Polar Mesospheric Clouds (PMCs) and Water Vapor
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Transcript of Polar Mesospheric Clouds (PMCs) and Water Vapor
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Polar Mesospheric Clouds (PMCs) and Water Vapor Mark Hervig
GATS Inc., Driggs, Idaho
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Overview
-Introduction to polar mesospheric clouds (PMCs)
-Connections between PMCs and atmospheric change
-HALOE measurements in the polar mesosphere
-The seasonal cycle in mesospheric water vapor, the role of PMCs
PMCs are historically known as noctilucent (or “night-shinning”) clouds (NLCs)
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PMC Characteristics
PMCs occur:
-Poleward of 50 latitude, in both hemispheres
-During summer
-Near 83 km altitude
photo by Pekka Parviainen
PMCs are composed of water ice
This was only recently confirmed by HALOE [Hervig et al., 2001]
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PMCs (NLCs) are visible from the ground
NLC over Finland
photo by Pekka Parviainen
Shortly after sunset, the observer is in darkness, but the NLC is still in sun light
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PMC measurementsIn Situ
Rockets
RemoteGround observers: visual, cameras
LIDARSatellite: solar source, limb, nadir
LIDAR
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A Connection Between PMCs and Atmospheric Change
• PMCs respond to atmospheric temperature and humidity
Temperature is affected by carbon dioxide (CO2)
Humidity is affected by methane (CH4):
• CO2 and CH4 are increasing due to human activities
• Increasing CO2: warms the troposphere, “greenhouse effect” cools the mesosphere
• Increasing CH4:
methane becomes water: CH4 + OH > CH3 + H2O
• PMCs are therefore a visible indication of climate change
“The minors canary” of climate change
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PMC patterns are changingPMCs are
occurring more often [Gadsen, 1997]
occurring farther from the poles [Wickwar et al., 2002]
getting brighter [Klostermeyer, 2002]
NLC photo by Timo Leponiemi
Are these changes telling us something?
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Temporal Change
NLCs occur nearly twice as often as they did 35 year ago
From Gaddsen, 1997
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Spatial Change
US Towns where NLCs were recently sighted:
Twin Falls, Idaho Logan, Utah Boulder, Colorado Glen Ullin, North Dakota McGuire, New Jersey
NLCs are occurring 700 miles farther south than ever before
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Are changing PMC patterns related to climate?
photo by Pekka Parviainen
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Increasing CO2 in the Atmosphere
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Temperature in the Mesosphere is decreasing
Temperatures near 80 km have cooled by almost 5 degrees Kelvin (K) every 10 years, since measurements were started in the 1950’s
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Increasing Methane in the Atmosphere
Some evidence suggests an increase in mesospheric H2O, but this is not yet clear.
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The Halogen Occultation Experiment (HALOE)
Measurements relevant to the polar mesosphere:
-Particle extinction at 6 wavelengths (2.45, 3.40, 3.46, 5.26, 6.26 m)
-Nitric oxide
-Temperature
-Water vapor
T & H2O are adversely affected by PMC signals
(we fixed that)
Coverage of polar summer in both hemispheres
12 years of measurements, and still going…
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PMC contamination was removed from HALOE
temperature and water vapor retrievals
Measured PMC extinction is extrapolated to the H2O and CO2 wavelengths using modeled PMC spectra, and then treated as an interfering absorber
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Some HALOE sampling issues
The effects on trend analysis
A synthetic PMC distribution
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The seasonal cycle in mesospheric H2O
HALOE, 83 km 65 - 80N
Microwave radiometer at ALOMAR, Norway 69N
Vertical transport is one driver behind this change
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The Seasonal Cycle in Mesospheric H2O A Relationship to PMCs ?
Upwelling cannot account for observed enhancements
Can PMC evaporation explain the seasonal increase in water vapor?
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Analysis of HALOE PMC measurements
PMC identification
compare modeled ice spectrum to HALOE measurements
Ice volume density (Vice) determined from HALOE extinctions ()
a fit to model calculations: Vice = A ()B
The equivalent gas phase H2O contained in PMCs
thermodynamics: H2O(ice) = Vice R* T ice / (P Mw)
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Seasonal Cross Sections of HALOE Measurements and Some Derived Quantities
Averages for 1992 – 2001, 65 – 80N
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The observed H2O increase compared to the PMC contribution
H2O increase since PMC onset
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Averages of HALOE measurements during 1992 -2001 at 65 to 80N latitude
Measured vs. modeled PMC volume
Early and late summer H2O
The seasonal H2O increase compared
to the H2O input from PMCs
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Seasonal Time Series
PMC volume density
water vapor
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Conclusions
Summer enhancement of mesospheric water vapor:
Upwelling contributes at altitudes from 50 to 88 kmPMC evaporation dominates from 83 to 88 kmUpwelling explains enhancement from 50 to 70 km
Enhancement at 70 to 82 km is a mystery, some possibilities:measurement errors? Possiblemolecular diffusion? Noeddy mixing? Inconsistent with secondary peak near 75 km
H2 + O on meteoric dust? Perhaps [Summers and Siskind, 1999]
PMC measurements vs. simulations:
support the growth/sedimentation theory also suggest the possibility of in situ PMC formation
By enhancing H2O, PMCs appear to be self-modifying