1. BACKGROUND

Extremes have always been a concern of GHP. At its first meeting in 1995, one of GHP's overall scientific issues was given as:

What feedback mechanisms affect the water cycle and how do these influence wet and dry periods?

Although some work on extremes has been conducted within the various components of GHP, a collective effort has not been established. The strategy of GHP has been to first of all concentrate on addressing water and energy budgets which consider longer-term averages as opposed to variability issues.

In addition, the phase II objectives of GEWEX now include objectives linked with the acceleration of the water cycle, climate variability, prediction, and water resource application. Each of these has an extremes link.

At the 2002 GHP meeting, the need to consider extremes in a collective manner was raised again and an initial, short workshop was held in conjunction with the 2003 meeting. The agenda of the workshop is shown in Appendix 1 below.

The main objectives of the workshop were to review individual activities within the CSEs and to consider future, collective efforts on this issue within GHP.

2. EXTREMES RESEARCH

Several brief presentations were made on extremes in different CSE regions. Bill Cotton discussed process, modeling and prediction studies on the 1988 flooding and 1993 drought over the Mississippi. Jose Marengo discussed wet and dry patterns over Brazil. Ron Stewart discussed low discharge over the Mackenzie basin and drought over the Canadian Prairies. Alan Seed talked about droughts over Australia and Tido Semmier discussed predictions of extremes over the BALTEX region as well as showing some of the changes in extremes over the last few decades. Hugo Berbery discussed flooding events in La Plata basin, and Tetsuzo Yasunari discussed some of he flooding and droughts over Asia.

3. EXTREMES-RELATED ACTIVITIES

It was pointed out that extremes research is being carried out within other groups and issues dealing with extremes are addressed within other groups.

Some groups working on extremes include CLIVAR and IAHS. CLIVAR's main interest, however, is on longer time frames than of interest to GHP. IAHS is of course very interested in floods and droughts.

Other organizations are dealing with the effects of extremes. This is a major issue within WMO itself and a new effort on disasters is very relevant to extremes.

4. DISCUSSION

A general sense was that extremes related activities within GHP should focus on two overall issues:

• To better understand, model and predict extremes
• To document their possible change over the last few decades

A number of specific suggestions for activities were made by the GHP meeting participants. Briefly, these fell into the following general categories:

• Define the types and definitions of extremes of interest to GHP
• Establish a web site on extremes
• Examine the acceleration of the hydrologic cycle with a focus on extremes
• Extremes and variability is a natural follow-on to WEBS and its averaging activities
• Concentrate on the similarities and differences in the processes leading to extremes such as droughts in different regions
• Examine the connectedness of extremes over and between continents, perhaps with CEOP
• Extremes activities should be linked to water resources through WRAP

In a follow-up meeting of the ad hoc group, a draft definition of extremes of interest to GHP was proposed. These will be updated in the future.

extended wet period: prolonged, substantial precipitation for 24 h to several days that affects basin's on scales of at least 10⁵ km².
drought: follow definitions made by other groups

Other actions to be done include:

• Web site - establish a small web site that provides links to other sites listing extremes and hazards
• Diagnose and compare factors and processes linked with 'extremes' in different regions (using present information)
  large scale forcing
  regional and internal scales
• Move towards capabilities of various models to simulate extremes linked with transferability
• Initial study of possible changes in extremes
  using ERA-40 information applied over Europe
  possible future application to other regions

5. CONCLUSIONS

An ad hoc group has been established to pursue this issue. The group for now will "promote extremes-related efforts within GHP and GEWEX".

Members include Ron Stewart, Daniela Jacob, Jose Marengo, Bill Cotton, Tetsuzo Yasunari, Hugo Berbery, Jun Matsumoto, Steve Williams and Alan Seed.

This ad hoc group has agreed upon some modest initial activities and has put forward some additional suggestions. The group will meet informally over the next few months to solidify its actual activities.

The ad hoc group will consider formalizing itself into a working group in the future but only if there is a consensus that significant progress can be made

Wednesday, September 24, 2003-09-24

Phase II Science Issues

• Are the Earth's energy budget and water cycle changing?
• Can these changes be predicted up to seasonal and interannual timeframes?
• How do global energy and water cycle processes contribute to climate feedbacks and natural climate variability?
• What is the impact of the variability in the global energy budget and water cycle on water resources?