Abstract
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.
Highlights
Noctilucent clouds (NLC) are the highest altitude clouds in the Earth’s atmosphere
The time series of number of noctilucent cloud (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
It should be noted that the “official” NLC observing period for the meteorological stations extended from day 146 to day 219 from the start in 1964, and this on its own is sufficient to detect most of the differences between the first two decades
Summary
Noctilucent clouds (NLC) are the highest altitude clouds in the Earth’s atmosphere. They lie at ∼82–85 km heights and are seen at high latitudes (mainly above 50◦) during summer. The uncertainties in the trend analysis from the latter measurements are, on the other hand large, due to the small number of observations available from just two locations, the latter varying between 5 and 20 nights per year from Moscow, and 3–14 from Denmark It is clear from the historical record that NLC occurrence rates have varied by large factors from year to year, during the late 19th and early 20th centuries, long before anthropogenic inputs are likely to have had any effect (Fogle and Haurwitz, 1966). NLC observations by observers at typically 30–40 different locations, covering the UK and Denmark, have been made and collected systematically for, so far, 43 summer seasons since 1964 This is a reasonably large, but still welldefined and restricted geographical region (confined in both latitude and longitude, with constant limits over time). This is less apparent in the core period, and not noticeable at all in the length of the season
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