1. Environmental Systems Science Centre, University of Reading
MRC, London, April 5th 2005
The atmospheric hydrological cycle and climate feedbacks: past, present and future
Richard Allan
Environmental Systems Science Centre, University of Reading

2. Climate Change
How the hydrological cycle responds to global warming is crucial to mankind (e.g. water supply, agriculture, severe weather, ecosystems)

3. Earth’s Radiation balance and climate
S~1367 Wm-2
Thermal IR/LW or Outgoing Longwave Radiation (OLR)
Solar/SW
(S/4)(1-Albedo) – OLR = 0 (at balance)
2xCO2  ~ 4 Wm-2 less OLR  heating
Feedback: response of system is crucial to magnitude and nature of climate change
Don’t care about dTs, BUT effect on hydrological cycle likely scales with magnitude of dTs
Need complex models to resolve myriad of regional interactions and feedbacks that determine response of the system, but…

4. Climate Sensitivity (K)
How will climate and global water cycle respond to increasing greenhouse gases?
IPCC(1990) 2xCO2 sensitivity dTs ~ 1.5 – 4.5 K
Stainforth et al.(2004) Nature
Need complex models of Earth-atmosphere system to resolve regional interactions between multiple feedback processes
Climate Sensitivity (K)

5. Aims and Strategy Aim Strategy:
to reduce uncertainty in climate prediction and climate impacts using EO data
to understand the likely response of the hydrological cycle to increased greenhouse gases
Strategy:
Monitor present-day and past changes in hydrological cycle
Identify physical models
Uncover previously unidentified processes

6. Previously unidentified process involving cloudiness and ocean heat storage?
An example of possibly unidentified process identified using EO and ocean data.
IPCC (2007) in preparation, provided by B.A. Wielicki

7. Decadal-scale passive microwave data: monitoring column integrated water vapour

8. Identify physical models
Note: Thermodynamic increase in WV  atmospheric LW radiative cooling  increase in global water cycle

9. water vapour and temperature
Increases in water vapour with temperature imply an enhanced atmospheric hydrological cycle
water vapour and temperature
atmospheric cooling
global water cycle

10. Concluding remarks Timeliness: why now? What is different?
EO-data beginning to resolve decadal time-scales
Apparent uncertainty in climate model predictions are becoming larger, not smaller!
What is different?
Encompass entire hydrological cycle – a 5-year challenge
Earth-system strategy: analysing interactions between global water cycle, radiative cooling, latent heating and atmospheric circulation using EO data
“Fast” and “Slow” feedbacks
Regional and regime-dependent interactions

11. Work plan
WP1 – Identification and intercomparison of datasets on hydrological cycle (year 1)
e.g. SSM/I, CERES, CMAP, daSilva, CLAUS…
Development of surface and atmospheric flux capability
WP2 – Spatial and temporal variability (years 1-2)
Spatial, daily, seasonal, interannual, ENSO, decadal
WP3 – Identification of regional and dynamical regime feedbacks + techniques (years 2-4)
WP4 – Identification of physical models (years 3-5)
Reduction in uncertainty of climate sensitivity

12. Links ESSC Meteorology Department – Shine, Hogan
Met Office, Hadley Centre
Griggs, Ringer
ESSC
Imperial College
Harries
GFDL -Ramaswamy
ECMWF -
Simmonds Morcrette
NASA Langley
Wielicki, Wong
Initial established links from ESSC as part of the project
University of Miami - Soden