Geoscience Reference
In-Depth Information
challenge for societies and ecosystems and impose substantial pressures on water manage-
ment, urban planning, agricultural production and tourism. Despite their obvious environ-
mental, societal and economic importance, our understanding of the causes and magnitude of
the variations in the GWC is still unsatisfactory. Key issues for climate change that hinge on
the global water cycle include: (1) the strength and variability of global and regional
hydrological cycles in a warmer world; (2) freshwater forcing and salinity budget of the
global oceans; (3) terrestrial ecosystems and their dependence on water availability; (4) the
fate of polar ice caps and glaciers with consequent sea level rise.
Uncertainties in hydrological predictions from the current generation of models pose a
serious challenge to the reliability of forecasts across timescale and space scale. On climate
timescales, the GWC is highlighted as a key source of uncertainty in the latest Intergov-
ernmental Panel on Climate Change (IPCC) report (Randall et al.
2007
), and problems with
forecasting precipitation continue to affect measures of forecast accuracy. We require a
more holistic approach to understanding, modelling and predicting the global and regional
terrestrial water cycle and its role in the impacts of hazardous weather, climate variability
and climate change. This must extend from the prediction of hydrological extremes (floods
and droughts) to an integrated assessment of water, food and natural resources.
The growth of systematic errors in models remains one of the central problems in
producing accurate climate change projections for the next 50-100 years. Although great
advances have been made in global modelling in recent decades, there are still large
uncertainties in many processes, such as clouds, convection, and coupling to oceans and
the land surface. Such uncertainties are related to different physical parametrisations and to
model horizontal and vertical resolution. Global precipitation distribution, intensity and
variation on a range of timescales are also not well observed, particularly over oceans.
Similar systematic errors are seen on a range of timescales from Numerical Weather
Prediction (NWP)/seasonal/decadal/climate. ''Seamless'' model development, where the
same model configuration is developed and tested across all timescales (e.g., Martin et al.
2010
), has been instrumental in diagnosing and reducing errors in tropical circulation and
precipitation. Increasing model resolution has also been shown to improve the distribution
of tropical precipitation. This will ultimately lead to better predictions and projections for
the GWC.
This paper provides an overview of the current issues and challenges in modelling
various aspects of the Earth's hydrological cycle. These include: the global water budget
and water conservation, the role of model resolution and parametrisation of precipitation-
generating processes on the representation of the global and regional hydrological cycle,
representation of clouds and microphysical processes, rainfall variability, the influence of
land-atmosphere coupling on rainfall patterns and their variability, monsoon processes and
teleconnections, and ocean and cryosphere modelling. We include examples of studies
being done around the world which shed light on the relevant processes and limitations in
their modelling. We conclude that continued collaborative activity in the areas of model
development across timescales, process studies and climate change studies will provide
better understanding of how and why the hydrological cycle may change, and better
estimation of uncertainty in model projections of changes in the global water cycle.
2 Scientific Challenges in Modelling the Global Water Cycle
The main scientific challenges in modelling the global water cycle are now discussed, with
reference to research currently underway in a number of institutes around the world.
