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 measurementsIn Situ

Rockets

RemoteGround observers: visual, cameras

LIDARSatellite: 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 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?

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 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

The effects on trend analysis

A synthetic PMC distribution

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

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 – 80N

The observed H2O increase compared to the PMC contribution

H2O increase since PMC onset

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

Seasonal Time Series

PMC volume density

water vapor

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