|
|
 |
|
|
Ann. Geophys., 26, 1243-1254, 2008
www.ann-geophys.net/26/1243/2008/
© European Geosciences Union 2008
Noctilucent clouds observed from the UK and Denmark – trends and variations over 43 years
S. Kirkwood1, P. Dalin1, and A. Réchou2
1Swedish Institute of Space Physics, Box 812, 98128 Kiruna, Sweden
2Laboratoire de l'Atmosphère et des Cyclones, Université de la Réunion – Faculté des Sciences et Technologies, BP 7151 – 15 avenue René Cassin – 97715 ST DENIS Messag Cedex 9, Ile de la Réunion, France
Abstract. The combined UK/Denmark record of noctilucent cloud (NLC) observations over
the period 1964–2006 is analysed. This data set is based on visual
observations by professional and voluntary observers, with around 40
observers each year contributing reports. Evidence is found for a
significantly longer NLC season, a greater frequency of bright NLC, and a
decreased sensitivity to 5-day planetary waves, from 1973–1982, compared to
the rest of the time interval. This coincides with a period when the length
of the summer season in the stratosphere was also longer (defined by zonal
winds at 60° N, 30 hPa). At NLC heights, lower mean temperatures, and/or
higher water vapour and/or smaller planetary wave amplitudes could explain
these results. The time series of number of NLC nights each year shows a
quasi-decadal variation with good anti-correlation with the 10.7 cm solar
flux, with a lag of 13–17 months. Using multi-parameter linear fitting, it
is found that the solar-cycle and the length of summer in the stratosphere
together can explain ~40% of the year-to-year variation in NLC
numbers. However, no statistically confidant long-term trend in moderate or
bright NLC is found. For NLC displays of moderate or greater intensity, the
multi-parameter fit gives a trend of ~0.08 nights (0.35%) per year
with a statistical probability of 28% that it is zero, or as high as 0.16
nights (0.7%) per year. There is a significant increasing trend in the
number of reports of faint or very faint NLC which is inconsistent with
other observations and may be due changes in observing practices.
Full Article in PDF (372 KB) Supplement (6 KB) |
|
|
