Transcript pmc_talk_ames - GATS West

Polar Mesospheric Clouds (PMCs) and Water Vapor
Mark Hervig
GATS Inc., Driggs, Idaho
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)
PMC Characteristics
PMCs occur:
-Poleward of 50 latitude, in both hemispheres
-During summer
-Near 83 km altitude
PMCs are composed of water ice
This was only recently confirmed
by HALOE [Hervig et al., 2001]
photo by Pekka Parviainen
PMCs (NLCs) are visible from the ground
Shortly after sunset, the observer
is in darkness, but the NLC is still
in sun light
NLC over Finland
photo by Pekka Parviainen
PMC measurements
In Situ
Rockets
Remote
Ground observers: visual, cameras
LIDAR
Satellite: solar source, limb, nadir
LIDAR
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
PMC patterns are changing
PMCs are
occurring more often [Gadsen, 1997]
occurring farther from the poles [Wickwar et al., 2002]
getting brighter [Klostermeyer, 2002]
Are these changes
telling us something?
NLC photo by Timo Leponiemi
Temporal Change
From Gaddsen, 1997
NLCs occur nearly twice as often as they did 35 year ago
Spatial Change
NLCs are occurring 700 miles
farther south than ever before
US Towns where NLCs were
recently sighted:
Twin Falls, Idaho
Logan, Utah
Boulder, Colorado
Glen Ullin, North Dakota
McGuire, New Jersey
Are changing PMC patterns related to climate?
photo by Pekka Parviainen
Increasing CO2 in the Atmosphere
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
Increasing Methane in
the Atmosphere
Some evidence suggests an
increase in mesospheric
H2O, but this is not yet clear.
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…
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
Some HALOE sampling issues
A synthetic PMC distribution 
The effects on trend analysis 
The seasonal cycle in mesospheric H2O
Vertical transport is one driver behind this change
Microwave radiometer at
ALOMAR, Norway 69N
HALOE, 83 km
65 - 80N
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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?
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)
Seasonal Cross Sections of HALOE Measurements
and Some Derived Quantities
Averages for 1992 – 2001, 65 – 80N
The observed H2O increase
compared to the PMC contribution
 H2O increase since PMC onset
Averages of HALOE measurements
during 1992 -2001 at 65 to 80N 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
Conclusions
Summer enhancement of mesospheric water vapor:
Upwelling contributes at altitudes from 50 to 88 km
PMC evaporation dominates from 83 to 88 km
Upwelling explains enhancement from 50 to 70 km
Enhancement at 70 to 82 km is a mystery, some possibilities:
measurement errors? Possible
molecular diffusion? No
eddy 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