1. Climate and the Global Water Cycle Using Satellite Data
John Bates, Chief
Remote Sensing Applications Division
National Climatic Data Center
NOAA NESDIS

2. Outline What is Climate? The Global Water Cycle
Remote Sensing of Atmospheric Water Variables
Scales of Water Cycle Variability
Climate Feedbacks
Conclusions

3. 1. What is Climate?
WMO Climate Defined as 30-year average on 10 year boundaries
‘Climate is what you expect, weather is what you get’
Climate scales of variability include:
Intraseasonal
Seasonal
Interannual
Decadal

4. 2. The Water Cycle

5. 3. Remote Sensing of Atmospheric Water
Water Vapor
Clouds
Precipitation

6. Atmospheric Water Vapor
The most important greenhouse gas
Lower tropospheric water vapor – flux is responsible for precipitation; strongly interacts with aerosol particles; strongly interacts with stratus clouds
Upper tropospheric water vapor – feedback may significantly increase warming; strongly interacts with cirrus clouds
Lower stratospheric water vapor – large chemical and radiative impacts

7. Satellite Remote Sensing of Water Vapor
One of many water vapor absorption bands are sensed in the visible, infrared, or microwave portion of the spectrum
Looking down from space, the center of the bands are opaque – hence we can sense a layer of water vapor high in the atmosphere
Conversely, low absorption at the edge, or wing, of a band allows us to sense a layer lower in the atmosphere

8. Upper Troposphere Water Vapor Image from GOES Shows Circulation Features

9. Satellite Remote Sensing of Clouds
In the visible portion of the spectrum, clouds reflect sunlight. The amount reflected is proportional to the optical depth of the cloud
In the infrared portion of the spectrum, clouds emit at the cloud top temperature. This is usually colder than the surface.
In the microwave portion of the spectrum, liquid clouds emit relative the background and ice clouds scatter

10. Remote Sensing of Precipitation
Using infrared data, cold (high) cloud tops are used in the tropics and summer hemisphere as a proxy for instantaneous rain rate
Microwave data penetrate the clouds to observe the emission from raindrops and, hence, are a more direct measure of rain rate

11. 4. Scales of Water Cycle Variability
Diurnal – the daily cycle
Intraseasonal – 10 days to 6 weeks
Seasonal – the four seasons
Interannual – Year-to-year variability usually associated with extremes in temperature and precipitation (e.g., El Niño)
Decadal – Multiyear variability of extremes in temperature and precipitation

12. Diurnal Changes in Visible Cloud Cover

13. Intraseasonal Variability in Tropical Deep Convection
Identify only the coldest infrared cloud tops in the tropics associated with thunderstorms
Reduce 3 dimensions to 2 by averaging latitudes from 7N-7S to obtain a time-space diagram
The resulting diagram shows tropical wave modes of characteristic speeds and directions: Madden-Julian oscillations (blue), Kelvin waves (green), and equatorial Rossby waves (black)

14. Seasonal Cycle of Upper Level Water Vapor and Clouds (Outgoing Longwave Radiation-OLR) Shows March of Monsoon-Desert System

15. Annual Mean of Global Precipitation
Highest values are found in the tropical monsoons, inter-tropical convergence zones, and mid-latitude storm tracks
Lowest values are found in the subtropical deserts

16. Interannual Variability – El Niño
El Niño is a coupled ocean-atmosphere phenomena
Sea surface temperatures warm in the central Pacific
Deep convection and precipitation moves from the western Pacific to the central Pacific
Upper level water vapor shows that the shift in convection is accompanied by a global teleconnection pattern

17. Interannual Variability – El Niño Impacts in North America
The shift of deep convection to the central Pacific (shaded region on equator) is accompanied by a wavetrain to the northeast
An alternating pattern of Hi’s and Low’s results
The polar jet stream (arrows) is amplified bringing warm air north into western North America and cold air into eastern North America
A strong subtropical jet stream brings moisture-laden storms into the southeastern U.S. and eastern seaboard

18. 5. Climate Feedbacks and Climate Change
In an attempt to separate the natural and anthropogenic effects of climate change, the concept of a feedback mechanism has arisen. For example, given the magnitude of a man-made greenhouse gas such as CO2, the relationship between the magnitude of this anthropogenic climate forcing and the magnitude of the climate change response in a general circulation model, perhaps a global warming of 2C, defines the climate sensitivity. Any process that changes the sensitivity of the climate response to the imposed anthropogenic forcing is called a feedback mechanism

19. Water Vapor Feedback
Assume (most models work this way) that the relative humidity remains constant as man-made greenhouse gasses initially warm the atmosphere and ocean
More moisture is evaporated from the surface and the total amount of water in the atmosphere increases
The increased water vapor traps more outgoing radiation thereby further increasing global temperatures
This is a huge effect – increasing the man-made warming by 2-3 times

20. Expected Decadal Scale Variations of Water Vapor Due to Anthropogenic Influences – Boundary Layer
Boundary layer water vapor responds to surface temperature with fixed relative humidity and thus follows Clausius-Clapeyron equation
There is good agreement between observations and models
Radiative effect is small, but effect on precipitation and circulation is uncertain
Model estimate increase of 1%/decade from

21. Upper Troposphere Water Vapor Feedback – Satellite Observations
Retrieve upper tropospheric humidity (UTH) or layer specific humidity (gm/km)
Issues with cloud and intercalibration of many instruments over time
Global trends small but significant regional trends related to circulation changes not simply thermodynamics

22. Cloud Feedbacks – Stratus Clouds
In the visible, stratus clouds are highly reflective of incoming solar radiation
In the infrared, stratus clouds are low level clouds that emit energy to space at nearly the same temperature as the surface
If stratus clouds increase, more incoming solar radiation is reflected back into space with no change in Earth emitted energy
The net result is a negative feedback or cooling

23. Why do extreme rain events increase in the models?