GAME Water and Energy Balance Study interim report

(25 June 2003, revised 18 July 2003)

Frontier Research System for Global Change
Yokohama 236-0001, Japan

Water and Energy Balance Study (WEBS) is an activity under GEWEX Hydrometeorological Panel (GHP) to integrate activities of Continental Scale Experiments (CSEs) of which GAME is a member. Since autumn of 2002, Dr. B.J. Sohn (Seoul National University, Korea) and I joined the GAME International Science Panel to promote WEBS.

The purpose of WEBS can be summarized as follows from my viewpoint augmented with ideas given by Dr. Sohn.
  1. We want to know the global energy and water cycle quantitatively. For that goal, we must use some global data sets. Most of them are not direct observations, but either compiled from satellite observations or produced by data assimilation with numerical weather prediction models. In order to have good global budgets, we should evaluate the global data sets with more direct observations. The comparison can be useful to both selection of global data sets to be used immediately, and to improvement of global data sets in the next generation.
  2. A CSE can reveal climatic and hydrological characteristics of that region much better than before. Then, it will be fruitful to compare CSEs in different climatic zones along the same axis of water and energy balance components, in order to build up knowledge about diversity and commonness of water and energy cycle over the globe.
  3. We recommend the synthesis of all available measured and numerically-derived values to understand water and energy balance over the monsoon region. This may be one way to utilize data collected during the GAME Phase 1 for seeking scientific outcomes. For example, we can calculate the continental-scale cloud-radiation interactions by combining the satellite observations such as ISCCP with GAME reanalysis products from which the evolution of radiation energy budget can be examined in conjunction with the monsoon progress.

WEBS activity in GAME has started about two years before that under the leadership of Dr. Toshio Koike (Univ. of Tokyo). We call this phase "GAME WEBS Phase 1" (and the present phase "Phase 2"). Status report in the early part of Phase 1 was made by Koike (2001). Since then, about fifteen researchers in Japan who study water and energy balance with GAME data met together and presented their results. I cannot mention all of them and just name a few of them.

GEWEX WEBS activity is chaired by Dr. John Roads (Scripps Institution of Oceanography, USA), who is now also the chairperson of GHP. Roads had played an important role in the synthesis of water and energy balance obtained during the GEWEX Continental-scale International Project (GCIP) conducted in the Mississippi river basin. He edited a CD-ROM for GCIP WEBS (Roads et al. 2002a), including water and energy balance products both global-scale and regional-scale, and both observational and model-derived. He participated in comparative studies for GCIP (e.g. Maurer et al. 2001). On the other hand, he also discussed water and energy balance of all CSE regions with global data only (Roads, 2002, Roads et al., 2002b).

A GEWEX WEBS workshop was held in New York in September 2002, in conjunction with GHP meeting. There, Roads suggested to all CSEs to do something like GCIP WEBS and then we should synthesize. It seemed me (and other participants) that we need a nearer target. At the end of that meeting, We agreed upon doing comparison between CSE data sets and some common set of global data. The concept was somewhat refined at the occasion of IUGG Assembly in Sapporo in July 2003. Though not officially claimed, I wish to call this part "GEWEX WEBS Initiative 1".

GEWEX WEBS Initiative 1 is a comparison between global data sets and CSE data sets with respect to water and energy balance, at time resolution of a month. The time frame is July 1996 - June 2001 (5 years), but periods of actual comparison depend on availability of both CSE and global data. The spatial resolution is not explicitly specified, but the effective resolution is limited by that of global data sets which are typically in 2.5 degree latitude/longitude grids. The principal budgets to be considered are those of water and energy in vertically integrated columns. For CSE data sets, not only direct observations but also assimilation with atmospheric and/or land models should be included.

I call it "GEWEX WEBS Initiative 1" because this is just the first short-term goal. It is obviously not very satisfactory.
So I hope for those who are interested in deeper studies to propose Initiatives 2, 3, ... not waiting for the end of Initiative 1. I will insist on the Initiative 1 until it produces some substantial results and also I would like for those who are interested in this initiative to participate.

Vertically integrated budget equations can be written as follows (though some are not exact):

  1. Mass balance of water on an area of land (preferably a river catchment)
    dS/dt = P - E - R
  2. Mass balance of water vapor in an atmospheric column
    dW/dt = E - P + C
  3. Energy balance at the surface
    G = Rnet,sfc - H - L E
  4. Total energy balance in an atmospheric column
    d(ed + L W)/dt = Rnet,top - G + (Cd + L C)
  5. "Dry" energy balance in an atmospheric column
    ded/dt = Rnet,top - Rnet,sfc + H + L P + Cd
where S is terrestrial water storage, W is water vapor content in the atmosphere, P is precipitation, E is evaporation from surface, R is runoff, C is convergence of horizontal transport of water vapor in the atmosphere, ) ed is "dry" energy content in the atmosphere, Rnet,top is net downward radiation at the top of the atmosphere, Rnet,sfc is net downward radiation at the surface, G is net downward energy flux at the surface (conduction below land surface), H is net upward sensible heat flux at the surface, Cd is convergence of horizontal transport of "dry" energy in the atmosphere, and L is latent heat of vaporization of unit mass of water.

The choice of global data set is based partly on the fact that this is a part of GEWEX, and partly from popularity among water and energy balance researchers. Of course, we can also compare other global data sets.
Now we should think about what comparison can be made with GAME data. I mainly list those which seem to be useful for "Initiative 1", but also include some other interesting ones.
The list is not exhaustive and probably I miss something relevant. Please let me know about your activities, comments, suggestions, etc.