A proposed replacement classification - type 'K' - for simultaenously-occurring, associated Type 2b and 3anoctilucent cloud.

John Rowlands FRAS

July 2015

Visual, ground-based observations of noctilucent cloud (NLC) are reported to a standard classification system that appears to date from the late 1980s to early 1990s.

Whilst superficially helpful in describing in words what any given NLC looks like to the ground-based observer, a long period of observing shows that those 'types' that may have been considered separate and somehow arising from different processes might, in some cases, be related, usually by gravity wave dynamics.

In essence, an NLC display is a layer composed of extremely small, nanometer-sized charged ice particles that reflects sunlight from around 82km above the surface. The layer does not remain static, but is modulated by gravity waves that propagate from below. It is these gravity waves that lead to the ethereal appearance of NLC displays which have, especially in the past, often been compared to the outstretched wings of angels, harp strings, ghosts, and so on. Drawing on first-hand accounts, the present author has previously advanced a considered argument that the Angel of Mons story from WW1 is attributable to a display of NLC over northern France.

Simply speaking, most NLC forms are the result of gravity wave disturbances of a basic ice layer.

This discussion poses the question: are the currently separate, type 2b and 3a NLC forms better considered and recorded as a single NLC form instead?

Type 2b (long, straight streaks) and 3a (shorter, ripple-like structure) as conventionally defined.

According to Gadsden and Parviainen1, a type 2b form is essentially a 'bright streak with sharply defined edges.' A type 3a NLC consists of short 'billows', which have a ripple-in-sand appearance. The term 'billow' is somewhat nebulous. Critically, the same authors provide an image to illustrate what they considered to be these two NLC forms - given as 'Photograph 5' from 1983 in their NLC observing guide.

In presenting photograph 5, the authors curiously failed to identify a much earlier, 1993 paper by Fritts, et al2, for which Parvianien had in fact provided an image of types 2b and 3a, (later cited by Fritts in a 2003 review of gravity wave dynamics3), where simulations successfully recreated a structure consisting of co-occurring types 2b and 3a.

In looking again at the evidence, it is clear that, rather than being two 'types' arising independently of one another, the NLC structures presented by Gadsden and Parvianien as separate, types 2b and 3a are better described and explained by Fritts' work as instead being simply two components of a single structure an ice layer modulated by a gravity wave exhibiting streamwise-aligned instability structures. Such structures are not limited to mesospheric NLC, being regularly observed in tropospheric cirrus cloud.

As a result, I propose that types 2b and 3a, where these occur in structural association, ought to be abandoned as a description of the NLC form, and instead assigned type 'K'. In doing so, the classification system might be rendered simpler, whilst bringing to an end an artificial and incorrect separation of a single,coherent structure (type 'K') into two, independent structures (hitherto known as types 2b and 3a.)

Curiously, when one looks at Gadsden and Parvianen's previous, photograph 4, they assign an entirely different, type 3b, to what they term 'diffuse billows', where these are, in their image, simply nascent, disturbed or decaying type 3a (as they would have it), which ought now to be considered as part of a single, unstable gravity wave structure - type 'K'.

A criticism of type 'K' might ask: do types 2b or 3a occur without the other form being present? Type 2b certainly do occur without the associated 3a instabilities, and so must be retained as a classification type. But, it is usually the case that even superficially isolated 2b show signs of developing or decaying 3a structure. The use of type 'K' where 2b and 3a occur together as a coherent structure appears to improve rather than reduce the information conveyed about the display.

A type 2b 'streak with sharply-defined edges' NLC structure. Even here, close examination shows the presence ofdeveloping 3a structure.

Type 3a can appear to form independently of type 2b. But there is more uncertainty as to whether this is simply a product of the viewing geometry; most 3a appearing to exist on their own in a photographic database spanning more than ten years do so when at high elevation angles and when the wavefronts are normal to the observer (i.e. wavefronts approaching or, unusually, receding directly to/away from the observer.)

Type 3b waves ('billows'), appearing to occur in the absence of any 2b type. Type 'K' would not apply in this case, theoriginal classification being retained. The lack of type 2b might be a product of viewing geometry or, in this case, loss

of detail due to a bright sky.

Type 3b, apparently without 2b form. Closer consideration reveals the existence of linear structures normal to thewavefronts, suggesting that a different viewing geometry might well give rise to 2b alongside (and thus a 'K'

classification.)

Type 3a waves or billows do not appear to exist as a separate NLC form if we accept that they are simply instabilities in type 2b waves forms. Type 3b remains a useful descriptor where there is no apparent association with type 2b streaks/waves (which would otherwise render them a single, type 'K' form.)

Further consideration of NLC types may yield further simplification, or indeed call into question the fundamental usefulness of the entire classification system.

References:

Gadsden, and Parvianien, Observing Noctilucent Cloud, The International Association of Geomagnetism andAeronomy, 2006: http://www.iugg.org/IAGA/iaga_pages/pdf/ONC_Sep06.pdf accessed 2015 July 06.

Fritts, D.C., J.R. Isler, G.E. Thomas and O Andreassen, Wave breaking signatures in noctilucent clouds, Geophys. Res. Lett., 20: 2039-2042, 1993b.

Fritts, David C. and M.J. Alexander, Gravity Wave Dynamics and Effects in the Middle Atmosphere. Reviews of Geophysics 41: 1/1003 2003.