Abstract
Abstract. The influence of gravity waves on noctilucent clouds (NLC) at ALOMAR (69° N) is analysed by relating gravity wave activity to NLC occurrence from common-volume measurements. Gravity wave kinetic energies are derived from MF-radar wind data and filtered into different period ranges by wavelet transformation. From the dataset covering the years 1999–2011, a direct correlation between gravity wave kinetic energy and NLC occurrence is not found, i.e., NLC appear independently of the simultaneously measured gravity wave kinetic energy. In addition, gravity wave activity is divided into weak and strong activity as compared to a 13 yr mean. The NLC occurrence rates during strong and weak activity are calculated separately for a given wave period and compared to each other. Again, for the full dataset no dependence of NLC occurrence on relative gravity wave activity is found. However, concentrating on 12 h of NLC detections during 2008, we do find an NLC-amplification with strong long-period gravity wave occurrence. Our analysis hence confirms previous findings that in general NLC at ALOMAR are not predominantly driven by gravity waves while exceptions to this rule are at least possible.
Highlights
Noctilucent clouds (NLC), named polar mesospheric clouds (PMC) when observed from space, are a remarkable feature of the polar summer mesopause
The strongest noctilucent clouds (NLC) occurred during times of lower gravity wave kinetic energy, whereas mostly weak NLC occurred during times of higher kinetic energy
In the higher energy range the NLC occurrence rates are based on datasets containing a factor of 100 less data points than in the maximum, so that single data points have a much greater influence
Summary
Noctilucent clouds (NLC), named polar mesospheric clouds (PMC) when observed from space, are a remarkable feature of the polar summer mesopause. These clouds, first documented by Leslie (1885), Jesse (1885) and Backhouse (1885), are the direct indication of extremely low temperatures enabling ice particle growth to visible sizes. Long-term trends have been identified in the brightness of polar mesospheric clouds (DeLand et al, 2007) as well as their occurrence frequency (Shettle et al, 2009). On the shorter end of scales, namely minutes to days, NLC are highly influenced by dynamical features of the mesopause.
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