SA33C-01 INVITED
Dynamical variations of the polar summer mesopause region and PMC occurrence
Satellite observations of the variation of PMCs during the summer season have long shown PMC occurrence to be characterized by sharp variations whereby clouds appear and disappear on a global scale over timescales of only a few days [e.g. Bailey et al., SNOE observations of PMCs, J. Geophys. Res.,doi:10.1029/2004JD005422, 2005]. The first summer season of the AIM mission was no exception and this period offers an excellent opportunity to understand the link between mesospheric meteorology and global PMC occurrence. In 2007, clouds maximized near solstice but then sharply decreased by the end of June. This appears to be linked to an unexpected warming of the mesopause region. Clouds increased again in July but showed considerable variation for the rest of the summer season. The end of the summer season also happened abruptly. Over a several day period, mesopause temperatures jumped by 15K and the H2O dropped sharply. We examine these features using analyses and forecasts from the NOGAPS-ALPHA model. In some cases, changes in the filtering of gravity waves by the lower atmosphere appear to be linked to changes in mesopause weather. This might suggest a direct link between tropospheric weather and mesospheric weather. However, in other cases, a simple forecast using parameterized gravity wave forcing appears to be inadequate to explain the observations.
SA33C-02
Physical Processes Controlling PMC Cloud Formation
Our understanding of Polar Mesospheric Cloud (PMC) environment and physical properties has been advanced by multiple recent developments. The Aeronomy of Ice in the Mesosphere (AIM) satellite is providing unprecedented information about water vapor concentrations in the mesopause region, as well as information about PMC particle sizes and mass. Recent two- and three-dimensional simulations of meteor smoke indicate that the concentration of particles large enough to act as ice nuclei in the summer mesopause region is far lower than previous PMC modeling studies have assumed. Lidar measurements have indicated that gravity waves may have a significant effect on PMCs at lower latitudes where the mesopause is relatively warm, whereas at the South Pole, where the mesopause is very cold, gravity waves have no discernable effect. We will describe a microphysical model that tracks the nucleation, growth, transport, and sedimentation of thousands of individual ice crystals to represent the PMCs. Background temperature and water vapor fields are extraced from a global three-dimensional model that includes a bin microphysics representation of PMCs. We will address the following science questions: With the reduced concentration of meteor smoke particles in the summer mesopause region, is heterogeneous nucleation on these particles still a plausible candidate for the source of PMC ice crystals? Are gravity-wave temperature perturbations required for nucleation of sufficient ice concentrations? How sensitive are PMC properties (at about 82-84 km) to the concentration of ice crystals nucleated near the mesopause (at about 88-90 km)? How broad are the simulated PMC ice crystal size distributions? How do gravity-wave effects on PMCs depend on the background temperature and water vapor profiles?
SA33C-03 INVITED
On the impact of energetic solar particles on Noctilucent Clouds
In this contribution we report on the effects of solar proton events (SPEs) on the occurrence and properties of noctilucent clouds (NLCs) or Polar Mesospheric Clouds (PMCs). Satellite observations of NLC properties made with the SCIAMACHY/Envisat, OSIRIS/Odin and SBUV instruments were used to investigate changes in NLC during the January 16 -- 22, 2005 SPE. The observations showed a severe depletion in NLCs during the SPE with occurrence rates decreasing from about 80% to less than 20% for the 70°S to 80°S latitude range. Simultaneous temperature measurements with MLS/Aura showed an increase of > 10 K in zonally averaged temperatures at 85 km altitude within the period of enhanced ionization, which is likely the direct cause for the disappearance of NLCs. Different possible mechanisms leading to the temperature increase will be discussed, but the exact mechanism is not yet fully understood. We will also present first results of an investigation to identify further examples for NLC depletion during SPEs using the SBUV NLC data set. The impact of SPEs on NLCs is relevant for evaluating the role of NLCs as early indicators of climate change, because it may be another important process that has a significant effect on NLCs.
SA33C-04
About particles forming noctilucent clouds above ALOMAR (69N)
Noctilucent clouds (NLC) are the visible manifestation of icy particles persistently present in the polar summer mesopause region. Their formation is a rather complicated physical process depending on atmospheric background parameters, such as temperature and water vapor, which are highly variable and hardly to measure directly at the altitudes of interest. The strong dependence on the atmospheric parameters and the fact that the clouds show variabilities at different time scales from minutes to several years make NLC an attractive tracer. The ALOMAR RMR-lidar, located in Northern Norway at 69N, is capable to observe NLC on a regular basis since 1997. Using the most sensitive wavelength of the lidar at 532nm, we have observed NLC signatures covering all local times. From the observations basic cloud parameters like brightness, altitude and occurrence frequency are derived. From the backscatter coefficients of three widely separated wavelengths (355nm, 532nm, 1064nm) we deduce particle properties like number density and size of the particles. Additionally the width of the size distribution of NLC particles is deduced. The combination of basic cloud parameters and microphysical particle properties allows detailed studies of the observed variations and helps to understand the physical processes involved. We investigate and compare variations from hourly to seasonal time scales as contained in our 12-years data set.
SA33C-05 INVITED
PMC Morphology and its Dependence on Water Vapor and Temperature: Highlights From the AIM Mission
The Aeronomy of Ice in the Mesosphere (AIM) mission has provided nearly two years of unprecedented data on polar mesospheric clouds (PMCs). This talk will provide an overview of AIM contributions to improving our understanding of PMC morphology and the dependence of PMCs on water vapor and temperature. Highlights from recent and ongoing investigations using measurements from the AIM Cloud Imaging and Particle Size (CIPS) experiment and Solar Occultation For Ice Experiment (SOFIE) will be described. We will show that AIM measurements of PMCs, water vapor, and temperature compare well with correlative data, and we will present determinations of SOFIE and CIPS particle sizes. Correlations between SOFIE measurements of water vapor, temperature, and PMCs, as well as between CIPS measurements of PMCs over the polar region and ancillary (global) measurements of water vapor and temperature, suggest mechanistic connections between the forcing variables and PMCs. These connections are explored further with two different models, the Navy Operational Global Atmospheric Prediction System (NOGAPS) and the National Center for Atmospheric Research Whole Atmosphere Community Climate Model (WACCM).
SA33C-06
In-Situ Evidence for a Strong Seasonal Variability of Meteor Smoke at High Northern Latitudes and Implications for Mesospheric ice Nucleation
We report on new in situ measurements of meteor smoke particles (MSPs) and their mesospheric environment from three international field campaigns at the North-Norwegian Andoya Rocket Range. These are the ECOMA campaigns in 2006 and 2008 and the joint ECOMA/MASS campaign in 2007. During these campaigns a total of 6 sounding rockets were successfully launched. The prime instrument of the ECOMA- payload is a detector making use of active photoionization of MSPs and the subsequent detection of corresponding photoelectrons. During the first flight in September 2006, MSPs were detected throughout the mesosphere with concentrations in reasonable agreement with predictions from microphysical models. All later flights with this detector under polar summer conditions, however, do not show any detectable MSP concentration at altitudes below ~75 km, whereas the detector did detect signatures of mesospheric ice particles at about 83 km during all flights. In each of these cases, the mesospheric ice detection is unequivocally confirmed by simultaneous photometer measurements of noctilucent cloud particles on the same rocket payloads. Also, the electrostatic mass spectrometer for nanometer-sized charged aerosol particles MASS which was launched close to the ECOMA-rocket in 2007 did not detect any particle signatures outside the mesospheric ice region. We critically discuss these observations in the context of MSP- and ice particle-microphysics of the mesosphere using simultaneous measurements of the ambient plasma from the same sounding rockets as well as satellite observations of nitric oxide from the Canadian ACE satellite. Taking further into account high resolution temperature measurements from each rocket payload, we tentatively draw conclusions on the feasibility of different potential nucleation pathways such as heterogeneous nucleation on neutral and/or charged MSPs, ionic nucleation, and homogeneous nucleation in the presence of pronounced temperature fluctuations owing to gravity waves.
SA33C-07
Two-day Planetary Wave Impact on Austral Polar Mesopause Temperatures: as Revealed by a January Diminution in PMSE above Davis, Antarctica
A new characteristic of the austral summer polar mesopause as revealed by ground MST radar charged ice- aerosol echoes and satellite MLS temperature is reported, that is plausibly linked to the low-latitude easterly wind jet. Four consecutive seasons of polar mesosphere summer echoes (PMSE) and mesosphere temperature observations above Davis, Antarctica (geographic: 68.6°S; 78.0°N) exhibit an annual mid-January diminution in PMSE occurrence rate that is correlated with a simultaneous mesopause warming by several degrees. Interestingly, the corresponding mesosphere meridional wind field changes from equatorward to a poleward flow at these times. Although displaying some inter-annual variation in the peak onset time, the mid-January mesopause warming correlates with the peak in the line-of-sight meridional wind and temperature enhancements (both poleward and in altitude) associated with the low-latitude 2-day planetary wave (PW). Periodogram analyses of PMSE and Aura MLS temperatures show the dominance of 5- day PWs throughout the austral summer coupled with pronounced 2-day PWs evident from early January to mid-February. Zonal spectral analyses of Aura MLS temperature records reveal the high-latitude 2-day PWs have zonal wavenumber (s) with both westward (s = -2, -3) and eastward (s = 2, 3) spectral features consistent with the periodogram results. Our polar observations support the evolution of a myriad of prodigy PWs spawned from the westward 2-day PW (s = -3) activity, originating from the low-latitude easterly jet, albeit not previously linked to the reported mid-January diminution of PMSE at southern latitudes.
SA33C-08
The Homogeneous Nucleation of Amorphous Water Particles in the Upper Mesosphere
Particles dominantly composed of water are known to exist in the high latitude upper mesosphere during the summer months. However, the mechanism through which they nucleate remains unknown. In the past the homogeneous nucleation of ice directly from the vapour phase has been ruled out on the basis of homogeneous nucleation theory. It is postulated here that particles of amorphous water, ultra-viscous condensed water with a liquid like structure, may nucleate directly from the vapour phase in the summer mesosphere. It is demonstrated here that the energy barrier to nucleation of an amorphous water particle may be much smaller than that to nucleate a crystalline ice particle. These amorphous water particles may later crystallise to form cubic crystalline ice particles. It is concluded that homogeneous nucleation of amorphous water particles can not be ruled out as a potential mechanism of condensed water cloud formation in the upper mesosphere.