NOCTILUCENT CLOUDSObservations, Frequency & Linkage to Climate Change

Jenell Walsh-ThomasCSI 655 Atmospheric PhysicsMonday, May 16, 2011

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Noctilucent Clouds taken from 36,000 feet above the South of Nunivak Island, Alaska on June 20, 2006.Image credit: John Boardman. http://www.nasa.gov/mission_pages/aim/multimedia/nocticulant_clouds.html

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Presentation Outline Introduction What are noctilucent clouds? Observations

Location, formation & propertiesObservation techniquesOccurrence & observation frequency

Linkage to Climate Change Conclusion

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Introduction Purpose: develop an understanding of

noctilucent clouds (NLC) First reported June 8, 1885

Timeframe when industrial revolution began…coincidence?

Increased frequency of occurrence/observationIndicator of global change in the middle

atmosphere?Linkage to climatic changes?

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What are noctilucent clouds? Clouds that form at

very high altitudes: 82 km

Primarily composed of microscopic ice crystals

Seen at twilight & glow a white to bluish color

NLC vs. (high) Cirrus clouds

Peak observations around summer solstice

NLCs/PMCs; “night shining clouds”

Atmospheric historical importance

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Formation & Properties Formation conditions

Atmospheric environment: extremely low T at mesopause, very low P, high amount of water vapor, presence of condensation nuclei

Time of day: twilight Size & shape of particles

Average radius 150 nm Actual shape speculated to be non-

spherical, possibly needle or cubic Cloud shape & form: veil, bands,

waves & whirls Bluish-white in color with

occasional red edge

Summary of average properties of NLCs (Kokhanovsky 2005).

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Observation Techniques Ground-based (NLCs)

Direct & photographic Space-based (PMCs)

2 important first observationsConclusions from space:

○ Asymmetrical in N & S hemispheres○ S hemisphere contain smaller particles

AIM

Geometry of observing NLCs (Avaste 1993).

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Occurrence & Observation Frequency Typical occurrence:

around the summer solstice at twilight

First observed in 1885

Provide insight into conditions of mesosphere

Asymmetrical observation numbers between N & S hemisphere

Determining frequency

Increase in frequency & brightness

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Linkage to Climate Change Two major greenhouse gases: CO2 & CH4

CO2: causes cooling CH4: dissociation occurs, adds H2O vapor Also influencing increase in brightness Indicators of change in upper atmosphere

indicator of future change in lower layers & at mid-latitudes?

July 2009 mid-latitude outbreak event

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Conclusion NLC/PMC rare & exquisite site for

observers Much more to be learned about their

properties, formation & possible foreshadowing for future changes

Limited time & geographic range Both ground & space-based improving

AIM satellite mission Standard need for observations

QUESTIONS? COMMENTS?

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Noctilucent cloud (NLC), polar mesospheric cloud (PMC) or "night-shining" cloud (Russell et al 2009). (Photographed by Tom Eklund, July 28, 2001, Valkeakoski, Finland.)