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 photo by Pekka Parviainen PMCs are composed of water ice This was only recently confirmed by HALOE [Hervig et al., 2001]

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

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 (CO 2 ) Humidity is affected by methane (CH 4 ): CO 2 and CH 4 are increasing due to human activities Increasing CO 2 : warms the troposphere, “greenhouse effect” cools the mesosphere Increasing CH 4 : methane becomes water: CH 4 + OH > CH 3 + H 2 O 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] NLC photo by Timo Leponiemi Are these changes telling us something?

Temporal Change NLCs occur nearly twice as often as they did 35 year ago From Gaddsen, 1997

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

Are changing PMC patterns related to climate? photo by Pekka Parviainen

Increasing CO 2 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 H 2 O, 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 & H 2 O 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 H 2 O and CO 2 wavelengths using modeled PMC spectra, and then treated as an interfering absorber

Some HALOE sampling issues The effects on trend analysis  A synthetic PMC distribution 

The seasonal cycle in mesospheric H 2 O HALOE, 83 km 65  - 80  N  Microwave radiometer at ALOMAR, Norway 69  N  Vertical transport is one driver behind this change

The Seasonal Cycle in Mesospheric H 2 O 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 (V ice ) determined from HALOE extinctions  ( ) a fit to model calculations: V ice = A  ( ) B The equivalent gas phase H 2 O contained in PMCs thermodynamics: H 2 O(ice) = V ice R* T  ice / (P M w )

Seasonal Cross Sections of HALOE Measurements and Some Derived Quantities Averages for 1992 – 2001, 65  – 80  N

The observed H 2 O increase compared to the PMC contribution  H 2 O increase since PMC onset

Averages of HALOE measurements during at 65  to 80  N latitude Measured vs. modeled PMC volume  Early and late summer H 2 O  The seasonal H 2 O increase compared to the H 2 O input from PMCs 

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 H 2 + 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 H 2 O, PMCs appear to be self-modifying