Water Vapour & Cloud from Water Vapour & Cloud from Satellite Satellite

and the Earth's Radiation Balanceand the Earth's Radiation Balance- Chris Nankervis- Chris Nankervis

University of EdinburghUniversity of Edinburgh

• Water vapour is the most important greenhouse gas- high concentration- broad absorption range and far infra-red rotational spectra.

• Absorbs and emits IR radiation upon changes in its vibrational state + H2O rotational state changes in the far-infrared• The saturated water vapour pressure in the atmosphere increases

exponentially with the temperature.

Overall +’ve FeedbackHigher Temperatures => more water vapour => reduced OLR

Effect of water vapour may be viewed on GOES satellite imagery- White bands represent colder (higher) emitting temperatures- Less outgoing radiation is emitted to space in these regions

Water Vapour as a Greenhouse Gas:

6.7m

What Satellite Instrument?- Microwave Limb Sounder (MLS), onboard Aura satellite

• Scans the edge of the atmosphere in the forward direction (above 2km altitude)• Detects passive radiation emitted natural from atmospheric gases.• Very good vertical resolution of 1.5km layers• Highly sensitivity to trace gases at high altitude (<0.1ppm)• It makes indirect measurements of water vapour and cloud•Aura observes at the same point above the Earth’s surface as Aqua (7 minutes later) •AIM: Using data from the precise orbital path as an input into radiation models, can the true outgoing radiation observed by Aqua instruments be matched.

MODELLED RADIANCE vs TRUE RADIANCE

Limb SoundingScan edge of atmosphere along the satellite track

NadirInstrument views directly downward

AURA

A-Train (Aqua & Aura measurements)

Instruments onboard..

Aqua Satellite

- MODIS. 16 IR bands

- AIRS. 2378 IR bands

- CERES. ENTIRE IR

Aura Satellite

-MLS. 3495 profiles.

Atmospheric absorber

concentrations.

Water & Cloud

NASA A-Train (Earth Observing System)

APPLY‘Cloud Mask’ & ‘Land Mask’

CORRELATIONOLR vs Water and Cloud

MODELLINGWater vapour and cloud

COLLOCATIONMLS – CERES

CERES – MODISCERES – AIRS

Closest Match of Measurement points between satellites on A-Train

(Same time, Same Location)

DOWNLOADMODIS: Radiance data,

cloud/land mask

DOWNLOADAIRS: Radiance data

METHODOLOGY:

Fast Radiation Models

• These models solve a radiative transfer equation in order to calculate the radiation leaving the atmosphere at set frequency bands. This gives an emitting temperature (BBT) or radiance.

• Model input comprises of 7 absorbers: H2O, CO2, O3, N2O,

CO, CH4, O2

• Temperature profile is also required

Optional inputs:•SST as an from MODIS (Aqua)•RTTOV (model) allows a cloud input, (MLS Ice Water Product)

Mean WV Profile

MO

DIS

(K

)MODEL (K)

MLS WV Profile

MODEL (K)

MO

DIS

(K

)

• Emitting temperature in Kelvin (related to radiance)• Lower temperatures = more absorbance by atmospheric gases

(more opaque)Better ‘fit’ between observed and modelled radiances for MLS water

vapour input, than ‘mean’ water vapour input, but far from perfect?

However … MODIS band sensitive to lower atmosphere. Here MLS retrieval is largely based on climatology.

Solution …Look in far-infrared (Pure rotational bands)

Satellite vs Model Radiances: