1. Water Vapour & Cloud from Satellite and the Earth's Radiation Balance
- Chris Nankervis
University of Edinburgh

2. Water Vapour as a Greenhouse Gas:
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 Feedback
Higher 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
6.7m

3. MODELLED RADIANCE vs TRUE RADIANCE
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
AURA
Limb Sounding
Scan edge of atmosphere along the satellite track
Nadir
Instrument views directly downward

4. A-Train (Aqua & Aura measurements)
Instruments onboard..
Aqua Satellite
MODIS. 16 IR bands
AIRS IR bands
- CERES. ENTIRE IR
Aura Satellite
-MLS profiles. Atmospheric absorber concentrations.
Water & Cloud

5. NASA A-Train (Earth Observing System)

6. MODIS: Radiance data, cloud/land mask ‘Cloud Mask’ & ‘Land Mask’
METHODOLOGY:
COLLOCATION
MLS – CERES
CERES – MODIS
CERES – AIRS
Closest Match of Measurement points between satellites on A-Train
(Same time, Same Location)
DOWNLOAD
AIRS: Radiance data
DOWNLOAD
MODIS: Radiance data, cloud/land mask
APPLY
‘Cloud Mask’ & ‘Land Mask’
CORRELATION
OLR vs Water and Cloud
MODELLING
Water vapour and cloud

7. 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)

8. Satellite vs Model Radiances:
MLS WV Profile
Mean WV Profile
MODIS (K)
MODIS (K)
MODEL (K)
MODEL (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)