Characteristics of ripple structures revealed in OH airglow images

Abstract

Small‐scale ripple structures observed in OH airglow images are most likely induced by either dynamic instability due to large wind shear or convective instability due to superadiabatic lapse rate. Using the data set taken in the mesopause region with an OH all‐sky imager at Yucca Ridge Field Station, Colorado (40.7°N, 104.9°W), from September 2003 to December 2005, we study the characteristics and seasonal variations of ripple structures. By analyzing the simultaneous background wind and temperature observed by the nearby sodium temperature/wind lidar at Fort Collins, Colorado (40.6°N, 105°W), and a nearby medium‐frequency radar at Platteville, Colorado (40.2°N, 105.8°W), we are able to statistically study the possible relation between ripples and the background atmosphere conditions. Characteristics and seasonal variations of ripples are presented in detail in this study. The occurrence frequency of ripples exhibits clear seasonal variability, with peak in autumn. The occurrence of ripples shows a local time dependence, which is most likely associated with the solar tides. The lifetime and spatial scale of these ripples are typically 5–20 min and 5–10 km, respectively, and most of the ripples move preferentially either southward or northward. However, more than half of the observed ripples do not advect with background flow; they have higher Richardson numbers than those ripples that advect with background flow. It is possible that they are not instability features but wave structures that are hard to be distinguished from the real instability features.

Gravity wave instability structures and turbulence from more than one and a half years of OH* airglow imager observations in Slovenia

“…However, it has been reported that ripples can be rotated by the background wind and that ripples may even be created by a combination of both dynamical and convective instability (Fritts et al, 1996;Hecht, 2004). Li et al (2017) report that ripples are hard to distinguish from small-scale gravity waves. Height-resolved measurements of the horizontal wind would be needed to determine the local wind shear and make a profound statement about atmospheric instability.…”

“…Gravity wave instability can also be of convective nature when growing wave amplitudes lead to a superadiabatic lapse rate (Fritts & Alexander, 2003). In general, atmospheric instabilities like KHIs often manifest as so-called ripples -periodic structures with small spatial dimensions and short lifetimes (Peterson, 1979;Adams et al, 1988;Taylor & Hapgood, 1990;Li et al, 2017).…”

acp-2018-1012 Wust et al

“….". Li et al (2017) report that less than half of the ripples examined moved with the background wind, and were in fact real wave structures that are difficult to distinguish from real instability features.…”

“…A really useful reference on this point is the recent paper by Li et al (2017), in which the authors study statistically the relation between ripples and the background atmosphere. Some of the statements made in the current manuscript are not supported in the work by Li et al (2017), e.g., line 30 on page 11 states (referring to ripples) "They move with the background wind . .…”

“…The authors should read Li et al (2017) and revise the current manuscript in the light of the results presented there.…”