gravity wave effects on clouds. Simultaneous ground based observations along with AIM has been planned and 204
is part of our future work. 205
References 206
Carbary, J. F., Morrison, D., Romick, G. J., 2000, Transpolar structure of polar mesospheric clouds, J. Geophys. 207
Res., 105, D20, 2476324769 208
Espy, P.J., R. Huppi, Manson, A., 1995, Large scale, persistant latitude structures in the mesospheric 209
temperature during ANLS93, Geophys Res Lett., 22, 28012804 210
Ern, M., P. Preusse, M. J. Alexander and C. D. Warner, 2004, Absolute values of gravity wave momentum flux 211
derived from satellite data, J. Geophys. Res., 109, D20103, doi:10.1029/2004JD004752 212
Fritts, D.C., and M. J. Alexander, 2003, Gravity wave dynamics and effects in the middle atmosphere, Rev. 213
Geophys., 41, doi:/2001RG00106 214
Fritts, D. C., Isler, J. R. and G. E. Thomas, 1993, Wave breaking signatures in noctilucent clouds, Geophys. 215
Res. Letters, 20, 19, 20392042 216
Fritts, D.C., Rastogi, P.K., 1985, Convective and dynamical instabilities due to gravity wave motions in the 217
lower and middle atmosphere: Theory and observations, Radio Sci., 12471277 218
Fritts, D.C., 1984, Gravity wave saturation in the middle atmosphere: a review of theory and observations, Rev. 219
Geophys., 22, 275308 220
Fritts, D. C., J. R. Isler, and G. E. Thomas, 1993, Wave breaking signatures in noctilucent clouds, Geophys, 221
Res. Lett., 20, 20392042 222
Gadsden, M., Schröder, W., 1989, Noctilucent Clouds, Springer Verlag, Berlin 223
Gadsden, M. and P. Parviainen, 1995, Observing Noctilucent Clouds, International Association of 224
Geomagnetism and Aeronomy 225
Gill, A. E., 1982, AtmosphericOcean Dynamics, Academic, San Diego, California 226
Haurwitz, B., Fogle, B., 1969, Wave forms in nocltilucent clouds, Deep Sea Res., 16, 8595 227
Holton, J. R., 1983, The influence of gravity wave breaking on the general circulation of the middle 228
atmosphere, J. Atmos. Sci., 40, 24972507. 229
Hines, C. O., 1968, A possible source of waves in noctilucent clouds, Space Res. IX, 170174. 230
Jensen, E. J., and G. E. Thomas, 1994, Numerical simulations of the effects of gravity waves on noctilucent 231
clouds, J. Geophys. Res., 99, D2, 34213430. 232
Manson, A. H., 1990, Gravity wave horizontal and vertical wavelengths: An update of measurements in the 233
mesopause region (~80100 km), J. Atmos. Sci., 47, 27652773. 234
Rapp, M., F.J. Lübken, and A. Müllemann, 2002, Smallscale temperature variations in the vicinity of NLC: 235
Experimental and model results, J. Geophys, Res., 107(D19), 4392, doi:10.1029/2001JD001241. 236
Torrence C., Compo G. P., 1998, A practical guide to wavelet analysis, Bulletin of the American 237
Meteorological Society, Vol 79,No 1, 61 – 78. 238
von Zahn, U., Berger, U., Singer, W., 2000, NLC and PMSE: Towards a unified view, EOS Trans. AGU, 239
81(19), Spring Meet. Suppl., SA32B03. 240
Witt, G., 1962, Height, structure, and displacements of noctilucent clouds, Tellus, 14, 18. 241
Figure Captions 242
243
Figure 1. The Morlet wavelet used for analysis with the real (solid) and imaginary (dashed) parts. 244
245
Figure 2. Figure 2 a. is a CIPS bowtie image clouds taken on July 22 nd , which shows very distinct wave 246
patterns in PMC’s. In Figure 2 a., a wave field can be seen extending close to 1000 km between 110 degrees 247
west and 120 degrees west and 65 67 degrees north. A wavelet analysis of the albedo variation from a 248
smoothed background fit for two sections shown in figure 2 a., indicates a very strong wavelength of 45 km as 249
shown in figure 2 b. Figure 2 c shows the geographical extent of the observed coherent wave pattern. 250
Figure 3. Figure 3 a. is a CIPS image taken at the start of the PMC season on June 5 over Northern Greenland 251
in orbit 611. It indicates the presence of a long wavelength gravity wave. This wave field can be seen in 252
multiple orbits. Figure 3 b. shows how the wave fields line up in consecutive orbits (denoted by the numbers on 253
top of the images) in images taken 96 minutes apart. The outline is the coast of Northern Greenland. 254
255
Figure 4. An analysis of the wave structures at three sections indicated by 1, 2 and 3 in figure 3 a. 256
257
Figure 5. Histogram plots of the occurrence of wave events observed in Northern hemisphere in the months of 258 June, July and August of 2007. 259
260
Figure 6. The geographical locations of the wave events observed during summer 2007 in the Northern 261 hemisphere. 262
263
Table Caption 264
265
Table 1. Summary of wave structures seen throughout the northern hemisphere summer season 266
267
268
269
270
271
272
273
Figures 274
Figure 1. The Morlet wavelet used for analysis with the real (solid) and imaginary (dashed) parts 275
276
277 Distance parameter, η (km)
278
Figure 2 a. is a CIPS bowtie image clouds taken on July 22 nd , which shows very distinct wave patterns in 279
PMC’s. In Figure 2 a., a wave field can be seen extending close to 1000 km between 110 degrees west and 120 280
degrees west and 65 67 degrees north. A wavelet analysis of the albedo variation from a smoothed background 281
fit for two sections shown in figure 2 a., indicates a very strong wavelength of 45 km as shown in figure 2 b. 282
Figure 2 c shows the geographical extent of the observed coherent wave pattern. 283
284
285
286
287
a b
c
Figure 3 a. is a CIPS image taken at the start of the PMC season on June 5 over Northern Greenland in orbit 288
611. It indicates the presence of a long wavelength gravity wave. This wave field can be seen in multiple orbits. 289
Figure 3 b. shows how the wave fields line up in consecutive orbits (denoted by the numbers on top of the 290
images) in images taken 96 minutes apart. The outline is the coast of Northern Greenland. 291
292
293
(a) (b) 294
295
296
1 2 3
Figure 4 shows an analysis of the wave structures at three sections indicated by i, ii and iii in figure 3 a. 297
298
299
Figure 5 shows histogram plots of the occurrence of wave events observed in Northern hemisphere in the 300 months of June, July and August of 2007. 301
302
303
304
Figure 6 showing the geographical locations of the wave events observed during summer 2007 in the Northern 305 hemisphere. 306
307
308
Table 1 summary of wave structures seen throughout the northern hemisphere summer season 309
Month λ <100 km 100< λ <200 km λ >200 km # of Events
June 40 20 8 68
July 69 22 11 102
August 40 19 11 70
149(62.08 %) 61(25.42 %) 30(12.50 %) 240
310
