1 The Cloud Imaging and Particle Size Experiment on the aeronomy of ice in the 2 Mesosphere mission: Cloud norphology for the northern 2007 season 3 4 Rusch, D. W1., G. E. Thomas1, W. McClintock1, A. W. Merkel1, S. M. Bailey2, J. M. 5 Russell III3, C. E. Randall1, C. Jeppesen1, and M. Callan1 6 7 Abstract 8 9 The Aeronomy of Ice in the Mesosphere (AIM) mission was launched from Vandenberg 10 Air Force Base in California at 4:26:03 EDT on April 25, 2007 becoming the first 11 satellite mission dedicated to the study of noctilucent clouds, also known as Polar 12 Mesospheric Clouds (PMC) when viewed from space. We present the first results from 13 one of the three instruments on board the satellite, the Cloud Imaging and Particle Size 14 (CIPS) instrument. CIPS has produced detailed morphology of the Northern 2007 PMC 15 season with 5 km horizontal spatial resolution. CIPS data yield panoramic views of 16 cloud structures at multiple scattering angles within a narrow spectral bandpass centered 17 at 265 nm. Spatial coverage is about 50% at the lowest latitudes where data are collected 18 (35o). Coverage increases with latitude to 100% about 70o, where camera views overlap 19 from orbit to orbit, and terminates at about 82o. Cloud structures have for the first time 20 been mapped out over the summertime Polar Regions completely free of slant-path 21 distortions and limited spatial coverage characteristic of single-station ground-based 22 imagery. These structures include 'ice rings', spatially small but bright clouds, and large 23 regions ( 'ice voids') in the heart of the cloud season essentially devoid of ice particles. 24 The ice rings bear a close resemblance to tropospheric convective outflow events, 25 suggesting a point source of mesospheric convection. These rings (often circular arcs) are 26 most likely Type IV NLC ('whorls' in the standard WMO nomenclature). Modeling of ice 27 particles in the general circulation model (WACCM) suggests that the voids are due to 28 warm patches of descending air. Surprisingly, in contrast to ground-based views from the 29 NLC zone (50-65o latitude zone) wave features are comparatively rare in the CIPS 30 images and are generally confined to the edge of the ice existence region. 31 32 1. Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 33 80303. 34 35 2. Bradley Department of Electrical and Computer Engineering, Virginia Polytechnical 36 and Stae University, Blacksburg, VA 24061, USA. 37 38 3. Center for Atmospheric Sciences, Hampton University, Hampton, VA 39 40 Keywords: Polar Mesospheric Clouds, Mesosphere, Dynamics, Gravity waves 41 1. Introduction 42 43 Polar Mesospheric Clouds (PMCs) are the Earth's highest clouds, occupying the 44 very cold atmospheric region below the summer mesopause with a latitude extent from 45 about 55° to the geographic pole. They occur largely within the Arctic and Antarctic 46 circles within each hemisphere. The duration of the cloud season in the north is from 47 about mid-May to mid August and in the south from mid-November to mid-February. 48 The polar mesopause region becomes the coldest place on earth around summer solstice, 49 when the temperatures may fall below 130K, possibly as low as 110K at times. First 50 identified over 120 years ago [Leslie, 1885], their nature as very small sub-micrometer 51 water-ice crystals was not established until recently [Hervig et al., 2001]. Interest in 52 PMCs has been generated because of they are a tracer of phenomena in this unusual part 53 of the upper atmosphere, and are changing in ways that are not understood. They are 54 appearing more frequently, are brighter and are being seen at lower latitudes than ever 55 before reported [Deland et al., 2004; 2007b; Taylor et al., 2002]. The AIM mission 56 [Russell et al., 2008 ( this issue)] was designed to study the relationship of PMCs to their 57 atmospheric environment, which could resolve the question of what causes the long-term 58 changes in PMC brightness and frequency [DeLand et al., 2007; Shettle et al., 2007] 59 Due to their extreme sensitivity to changes in the environment (changes in 60 temperature as small as 3K may have profound effects on PMC growth and brightness, 61 see Merkel et al. [2006]), PMCs are expected to be a particularly sensitive indicator of 62 long-term global change. Indeed, PMCs may be a modern phenomenon associated with 63 the rise of certain greenhouse gases in the industrial era [Thomas, 1996]. A significant 64 increase of ~5%/decade in PMC brightness has occurred over the past 27 years [DeLand 65 et al., 2007b]. 66 At least four factors are believed to control PMC formation: (1) temperature, (2) 67 water vapor, (3) cosmic dust influx [Thomas, 1996], and (4) mesospheric dynamics 68 [Hines, 1960; Turco et al, 1982]. However, there are many open questions regarding 69 PMC formation and destruction [Rapp and Thomas, 2006], heterogeneous destruction of 70 trace metals [e.g. Plane and Murray, 2004], the influence of gravity waves, water 71 sequestration, solar cycle effects, and bulk transport. Despite advanced present-day 72 modeling [e.g. Berger and Lübken, 2006, Lübken and Berger, 2007], many uncertainties 73 relating to time dependence of cloud formation, destruction, and dynamics hamper our 74 ability to understand the observed changes. Recent progress has been made in 75 understanding global scale properties of PMCs and the related radar phenomenon, Polar 76 Mesospheric Summertime Echoes (PMSEs). For a recent review of these developments, 77 see Lübken and Berger [2007]. 78 A relatively uncharted territory is the spatial distribution of PMC on spatial scales 79 comparable to those of gravity waves from a few km to hundreds of km [see Chandran et 80 al., 2008 (this issue)]. There are many thousands of ground-based photographs of NLC 81 taken over the years, beginning with Jesse [1896]. These photographs, and naked-eye 82 reports, reveal complicated spatial structures at sub-km resolution. A morphological 83 classification has been established for many years [WMO, 1970], systematizing the 84 various cloud forms into five main types, I through V. The dominant wave features 85 appear as two types: bands (Type II) which are periodic structures with horizontal 86 wavelengths ~10-100 km, and are many hundreds of km long; and billows (Type III) 87 which are closely-spaced bands of 10-20 km wavelength or less, but longitudinally much 88 shorter than bands. More detail can be found in Gadsden and Schröder [1989]. 89 At any given moment, the bands and billows appear to be static. In time-lapse 90 movies, they are found to move rapidly and last for many hours with wave periods of 91 several hours. Their apparent drift speeds (the vector sum of the phase velocity and bulk 92 wind velocity) can, at times, exceed 100 m/s [Haurwitz and Fogle, 1969]. The billows 93 are much more transient, and may disappear over time scales of minutes to tens of 94 minutes. The billow motions follow the bulk wind speed, since their phase velocities are 95 small. Waves seen near the horizon can be contrast-enhanced due to strictly geometrical 96 effects. To quote Jensen and Thomas [1994], "The geometrical effect of viewing an 97 undulating cloud layer at a low elevation angle is maximized when the elevation angle is 98 equal to the phase tilt of the wave.” Hence, as long as the wave trains are perpendicular 99 to the line of sight, NLC bands should have a maximum contrast at elevation angles of 5- 100 10o, and billows should show up best at 30-40o elevation angle. Thus a 'ripple' in the 101 height of a uniform layer would cause a corresponding brightness ripple, which does not 102 necessarily reflect any true variations of ice particle properties over the field of view. 103 However, it is known that true horizontal variations of ice properties occur, since 104 waves are seen when the line of sight is along the wave fronts. Multi-station stereography 105 is capable of sensing true spatial variability. Instructive stereo observations of NLC 106 distributions were reported by Witt [1950], who showed how cloud properties vary over a 107 3D spatial domain. Indeed, the stereo (two-camera) technique was used 117 years ago by 108 Jesse [1896] to demonstrate that NLC exist at what was then an extraordinary altitude of 109 82 km. It is remarkable that this value is still valid within the much smaller errors 110 characteristic of lidar measurement. 111 The measurement of changes in PMC structure is important because it indicates 112 upper atmospheric variability around the mean state that is critical in determining the 113 occurrence and morphology of cloud structure, particularly in regions where temperature 114 is marginal for ice production. For example, Gerding et al [2007] have shown that at 115 54oN (Kühlungsborn, Germany), deviations of temperatures down to 20 K below the 116 climatological average are necessary for NLC to occur in this comparatively warm 117 region, at the edge of the NLC zone of occurrence. Such large deviations occur at the 118 troughs of atmospheric gravity waves and tides. One of the scientific objectives of the 119 AIM mission is to determine the importance of gravity waves in influencing PMC. It is 120 clearly important to understand this relationship quantitatively, since if long-term 121 changes in waves were to occur, this could influence long-term variability of PMC. 122 Modeling the influence of waves on PMC production and loss began with Turco 123 et al. [1982] using an early 1D version of the Community Aerosol and Radiation Model 124 for Atmospheres (CARMA) model. Later work by Jensen and Thomas [1994] and Rapp 125 et al [2007] has shown that, using 2D versions of the CARMA model, the effects on PMC 126 of variable temperature and water vapor depend sensitively on the wave period. For 127 wave periods less than about 7 hours, waves have a destructive effect on PMC brightness, 128 since the ice particles disappear during the warm phase more rapidly than they grow 129 during the cold phases. For longer wave periods, there is the possibility of a temporary 130 enhancement of cloud brightness due to the fact that the cold phase lasts longer than the 131 particle growth time of several hours. However, these time-dependent studies did not 132 examine the full range of possible initial conditions of pre-existing ice particles, and the 133 role of different background temperature. 134 Here we briefly review lidar measurements, which have very high spatial 135 resolution because of the small size of the illuminated spot. Lidar studies generally yield 136 ambiguous information on spatial structure because of the unknown speed of the 137 structures through the stationary spot. However, the one exception is the two-lidar 138 measurements [Baumgarten et al., 2002] that provided limited information on 2D NLC 139 structure. The illuminated area of backscattering at mesospheric heights may be as small 140 as 15 meters. The necessity to integrate the signal over several minutes to improve the 141 S/N ratio, and the always present motion of the NLC structure, results in a larger 142 effective horizontal resolution. For three-color lidar observations at the ALOMAR 143 observatory (69.3oN), the analysis requires longer integration times to obtain accurate 144 color ratios. Baumgarten et al. [2007] adopted a 14 minute integration time, and for the 145 spot size of the illuminated area, this implies a horizontal resolution of 34 km. The 146 published time series of lidar data with integration times of several minutes show back- 147 scattering brightness variations consistent with highly variable spatial structure, as well as 148 time-changing heights. For example, Figure 6 from Baumgarten et al. [2007] shows 149 variations of two to three over a time scale of three minutes. These very rapid 150 enhancements may be manifestations of the bright spots seen in CIPS images, as 151 discussed in Section 3. 152 153 154 2. The Cloud Imaging and Particle Size Experiment 155 The Cloud Imaging and Particle Size (CIPS) experiment on AIM is a wide angle 156 (120° along track by 80° across track) UV imager consisting of four identical cameras 157 arranged in a cross pattern (Figure 1). CIPS provides images of PMCs with a spatial 158 resolution of 1 x 2 km in the nadir and about 5 km at the edges of the forward and aft 159 cameras. The spatial resolution of CIPS provides a 100-fold increase in horizontal 160 resolution over previous limb-viewing space experiments (see DeLand et al. [2006] for a 161 recent review of space-based PMC observations). For a complete description of the CIPS 162 instrument, see McClintock et al. [2008 (this issue)]. The CIPS instrument is fully 163 operational on orbit, with all four cameras performing flawlessly. The brightness and 164 occurrence frequencies of PMCs inferred from CIPS data is in excellent agreement with 165 concurrent measurements from the Solar Backscatter Ultraviolet (SBUV/2) instruments 166 [Benze et al., 2008 (this issue)] 167 168 In our standard analysis, the four CIPS camera images are merged to form a 169 single display we call a scene with a spatial resolution of 5 x 5 km. To achieve a uniform 170 spatial coverage, the resolution is intentionally degraded to match the geometrical 171 smearing at the camera edges. A scene is depicted in Figure 1, showing how the native 172 rectangular images appear as projected on a spherical earth at the normal PMC height of 173 83 km. The cameras are marked as Px, the fore camera; Mx, the aft camera, and the nadir 174 cameras, My and Py. The orbital direction is to the right of the scene. The scene has 175 dimensions 120o by 80o, as measured from the nadir direction. This results in spatial 176 coverage of approximately 2000 km along the satellite track and 1000 km across track. 177 As the satellite moves in orbit, the object is viewed seven times at a large range of 178 scattering angles. The time interval over which the multiple scenes are taken is 258 179 seconds. 180 181 3. Initial Imaging Results 182 183 Since the cloud scattering signature is an albedo enhancement above a 184 comparatively bright Rayleigh scattered background, cloud detection and retrieval of 185 cloud properties require careful removal of the background. This background varies over 186 the viewed area due to geometrical effects of incoming and outgoing solar rays passing 187 through mesospheric air and ozone. An important aspect with respect to the CIPS 188 technique is the necessity to specify this Rayleigh scattering background accurately along 189 the orbit. The air and ozone densities responsible for the value of the background 190 radiance may vary in time and space in ways that are not known a priori. It is necessary 191 therefore to derive the ozone mixing ratios in the 50-65 km region (where the 192 contribution function maximizes). The threshold at which clouds may be detected is 193 determined in part by our ability to accurately simulate the background. For more detail 194 on how clouds are separated from the background, see Bailey et al. [2008 (this issue)]. 195 The cloud albedo is defined as the ratio of the scattered radiance (after removal of 196 background) to the incoming solar irradiance, averaged over the bandpass of the 197 instrument (see McClintock et al., [2008], this issue). The units of albedo are sr-1. Here 198 we describe the method that results in the determination of cloud albedo. 199 200 To begin, we show a cloud-free, Rayleigh scattered background in Figure 2a for a 201 scene taken at 82oN prior to the beginning of the cloud season. The associated line plots 202 below the figure display the albedo as a function of pixel number across the center of the 203 scene (figure 2b) from the edge of the Mx camera (on the left) to the edge of the Px 204 camera, and across the center of the nadir cameras from the edge of the My to the edge of 205 the Py cameras (figure 2c). (One Gary (G) is defined as an albedo of 1x10-6 sr-1.) The 206 pixel size projected to an 83-km height is 5 km. The variation in the Rayleigh scattered 207 albedo across the scene is due to both the changing scattering and view angles. (The 208 scattering angle is defined as the angle between the incoming solar ray and the scattered 209 radiance vector.) The background albedo varies from about 440 to 130 G from the edge 210 of the aft camera through the nadir and then increases to about 340 G at the edge of the 211 forward camera. The scan across-track of the nadir cameras varies from about 180 to 150 212 G and then increases to about 185 G at the edge of the other camera. The small scale 213 variations are consistent with instrument noise [McClintock et al., 2008 (this issue)] or 214 small scale ozone variations. The background is a result of Rayleigh scattering from air 215 molecules modified by ozone absorption in the 50-65 km height region, where optical 216 depth unity is reached. Ozone absorption in the Hartley bands, combined with Rayleigh 217 scattering causes a minimum in the Earth’s albedo near 260 nm [McClintock et al., 2008 218 (this issue)]. The spectral region chosen for the CIPS bandpass (258-274 nm) provides 219 the maximum contrast of PMC to background scattering. 220 221 222 In Figure 3a we display a scene taken at similar latitude as that in Figure 2a, but for data taken in the heart of the PMC season. A comparison with Figure 2a clearly 223 shows the presence of PMCs with highly-variable albedo across the scene, that is 224 obviously distinct from the relatively smooth background albedo. The line plots (Figures 225 3b and 3c), generated as in Figures 2b and 2c, show the PMC enhancements to the 226 Rayleigh scattered background. The PMC albedo for this particular set of scattering 227 angles is highly variable, with values from a few Gs to 50 or 100 Gs. This scene is 228 typical of those taken at high latitudes during the cloud season. 229 230 Figure 4a displays the scene that results from subtracting the Rayleigh 231 background from the albedo shown in Figure 3a. The Rayleigh background is 232 determined by retrieving the atmospheric ozone from the non-cloud regions of the scene 233 and forward-modeling the values of Rayleigh scattering in cloudy areas [Bailey at al., 234 2008 (this issue)]. In addition, to account for the increase in brightness when viewing the 235 clouds at small scattering angles in the forward camera, the scene is approximately 236 compensated for forward-scattering behavior of the ice particles. This procedure is not 237 intended to be rigorous, but is a qualitative attempt to represent "normalized" cloud 238 brightness. By trial and error, we adopted a phase function for a Gaussian distribution 239 with a mode radius of 60 nm and a width of 12 nm. This set of parameters applies to a 240 bright PMC seen at the ALOMAR observatory [Baumgarten et al., 2007]. We arbitrarily 241 chose to normalize all observations to one at a 90o scattering angle. Each pixel is then 242 ‘corrected’ by dividing its albedo value by the value of the phase function at the 243 appropriate scattering angle. This brings out cloud features across the backward- 244 scattering portion of the scene that would normally be suppressed. The result shows a 245 highly variable and complicated cloud structure with bright spots, regions where clouds 246 are dim or even absent, and what appears to be chains of spatially small and dim regions. 247 The line plots (Figures 4b and 4c) are the same as those in Figures 3b and 3c. They show 248 highly variable and complicated structures over the image. 249 250 Although these complicated features no doubt are present in ground-based views, 251 this is the first time that they have been seen from above, undistorted by the large slant 252 paths that are characteristic of NLC views near the observer's horizon. Indeed, the slant 253 views of NLC more clearly reveal the wave structure because of the horizontal variation 254 of cloud height induced by gravity waves (see Jensen and Thomas [1994]). The ground- 255 based perspective can distort the true variation of cloud properties across the wave field. 256 CIPS viewing geometry allows a view of the 'true' spatial variability. This difference in 257 view angles should be borne in mind when comparing NLC photographs with CIPS 258 images. CIPS images are essentially free of effects of ‘rippling’ by variations in the 259 cloud height which are the order of 1-3 km. NLC images frequently distort the brightness 260 variation, at least whenever waves are present. 261 262 The CIPS data collected for 15 orbits each day are merged to form orbital strips 263 that are then merged into a daily asynoptic display of cloud occurrence. An example of a 264 picture of daily cloud occurrence is shown in Figure 5 for July 9, 2007 (Day 190), the day 265 on which the data displayed in Figures 3 and 4 were taken. The blue background shows 266 the orbit by orbit coverage of the CIPS instrument. The clouds are displayed white on the 267 blue background where pure blue indicates no clouds were detected at that location. The 268 image shows nearly continuous cloud presence at the higher latitudes, but also contains 269 many cloudless regions interspersed across the image. The minimum brightness 270 displayed in these images is 5 G. The spatial variation of the clouds appears random and 271 does not generally resemble the features seen in most ground based photographs. The 272 relative absence of wave-like features in CIPS images will be discussed in Section 4. 273 274 Cloud structures, as viewed by CIPS, are highly variable from scene to scene and 275 orbit to orbit and contain interesting features never before seen from space. These 276 features include ‘ice rings’ in which regions of dim clouds are surrounded by a narrow 277 ‘ring’ of brighter clouds. A second type of structure we have identified is spatially small, 278 but bright clouds that have radii of approximately 10 to 20 km. These bright spots are 279 significantly brighter than the surrounding clouds and, at times, even the Rayleigh 280 scattered background. 281 282 An example of an ‘ice ring’ is shown in Figure 6 in the fore camera image that 283 includes the Rayleigh scattered background. As delineated by the imposed box, it has a 284 diameter of about 250 km. The ring is about twice as bright as the enclosed region. 285 Ringed features, including 'broken' rings (circular arc-like structures) are a common 286 morphological feature and occur in many sizes from ten's of km up to nearly 1000 km in 287 diameter. An additional image contains a different presentation of this feature. Figure 7 288 shows a CIPS view of PMCs on day 190 (July 9, 2007). The circular features show 289 regions of low cloud intensity surrounded by brighter clouds. The diameters of these 290 features are about 100 km. These features must be formed by complicated upper 291 atmospheric dynamics. 292 293 As discussed above, spatially small but very bright features also commonly occur. 294 Examples are shown in the images and plots shown here. Figure 8a displays a forward 295 camera image containing two bright features. These 'spots' are well defined spatially by 296 the CIPS camera with 5 km resolution at the cloud height. Although they may have been 297 within the field of view of other space-based instruments, the larger volumes sampled did 298 not allow them to spatially resolve these features. However, they may have been present 299 in lidar time series, since the typical size of the illuminated region is only a few hundred 300 meters. They would show up as temporary enhancements lasting only a few minutes as 301 they pass over the lidar site. For example, Figure 6 of Baumgarten et al [2007] exhibits a 302 rapid brightening of the backscatter coefficient from a typical value of 10 units up to 40 303 units. The features seen in CIPS are equivalent to or brighter than the Rayleigh 304 background at the small scattering angles seen in the forward camera. A line plot of the 305 albedo across the two bright spots is shown in figure 8b. The plot shows that the features 306 are bright reaching 1,100 G for one and 1,200 G for the other. Each bright feature is 307 about 25 km wide. 308 309 The existence of these two classes of cloud features, the ice rings and the small, 310 bright clouds, seem in conflict with the current understanding of cloud structures that rely 311 on gravity wave temperature modulations, breaking, and momentum deposition in the 312 mesosphere. It appears that under the cold conditions that occur in the middle of the 313 season at the high latitudes, ice formation may be dominated by additional dynamical 314 influence. 315 316 Thus we are finding that deep in the polar region and in the heart of the season, 317 PMC are characterized by what appears to be convective patterns, rather than by the 318 bands and billows usually seen at the lower latitudes, or at the extremes of the cloud 319 season. Wave features are not rare, but more often we see rather complicated structures 320 that perhaps overwhelm the visible effects of gravity waves. The origin of this convection 321 is not understood, but a reasonable hypothesis is that the convection it would be favored 322 by the steeper latitudinal temperature gradient present in the summer mesosphere. These 323 gradients approach the adiabatic lapse rate which may drive the region close to 324 convective instability. For example, rocket-borne measurements during summer of 325 temperature profiles at Andöya, Norway often reveal temperature gradients close to - 326 10K/km. The resulting instability causes a deposition of momentum and energy and has 327 long been believed to be the driving force behind the closing of the mesospheric jet and 328 the cold summertime temperatures. However, this process is not confined solely to the 329 Polar Regions. Thus the change of character of the ice structures with latitude may be due 330 to a transition to a more convective regime in the ice formation region (~80-90 km). 331 Alternatively, the effect of the lower temperature at these high latitudes (< 130k) on the 332 ice formation may change qualitatively as suggested by the CARMA model sensitivity 333 calculations. At temperatures below 140K, the results of Rapp and Thomas [2006] (see 334 their figure 20) indicate the UV brightness could be controlled more by water vapor 335 variation than temperature. This resolution of this question poses a challenge to models 336 which couple ice formation with the circulation [e.g. Berger and Lübken, 2007]. 337 338 In addition, the CIPS data have revealed the presence of large regions that are 339 virtually ice free. Many of these regions encompass thousands of square kilometers and 340 are distinct from the ice rings described above in that they are not surrounded by a more 341 dense cloud ‘ring’ and are essentially absent of clouds in the interior. They could be due 342 to large regions of warm air, which are predicted by a recent general circulation model 343 which contains mesospheric ice formation [Bardeen et al., 2007]. An example of an 'ice 344 void' is seen in the daily view in Figure 9. The large dark area near 85N and 135W has 345 an area of several thousand square km2. This particular region was recognized in several 346 consecutive orbits where the orbital coverage overlaps. Its motion was calculated to be 347 northward into the polar region, contrary to the general view that air always flows out of 348 the pole during the summer. It is not unreasonable to suppose that this could be due to the 349 presence of a tidal wind which can dominate the zonally-averaged general circulation. A 350 visual scan of all daisies for the northern 2007 season reveals 19 large ‘ice voids’ spread 351 approximately evenly throughout the season, or about 1 every 4 days. 352 353 To further illustrate the contrast seen in the cloud features as seen from the ground 354 and from space, a daily view of the entire cloud region is shown in Figure 10. In this 355 view, taken near the end of the cloud season on August 13, 2007 (DOY 225), two distinct 356 types of cloud features are seen. These regions, one near longitude 135° and one near 357 longitude of -45, are enlarged in the following figures. In the first (Figure 10a), we 358 clearly see many bands with lengths of > 100 km and with regular separation typical of 359 gravity wave control [see Chandran et al., 2008 (this issue)]. These are the type of 360 gravity wave controlled bands that are seen regularly in ground based images. Figure 361 10b shows the more common cloud attributes seen at high latitudes in during most of the 362 cloud season, complicated structure with embedded features distinct from those seen in 363 Figure 10a. These types of cloud structures are more typically seen in tropospheric 364 clouds generated by large scale convection. A detailed analysis is beyond the scope of 365 this paper but is in progress. 366 367 4. Discussion 368 369 The CIPS instrument provides the first look from space at PMC with 5 km 370 resolution for an entire PMC season. The images reveal details of cloud structure either 371 never before identified (‘bright spots’) or define features in more detail than before (‘ice 372 rings’). Perhaps the most surprising discovery is the nearly ubiquitous presence of 373 amorphous structures extending nearly to the pole. Conspicuous by their rarity are waves, 374 which are quite common in ground-based photographs. It must be noted that waves are 375 not as prominent as might be thought from inspection of historical collections of NLC 376 photographs. Photos with prominent waves are selectively chosen for distribution 377 because of their aesthetic appeal. To our knowledge, there is no published information on 378 the occurrence frequency of the various classes of NLC forms (including waves) in 379 ground-based views. Also, to our best knowledge they are common in the NLC zone (50- 380 60o), but are by no means ubiquitous. We should also note that Carbary et al. [2000] in 381 their more limited mapping of PMC structures identified only one feature (their Figure 8 382 containing what they called a 'repetitive feature') that appeared wave-like, and certainly 383 does not resemble the familiar ground-based views. 384 385 The bright spots may have been detected in lidar data, but the lidar only provides a 386 microscopic view along an unknown cross-section, whereas CIPS reveals the 2D 387 structure. These regions may be evidence of very localized upwelling, in which cooling 388 occurs in a small area. Our preliminary analysis using the scattering phase function in 389 two of these features does not show any unusual values of ice water content. 390 391 The ice rings may be the ground-based Type IV ('whorls'), which to quote Gadsden 392 and Parvianien [1995] exhibit "partial or, on rare occasions, complete rings of cloud with 393 dark centers". The CIPS images in the heart of the season and at high latitudes reveals 394 cloud morphology that resembles features in the lower atmosphere convection patterns 395 more than the striated or gravity wave like features seen from ground-based observations. 396 For example, Figure 11 shows the image of a 'convective outflow boundary', taken by the 397 GOES-12 geostationary weather satellite. A time-lapse movie of this event ( 398 http://www.nasa.gov/mov/204839main_GOES_CloudRings.mov) depicts a nearly- 399 circular cloud-free region emanating from a region of intense thunderstorm activity off 400 the coast of Cuba. The tropospheric cloud 'ring' at the boundary of this wind outflow 401 resembles the CIPS ice rings (see figures 6 and 7). These tropospheric structures are not 402 uncommon in the vicinity of intense convection, and suggest that the ice rings seen by 403 CIPS are a result of a localized convective source of momentum, generating an outflow 404 of wind. It is also possible that a localized source of gravity waves could cause such a 405 structure. Taylor and Hapgood [1988] presented evidence for radially-outflowing gravity 406 waves which apparently had their source near the ground. Through ray tracing they 407 attributed the source to be a thunderstorm. Sentman et al [1993] have described circular 408 features in the OH airglow (originating at ~95 km) which apparently result from a 409 mesospheric pressure surge caused by a sprite, which was associated with intense 410 thunderstorm activity. 411 412 We note that a ring of gravity waves could also cause a circular ring to occur. If 413 the wave caused a temperature perturbation 3K or greater in a region whose temperature 414 is high enough to be close to saturation, then as the warm crest passes through the region, 415 the ice particles would vanish quickly. In the cold phase, the time scale for growth is 416 much longer (the order of hours) so new particles would not form within a wave period. 417 The net result would be a circular region 'swept clear' of ice particles. The outer boundary 418 (the ice ring) could be a region of convergence of ice particles (a 'snow shovel' effect 419 perhaps). Since the ring itself is not always detectable (in which case, we have dubbed 420 them 'ice holes' or 'ice voids') the more fundamental property is probably the large near- 421 circular ice-free area. 422 423 Other possibilities for these large ice-free regions are suggested by modeling in 424 which the circulation is coupled to the cloud microphysics [Berger and Baumgarten, 425 private communication, 2007; Bardeen et al., 2007]. These models, although not having 426 the resolution to resolve the narrow rings, nevertheless simulate large ice-free regions, 427 that are warmer than their surroundings. The causes of these features in the model are 428 being investigated. These processes may be generated by steep temperature gradients in 429 the summer mesosphere that drive the region into convective instability. A further 430 possibility is that the ice formation process may differ between the latitudes where ice 431 formation is marginal (in the 50-60o zone) and deep in the polar regions where 432 temperatures can dip to as low as 110K. 433 434 Signatures of gravity wave processes in the cloud structures are observed, but for 435 the most part they are limited to the southern edge of the cloud deck or to the early and 436 late parts of the season [Chandran et al., 2008 (this issue)]. In the heart of the season, the 437 signatures of gravity waves are either absent or overwhelmed by what appears to be 438 convective processes that may be generated by steep temperature gradients in the summer 439 mesosphere that drive the region into convective instability. A further possibility is that 440 the ice formation processes may differ between latitudes where ice formation is marginal 441 (in the 50-60° zone) and deep in the polar regions where temperatures can reach as low as 442 110K. 443 444 Work in progress that relates to cloud morphology includes the determination of 445 systematic longitudinal variations, and correlations with concurrent measurements of 446 temperature by the SABER instrument on the TIMED satellite. In addition, coordination 447 of CIPS data with data from the high-precision SOFIE solar occultation instrument on 448 AIM [Hervig et al., 2008 (this issue)] is under study. SOFIE measurements are made 449 within in a common volume viewed by CIPS. At this time, this latter task is hindered by 450 the comparatively weak clouds present in the CIPS images at the latitudes observed by 451 the SOFIE instrument. We anticipate making further progress in reducing the cloud 452 detection threshold in the CIPS images, so that we can extend the morphological studies 453 to lower latitudes. This will allow correlation of these newly-discovered structures with 454 atmospheric temperature, water vapor and other constituents measured by SOFIE. 455 456 Acknowledgements: 457 458 We gratefully acknowledge the tremendous effort of the engineering and mission 459 operation teams whose dedication and skill resulted in the success of the CIPS 460 instrument. We thank the entire AIM science team for helpful input. The AIM mission 461 is sponsored by NASA. 462 463 References 464 465 Bailey, S. M., G. E. Thomas, D. W. Rusch, A. W. Merkel, C. Jeppesen, J. N. Carstens, C. 466 E. Randall, W. E. McClintock, J. M. Russell, III, 2008, Phase Functions of Polar 467 Mesospheric Cloud Ice as Observed by the CIPS Instrument on the AIM Satellite 468 , J. Atmos. Sol., Terr. Phys, (this issue). 469 470 Bardeen, C., Numerical simulations of the three-dimensional distribution of polar 471 mesospheric clouds, 2007, 8th International Workshop on Layered Phenomena of the 472 Mesopause Region, Geophysical Institute, University of Alaska, Fairbanks, AK. 473 474 Baumgarten, G., F.-J. Lübken, and K. H. Fricke, 2002, First observations of one 475 noctilucent cloud by a twin lidar in two different directions, Annales Geophysicae, 20, 476 1863-1868. 477 478 Baumgarten, G., J. Fiedler, and G. vonCossart, 2007, The size of noctilucent cloud 479 particles above ALOMAR (69N, 16E): Optical modeling and method description, Adv. 480 Space Res., 40, 772-784. 481 482 Benze, S. C. E. Randal, M. T. DeLand, G. E. Thomas, D. W. Rusch, S. C. Bailey, J. M. 483 Russell III, W. E. McClintock, A. W. Merkel, C. Jeppesen, 2008, Comparison of 484 Aeronomy of Ice in the Mesosphere to the Solar Backscatter Ultraviolet Instrument, J. 485 Atmos. Sol. Terr. Phys., this issue. 486 487 Berger, U., and F.-J. Lübken, 2006, Weather in mesospheric ice layers, Geophys. Res. 488 Lett., 33. 489 490 Berger, U., 2007. Modeling of middle atmosphere dynamics with LIMA. J. Atmos. Sol., 491 Terr. Phys, submitted for publication. 492 493 Carbary, J. F., D. Morrison, G. J. Romick, 2000, Transpolar structure of polar 494 mesospheric clouds, J. Geophys. Res., Vol. 105, NO D20, 2000JD900307. 495 496 Chandran, A., D. W. Rusch, S. E. Palo, G. E. Thomas, annd M. Taylor, (2008), Gravity 497 Wave observations from the Cloud Imaging and Particle Size (CIPS) Experiment on the 498 AIM spacecraft, J. Atmos. Sol. Terr. Phys., this issue. 499 500 Deland, M.T., E.P. Shettle, G.E. Thomas, and J.J. Olivero, 2007a, A quarter-century of 501 satellite polar mesospheric cloud observations,., 68, 9-29. 502 503 DeLand M. T., E. P. Shettle, G. E. Thomas, J. J. Olivero, 2007b, Latitude-dependent 504 long-term variations in polar mesospheric clouds from SBUV version 3 PMC data, J. 505 Geophys. Res., 112, D10315, doi:10.1029/2006JD007857. 506 507 Gadsden, M. and P. Parvianien, Observing Noctilucent Clouds, International Association 508 of Geomagnetism and Aeronomy, 1995 (available on 509 http://www.iugg.org/IAGA/iaga_pages/pdf/ONC_Sep06.pdf) 510 511 Gadsden, M. and W. Schröder, Noctilucent Clouds, Springer-Verlag Berlin, Heidelberg, 512 New York, 165 p, 1989. 513 514 Gerding, M., J. Höffner, M. Rauthe, W. Singer, M. Zecha, and F.-J. Lübken, 2007, 515 Simultaneous observation of noctilucent clouds, mesospheric summer echoes, and 516 temperature at a mid-latitude station (54oN), J. Geophys. Res., 112, D12111 517 doi:10.1029/2006JD008135. 518 519 Haurwitz, B, and B. Fogle, Waveforms in noctilucent clouds, 1969, Deep Sea Res., 16, 520 85-95. 521 522 523 Hervig, M. R., E. Thompson, M. McHugh, L. L. Gordley, J. M. Russel III, and M. E. 524 Summers, 2001, First confirmation that water ice is the primary component of polar 525 mesospheric clouds, Geophys. Res. Lett., 28, 971-974. 526 527 Hervig, M.E., L.L. Gordley, M. Stevens, J.M. Russell, and S. Bailey, Interpretation of 528 SOFIE PMC measurements: Cloud identification and derivation of mass density, particle 529 shape, and particle size, J. Atmos. Solar-Terr. Phys., in review, 2008. 530 531 Hines, C. O., A possible source of waves in noctilucent clouds, 1968, J. Atmos. Sci., 25, 532 937-942. 533 534 Jensen, E. J., and G. E. Thomas (1994), Numerical simulations of the effects of gravity 535 waves on noctilucent clouds, J. Geophys. Res., 99(D2), 3421–3430. 536 537 Jesse, O., 1896, Die höhe der leuchtenden nachtwolken, Astron. Nacht., 140, 161-168. 538 539 Leslie, R. J., 1885, Sky Glows, Nature (London), 34, 264. 540 541 Lübken, F.-J., and U. Berger, 2007, Interhemispheric comparison of mesospheric ice 542 layers from the LIMA model, J. Atmos. Sol. Terr. Phys.. doi: 543 10.1016/j.jastp.2007.07.006. 544 545 McClintock, D. W. Rusch, G. E. Thomas, A. W. Merkel, S. M. Bailey, J. M. Russell III, 546 2008, The Cloud Imaging and Particle Size Experiment on the Aeronomy of Ice in the 547 Mesosphere Mission; Instrument concept, design, calibration, and on-orbit performance, 548 J. Atmos. Sol. Terr. Phys., this issue. 549 550 Merkel, A. W., Bailey, S. M., Rusch, D. W., Thomas, G. E., and Russell, J. M. III, 2006, 551 Seasonal variability of Polar Mesospheric Cloud altitude, particle size and observation 552 frequency in relation to the frost point temperature., AGU Fall Meeting, San Francisco. 553 554 Plane, J. M. C., B. J. Murray, X. Chu, and C. S. Gardner, 2004, Removal of meteoric iron 555 on Polar Mesospheric Clouds, Science, 304, 426-428. 556 557 Rapp, M., and G. E. Thomas, 2006, Modeling the microphysics of mesospheric ice 558 particles: assessment of current capabilities and basic sensitivities, J. Atmos. Sol. Terr. 559 Phys.. Vol. 68, 715-744. 560 561 Rapp, M., G. E. Thomas, and G. Baumgarten, 2007, Spectral properties of mesospheric 562 ice clouds: Evidence for nonspherical particles, J. Geophys. Res., VOL. 112, D03211, 563 doi:10.1029/2006JD007322. 564 565 Russell III, James M., Scott M. Bailey, Mihály Horányi, Larry L. Gordley, David W. 566 Rusch, Mark E. Hervig, Gary E. Thomas, Cora E. Randall, David E. Siskind, Michael H. 567 Stevens, Michael E. Summers, Michael I. Taylor, Christoph R. Englert, Patrick J. Espy, 568 William E. McClintock and Aimee W. Merkel, 2008, Aeronomy of Ice in the 569 Mesosphere (AIM): Overview and early science results, J. Atmos. Solar-Terr. Phys., in 570 review. 571 572 Sentman, D.D. et al., 1993, Simultaneous observations of mesospheric gravity waves and 573 sprites generated by a midwestern thunderstorm, J. Atmos. Solar-Terr. Phys., 537-550,. 574 575 Shettle, E.P., M.T. Deland, G.E. Thomas, ad J.J. Olivero, 2007, Long-term variations in 576 PMC frequency as a function of latitude, 8th International Workshop on Layered 577 Phenomena of the Mesopause Region, Geophysical Institute, University of Alaska, 578 Fairbanks, AK. 579 580 Taylor, M.J., M. Gadsdsen, R.P. Lowe, M.S. Zalcik, and J. Brausch, 2002, Mesospheric 581 cloud observations at unusually low latitudes, J. Atmos Sol.-Terr. Phys., 64, 991-999. 582 583 Taylor, M. J. and M. A. Hapgood, 1988, Identification of a thunderstorm as a source of 584 short period gravity waves in the upper atmospheric nightglow emissions, Planet Space 585 Sci., 36, 975-985. 586 587 Thomas, G. E., and J. J. Olivero, 1989, Climatology of polar mesospheric clouds, 2. 588 Further analysis of Solar Mesospheric Explorer data. J. Geophys. Res., 94, 14,673- 589 14,702. 590 591 Thomas, G. E., J. J. Olivero, E. J. Jensen, W. Schröder, and O. B. Toon, 1989, Relation 592 between increasing methane and the presence of ice clouds at the mesopause, Nature, 593 338, 490-492. 594 595 Thomas, G. E., 1996, Is the polar mesosphere the miner’s canary of global change?, Adv. 596 Space. Res., 18, 149-158. 597 598 Turco, R.P.,O.B. Toon, R. C. Whitten, R. G. Keesee, and D. Hollenback, 1982, 599 Noctilucent clouds: Simulation studies of their genesis, properties, and global influences, 600 Planet. Space Sci., 30, 1147-1181. 601 602 Witt, G., 1962, Height, structure and displacements of noctilucent clouds, Tellus XIV, 1, 603 465 1-18. 604 605 WMO Report (1970) International Noctilucent Cloud Observation Manual: WMO - No 606 250.TP.138. 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 FIGURE CAPTIONS 627 628 Figure 1. The overlapping fields of view of the four CIPS camera images projected on 629 the PMC cloud height at 83 km. The camera names are identified. Although the focal 630 planes are rectangular, their projection on the curved earth results in a 'bowtie' shaped 631 coverage. 632 633 Figure 2. (a). A cloud-free scene (only Rayleigh scattering) near 82°N taken before the 634 onset of the PMC season. The pixel size is 5km. Contour lines of latitude and longitude 635 are shown; (b) Albedo (Garys) vs pixel number for a scan across the center of the scene; 636 (c) Albedo vs pixel number for a scan across the nadir camera from top to bottom. 637 638 Figure 3. a. A scene with Rayleigh and PMC scattering near 82°N taken during the PMC 639 season. The scales are identical to those in Fig. 2. b. Albedo (Garys) vs pixel number for 640 scan across the center of the scene; c. Albedo vs pixel number for a scan across the nadir 641 camera from top to bottom. 642 643 Figure 4. a: PMC albedo, depicted the same as Figure 2a with the Rayleigh background 644 subtracted and phase-compensated (see text); (b): Horizontal scan of albedo across the 645 middle of the scene. c. Vertical scan of albedo across the center of the nadir cameras. 646 647 Figure 5. Daily cloud cover of phase-compensated PMC albedo where 15 orbits of 648 images are merged. When overlapping occurs, the most recent part of the image is 649 substituted. The Greenwich meridian is at 6 o'clock, and longitude is measured from this 650 meridian positive in the counterclockwise direction, and negative in the clockwise 651 direction. The outlines of the continents are shown as white lines. The latitude circles are 652 shown as white circles. The circle of constant daylight is shown as the dashed circle, 653 inside of which there is 24 hours of daylight. The dark circle surrounding the pole 654 indicates no data. 655 656 Figure 6. a. A CIPS 'bowtie' scene with an ‘ice ring’. The scales are similar to Figure 3. 657 658 Figure 7. a. A CIPS scene showing a series of ice rings with diameters of 50 to 100 km. 659 The scale is identical to Figure 3. 660 661 Figure 8. (a) A CIPS scene depicting raw albedo with two bright spots in the fore camera 662 (on the right). The scales are similar to Fig. 2. (b) A scan across the bright spots within 663 the box shown in Figure 8(a) showing the PMC albedo associated with the bright spots 664 and the surrounding region. 665 666 Figure 9. A CIPS daily cloud occurrence plot for DOY 184 showing a large ‘ice void’ at 667 about 85N and 135W. Scale identical to Fig. 5. 668 669 Figure 10. a: A CIPS daily cloud composite image near the end of the northern season, b: 670 A sector of the daily image from the upper left area showing wave streak features, and c: 671 A sector of the daily image from the lower right area showing the complicated cloud 672 structures similar to those seen during the majority of the season. 673 674 Figure 11. A grey-scale visible image of tropospheric convective outflow events taken by 675 the GOES 12 geostationary weather satellite on June 8, 2007. The blue lines indicate the 676 coastline of Cuba. A nearly-circular cloud ring is seen at the lower left, centered on the 677 western coast of the island, where an intense thunderstorm was occurring. A time-lapse 678 movie of this event shows clearly the evolution of the ring emanating from the region of 679 intense convection. 680 681 682 683 684 685 686 687 688 689 690 Figure 1 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 Figure 2a Figure 2b Figure 2c 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 Figure 3a Figure 3a Figure 3b 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 Figure 4a 753 754 755 Figure 4b Figure 4c 756 757 758 759 Figure 5 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 Figure 6 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 Figure 7 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 Figure 8a Figure 8b 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 Figure 9 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 Figure 10a Figure 10b Figure 10c 891 892 893 Figure 11