Phase 1: Continental North America During the Warm Season
The Earth's hydrologic cycle has an enormous impact on human activity and on economic prosperity,
especially through precipitation. But understanding the complex interactions among the various processes that
drive the hydrologic cycle remains a major scientific challenge. Part of this uncertainty is due to poor
measurements of the distribution of water vapor in the atmosphere and inadequate information on the accompanying
sources and sinks. Basic issues stem from the chaotic nonlinear properties of fluid motion, especially those involving
transitions among the three phases of water.
While the hydrologic cycle is a product of multiscale interactions among cloud-microphysics, radiation,
convection, and boundary layer and surface hydrology, none of these processes is dominant and, moreover, all are
inherently sub-grid scale. The dynamical interactions embodied in the attendant parameterizations are not well
understood and are poorly represented in large-scale models. The fact that weather prediction models, even the
most advanced ones, have great difficulty in estimating precipitation underscores this point. The absence of precipitation
is just as important as its presence, and extremes in either can have major consequences for society and the environment.
This is an opportune time to advance our knowledge in the water cycle, both from the standpoint of the state of the
science as well as the growing needs of society. Specifically, much-improved satellite- and radar-based estimates of rainfall
rate and distribution are becoming available; the ability to conduct surface and satellite-based water vapor measurements has
dramatically evolved; sophisticated atmospheric and surface-hydrologic models exist; and computing costs are dropping rapidly.
NCAR has expertise in cloud-system, land-surface and biospheric and climate modeling, has extensively developed community models
for the atmosphere and surface hydrology, and also has expertise in remote sensing.
Our initiative represents a significant contribution to an unprecedented set of national and international programs
in the atmospheric and hydrologic sciences. These contributions will be made through focused research involving the moist
physical processes where our collective proficiency is strong, and where the truly fundamental issues in weather and climate reside.
The focus of Phase I is on warm season convection in the North American continent. Four main areas of emphasis are:
- diagnostic studies of the diurnal cycle of precipitation and improving its representation in models,
- cloud systems and their simulation, with a focus on improving the initiation and evolution of clouds and their
parameterization through cloud resolving model simulation,
- 3) water vapor studies in the context of the
International H2O (IHOP) field program with a goal of improving its measurement and its role in convective initiation, and
- the role of land-atmosphere interactions in controlling latent and sensible heat fluxes to the atmosphere. Of particular focus
will be the role of soil moisture during IHOP.
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