Institute of Hydrological and
Oceanic Sciences Research
Modeling
Taiwan's Hydro-Environment
In the island of Taiwan and the surrounding seas, human, industrial and
economic activities are intimately linked to the water cycles and
associated resources and disasters. It is therefore important to
establish ocean-atmosphere-land models to make short-to-long-term
predictions of winds, sea levels, ocean currents, hydrography, and
water cycles. The short-term (nowcast) system can be used for
disaster mitigation, rescue and search, and fishery and marine
resources management. The long-term system can be used for
climate prediction, drought warning, and water resources managements.
The atmosphere
models
are based on existing regional models. Currently, a version of
the
model nesting down to 2.2-km horizontal grid spacing with a proper
treatment of topographic forcing, ice-phase microphysics, and the
assimilation of radar data can successfully predict 24-hour rainfall in
selected typhoons. The model will be furthered modified by
improving
the treatment of precipitation, boundary layer, and land surface
processes. (M.-J. Yang)
The ocean models
will be developed in house. The validation and improvement
requires
ship observations, moored and satellite instrumentmeasurements. These will be
done jointly with collaborating scientists. (S. Jan)
Distributed basin-scale
runoff models are also being developed to estimate the river
flow in response to typhoon rains. The upstream basin of
Reservoir Shihmen is selected in the figure as a target domain of
interest. Ground-based hourly observations of both stream flows
and gage rainfalls are crucial measurements. Selected
typhoons are used for calibrating the runoff parameters and the
basin landuse conditions of the distributed model. (M.-H. Li)
Air-Sea
Exchanges
High- frequency
(daily) momentum and heat exchanges are important for coupled
ocean-atmosphere variability but are generally treated
insufficiently in many models. The study from an ocean circulation
model shows such exchanges primarily enhance vertical mixing that
reduces the vertical shear of the upper ocean. It also changes
the net heat into the ocean through two contrasting processes: one is
the increased surface latent heat loss induced by transient winds and
the other is colder SST due to stronger mixing, which reduces further
heat loss at the surface. As a result, the momentum and heat
balance in the upper ocean are affected by both local and remote
responses. (C.-H. Sui)
The effect of ais-sea
exchanges in typhoons, intraseasonal oscillations, and the oceanic
mesoscale eddies may cause locally large variations in the Kuroshio
transport as those observed at the World Ocean Circulation Experiment
PCM-1 section east of Taiwan. An eddy-resolving, general
circulation model, with a horizontal resolution of 1/6 degrees, is
adopted to study the Kuroshio-eddy interactions. The figure shows
flow fields before and after a westward-propagating anticyclonic eddy
impinges to the Kuroshio. The scenario runs qualitatively mimic
observational evidences, suggesting mesoscale eddies with a relative
vorticity of 0.0001/s would considerably affect the path, flow volume
transport and vertical hydrographic structure of the Kuroshio.
(S. Jan)
A
"numerical laboratory" of the surface ocean driven by winds shows the
thermal structure within the aqueous boundary layer. The picture
enables a visualization of the "surface renewal events" which have been
widely employed in estimating the air-sea fluxes (so called bulk
parameterizations) but remain to be conceptual. Such a thermal
structure also produces the "cool-skin effect" important in validating
satellite-derived sea surface temperature using the in situ bulk
temperature. (W.-T. Tsai)
The ubiquitous physicochemical effect of surfactants in the global
oceans is another process of micro length-scale but can substantially
influence the global air-sea exchanges. The figure shows the
distribution of the annually-averaged global atmosphere-ocean CO2
flux with the transfer reduction by the presence of the surfactants
considered in the calculations. The major reduction in absorption
fluxes occurs in the northern Pacific and Atlantic, where the water is
known to be highly productive, while the decrease in outgassing occurs
in the eastern equatorial and subtropical Pacific and Atlantic, where
nutrient-rich water wells up. (W.-T. Tsai)
Biogeochemical
Cycles in Natural Environment
These cycles affect water quality in fresh waters and coastal waters on
a local scale; they also affect ocean productivity and climate on the
regional to global scale. The approaches include field surveys,
isotopic and elemental analyses of water samples, and numerical
modeling to investigate the coupled physical-biogeochemical processes
controlling the elemental cycles. The example shows the
comparison between the modeled chlorophyll distribution and the image
of pigment concentration derived from satellite ocean color data in the
South China Sea in winter. The patches of high chlorophyll result
from upwelling of nutrient-rich seawater from the deeper ocean, which
was induced by the strong northeast monsoon. (K.-K. Liu)