Global Water and Energy Budget Study
NEWS
PI: John O. Roads
UCSD, Scripps Institution of Oceanography
9500 Gilman Drive
La Jolla, CA 92093-0224
ph: 858 534 2099
fax: 858 534 8561
jroads@ucsd.edu
1. Background
The goal of our NEWS effort is to develop a Water and Energy Budget Study (WEBS) over land
regions in general with a focus on the Global Energy and Water Cycle Experiment (GEWEX)
Continental Experiments (CSEs) in particular. WEBS will involve a comparison of a number of
observationally based (mostly GEWEX) water and energy budget processes and variables with
corresponding processes and variables from global reanalyses and Global Land Data
Assimilation Systems (GLDAS). Such a comparison will provide an assessment of the accuracy
to which we can quantitatively characterize bulk water and energy cycle processes, including
understanding their potential error and the potential error of the overall water and energy budget
“closure”. These “closure” errors will depend on time and space scales. We are focused here on a
2 deg. grid, aggregated to CSE basin and global land regions, and monthly to 10-year time
scales. The time period, 1986-1995, chosen for the initial WEBS, takes advantage of the global
data sets available from the GEWEX International Satellite Land-Surface Climatology Project
(ISLSCP) Initiative I and II data sets.
As an example of the WEBS comparisons being developed (e.g. Roads 2006), Fig. 1
demonstrates our current uncertainty in estimating the annual latent heat flux (evaporation) over
large continental-scale regions. There are currently no well-established global land latent heat
flux data sets, although ocean data sets are slowly becoming available. Land evaporation is
complicated by soil and vegetation heterogeneity and currently only model estimates are the
“best available.” Since atmospheric reanalyses must use soil and vegetation properties influenced
by model precipitation and radiation, it might be assumed that currently available off-line
GLDAS products using a single observationally based forcing could provide the most reliable
estimates. However, an intrinsic advantage of atmospheric reanalyses is that they are a coupled
land-atmosphere solution, whereas current GLDAS models use uncoupled atmospheric forcings
(e.g., uncoupled surface winds, temperatures, humidities), which likely produce some unknown
error. Future coupled atmospheric reanalyses, which assimilate precipitation and radiation are
expected to provide better estimates, as are promising experimental remote sensing techniques.
2. References
Roads, J., 2006: GHP Water and Energy Budget Study. GEWEX Newsletter, Vol. 16, 6-8
3. Presentations
2005/09 NEWS kickoff meeting, New York, New York
2006/01 GEWEX SSG, Dakar Senegal
2006/09 NEWS annual meeting, Washington D.C.
2006/10 Pan GEWEX meeting, Frascati, Italy
2007/01 AMS annual meeting, San Antonio, Texas
Fig. 2 1986-1995 annual latent heat flux (W/m2) means from R1, R2, ERA40, JRA, Noah, CLM,
Mosaic and the atmospheric and land reanalyses ensemble means for GHP CSE regions as well
as for the Global Land (-60 to + 60), Ocean (-90 to 90), and entire Globe. The areas are ordered
from left to right by their annual mean surface air temperatures in the R1. Note the dry MDB and
AMMA areas bracketing the wetter tropical areas.