Noctilucent clouds observed from the ground: sensitivity to mesospheric parameters and long-term time series

Abstract

Long-term systematic observations of noctilucent clouds in the regions of Moscow (Russia), Vilnius (Lithuania), and La Ronge (Canada) are considered. Variables, describing the seasonal activity of noctilucent clouds, are discussed. It is shown that there are no statistically significant trends within time intervals of several recent decades. This result is compared to other known findings on trends in mesospheric clouds. Based on the data of the modern ground-based noctilucent cloud observing network in the northern hemisphere and simultaneous satellite data on mesospheric temperature and humidity, we estimate sensitivity of noctilucent clouds to the relative humidity of the upper mesosphere. Such an approach allows us to discuss possible changes of the upper-mesospheric relative humidity, which are consistent with a zero secular trend in noctilucent cloud activity.

Updated Long‐Term Trends in Mesopause Temperature, Airglow Emissions, and Noctilucent Clouds

“…We have demonstrated the presence of about zero and small positive long‐term trends in the corrected NLC number and brightness, respectively, for the period of 1968–2018 (the continuous time series for 51 years); both trends lack statistical significance, meaning that the trends may readily change at other time intervals. These results are consistent with previously established long‐term trends by ground‐based NLC observations at middle latitudes around the globe, which were found to be either about zero or slightly positive and statistically insignificant (Dalin et al, ; Dubietis et al, ; Kirkwood et al, ; Kirkwood & Stebel, ; Pertsev et al, ; Romejko et al, ; Zalcik et al, , ). Lübken and Berger () have also found about zero trend in the NLC occurrence rate and a small positive trend in the NLC brightness at the midlatitude of 54°N for 1961–1994 using LIMA model simulations.…”

“…At the second step, the following multiple regression analysis (MRA) was applied to all analyzed time series: where C 0 is the constant, C 1 and C 2 are the regression coefficients characterizing the linear long‐term trend (Value/year, V/yr) and solar activity term (Value/SFU, solar Ly‐α flux units, 1 SFU is 10 11 photons · s −1 · cm −2 ), t lag is the phase time lag between an atmospheric parameter and solar activity predetermined based on the cross‐correlation analysis, and F Lya is the Lyman α flux averaged over either the winter or summer season. The same MRA technique has been frequently utilized in geophysical data analysis (e.g., DeLand & Thomas, ; Dubietis et al, ; Kirkwood et al, ; Pertsev et al, ).…”

“…Based on this revised data, we have developed new estimations for the NLC occurrence number and NLC brightness. Traditionally, there have been used the relative NLC number (NLC occurrence probability on a clear/semiclear night) and relative NLC brightness (NLC brightness estimation on a clear/semiclear night; see Romejko et al, ; Pertsev et al, ). These estimations suffered from different numbers of observational nights for visual summer seasons (1962–2002) as well as did not take into account a seasonal variation of NLC average occurrence probability (averaged over all available summer seasons) on a clear/semiclear night.…”

“…Thus, investigations made from space have been reported to demonstrate increasing PMC brightness and frequency at subpolar and polar latitudes in both hemispheres over the past four decades (DeLand et al, , , ; DeLand & Thomas, ; Shettle et al, ). On the other hand, ground‐based observations show no statistically significant increases in either the NLC brightness or NLC occurrence frequency (Dalin et al, ; Dubietis et al, ; Kirkwood et al, ; Kirkwood & Stebel, ; Pertsev et al, ; Romejko et al, ; Zalcik et al, , ). This very important question should be further carefully studied because it may help understand the problem of how global change affects middle atmosphere long‐term changes.…”

On the Anthropogenic Impact on Long‐Term Evolution of Noctilucent Clouds

“…The observational database on NLC/PMC including microphysical properties, variations with latitude and longitude, solar cycle, and tides has improved significantly in recent decades (see, e.g., Baumgarten et al, 2012;Fiedler et al, 2009;Gerding et al, 2013;Russell et al, 2015). However, the unequivocal detection of trends in NLC is still challenged due to the impact of various processes varying mainly in the last decades, for example, stratospheric ozone mixing ratios or water vapor transport through the tropical tropopause into the stratosphere (Brinkop et al, 2016;Lübken et al, 2013;Pertsev et al, 2014;WMO, 2011). However, the unequivocal detection of trends in NLC is still challenged due to the impact of various processes varying mainly in the last decades, for example, stratospheric ozone mixing ratios or water vapor transport through the tropical tropopause into the stratosphere (Brinkop et al, 2016;Lübken et al, 2013;Pertsev et al, 2014;WMO, 2011).…”