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	1430-1450 Uwe Berger, IUP, Germany,Solar variability and trend effects in mesospheric ice layers		1430-1450 Uwe Berger, IUP, Germany,Solar variability and trend effects in mesospheric ice layers
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	1450-1500 Q&A, and discussion		1450-1500 Q&A, and discussion
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	1500-1520 Aimee Merkel, LASP, USA,WACCM-PMC simulations of long-term trends of PMC		1500-1520 Aimee Merkel, LASP, USA,WACCM-PMC simulations of long-term trends of PMC
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	1520-1530 Q&A, and discussion		1520-1530 Q&A, and discussion
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	1530-1600 Coffee Break		1530-1600 Coffee Break
	1600-1620 David Siskind, NRL, USA,The PMC region as an integrator of coupling processes: Implications for trend studies from AIM and other missions		1600-1620 David Siskind, NRL, USA,The PMC region as an integrator of coupling processes: Implications for trend studies from AIM and other missions

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Revision as of 00:44, 13 March 2012

• Project leaders: G.Thomas (USA), U.Berger (Germany)
• Project members: S. Bailey (USA),G. Baumgarten(Germany),M. DeLand (USA), J. Fiedler (Germany),B. Karlsson (Sweden), S. Kirkwood (Sweden), A. Klekociuk (Australia), F.-J. Lübken (Gemany), A. Merkel (USA), N. Pertsev (Russia), J. Russell III (USA), E. Shettle (USA)

Contents 1 Introduction 2 References 3 Workshops and Meetings 3.1 Second CAWSES-2 Task 2 Workshop: Modeling Polar Mesospheric Cloud Trends, May 3-4, 2012 4 AM Thursday, May 3 4.1 Long-term Trends: PMC Observations 4.2 Long-term Trends: Temperature & Water Vapor Observations 5 PM Thursday, May 3 5.1 Long-term Trends: Temperature & Water Vapor Observations (cont.) 5.2 Long-term Trends: Modeling 5.3 Inter-annual and seasonal variability: Observations= 5.4 Inter-annual and seasonal variability: Modeling 5.5 Properties of PMC and their Environment: Observations 5.6 Properties of PMC and their Environment: Modeling 6 Local Organizing Committee: Aimee.Merkel@lasp.colorado.edu,Dan Marsh (marsh@ucar.edu) and Charles Bardeen (bardeenc@ucar.edu) 6.1 NOTE: the Particle Size Workshop,originally scheduled for May 2, has been postponed to a later date. 7 SPECIAL JOURNAL ISSUE ON UPPER ATMOSPHERIC TRENDS NOW AVAILABLE. The joint JGR/Space, JGR/Atmospheres special section on upper atmospheric trends is now complete, and can be accessed at http://www.agu.org/journals/ja/special_sections.shtml?collectionCode=UATREND1&journalCode=JA 8 Observing Facilities 8.1 Aeronomy of Ice in the Mesosphere (AIM), a NASA satellite mission (2007-) 8.2 Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) 8.3 Visual Observations of Noctilucent Clouds 8.4 Additional notes about NLC long-term behavior according to visual observations

Introduction

The 20-yr old speculation that high-altitude summertime ice clouds (polar mesospheric clouds or noctilucent clouds, here denoted MC) are affected by anthropogenic activities has recently received support from a 30-year time series of SBUV (Solar Backscatter Ultraviolet) satellite measurements (see Figure 1). SBUV data reveal a significant trend in bright MC properties. However, the robustness of the trend, extracted from interannual, local-time and solar-cycle variability, and its underlying causes remains debatable. It is important to understand the relative roles of these three factors (solar, inter-annual and long-term forcing) before a definitive long-term trend can be evaluated. Furthermore the problem of attribution, that is, the nature of the various forcings on ice formation is not yet understood. For example with respect to the long-term changes, is a lowered temperature due to higher carbon dioxide responsible for the observed increase in brightness and occurrence frequency of MC? Or are water vapor changes due to oxidation of methane responsible, since we know that lower atmospheric methane has more than doubled over the past 120 years, and methane oxidation leads to upper atmospheric water vapor. The failure to detect any changes in the altitude of MC since the first measurement made by Otto Jesse in the late nineteenth century has provided an important constraint, since water vapor changes and temperature changes affect cloud altitude in different ways. Fortunately, the state of the art in modeling has now reached a point where ice formation is coupled with general circulation models. In a recent study Berger and Lübken (2011) showed that in the summer period 1979 -1997 at mid-latitudes strong cooling of up to 3-4 K/decade occurs in the middle mesosphere, in the period 1961-1979 the middle atmosphere cooled significantly less, and for the period 1997-2009 they find a warming of ~1 K/decade. For the first time, modeled temperature trends confirm the extraordinarily large temperature trends observed at mid-latitudes during the period 1979-1997 derived from lidar measurements, satellite data, and phase height measurements. The differences in temperature trends in the mesosphere originate from the evolution of stratospheric ozone in the past 50 years, e.g. the observed reversal of both stratospheric and mesospheric temperature trends in the mid 1990s is caused by the recovery of stratospheric ozone (WMO report 2011). Therefore a new research question arises: does any trend in MC show a similar behavior? Relevant publications on this subject are reported in the references below:

References

1. Luebken, F.-J., U. Berger, and G. Baumgarten (2009), Stratospheric and solar cycle effects on long-term variability of mesospheric ice clouds, J. Geophys. Res., 114, D00I06, doi:10.1029/2009JD012377
2. Merkel, A. W., Marsh, D. R., Gettelman, A., and Jensen, E. J.: On the relationship of polar mesospheric cloud ice water content, particle radius and mesospheric temperature and its use in multi-dimensional models, Atmos. Chem. Phys., 9, 8889-8901, 2009
3. Merkel, A. W., D. Marsh, G. E. Thomas, C. Bardeen, M. Deland, WACCM simulations of long-term changes in polar mesospheric clouds, Layered Phenomenon in Mesospheric Regions (LPMR) Conference, Stockholm, 2009
4. Marsh, D and A. W. Merkel, 30-year PMC variability modeled by WACCM, SA33B-08, Fall AGU Meeting, San Francisco, 2009
5. Shettle,E. P.,M. T. DeLand, G. E. Thomas, and J. J. Olivero (2009), Long term variations in the frequency of polar mesospheric clouds in the Northern Hemisphere from SBUV, Geophys. Res. Lett., 36, L02803, doi:10.1029/2008GL036048.
6. Thomas, G. E., D. Marsh and F.-J. Lübken, Mesospheric ice clouds as indicators of upper atmosphere climate change, EOS, Transactions, American Geophysical Union, 91, No. 20, 18 May 2010, p. 183.
7. Berger, U., and F.-J. Lübken (2011), Mesospheric temperature trends at mid-latitudes in summer, Geophys. Res. Lett., 38, L22804, doi: 10.1029/2011GL049528.
8. WMO (2011), Global ozone research and monitoring project-Report no. 52, Scientific assessment of ozone depletion: 2010, World Meteorological Organization

Workshops and Meetings

The first workshop under the auspices of this working group was held in Boulder, Colorado on December 10 & 11, 2010, entitled Modeling Trends in mesospheric clouds (Thomas et al, 2010).

Second CAWSES-2 Task 2 Workshop: Modeling Polar Mesospheric Cloud Trends, May 3-4, 2012

Laboratory for Atmospheric and Space Physics, University of Colorado, USA.

We announce here a forthcoming workshop to be held in Boulder, CO USA on May 3-4, 2012. This is the 7th workshop sponsored by IAGA/ICMA/CAWSES and focusses on global change in the upper mesosphere, specifically decadal-scale trends in Polar Mesospheric Clouds, PMC (or Noctilucent Clouds, NLC, as they are traditionally called when observed from the ground at twilight).

The issue of long-term changes in PMC was addressed in the first workshop in Boulder, Colorado, 10-11 December, 2009 (Thomas et al, 2010). A long suite of satellite measurements by the Solar Backscatter Ultraviolet Spectrometer (SBUV) dating back to 1979 shows a significant long-term trend in mesospheric cloud activity and brightness. Attribution of trends in these high-altitude ice clouds (~83 km) includes: (1) increasing carbon dioxide concentrations which tend to cool the upper mesosphere (the 80-90 km 'mesopause region'); (2) increasing water vapor due to growing concentrations of methane, in addition to changing efficiency of tropospheric water vapor entry through the tropopause; (3) shrinking of the upper atmosphere arising largely from decreased ozone levels and subsequent stratospheric cooling; (4) increased concentrations of water vapor due to the rise in space traffic over the last 30 years. Other possible causes are long-term changes in winds and wind filtering of gravity waves which dominate the dynamics of the high-latitude summertime mesopause region. An even larger (cyclic) trend is superimposed on the long-term record, believed to be a result of the solar cycle, but not well understood.

General circulation models coupled with ice formation are now capable of simulating the SBUV trends to a remarkable degree, as discussed in the first workshop. However, new modeling results have brought out the importance of ozone trends, which affect the heat balance of the entire upper atmosphere. The LIMA/ICE model predicts a 'break-point' in the time series of PMC trends, due to the turn-around of the ozone trend in the mid-1990's. A wealth of new data from several satellite missions (AIM, ODIN, TIMED) are available. The Aeronomy of Ice in the Mesosphere (AIM) directly addresses the processes which control ice cloud evolution. In addition, more information is now available on diurnal variations in ice water content, which in principle affects the analysis of trends made from satellites in sun-synchronous orbits with slowly-varying local time coverage. These and other advances in modeling, data availability and trend analysis are motivations for a coming together of both modelers and data analysts to assess the current state of knowledge. A goal of the workshop would be to make recommendations for future work to resolve many of the outstanding issues.

The PMC Trend Workshop will be held at the Laboratory for Atmospheric Physics, University of Colorado, Boulder, Co, USA on May 3 and 4, 2012. All interested researchers, and particularly students, are invited to the meeting. Our current plans are that no registration fees will be charged. We have now committed the travel funds allocated to us by CAWSES.

Gary Thomas (thomas@lasp.colorado.edu) and Uwe Berger (berger@iap-kborn.de), co-chairs of CAWSES-II, Task 2, Project 3

       The following are the names and affiliations of invited and contributed speakers (in alphabetical order), and the preliminary titles of their talks:

• Charles Bardeen, NCAR, USA, Simulations of PMC during the AIM time period using WACCM/CARMA
• Gerd Baumgarten, IUP, Germany, Decadal observations of particle sizes and water vapor content of NLC
• Uwe Berger, IUP, Germany, Solar variability and trend effects in mesospheric ice layers
• Matthew DeLand, SSAI, USA, Current PMC Trends Derived from SBUV Measurements
• Alain Hauchecorne & Philippe Keckhut, LATMOS, Laboratoire ATmosphères, France,Temperature trends in the stratosphere and in the mesosphere as seen from Rayleigh lidar observations
• Dan Marsh, NCAR, USA,Climate change in the mesosphere from 1850 to 2100 in CESM-WACCM
• Aimee Merkel, LASP, USA, WACCM-PMC simulations of long-term trends of PMC
• Gerald Nedoluha,NRL,USA,Long-Term Ground-based Microwave Measurements of Middle Atmospheric Water Vapor from NDACC sites
• Kota Okamoto, U. of Tokyo, Japan,On the dynamical responses in the middle atmosphere to ozone recovery and CO2 increase
• John Olivero, Embry-Riddle University, USA, Some Historical Notes on Noctilucent Cloud Studies
• Cora Randall, LASP, USA, AIM/CIPS Observations of PMC Variability
• Karen Rosenlof, NOAA, USA, A new satellite based zonally averaged time series of stratospheric water vapor
• James Russell III, Hampton Univ, USA, AIM science results and their significance for PMC long-term change studies
• David Siskind, NRL, USA, The PMC region as an integrator of coupling processes: Implications for trend studies from AIM and other missions
• Michael Stevens, NRL,USA,The impact of space shuttle main engine exhaust on PMCs and implications to trend studies
• On the dynamical responses in the middle atmosphere to ozone recovery and CO2 increase"
• Stan Solomon and L. Qian,NCAR, USA, Thermospheric Temperature Trends:Modeling and Observations

Contributed Speakers (as of Feb.29)

• Susanne Benze, LASP, USA,On the onset of polar mesospheric cloud seasons
• P. Dalin, Swedish Institute of Space Physics, Sweden, S. Kirkwood, N. Pertsev, V. Perminov, A. Dubietis, R. Balciunas, K. Cernis, V. Romejko, On the long-term trends in noctilucent clouds as observed from the ground (Moscow and Lithuania) and on the trends in the OH summer temperature as measured in Moscow.
• Hanli Liu, NCAR,USA, Fast transport of the Columbia Space shuttle exhaust plume by the quasi-two-day wave
• E.J. Llewellyn, L. Rieger, A.E. Bourassa, P.E. Sheese, R.L. Gattinger, N.D. Lloyd, I.C. McDade, W.F.J. Evans, and D.A. Degenstein, University of Saskatchewan, Canada,Special Observational Opportunities Offered by PMCs - Nadir Observations in the Limb
• Richard Goldberg,GSFC,USA,Study of an IR limb emission anomaly observed by SABER/TIMED in the mesosphere-lower thermosphere (MLT) region
• Mark Hervig, GATS, Inc., USA,The Smoke Content of Ice: Interpretation of SOFIE Results
• Kristell Pérot, LATMOS Laboratoire ATmosphères, France, PMC Particle Size Retrieval from GOMOS / ENVISAT Observations
• Scott Robertson, U of Colorado, USA, Shannon Dickson, Mihaly Horanyi, Zoltan Sternovsky,Martin Friedrich, Diego Janches, and Biff Williams, Detection of Meteoric Dust in Mesosphere by the CHAMPS Rockets
• Rachel Ward, Utah State University, USA,Comparison of Northern and Southern Hemisphere Mesospheric Gravity Waves using CIPS PMC Data
• Mark Zalcik,Coordinator,NLC Can Am Network, Canada, Two Decades of Noctilucent Cloud Monitoring in North America

MEETING AGENDA

AM Thursday, May 3

0830-0840: Gary Thomas, LASP Welcome & Summary of Workshop Objectives

Long-term Trends: PMC Observations

0840-0900 John Olivero, Embry-Riddle University, USA, Some Historical Notes on Noctilucent Cloud Studies 0900-0910 Q&A, and discussion

0910-0930 Matthew DeLand, SSAI, USA,Current PMC Trends Derived from SBUV Measurements 0930-0940 Q&A, and discussion

0940-1000 Mark Zalcik, Coordinator, NLC Can Am Network, Canada,Two Decades of Noctilucent Cloud Monitoring in North America 1000-1010 Q&A, and discussion

1010-1025 P. Dalin, Swedish Institute of Space Physics, Sweden,On the long-term trends in noctilucent clouds as observed from the ground and on the trends in the OH summer temperature as measured in Moscow and Lithuania

1025-1030 Q&A, and discussion

1030-1100 Coffee Break

1100-1120 Gerd Baumgarten, IUP, Germany,Decadal observations of particle sizes and water vapor content of NLC

1120-1130 Q&A, and discussion

Long-term Trends: Temperature & Water Vapor Observations

1130-1150 Alain Hauchecorne, Laboratoire ATmosphères, France, Temperature trends in the stratosphere and in the mesosphere as seen from Rayleigh lidar observations

1150-1200 Q&A, and discussion

1200-1220 Karen Rosenlof, NOAA, USA , A new satellite based zonally averaged time series of stratospheric water vapor

1220-1230 Q&A, and discussion

1230-1330 Lunch Break

PM Thursday, May 3

Long-term Trends: Temperature & Water Vapor Observations (cont.)

1330-1350 Gerald Nedoluha, NRL, USA , Long-Term Ground-based Microwave Measurements of Middle Atmospheric Water Vapor from NDACC sites 1350-1400 Q&A, and discussion 1400-1420 Michael Stevens, NRL, USA ,The impact of space shuttle main engine exhaust on PMCs and implications to trend studies 1420-1430 Q&A, and discussion

Long-term Trends: Modeling

1430-1450 Uwe Berger, IUP, Germany,Solar variability and trend effects in mesospheric ice layers

1450-1500 Q&A, and discussion

1500-1520 Aimee Merkel, LASP, USA,WACCM-PMC simulations of long-term trends of PMC

1520-1530 Q&A, and discussion

1530-1600 Coffee Break 1600-1620 David Siskind, NRL, USA,The PMC region as an integrator of coupling processes: Implications for trend studies from AIM and other missions 1620-1630 Q&A, and discussion 1630-1650 Stan Solomon, NCAR, USA,Thermospheric Temperature Trends: Modeling and Observations 1650-1700 Q&A, and discussion 1700-1720 Dan Marsh, NCAR, USA,Climate change in the mesosphere from 1850 to 2100 in CESM-WACCM 1720-1730 Q&A, and discussion 1730-1750 Kota Okamoto, The University of Tokyo, Japan, On the dynamical responses in the middle atmosphere to ozone recovery and CO2 increase 1750-1800 Q&A, and discussion 1800 Adjourn 1900-2100 Group Dinner (TBD)

AM Friday, Friday, May 4:

Inter-annual and seasonal variability: Observations=

0830-0850 James Russell III, Hampton Univ, USA, AIM science results and their significance for PMC long-term change studies 0850-0900 Q&A, and discussion 0900-0920 Cora Randall, LASP, USA, AIM/CIPS Observations of PMC Variability 0920-0930 Q&A, and discussion 0930-0945 Rachel Ward, Utah State University, USA, Comparison of Northern and Southern Hemisphere Mesospheric Gravity Waves using CIPS PMC Data 0945-0950 Q&A, and discussion

Inter-annual and seasonal variability: Modeling

0950-1005 Susanne Benze, LASP, USA, On the onset of polar mesospheric cloud seasons 1005-1010 Q&A, and discussion 1010-1025 Jia Yue, NCAR, USA ,Fast meridional transport in the lower thermosphere by planetary-scale waves 1025-1030 Q&A, and discussion 1030-1050 Coffee Break

Properties of PMC and their Environment: Observations

1050-1105 E.J. Llewellyn, University of Saskatchewan, Canada, Special Observational Opportunities Offered by PMCs - Nadir Observations in the Limb 1105-1110 Q&A, and discussion 1110-1125 Richard Goldberg, GSFC, USA, Study of an IR limb emission anomaly observed by SABER/TIMED in the mesosphere-lower thermosphere (MLT) region 1125-1130 Q&A, and discussion 1130-1145 Scott Robertson, U of Colorado, USA, Detection of Meteoric Dust in Mesosphere by the CHAMPS Rockets 1145-1150 Q&A, and discussion 1150-1205 Kristell Pérot, LATMOS Laboratoire ATmosphères, France, PMC Particle Size Retrieval from GOMOS / ENVISAT Observations 1205-1210 Q&A and discussion 1210-1310 Lunch

PM Friday, Friday, May 4:

Properties of PMC and their Environment: Modeling

1310-1330 Charles Bardeen, NCAR, USA, Simulations of PMC during the AIM time period using WACCM/CARMA 1330-1340 Q&A, and discussion 1340-1355 Mark Hervig, GATS, Inc., USA, The Smoke Content of Ice: Interpretation of SOFIE Results 1355-1400 Q&A, and discussion 1400-1415 Mark Hervig, GATS, Inc., USA, The 0D Model of PMC: Comparisons with AIM SOFIE data 1415-1425 Q&A, and discussion 1425-1530 Panel and group discussion 1530-1600 Concluding remarks & summary from the project co-chairs 1600 Adjourn

Local Organizing Committee: Aimee.Merkel@lasp.colorado.edu,Dan Marsh (marsh@ucar.edu) and Charles Bardeen (bardeenc@ucar.edu)

NOTE: the Particle Size Workshop,originally scheduled for May 2, has been postponed to a later date.

SPECIAL JOURNAL ISSUE ON UPPER ATMOSPHERIC TRENDS NOW AVAILABLE. The joint JGR/Space, JGR/Atmospheres special section on upper atmospheric trends is now complete, and can be accessed at http://www.agu.org/journals/ja/special_sections.shtml?collectionCode=UATREND1&journalCode=JA

Observing Facilities

Aeronomy of Ice in the Mesosphere (AIM), a NASA satellite mission (2007-)

AIM was launched from Vandenberg Air Force Base on April 25, 2007 becoming the first satellite mission dedicated to the study of Polar Mesospheric Clouds (PMCs). A Pegasus XL rocket placed the AIM satellite into a near circular (601 km apogee, 595 km perigee), 12:00 AM/PM sun-synchronous orbit. By measuring PMCs and the thermal, chemical and dynamical environment in which they form, AIM will quantify the connection between these clouds and the meteorology of the polar mesosphere. In the end, this will provide the basis for study of long-term variability in mesospheric climate and its relationship to global change. The results of AIM will be a rigorous test and validation of predictive models that then can reliably use past PMC changes and current data to assess trends as indicators of global change. This goal is being achieved by measuring PMC densities, spatial distribution, particle size distributions, gravity wave activity, meteoric smoke influx to the atmosphere and vertical profiles of temperature, H2O, O3, CH4, NO, and CO2.

The overall goal of AIM is to resolve why PMCs form and why they vary. It has been suggested that the observed changes in the clouds are related to increased concentrations of greenhouse gases. This suggestion is plausible because an increase in carbon dioxide, while warming the surface of the Earth, cools the upper atmosphere which can facilitate mesospheric cloud formation there. Additionally, increases in methane at the surface of the Earth lead to increases in water vapor at high altitudes through chemical oxidation processes, which further facilitates cloud formation and growth. While plausible, this greenhouse gas hypothesis has not yet been proven.The AIM webpage is at [2]

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Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR)

Fig. 2:Year-to-year variability of seasonal mean NLC occurrence and altitude for two different cloud classes. The blue curves contain all measurements having a sensitivity above the long-term detection limit, whereas the green curves show results for strong clouds only. The vertical bars indicate 95% confidence limits for the occurrence and errors of the mean altitudes. For more information see text and references.

The Rayleigh/Mie/Raman (RMR) lidar at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) is located on top of a ≈400 m high mountain in Northern Norway (69.3°N, 16.0°E). It was installed in 1994 and designed for multi-parameter investigations of the Arctic middle atmosphere on a climatological basis. Because of its Arctic location, the lidar is optimized for measurements during full sunlight. Appropriate technical solutions, laser wavelength stabilization in combination with strong spectral as well as spatial filtering at the receiving system, have been implemented. The lidar is a complex twin-system consisting of two power lasers, two receiving telescopes, and one optical bench for spectral separation and filtering of the light received from the atmosphere. The lasers emit pulses at three wavelengths (355 nm, 532 nm, 1064 nm) simultaneously with an overall peak power of 150 MW per laser. The backscattered light from the atmosphere is collected by telescopes with a diameter of 1.8 m each. They can be tilted up to 30° off-zenith to allow different viewing directions (Fig. 1), which separates the sounding volume up to 90 km at an altitude of 80 km. After spectral and intensity separation the light is analyzed by 15 channels and recorded by single photon counting detectors. One main objective of the RMR-lidar is the observation of ice layers in the mesopause region, which are known as noctilucent clouds (NLC) and polar mesospheric clouds (PMC). Throughout the NLC season (1 June to 15 August) the lidar is operational for 24 hours per day to measure whenever permitted by the local weather conditions. This yielded a total of more than 4000 measurement hours from 1997 to 2009. NLC were observed during ≈1700 hours which is the largest NLC data base acquired by lidar. The data are used to investigate decadal scale changes of NLC parameters, size and number density of the ice particles, as well as small scale structures which are often observed in the cloud layers. Fig. 2 shows the time series of NLC occurrence and altitude above ALOMAR covering one solar cycle (taken from reference 2). During these 11 years there is no statistical significant anti-correlation between cloud occurrence and solar activity, which is partly in contrast to other data sets. The mean cloud altitude is 83.2 km and appears to remain nearly unchanged since the first NLC observations more than 100 years ago.

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Visual Observations of Noctilucent Clouds

Scientific interest in noctilucent clouds (NLC) can be traced back to 1884 when many observers watched the twilight sky to see the dramatic sunsets caused by dust from Krakatoa, which had erupted during the previous year. Captivating displays of 'night shining' clouds, were seen and quickly recognized as lying at much higher altitude than normal clouds. For much of the 20th century, visual and photographic observations were the only methods available for systematic monitoring of NLC characteristics. By the early 1960's it was clear that their occurrence rates varied enormously from year to year. Widespread, intense displays of NLC were seen for a few consecutive summers, with NLC reported from somewhere around the 50 - 60 N latitude band almost every night over the summer season. These periods were followed by years when almost no NLC at all were seen. The most notable 'high spots' for NLC were the years 1885-1890 and 1963-1968, which were each followed by strong declines in NLC reports even though the same observers as during the 'hot spots' continued to look for them (reference 1).

Starting with the International Geophysical Year in 1958, professionally organized observing networks started to gather systematic records of visual sightings of noctilucent clouds (see references). Even though professional involvement has generally ended, or become sporadic, systematic NLC observations are still collected by networks of enthusiasts, in Europe, Russia, Canada and North America. Since 1996, visual observations by members of the public are collected at a number of regional or national centers [3], and since 1996 at the 'Noctilucent Cloud Observers Homepage' [ http://www.kersland.plus.com/]. NLC observations from the UK and Denmark since 1964 form the longest continuous record (observing latitudes from 51 - 61 N). These show how NLC are most common in years of low solar activity, and rare in years of high solar activity. They do not show any significant increase over the last 45 years although a few percent increase (or decrease) cannot be ruled out (Fig. 2, reference 3).

Although monitoring of large-scale structures and possible trends in NLC is being taken over by satellite measurements, visual and photographic observations still have an important role to play. For example, they are the best method available for studying fine-scale structure, on scales of 10s of km or less, and they can be instrumental in identifying NLC at unusually low latitudes where satellites observations and ground-based remote sensing instruments are not available.

1. Fogle,B.and B.Haurwitz,Long term variations in noctilucent cloud activity and their possible cause, in Climatological Research,edited by K.Fraedrich, M.Hantel,H.Claussen Korff,and E.Ruprecht,pp. 263–276,Heft 7,Bonner Meteorologische Abhandlungen.,Bonn,Germany,1974.
2. Romejko, V.A., P.A. Dalin, N.N. Pertsev, Forty years of Noctilucent Clouds observations near Moscow: database and simple statistics, J. Geophys. Res., 108, D8, 8443, doi: 10.1029/2002JD002364, 2003.
3. Kirkwood, S., P. Dalin, A. Rechou, Noctilucent clouds observed from the UK and Denmark – trends and variations over 43 years, Annales Geophysicae, 26, 1243-1254, 2008.

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Additional notes about NLC long-term behavior according to visual observations

1. There is no distinct contradiction when comparing a trend in the NLC occurrence frequency from ground-based observations and a trend in the PMC satellite observations for lower latitudes (50-64°N), if we take the same years for an analysis. The difference between the NLC and PMC trend is rather in their significance probability. However, if we consider ground-based NLC observations for more than 40 years (which are longer than space-borne measurements) we arrive at almost zero trend in the NLC occurrence frequency and at very small positive trend in the NLC brightness which have no statistical significance.

2. NLC most frequently occur in 1-2 years after the sunspot minimum and this delay is statistically significant. The PMC observations show a smaller delay of 0.5±0.5 year.

3. Concerning the year of the NLC discovery, we should note the following. The majority of papers, devoted to first observations of noctilucent clouds, refer not to 1884 but to 1885 as a year of first reliable descriptions undoubtedly concerning noctilucent clouds. Leslie (1884) did describe some sky phenomenon, resembling noctilucent clouds, but in his successive papers, Leslie (1885) and (1886), devoted to luminous clouds as a new phenomenon, he wrote nothing about priority of that observation of 1884; moreover, he did not refer to it at all. So, the description of Leslie's observation of 1884 cannot be regarded as reliable. Summarizing, Gadsden and Schröder (1989) wrote in their canonical book: “…It is certain that at the times of coloured twilight appearances of 1883/1884, no noctilucent clouds were discovered. Various reports also exist which could be interpreted as noctilucent clouds, but this will always remain uncertain (Pernter 1889; Schröder 1975; Gadsden 1985).”.

References:

1. Dalin, P., S. Kirkwood, H. Andersen, O. Hansen, N. Pertsev, V. Romejko, Comparison of long-term Moscow and Danish NLC observations: statistical results, Annales Geophysicae, 24, 2841-2849, 2006.
2. Leslie, R., 1884. The sky-glows. Nature 30, 583.
3. Leslie, R., 1885. Sky glows. Nature 32, 245.
4. Leslie, R., 1886. Luminous clouds. Nature 34, 264.

P. Dalin, N. Pertsev