MEETINGHIGHLIGHTS
After two and one-half days of oral and poster presentations, the group convened to discuss issues raised during the meeting. These issues are all outstanding problems and important topics in the field.

Secondary maximum in PMC occurrence
S. Bailey presented SNOE satellite results that demonstrated a reduction in cloud occurrence and brightness (of varying degree with different years) recurring near 15–30 July for the past four years. This decrease was followed by a second maximum in cloud occurrence and brightness near 10 August followed by the final reduction in cloud occurrence in late August. This behavior was confirmed by E. Shettle and M. Deland at NRL from other spacecraft observations but was not seen in the earlier SME observations.

Some theories were broached but an understanding of this behavior was not achieved.

Solar proton events/auroral precipitation
Solar proton events and their effects on PMCs were discussed, as the PMCs detected by SNOE essentially disappeared over the entire polar region on the solar proton event of 14 July 2000. Mixed comments followed as others have observed little impact of solar proton events on NLC formation. A more detailed study of this observation was requested so that others can correlate their observations with the SNOE measurements and the precise timing of the well-documented 14 July solar proton event. U. von Zahn pointed out that the Alomar lidar during that time observed NLCs, and on 14 July observed a particularly high-altitude aerosol layer (NLC?) above 85 km.

S. Bailey presented a correlation between aurorally produced nitric oxide and PMC brightness. He found positive correlations in 1999 and 2001 but little correlation in 1998 and 2000. One possible explanation of the positive correlation with nitric oxide (which is a proxy for energetic particle flux when the values are high) is that increased electron densities lead to increased recombination of heavy proton hydrate ions. If these ions are responsible for nucleation of ice particles, then a reduction in ion density would cause each ice particle to increase in size, because of the decreased competition for the available water vapor. The increased size more than compensates for the smaller number of particles in the scattered brightness. Another possibility is that the relationship is not cause and effect, but reflects a physical process affecting both PMC brightness and nitric oxide, an example being an enhanced vertical transport process. The question of why the process is biannual is not understood, but could be related to the QBO wind oscillation in the stratosphere.

Dynamical forcing of PMC structures
D. Fritts addressed issues regarding dynamical influences on PMCs, particularly the production of turbulence at mesopause heights both through shear instabilities (the K-H effect) and through convective instabilities. He also emphasized the importance of determining how often large amplitude, short period waves occur. As these waves can provide significant momentum flux to the region, NLC observations of centroid height variations could help determine whether these waves occur regularly or more sporadically.

J. Thayer’s Greenland lidar results and M. Rapp’s CARMA microphysical modeling showed that short period gravity waves are acting to reduce cloud width and backscatter strength. However, these same short period waves are most likely responsible for driving the cold summer mesopause that leads to cloud formation. It was then emphasized that if the background conditions are cold enough, the wave effects may not be as detrimental as indicated by the results for nominal conditions. This will have to be borne out from more observations and modeling. An interesting result presented by Rapp was the reanalysis of ion gauge data illustrating the presence of NLCs within local temperature minima presumably induced by gravity waves. M. Rapp showed that low-frequency gravity waves can maintain local temperature minima that are in phase with NLC formation.

E. Llewellyn, speaking for W. Evans, presented evidence from WINDII data on PMCs, indicating that Southern Hemisphere PMCs were more extensive over the Palmer Peninsula. WINDII data also show that the lower in altitude the PMCs, the brighter they are, on the average.

Modeling issues of background mesospheric properties
R. Walterscheid reviewed wave forcing on the general circulation and the dynamical drivers that lead to a cold summer mesopause. He attributed the dynamical influences to gravity and planetary wave activity and raised issues regarding interhemispheric differences in wave propagation. A provocative result from R. Roble’s talk on early WACCM 3-D coupled ocean–atmosphere model pertained to the connection of sea surface temperature changes in El Niño and La Niña. A sea surface warming during El Niño conditions led to a summer mesopause temperature change of 8 degrees. The 1991 El Niño might have contributed to the small number of NLCs seen in the summer of 1992 in both Europe and Canada. This illustrates the complex coupling that may be at play in describing the background conditions of the summer mesopause.

R. Walterscheid also raised the issue concerning the local heat budget of the mesosphere. Adiabatic cooling, gravity wave heat flux, chemical heating, turbulent heating, and radiative cooling/heating are all quantities that can have a significant impact on the background temperature equilibrium of the mesopause region—their relative influence on the mesopause region is still an open question.

M. Stevens presented the MAHRSI and HALOE evidence for enhanced water vapor in the lower thermosphere. He demonstrated that shuttle exhaust is most likely responsible and by extrapolating shuttle orbits, he was able to infer a meridional wind velocity of 34 m/s in the lower thermosphere.

PMC/NLC height determination
South Pole lidar observations presented by X. Chu determined typical mesospheric cloud heights near 85 km. These clouds are 2 to 3 km higher than in the northern hemisphere, as demonstrated by U. von Zahn’s lidar observations from ALOMAR and J. Thayer’s lidar observations from Sondrestrom, Greenland. Three different satellites over the years have analyzed Southern Hemisphere PMCs and have found their heights to be similar to the Northern Hemisphere. It should be noted that MSX observations determined PMC altitudes to be 0.6 km higher in the Southern Hemisphere than in the Northern Hemisphere.

The South Pole lidar observations also show clouds to be their brightest at their highest altitude. This also contradicts satellite observations and Northern Hemisphere lidar observations of an anticorrelation in cloud height and brightness. It highlights the possibility that the South Pole contains a unique environment for PMC evolution. This is an interesting possibility that will need further assessment.

Particle size distributions and shape
U. Berger described a numerical study using the IAP’s COMMA model to simulate cloud particle formation on smoke particles. The simulation showed strong removal of water vapor by the freeze-drying process. Eddy diffusion processes included in the model also illustrated a departure in the size distribution function from the often-assumed lognormal to a more normal (Gaussian) distribution.

G. Witt, in his invited talk, discussed the use of linear and circularly polarized light to determine the Stokes parameters for best characterizing the NLC shape. He also emphasized that near infrared measurements may be the best spectral region to optically sense ice crystals, because of the dependence of IR cross sections on particle volume and hence mass. U. von Zahn presented the first depolarization lidar measurements that indicated the cloud particles were not spherical in the region above the peak of the cloud. The NLC shape will continue to be an issue, but future observations may help resolve this important aspect of cloud structure.

IR effects on ice particle heating
G. Witt and P. Espy discussed the absorption of IR radiative flux by ice particles that could lead to the ice particles heating above ambient. P. Espy's model estimates suggested an 8 to 10 K increase in temperature over ambient for ice particles in excess of 100 nm. G. Witt illustrated that, based on the volume to surface ratio of the ice particle, the temperature enhancement is linear to the particle radius. This prompted a discussion of feedback to the ambient temperature resulting in local heating of the background atmosphere.

Laboratory evidence and condensation nuclei
In J. Gumbel’s contributed talk, cloud condensation nuclei was discussed with an emphasis on the growth of cluster ions. He presented a model (OASIS) that for the first time couples detailed ion chemistry and the charging of ice particles. It is shown that the presence of ice particles generally enhances the growth of cluster ions (proton hydrates). As these cluster ions are potential nuclei for the formation of new ice particles, positive feedback can cause local enhancements of both particle and charge density. The required electron density in the region was on the order of 500 electrons per cc. The effects of this feedback on particle layering should be further explored.

J. Plane presented laboratory evidence showing a strong temperature dependence of the uptake coefficient of ice for atomic oxygen. He showed that past measurements of atomic oxygen depletion near PMCs are consistent with a destruction on ice particle surfaces, if the effective surface area of the particles is about 10 times the geometric cross section.

Long-term NLC and water vapor trends
N. Pertsev presented 40 years of NLC observations from Moscow and indicated that a clear solar cycle periodicity was observed in cloud occurrence but that no observable long-term trend in cloud occurrence frequency was detected over the past 40 years once solar cycle and weather effects were removed. However, the average brightness of the cloud was found to have a significant upward trend, a result predicted by previous models of Jensen and Thomas. An additional result is that with the removal of ground-based weather effects, the periodic variations of NLCs have periods of 9 to 9.5 years, not the canonical 11 years. G. Thomas presented a composite data set derived from long-term observations from the Canadian, northeast European, and USSR observing networks. In agreement with the published data of Gadsden, these data indicate an increasing trend of noctilucent cloud occurrences with time, extending backward to the 1950's. M. Taylor described midlatitude observations for the past three years from Colorado and Utah, raising the question of whether these events are an indicator of change. A. Merkel described SNOE satellite data that reveal occasional forays of PMCs into the 40 to 45 degree latitude band.

E. Shettle provided a review of satellite evidence of long-term trends of PMCs from the available data sets (including SME, WINDII, SAGE-II, SBUV/NOAA, POAM II and III). There is a clear 11-year modulation that is nearly out of phase with the solar Lyman-alpha flux, and also a definite brightening of PMCs from the 1980's to the 1990's.

The 10-year data set for water vapor at midlatitudes from HALOE (on the UARS spacecraft) was reported by Jim Russell to show a 1% per year trend in the mid-mesosphere, where solar Lyman-alpha variations are minimum and where the data are of high-enough quality to detect small trends in a data set that reveal important year-to-year variability.

Latitudinal effects
F-J. Lübken described recent results of a campaign conducted at Svalbard, 78N, consisting of temperature measurements made by falling spheres launched from rockets, and potassium lidar, and NLC measurements made by lidar. PMSE was observed by radar. It was found that PMSEs and NLCs are nearly ubiquitous at these high latitudes and that PMSEs often occur in double or even triple layers. This contrasts with measurements at ALOMAR, 69N, where the PMSEs and NLCs only occur in parts of the saturated region. The temperature structures often show these mulitple-layering effects, although they do not seem highly spatially correlated with the PMSE and NLC layers.

PMSE issues
P. Hoffmann showed PMSE observations at Andenes, Norway, that demonstrate an asymmetry in the mean seasonal occurrence rate (steep increase in May and a more gradual decrease in August), which is not yet explained. It was pointed out in several presentations (e.g., by E. Thrane and F-J. Lübken) that neutral air turbulence is most likely not the main cause for PMSE. However, new data from radars at Svalbard and from Tromsö demonstrate that the reduction of electron diffusivity is a necessary condition for the existence of PMSE. A proxy for PMSE was suggested in the paper by M. Rapp, which considers the aerosol charge and the electron diffusion reduction. J. Gumbel pointed out the potential role of heterogeneous chemistry for the existence of charged ice particles. Still, the main scientific question regarding PMSE, namely what causes the echoes, remains unanswered.

It was shown that the mean diurnal variation of PMSE is markedly influenced by atmospheric dynamics and by solar radiation. At midlatitudes mesosphere summer echoes (MSE) exist in an environment where the mean neutral air temperature is significantly above the frost point temperature of water ice (assuming reasonable water vapor concentration). The explanation for this observation is pending.

In private communication with R. Woodman (October 2001 and Boston Spring AGU Meeting 2001) the PMSE backscatter strengths in the Southern Hemisphere were recently measured using a different antenna array. The previous antennas were determined to be inefficient and the new array has detected stronger PMSEs, but they remain to be about a factor of 100 less than observed at Poker Flat. The low backscatter power observed at Resolute Bay seems to remain an open question.


FUTUREPLANS

Next meeting of the working group
It was tentatively decided that the BAS people would host the next workshop in Cambridge, England, in September 2004. Further developments are forthcoming.

Satellite missions
The Odin satellite is healthy and providing unique observations of water vapor and PMC detections. We expect to hear a lot more from the Odin team over the next year.

If funding is available, the SNOE should continue providing high quality PMC and nitric oxide data until 2007.

The NASA TIMED spacecraft is slated for a 7 December launch. SABER onboard TIMED will help provide further water vapor observations and PMC detections in addition to monitoring the radiative heat budget of the mesopause and the concomitant minor species.

AIM (Aeronomy of Ice in the Mesosphere) is a NASA Small Explorer mission that is near completing its Phase A study. If approved it will be a dedicated mission specifically for unraveling many of the issues regarding PMC science.

Rocket campaigns
A rocket project called MAC/WAVE is scheduled for summer 2002 from Andoya and January 2003 from Kiruna. Several meteorological and sounding rockets will be launched including the MIDAS payload of FFI (Oslo) and IAP (Kühlungsborn) with fine scale measurements of plasma and neutral air parameters. Further MIDAS launches are planned for Spitsbergen in summer 2003.

In the next 2 years a series of meteorological rockets, namely falling spheres and foil clouds, will be launched within the ROMA project from Andoya. The main scientific aim is the study of the thermal and dynamical state of the neutral atmosphere near layered structures especially in the transition periods to/from the summer season.

Laboratory work
Further laboratory experiments are planned at both the University of East Anglia (UEA) (J. Plane and B. Murray) and SRI International (J. Marschall and J. Boulter) to study oxygen atom uptake. Particular questions are the nature of the products (O₂(³S_g– or ¹D_g) or O₃), and the effect of chemical heating due to surface recombination on the growth of small particles. A new laboratory experiment at UEA (J. Plane, S. Meech, and T. Vondrak) will study photoelectric emission from ice that has been doped with atomic Fe and Na, since rocket experiments have revealed the presence of positively charged particles. It was clear from discussion during the workshop that experiments also need to be carried out to study the physical chemistry of ice condensing on 1 nm particles that mimic meteoric "smoke," with a view to establishing whether the Kelvin effect and other thermodynamic properties still hold.

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