Geophysical Fluid Dynamics
and Scalar
Transport in the Tropics
(13 Nov - 8 Dec 2006)
~ Abstracts ~
Large-scale tropical atmospheric
dynamics: waves or balance?
Jun-Ichi Yano, Meteo France
It is often considered that the large-scale tropical
atmosphere is not in any balanced state but characterized by
an ensemble of equatorial waves. Observationally speaking,
various longitudinally-propagating moist convective
coherencies are seen with the Madden-Julian wave probably,
as the best known example. These coherencies are often
interpreted as equatorial waves generated under a coupling
with moist convective processes. However, a conventional
scale analysis (as used for deriving the midlatitude quasi-geostrophy)
fail to convincingly support this view.
The present talk proposes an alternative possibility that
the tropical large-scale is dominated by a balanced
dynamics. The proposed balanced dynamic consists of the two
parts. The first and a more dominant balance is the one
between the vertical advection and the diabatic heating in
the thermodynamic equation (the thermodynamic balance). An
equivalent balance condition may also be posed on the
moisture equation. These are well known dominant balances in
the tropical convective observations, and applied to the
large-scale dynamic context as a weak-temperature gradient (WTG)
approximation by Sobel et al. The second one is the
nondivergence condition to the leading order, as originally
proposed by Charney (1963).
The formulation for the tropical balanced dynamics,
observational evidences, and the implications will be
discussed.
Vortex interactions and the barotropic
aspects of concentric eyewall formation
Hung-Chi Kuo, National Taiwan University
As an idealization of the interaction of a tropical
cyclone core with nearby weaker vorticity of various spatial
scales, the authors consider nondivergent barotropic model
integrations of the vortex interactions with modified
Rankine vortices. We address the issues of why there is a
wide range of radii for concentric eyewalls and roles of the
moat and the turbulent vorticity scale in the formation of
concentric eyewall structure. An important parameter in the
classification
of the resulting interactions is the vorticity strength
ratio. Variation of this parameter can lead to end states
that can be classified as vortex mergers, tripole
structures, or concentric eyewall structures. Concentric
structures result from binary interactions in which
the small vortex is at least 4 to 6 times stronger than the
larger companion vortex. An additional requirement is that
the separation distance between the edges of the two
vortices be less than six times the smaller vortex radius.
In general, when the companion vortex is three time larger
than the core vortex in radius, a core vortex with a
vorticity skirt produce concentric structures in the region
where the separation distance is 4 times greater than the
smaller vortex. The region is where the Rankine vortex has
elastic interaction. Thus, a Rankine vortex favors the
formation of a concentric structure closer to the core
vortex, while the skirted vortices favor the formation of
concentric structures farther from the core vortex. This may
explain the satellite microwave observations that suggest a
wide range of radii for concentric eyewalls. The numerical
results also indicate that a strong tropical cyclone with a
moat of 10--20 km width is able to organize a stirred
vorticity field with 40--50 km spatial scale into a
concentric structure with magnitude similar to those formed
in binary vortex interactions. Both binary vortex
interaction experiments and turbulent background vorticity
experiments highlight the pivotal role of the core vorticity
strength in maintaining itself, and in
stretching, organizing, and stabilizing the outer vorticity
field. Furthermore, these experiments depict the shielding
effect of the moat in preventing further merger and
enstrophy cascade processes during
concentric eyewall formation. Finally, the results support
the notion that concentric eyewalls form only in strong
tropical cyclones.
Development and numerical results
of a global cloud resolving model and its potential
application to numerical weather forcasting of tropics
Masaki Satoh, University of Tokyo
We review simulation results and development of the Nonhydrostatic ICosahedral Atmospheric Model (NICAM), which achieved the first global cloud resolving simulations (GCRM) with 3.5km-mesh horizontal grid interval using the Earth Simulator.
NICAM is developed at Frontier Research Center for Global
Change, JAMSTEC, and Center for Climate System Reseach,
Univ. of Tokyo. It is aimed to simulate more reliable
climate states by representing explicit deep convective
circulations instead of using ambiguous cumulus
parameterizations. Toward this aim, we are testing the model
in various kinds of experimnets. We conducted three GCRM
runs: 1. the aqua planet experiment, 2. a short-term (one
week) experiment with the realistic land/sea distribution
for Apr.2004 condition, and 3. a long-term (a few months)
climatological statistical experiment for perpetual July
condition.
The GCRM captures behaviors of cloud systems in the tropics:
diurnal cycles of convection, land/sea contrast of cloud
clusters, and super-cloud clusters. It is expected that GCRM
will give better simulations of the intraseasonal
variability incuding the Madden Julian Oscillation,
cyclogenesis of tropical cyclones, and more reliable climate
sensitiviy without ambiguity of cumulus parameterization.
These will lead to better numerical weather forcasting in
the tropics. We will argue some of the issues of development
of GCRM.
Cloud-resolving modeling of
cloud
system during Jakarta flood event for period of january 2002
Nurjanna Joko Trilaksono, Institut Teknologi Bandung
Numerical simulations of cloud systems for the period of
26–30 January 2002 for area of Indonesia and Java are
performed using Weather Research and Forecasting (WRF) -
Advance Research WRF (ARW) model. The focus area will be in
West Java. The aim is to reproduce cloud systems over the
Jakarta area that brought torrential rain. The current
result shows domination of tropical cyclone in the Indian
Ocean to precipitation over western West Java is strong as
seen on the day one. On the other day, even the cyclone has
disappeared precipitation tendency still high in northern
Jakarta. Blocking zones were found to prevent the growth of
cloud system. The blocking zones were possible explanations
why the high amount of precipitation only occurs in Jakarta
and caused the flood.
The influence of
meteorological conditions on the distribution of smoke haze
distribution over Indonesia 1997/1998
Kadarsah Bin Sukandar Riadi, Indonesian National
Institute of Aeronautics and Space
Numerical modeling of fire-related smoke haze episodes in
Southeast Asia is important for both prediction and
assessment of atmospheric impacts, especially when
observational data are fragmentary, as is the situation in
Indonesia. This research is aimed to analyze the influence
of meteorological conditions on the distribution of the
smoke haze from the forest fire in Indonesia with a regional
climate model (REMO) that has been modified into REMO with
Tracer Extension (REMOTE) from the Max Planck Institute.
This model includes an estimate of the gaseous and
particulate matter emission from vegetation and peat fires
in Indonesia. Forest fire emission inventory of July to
December 1997 has been used as main data to simulations that
used meteorological condition of 1997 or the El Nino period,
condition on the Normal period (July to December 1996) and
the La Nina period (July to December 1998). We investigate
the role of different meteorological condition in setting up
the smoke haze distribution. The result of simulations over
those three periods will be compared among each other to
analyze the influence of the respective meteorological
condition toward regional smoke haze distribution vertically
and horizontally. We simulated those scenarios using REMOTE
under a Linux operating system with main forcing data from
the European Centre for Medium range Weather Forecast and
forest fire emission inventory from the ATSR satellite
derivation database. The major parameter measured is PM10
(particulate with a diameter below 10 μm). We found out that
the smoke haze distribution were mainly influenced by the
wind and sea surface temperature anomaly in other years that
consequently brings less distributed area in the normal and
in La Nina period. The water content level will bring
implication on the precipitation processes and the wet
deposition that is larger and the accumulated PM10 will
reach the maximum during the La Nina period. Furthermore,
the smoke haze distribution is influenced by the wind that
reached maximum during the El Niño period, especially on
September 1997. Strongly reduced rainfall and generally
stronger southeasterly winds during El Niño years provide
favorable conditions for larger scale smoke haze pollution.
In years with normal meteorological conditions, intermittent
precipitation and associated wet deposition during the dry
season is predicted to remove most of the particulate
emissions close to the sources. The emission data inventory
emission and the simulation result shows that Kalimantan,
Sumatra, and Papua are major source of smoke haze of forest
fire in Indonesia and the distribution of the smoke haze
will be larger during the La Niña, Normal, and El Niño
period, consecutively. Meanwhile, the concentration becomes
larger from the El Niño, La Niña, and normal periods,
consecutively. The concentration of PM10 reaches maximum on
October 1996 and 1998 as much as 35,000 μg/m3 or during La
Niña and Normal periods. The extent of the smoke
distribution reaches the maximum on September 1997 in
Kalimantan of about 2.904.000 km2.
Keywords: Regional Model, PM10, El Niño, La Niña
Application of
geostatistical techniques in satellite-based rainfall
retrieval
Chee Kiat Teo, Temasek Laboratories, NUS
Some geostatistical techniques applicable for
interpolating meteorological fields are discussed in the
context of spatial interpolation of rainfall fields and
rainfall retrieval from satellite images. Kriging as an
objective method in spatialising rainfall field from
rainfall gauge observations inclusive of a treatment of
intermittent rainfall fields is first discussed. This is
followed by a description of an adoption of the Sequential
Simulation technique to stochastically generate spatial
rainfall fields given satellite images, useful for
sensitivity studies or ensemble methods in upstream
hydrological and agricultural applications.
Impacts of the tibetan plateau on
the climate in tropics and subtropics
Guo Xiong Wu, Institute of Atmosphreic Physics, Chinese
Academy of Sciences
Recent progress from the study of the mechanical as well
as thermal effects of the Tibetan Plateau (TP) on the
circulation and climate in Asia are summarized as a
complement to the newly published history review of Yanai
and Wu (2006). Efforts are made to provide new understanding
through diagnosis and numerical experiments. The air column
over the TP descends in winter and ascends in summer each
year, working as an air-pump. It is shown that such an
air-pumping can regulate the surface Asian monsoon flow
effectively only because it is driven by the surface
sensible heating on its sloping lateral surfaces. It is
therefore named as the Tibetan Plateau Sensible Heat Driving
Air-Pump (TP-SHAP). The retarding and deflecting effects of
the TP in winter generate an asymmetric dipole
zonal-deviation circulation with a large anticyclone gyre to
the north and a cyclonic gyre to the south. This enhances
cold outbreaks from the north over East Asia, results in a
dry climate in South Asia and a moist climate over the
Indochina Peninsula and South China, and forms the
persistent rainfall in early spring (PRES) in South China.
In summer the TP-SHAP generates a cyclonic spiral
zonal-deviation circulation in the lower troposphere, which
converges towards and rises over the TP. It is shown that
because the TP is located on the east of the Eurasian
Continent, in summertime the meridional wind and vertical
motion forced by the Eurasian continental-scale heating and
the TP local heating are in phase over the eastern part of
the continent. The monsoon in East Asia and the dry climate
in the Middle Asia are therefore intensified. It is also
shown that the thermal contrast between the TP and Iran
Plateau contributes to the presence of the bimodality of the
South Asian High with regard to the longitude location of
its center and the occurrence of large-scale drought and
flood in Asia.
Land-sea distributions and modeling
of the asian monsoon
Yimin Liu, Institute of Atmosphreic Physics, Chinese
Academy of Sciences
The roles of land-sea contrast and the associated
diabatic heating in the formation of the Asian summer
monsoon and the subtropical anticyclone are examined through
data diagnoses and numerical experiments. Results show that
the existence and geometric shape of land-sea distribution
crucially affect the Asian summer monsoon. In an aqua-planet
case, no monsoon is observed. In an experiment in which only
the subtropical Eurasian landmass exists, there is a weak
summer monsoon over its southeastern corner, but there is no
tropical summer monsoon. The existence of tropical lands
induces cross-equatorial flows and strong low-level
southwesterlies over the tropical regions, leading to the
formation of the Asian summer monsoon over India, the Bay of
Bengal, and the South China Sea. The extension of the
subtropical continent into the tropics greatly enhances the
“East Asian monsoon”.
The relative impacts of different diabatic heating which is
associated with the land-sea distribution and their
synthetic effects on the formation of the summertime
subtropical anticyclones will also be discussed. Results
show that the strong land-surface sensible heating (SE) on
the west and condensation heating (CO) on the east over each
continent generate cyclones in the lower layers and
anticyclones in the upper layers, whereas radiation cooling
over oceans generates the lower layer anticyclone and upper
layer cyclone circulations. Such circulation patterns are
interpreted in terms of the atmospheric adaptation to
diabatic heating through a PV- view. A Sverdrup balance is
used to explain the zonal asymmetric configuration of the
surface subtropical anticyclones. The center of the surface
subtropical anticyclone is then shifted towards the eastern
ocean, and its zonal asymmetry is induced. We concludes that
in the summer subtropics over each continent and its
adjacent oceans, LO, SE, CO and of a double-dominant heating
(D) from west to east compose a LOSECOD heating quadruplet.
A specific zonal asymmetric circulation pattern is then
formed in response to the LOSECOD quadruplet heating. The
global summer subtropical heating and circulation can then
be viewed as “mosaics” of such a quadruplet heating and
circulation patterns, respectively.
Simulating Sumatra squall lines
using COAMPS®
Lan Yi, Nanyang Technological University
The US Navy’s mesoscale numerical weather prediction
model COAMPS* was employed to simulate the Sumatra squall
lines. These impactive convective storms frequently form
along the Melaka Straits and sweep across Malaysia and
Singapore in the early morning hours from April to October
each year. In order to understand and better predict this
weather system, investigations on dynamics of the convection
self organization, and mechanisms of boundary forcing and
large scale control, were carried out through idealized and
semi-idealized model experiments. Real case simulations and
Ensemble Kalman Filtering data assimilation coupled with
COAMPS model were performed as well.
* COAMPS stands for Coupled Ocean/Atmosphere Mesoscale
Prediction System. COAMPS is a registered trademark of US
Naval Research Laboratory.
Current problems in tropical
meteorology
Adam Sobel, Columbia University
I will discuss some open problems in tropical
meteorology. No attempt will be made to be exhaustive. I
will address issues of observations, modeling, and theory,
but with an emphasis on places where theoretical
understanding is needed, and idealized models may have a
role to play in interpreting results from observations and
more comprehensive simulations. Time permitting, I will
discuss the mechanism of the Madden-Julian oscillation, the
double ITCZ problem in general circulation models, and the
relationship between tropical cyclones and climate.
ENSO and western north pacific
tropical cyclones
Adam Sobel, Columbia University
I will discuss various aspects of the relationship
between the El Nino-Southern Oscillation (ENSO) phenomenon
and tropical cyclones in the western north Pacific. Topics
to be discussed include: the relationship between ENSO and
TC intensity; attribution of the relationship between ENSO
and mean genesis location to specific physical factors; and
the possibility that TCs may feed back to influence the
dynamics of ENSO itself.
Assimilation of satellite altimetry
data into ocean circulation model
Vladan Babovic, National University of Singapore
Common problem in oceanographic modelling is that areas
of interest are scarcely covered with in-situ tide gauges.
Furthermore, most of the tide gauges are situated in shallow
areas directly along the coast or in estuaries, not
representing the offshore hydrodynamics in deeper open
waters. One option is to match the hydrodynamic model with
the tide gauge data, leaving it up to the model dynamics to
resolve sea level heights and current predictions in the
deeper parts. Another option is to utilise modern satellite
altimetry data in order to accurately model dynamics of deep
ocean. This presentation explores utilisation of satellite
altimetry in connection with oceanographic models and
particularly in data assimilation sense.
The altimeter signal can be further analysed in order to
obtain sea surface anomaly patterns (SSA). With SSA it is
possible to take into account seasonal water level
variations due to long term meteorological setup, such as
monsoon. The contribution describes an application of data
assimilation to combine satellite altimetry with
oceanographic models in order to arrive at more accurate
oceanographic models.
Numerical models are far from being perfect. A numerical
model is indeed only a model of reality. It employs a number
of simplifying assumptions, such as depth averaging of
velocities in vertically integrated two-dimensional models,
which inevitably produce inaccuracies. In a numerical model,
one also discretises the domain and is therefore not able to
resolve numerous subgrid-scale phenomena. Errors in the
model parameterisation (because most model parameters cannot
be directly measured) may contribute significantly to the
overall error in a numerical model. It is also impossible to
precisely define initial conditions and forcing terms over
the entire computational domain. All of these inaccuracies
and uncertainties could accumulate to produce poor model
results, despite our perfect knowledge of the governing
laws.
To combat the inevitable presence of such model errors, a
number of approaches for correcting the model results are
employed. Data assimilation is a methodology that utilizes
information from observations, and combines it with (or
assimilate it into) numerical models. A number of different
data assimilation procedures can be adopted. These are
designed to either improve description of initial conditions
at the time of forecast or provide correction of model
predictions during a forecast period. The data assimilation
procedures may be classified as follows: (i) updating of
input parameters:, (ii) updating of state variables, (iii)
updating of model parameters and (iv) updating of output
variables (error prediction).
In model error prediction techniques such as artificial
neural networks, genetic programming or an approach based on
chaos theory have demonstrated good forecast skill. By using
these techniques, one can combine the forecast of the
numerical model (model output) at the point of interest with
the latest observed data in order to obtain an improved
forecast. Another advantage of such an approach is that it
allows the combination of different variables (for example,
atmospheric data such as wind speed) to improve the
accuracy. This cannot be done in conventional data
assimilation methods where the data has to be introduced in
the model state in order to be assimilated.
In addition to providing theoretical background, the
presentation will also show an application of the approach
on modelling residual currents in Malacca and Singapore
Strait.
Separation, re-attachment,
transport, and mixing in Rayleigh-Benard convection cells
Francois Lekien, Princeton University
Numerous experiments have revealed the presence of
hyperbolic manifolds that govern mixing in time-chaotic
systems. These structures represents alleyways and barriers
to transport, and provide a geometric description of the
mixing processes in the system. Near the surface of an
airfoil, a container boundary, or a coastline, hyperbolic
manifolds correspond to separation and re-attachment
profiles. Recent exact criteria are used to detect and
extract separation points and related separation profiles.
The existence of such analytic criteria permits the
development of efficient algorithms to detect and control
the separation points in systems such as randomized
Rayleigh-Benard convection cells.
Numerical experiments on the
layered structures in the mid-troposphere over the
equatorial Pacific with a mesoscale model
Shigeo Yoden, Kyoto University
Three-dimensional structures and transport processes
associated with layered structures in the tropical
mid-troposphere over the eastern Pacific, which are often
observed by radiosonde and airborne measurements as
anti-correlation between humidity and ozone, are
investigated numerically with a mesoscale limited-area
model. We reproduced a thin layered structure of high
humidity, which has a horizontal scale of about 1000 km and
a vertical extent of about 1 km, in the simulation for
September 1999. Particle transport experiments around the
layered structure show that the wet part of the layered
structure is advected from the intertropical convergence
zone by northwesterly winds, while dry parts above and below
it come from the southeast direction. Streamline analysis
shows that the thin layered structure is produced by the
vertical shear of a horizontal wind component. This shear is
associated with longitudinal displacement of a stagnation
point, which is located between two synoptic-scale vortices,
with height.
We also performed an experiment on year-to-year variation of
the layered structures over the tropical eastern Pacific for
Septembers from 1999 to 2005. All the years, layered
structures are obtained to the south of the intertropical
convergence zone. Large year-to-year variation in the
occurrence ratio of the layered structures is found to have
a good correlation with that of the Southern-Ocillation
index known as an index of El Nino.
Ocean hydrodynamics model with
tidal forcing derived using an Artificial Neural Network
My Ha Dao, National University of Singapore
The prescription of open boundary conditions is one of
the greatest challenges in ocean and coastal hydrodynamic
modeling. It requires extensive information on current
velocity, tidal elevation, temperature and salinity at
specific points in time and space. Once boundary conditions
are known and correctly prescribed, one can solve a set of
governing equations to simulate the dynamics of the state
variables in a domain of interest. In this work, we compare
the performance of a hydrodynamic model using two different
sets of tidal boundary conditions derived from harmonic
analyses in one case and an Artificial Neural Network (ANN)
in another.
The hydrodynamic model used in this study is Tropical Marine
Hydrodynamics (TMH) of TMSI. The study domain includes the
entire Singapore Strait and has six open boundaries. Since
no consistent current velocity measurements are available at
the moment, only tidal elevations are prescribed at the
boundaries. In one approach, tidal boundary conditions are
predicted by applying harmonic analysis on a set of known
harmonic constituents. These harmonic constituents are
extracted from JTS, ATT tide tables (UK), or TotalTide (UK).
This technique requires information from only tidal stations
nearest the open boundaries. On the hand, the ANN can make
full use of all available stations nearby the open
boundaries. The database is divided into 2 parts, one is for
training and other is for validation. Upon achieving
satisfactory performance, the trained ANN is then used to
generate tide elevation data at specific boundary grid
points.
TMH is then run for the two cases of boundary conditions,
and the results are compared between each others, and both
against measurements. The results from ANN methods are
capable to improve accuracy of tidal hydrodynamic
predictions.
Air pollution dispersion modeling
in Singapore
Martin Skote, Institute of High Performance Computing
In Singapore, the National Environment Agency (NEA) has generated an efficient air quality monitoring network for real time studies. However, the numerical model for prediction/analysis has previously been limited to a steady state Gaussian model, which mainly has been used for air pollution impact evaluation based on local historic wind and emission data. Recently, a simulation package has been implemented to analyze the detailed three dimensional spatial pollutant distributions and time variance over the country. In this new model, the chemistry transport equations are solved by CMAQ, a code developed by U.S. Environmental Protection Agency. These simulations require input of weather and emission data, which are produced by separate simulation codes (MM5 and SMOKE respectively).
MM5 provides weather data based on large scale observations
(taken from a global data base) and fine scale boundary
conditions (terrestrial data). Typically five nested
simulations are performed with MM5 at 81, 27, 9, 3, 1 km
resolutions respectively.
The first level covers the Southeast Asia region while
the fifth and last level with the finest resolution covers
Singapore. The emission inventory consists mainly of power
plants and oil refineries, and is converted to source terms
using SMOKE. The dispersion simulations are then performed
with a time step of one hour, and with the finest resolution
of one kilometer retained. The physical modeling, especially
the parameterization of the planetary boundary layer, will
be discussed. Furthermore, brief overview of the cloud
dynamics, the land surface modeling, and atmospheric
chemistry will be covered. The particular problems related
to the tropical atmosphere and its pollutants will be
discussed. Results from the most interesting simulation for
the south-west monsoon period will also be presented during
the workshop. In particular the SO2 simulation results,
which are compared in detail with data from the monitoring
station network, will be discussed.
Hopefully both the audience and the speaker will learn
something from this interactive workshop talk.
Secret of Tropical Cyclone
Huashu Dou, Temasek Laboratories, NUS
Tropical cyclone is one of the most significant disasters
to mankind. For example, Hurricane Katrina in the autumn of
2005 killed more than 1200 persons and lead to damage of
$81.2 billion (2005 USD) in New Orleans area and over
southern America, and Typhoon Saomai in August of 2006
resulted in at least 441 deaths, mostly in China, and $1.5
billion (2006 USD) in damage. It is very important to study
the principle and the behaviour of tropical cyclone since
this would help us to avoid losses and to get control of the
nature. However, our understanding of tropical cyclone is
still poor and the secret of tropical cyclone has not been
clarified so far. Now, it is well known that tropical
cyclone (tornado like) is a vortex which can be expressed by
the combined Rankine vortex. In this vortex, the velocity
distribution is represented by a free vortex and a sink
except in the core area which can be represented by a linear
vortex with updraft. Recently, we developed a theory (named
as energy gradient theory) which is well valid for flow
instability and turbulent transitions. This theory obtains
very good agreement with experiments for various flows (pipe
Poiseuille flow, plane Poiseuille flow, plane Couette flow,
and Taylor-Couette flow). According to this theory,
potential flow is of a uniform energy field and is always
stable, no matter how large the disturbance amplitude.
Rankine vortex is one kind of potential flow (except the
core area), in which the energy is uniform in the flow
field. Any disturbance could not be amplified in a constant
energy filed so that the Rankine vortex is always stable and
is able to last very long time. This is the secret of
tropical cyclone. In comparison, trailing vortices generated
behind electric poles, bridges, and buildings are so
unstable that they could break very quickly after they are
formed. This is because that the energy in these vortices is
very non-uniform. In this study, we will describe the
principle of the vortex stability, and give out
formulations, and then apply the theory to the stability to
vortices. Final, we demonstrate why the tropical cyclone is
so stable and when the tropical cyclone becomes unstable.
Numerical modeling of coastal
processes with SLON model
Oleksandr Nesterov, Tropical Marine Science Institute,
NUS
This presentation will cover topics related to coastal
hydrodynamics problems. It can be interesting for modelers
dealing with parallel computations as well. Simulation of
hydrodynamics, water quality and sediment transport in
coastal areas is one of the most important and challenging
task of environmental hydrodynamics. In contrast to models
dealing with open seas, coastal models often include large
number of state variables, which are dependant on each other
and therefore should be computed simultaneously. Also
coastal models often require high horizontal and vertical
resolutions to simulate large amount of small eddies arising
due to coastal line or rapid changes in bottom topography.
Rivers, artificial outlets or water intakes also require
high horizontal resolution. The same time, fine vertical
resolution is often necessary in order to simulate
stratification and relevant problems arising due to this
phenomenon.
Numerical model SLON (Sigma Layers Offing Numerics) has been
developed in TMSI as further version of the model Threetox (IMMSP,
Kiev, Ukraine) to fulfill such requirements. At the present,
SLON is applied to various problems relating to
hydrodynamics, water quality and sediment transport
simulations in Singapore Straits. Optionally, the model can
employ double sigma vertical coordinate system, which
appears very useful for simulations of baroclinic motions,
especially in estuaries, coastal areas and lakes with
artificial ship channels.
SLON uses comparatively simple method, which is easy in
programming implementation, of splitting the entire
computational domain into several sub-domains to allow
parallelization of computations. Implicit scheme for surface
elevation calculations involves finding of the solution of
the system of linear equations with large sparse matrix,
also computed in parallel by means of the model. Dynamic
data exchange (using MPI) at each integration time step and
iteration ensures the same solution as in the case of serial
computation. It is shown that such method is quite efficient
for computations using small network consisting of several
high-end PCs. Such network is usually affordable for small
companies and labs in contrast to dedicated large clusters
consisting of hundreds of CPUs.
Presently 2-D version of the model uses grid 2700 x 1500
numerical cells to cover entire Singapore Strait with
resolution 50m. Modeling shows generation and existence of
many eddies, which could not be modeled at coarser
resolutions. 3-D model is used for hydrodynamics and
sediment transport modeling in Singapore waters covering
smaller domains. Examples of applications will be shown and
discussed.
Forced and spontaneously generated
tropical precipitation patterns in an aqua-planet
Yoshi-Yuki Hayashi, Hokkaido University
Aqua-planet GCM experiments have been performed for the
purpose of investigating precipitation and circulation
features in the tropics. The motivation was originally to
search and/or reveal possible fundamental dynamical
structures which might be embedded in the real tropics by
performing numerical experiments with some idealistic setups
reducing the complexity of real situations (e.g. boundary
conditions, physical processes, and so on). However, it has
been recognized that the appearances of tropical
precipitation patterns depend rather severely on particular
model implementations. Searching for the possible varieties
of appearance in the models and understanding how and why
they appear now become an aim of investigation. APE (http://www-pcmdi.llnl.gov/projects/amip/ape/),
the international aqua-planet comparison project, is an
example of such an activity.
Here, a variety of tropical precipitation patterns which
have been obtained in the aqua-planet GCM experiments so far
performed in our group are presented. There are basically
two types of experiments.
One is on the line of Hayashi and Sumi 1984, that is, to
consider spontaneous appearance of tangent and coherent
precipitation features on a SST distribution which is
zonally uniform and symmetric around the equator. The other
is on the line of Hosaka et al. 1998, that is, to consider a
remote response of precipitation and circulation patterns
caused by a warm SST anomaly placed on a simple SST
distribution like that of the former type. The appearance or
disappearance of coherently eastward moving precipitation
areas are considered from the old concept of wave-CISK, with
some additional results by two-dimensional cumulus ensemble
model of Nakajima. Remote response to the existence of a
warm SST area is considered from the moisture convergence in
the planetary boundary layer. The behaviors of precipitation
patterns, however, have not been very well understood yet.
The study of the effect of cloud
inhomogeneity in AGCM
Feng Zhang, Institute of Atmospheric Physics, Chinese
Academic of Sciences
By using the mesoscale cloud inhomogeneity data obtained
from ISCCP, the effects of cloud inhomogeneity in IAP AGCM
have been investigated. Numerical simulations have shown
that cloud inhomogeneity has obvious influence on the
simulated climates, such as precipitation, ground surface
air temperature, and sea level pressure. The effect of
optical depth of inhomogeneous clouds on solar radiation
fields is shown to be opposite to those of the
single-scattering albedo and asymmetry factor of
inhomogeneous clouds, which has led to a small overall
direct influence on radiation field. In AGCM, cloud
inhomogeneity is mainly through the cloud water field to
indirect influence the radiation field. Our research has
clearly shown the importance of cloud-radiation interaction,
and the importance of cloud parameterization for the
improvement of the performance of global climate models.
The developments and applications of
LASG/IAP climate system ocean model (LICOM)
Hailong Liu, Institute of Atmospheric Physics, Chinese
Academic of Sciences
The LASG/IAP Climate system Ocean Model (LICOM), which is
the ocean component of the LASG climate system, has been
continuously developed during the past twenty five years.
The primary features of the model include a free surface, an
energy conserving numerical differential scheme, and the
h-coordinate in the vertical. The developments of LASG/IAP
climate ocean model are motivated for climate variability
and climate change studies as well as large-scale ocean
circulation simulations. The main developments and
applications of LICOM during the past five years are
presented here.
Sensitivity of tropical depression
due to surface forcing in the gulf of Thailand during 22-24
October 2003
Kamol Promasakha, Thai Meteorology Department
Observations within the planetary boundary layer in the
inner-core region of storm are rarity. They are available
only from isolated radiosonde or buoy. This lack of data
forces modelers to use boundary layer parameterizations that
have largely been developed for lower wind speed conditions.
Assumptions about boundary process are particularly
important to models attempting to simulate the convective
scale to mesoscale processes responsible for the evolution
and maintenance of storm. This research is to study the
sensitivity of tropical depression due to surface forcing in
the Gulf of Thailand with planetary boundary layer
parameterization during 22-24 October 2003. This research
performed experiment with four the planetary boundary layer
parameterization schemes: Blackadar, ETA, the Medium Range
Forecast (MRF) and the Burk-Thomson schemes. The experiments
were conducted by using of the Pennsylvania State University
and National Center for Atmospheric Research (PSU/NCAR)
Mecroscale and Microscale Model system (MM5) version 3.7.0
to investigate the sensitivity of tropical depression due to
surface forcing. The model runs with two domains as grid
resolution 15-km and 5-km in which 31 levels in vertical.
This experiment consists of two parts. Results showed part
one is to study the suitable four the planetary boundary
layer parameterization schemes. Results showed that MRF
model scheme produced the stronger storm which Blackadar
scheme produced the weaker storm. Reason, in different two
schemes, PBL dept in MRF scheme determined from shear and
temperature profile, and PBL data of MRF model scheme is
nonlocal mixing while PBL dept of Blackadar scheme
determined from only temperature profile. However, as
compared all products with observation. ETA scheme was
better product than others. Reason, in this scheme used
kinetic energy calculation and data is the local mixing.
Results showed part two is to select ETA scheme to study a
simulation of fictitious parameters (soil temperature and
soil moisture) by investigating the effects of surface
forcing on tropical depression development. Results showed
that increasing soil temperature and soil moisture effected
to the surface forcing (surface flux, moisture flux and
momentum flux) is increasing. Surface forcing related to
wind speed at nearly inner-core of storm, and the wind speed
related to the upward mass flux. Therefore, surface forcing
changed to increase then the wind speed and to the upward
mass flux are increased then tropical storm is strong
development. In the other word, the storm development
changing is due to surface forcing.
Development and implementation of
the lagrangean scalar transport model for environmental
problems in Singapore strait
Pavlo Zemskyy, Tropical Marine Science Institute, NUS
The talk reviews theoretical principles of scalar
transport modeling using the Lagrangean approach. Random
walk technique is utilized to account for turbulent
diffusion in the water column, and a quantitative
consistency between Eulerian and Lagrangean methods is
demonstrated.
Several case studies are considered, involving contaminants,
sediments and larvae population dynamics.
Three dimensional temperature
modeling for the South China Sea using remote sensing
techniques
Daniel Twigt, WL | Delft Hydraulics, The Netherlands
The South China Sea’s (SCS) seasonal, large-scale
temperature cycle is governed to a large extend by the
monsoon. This phenomenon modulates the large-scale
circulation, transport and mixing as well as the exchange
processes with the Pacific Ocean and the East China Sea.
Also, significant variations in net surface heat flux will
contribute to the large-scale, seasonal temperature cycle.
As a result, a seasonal mixed layer temperature cycle of
over 6oC occurs in the northern SCS regions and between 2oC
and 4oC in the southern regions. Over the central
SCS temperature stratification is observed throughout the
year, while over the shallow northern and southern regions
atmospheric forcing and large-scale transport will attribute
to a seasonal breakdown of the stratified system.
The objective of this study is to assess the large-scale
three-dimensional temperature cycle of the SCS and to
develop a corresponding hydrodynamic model that is resolving
the monsoonal response. Due to the significant spatial and
temporal scales, sea level anomalies observed by satellite
altimetry and Sea Surface Temperature (SST) observed by
satellite radiometer play an essential role in this study,
both to assess the SCS physical system and for modelling
applications. The model is setup using the Delft3D-FLOW
hydrodynamic modelling package and applies an orthogonal
spherical-curvilinear and boundary fitted grid in the
horizontal. In the vertical a sigma-layer approach is
applied. In the deep SCS regions the model depth is
truncated based on a reduced depth approach (Gerritsen et
al, 2001). For surface heating the so-called Ocean heat flux
model of Delft3D-FLOW is used. At the open model boundaries
water level and lateral transport forcing is applied. The
model does not resolve tidal forcing. An extensive
sensitivity analysis is performed, with model forcing and
validation data both for a climatological year and for the
year 2000. The models temperature accuracy is subsequently
improved by assimilating remotely sensed SST data using a
nudging method. On seasonal scales, the model represents the
large-scale transport, surface heating and stratification
with reasonable accuracy. Without SST nudging a mean
difference of 1.75oC is observed with respect to
validation data. By nudging SST the mean difference
decreases with 15% to 1.5oC.
Review of the squall line convection
theory
Tieh-Yong Koh, Nanyang Technological University
Drawing mainly upon 3 papers (Rotunno, Klemp and Weisman,
1988; Weisman and Rotunno 2004; Bryan, Knievel and Parker
2006), we review the RKW theory for squall line convection.
The essential idea is that the negative buoyancy of the cold
pool in a squall line is balanced by low-level wind shear,
leading to an optimal structure where updrafts are vertical
squall intensity is maximized. The controversy surrounding
these claims of the theory are examined in the light of
recent multi-model comparison in the literature.
How to "compress" convective
tropical atmosphere
Jun-Ichi Yano, Meteo France
The climate system is extremely complex. Simple
analytical theories often do not offer more than
physical insights. More than often their relevance in a
realistic system is hard to decide. In this
talk, I propose "compression" as an alternative approach
than to traditional “simplification”
approaches for disentangling the complexity of the climate
system by taking the tropical convective
atmosphere as an example. The notion of the "compression" is
taken from the multi-resolutional
analysis in mathematics with the wavelet as their principal
tool. The image compression is the most
intuitive example that we see in using the computers: images
can be compressed in the wavelet space
by dismissing the small expansion coefficients. As a result,
the image data size can substantially
be reduced without essentially scarifying the image quality.
The same principle can be applied to a set of partial
differential equations (e.g., primitive
system) describing the climate system. From a diagnostic
point of view, essential physical processes
working in a GCM simulation can be extracted by this
compression approach. I take a GCM-simulated
MJO as a example to explain how this approach works with use
of a discrete orthogonal wavelet set.
This approach can furthermore be employed in the prognostic
sense: the model equation system itself
can be "compressed", and the system can be time-integrated
in this "compressed" space.
The parameterization problem can be solved in a much more
logical manner under this principle: in
essence, a compression of the original full system provides
a parameterization. A compression of a
cloud-resolving model by a finite element approach (segmentally-constant
approximation) will be
presented as such an example.
Seasonal climate predictability
over tropical areas
Zhaohui Lin, Institute of Atmospheric Physics, Chinese
Academy of Sciences
Climate predictions become more and more important in the
planning of economical development and protection of the
people from disasters, so many international research
instituitons and projects have been deeply involved in the
exploitation of predictability and the making of predictions
of seasonal-to-interannual climate variation.
In this talk, the dynamical seasonal prediction system
developed at the Institute of Atmospheric Physics (IAP-DCP)
will firstly be descried. Then the hindcast experiments for
the period 1980-2000 by using IAP DCP will be introduced,
and the seasonal predictability of Asian monsoon system will
be investigated by using the hindcast results. The variance
analysis method proposed by Rowell (1998) will be adopted
for the study of potential predictability of atmospheric
general circulation, where the ratio of external variance to
internal variance (Re) has been calculated from the ensemble
hindcast experiment, with high value of Re for higher
predictability. Then the potential seasonal predictability
of 500hpa geopotential height, surface air temperature and
rainfall over tropical region has been evaluated, together
with the seasonal predictability of East Asian winter
monsoon.
Meanwhile, the hindcast results from the DEMETER project and
APCC project have also been adopted for the study of the
predictive skills for different climate variables,
especially over tropical regions.
Temporal intermittency of
passive-scalar decay
Jacques Vanneste, University of Edingburgh, UK
In smooth random flows, the concentration of passive
scalars decays exponentially in the long time limit. The
corresponding decay rate is deterministic and can be
interpreted as a Lyapunov exponent for the
advection-diffusion operator; it provides what is arguably
the most basic characterisation of the decay. However,
theoretical studies of passive scalar decay typically give
predictions not for this Lyapunov exponent but instead for
the decay rate of ensemble-averaged quantities. Temporal
intermittency, when it is significant, makes the
relationship between the two types of decay rates highly
non-trivial, so that it becomes difficult to infer the
Lyapunov exponent from the decay rates of ensemble-averaged
quantities. We illustrate this point using the simple
example of random (unidirectional) sinusoidal flows. The
case of random phases provides an extreme example of
intermittency: the Lyapunov exponent and ensemble-averaged
decay rates scale with the diffusivity in a different manner
because the ensemble-averaged decay is controlled by rare,
highly atypical realisations of the flow.
Quantifying transport and mixing in
idealised flows and in the atmosphere
Peter Haynes, University of Cambridge, UK
Transport and mixing of atmospheric chemical species is a
vital part of the chemical-climate system. Transport and
mixing processes in the atmosphere operate on scales from
millimeters to thousands of kilometers. In certain parts of
the atmosphere the large-scale `layerwise two-dimensional'
flow appears to play the dominant role in transport and in
the stirring process that leads ultimately to true
(molecular) mixing at very small scales. There is therefore
much in common with the fluid dynamics of `chaotic
advection' or `Batchelor-regime turbulence'. My talk will
describe some of the theoretical tools have been developed
in trying to understand transport and mixing in idealised
chaotic-advection flows. I will also discuss the extent to
which these theoretical tools can be used in conjunction
with observational data on large-scale velocity fields to
quantify different aspects of transport and mixing in the
atmosphere.
Local diagnostic of mixing and the
distribution of dynamical barriers
Bernard Legras, Ecole Normale Supérieure, France
Based on the assumption of a zonal structure for the
large scale flow, mixing barrier properties at the
tropopause and in the lower stratosphere are often
elucidated by diagnostics based on zonal means. The lack of
event resolution along the longitude hinders such
diagnostics in detecting local mixing events like barrier
breaks. For
such events, their spatial localization is nevertheless the
most important feature affecting tracer distribution. Here
we propose the derivation of a local diagnostic for mixing.
The diagnostic combines forward Lyapunov calculation
(describing the stretching experienced by a tracer in the
future) with backward Lyapunov calculation (containing the
information on the tracer local gradient orientation due to
the previous advection). The diagnostic is shown to be
linearly correlated with the effective diffusivity in zonal
means, so that it can be expressed in diffusion units. The
application of this new diagnostic shows the modulation of
the subtropical barrier in connection with ENSO and NAO,
focusing on barrier breaks, and maps the threee dimensional
structure of mixing in the lower stratosphere.
Wave modelling and data
assimilation scheme for wave forecasting
S. A. Sannasiraj Indian Institute of Technology Madras,
India
The importance of wave prediction has been greatly felt
during world war period and subsequently, lot of efforts was
made for the development of the wave predictive models.
Currently, third-generation wave models are in practice for
wind-wave modeling. WAM, a third generation wave model has
been successfully implemented at various regional seas
worldwide. In our application, we have setup WAM in the
South China Sea and Indian seas. The model was implemented
to supplement the circulation and transport modeling
efforts, especially in predicting and assessing the
structure of physical parameters in the water column.
To enhance the wave predictive capability of the wave model,
WAM, a data assimilation module along with a forecasting
model is built over the WAM. Majority of the popular data
assimilation techniques in use today would provide an
improved estimate of the system state up to the current time
level based on measurements. From a forecasting viewpoint,
this corresponds to an updating of the initial conditions of
a numerical model. The standard forecasting procedure is
then to run the model into the future, driven by predicted
boundary and forcing conditions. The problem with this
methodology is that the updated initial conditions quickly
disappear. Thus, after a certain forecast horizon the model
predictions are no better than from an initially uncorrected
model. Our study considers a novel approach to wave data
assimilation and demonstrates that through the measurement
forecast (made using so-called local models), entire model
domain can be corrected over extended forecast horizons
(i.e. long after updated initial conditions have become
disappeared), thus offering significant improvements over
the conventional methodology.
Thoughts on the chaotic advection
vs turbulent mixing in the context of tropical convection
Tieh-Yong Koh, Nanyang Technological University
I shall attempt to discuss our concepts of passive tracer transport in tropical convection along two lines of thought: (1) chaotic advection by a regular flow, as detailed by Julio Ottino, Stephen Wiggins, George Haller and Tamas Tel; (2) mixing by a turbulent process, such as in classical notions of turbulent plumes and non-precipitating cumuli. Some (limited) observations on mixing properties obtained from the literature will be presented. A synthesis is sought to accommodate both views and hopefully (or ambitiously) a direction can be found to crystallize numerical experiments that will dilineate both regimes in tropical convection.
Haze episode in Singapore - Oct
2006
Chui Wah Yap, The National Environment Agency, Singapore
Smoke haze episodes have been a recurrent environmental phenomenon in the Southeast Asian region for many years. The most recent haze episode occurred in 2006, during which weak El Nino conditions aggravated the haze situation. This haze episode lasted longer and affected a much larger area than the one that occurred in August 2005. Moderate resolution data from polar orbiting satellites has become critical to operational haze monitoring efforts. The wide coverage of the satellite images was able to augment sparse ground observations in this region.
Turbulence closure problem for
stably stratified flows
Sergej Zilitinkevich, University of Helsinki, Finland
We propose a new turbulence closure model based on the budget equations for the basic second moments: turbulent kinetic and potential energies: TKE and TPE, which comprise the turbulent total energy: TTE = TKE + TPE; and vertical turbulent fluxes of momentum and buoyancy (potential temperature). Besides the new concept of the TTE, other key points are: non-gradient correction to the traditional formulation for the flux of buoyancy (potential temperature), and advanced analysis of the stability dependence of anisotropy of turbulence. The proposed model affords the existence of turbulence at any gradient Richardson number, Ri. Instead of the critical value of Ri separating the turbulent and the laminar regimes, the model includes its threshold value, between 0.2 and 0.3, which separates two turbulent regimes of essentially different nature: fully developed, chaotic turbulence at low Ri and weak, strongly anisotropic turbulence at large Ri. Predictions from the proposed model are consistent with available data from atmospheric and lab experiments, DNS and LES.
The influence of large,
semi-organised eddies on the convective heat / mass
transfer: revision of the classical theory and environmental
modelling applications
Sergej Zilitinkevich, University of Helsinki, Finland
Close to the surface large coherent eddies consisted of
plumes and downdraughts cause convergence winds blowing
towards the plume axes, which in turn cause wind shears and
generation of turbulence. This mechanism strongly enhances
the convective heat/mass transfer at the surface and, in
contrast to the classical formulation, implies an important
role of the surface roughness. In this context we introduce
the stability dependence of the roughness length. The latter
is important over very rough surfaces, when the height of
the roughness elements becomes comparable with the
large-eddy Monin-Obukhov length. A consistent theoretical
model covering convective regimes over all types of natural
surfaces from the smooth still sea to the very rough city of
Athens is developed and comprehensively validated against
data from measurements at different sites and also through
large-eddy simulation (LES) of the convective boundary layer
(CBL). Good correspondence between model results, field
observations and LES is achieved over a wide range of the
surface roughness lengths and CBL heights. The proposed
theory opens new opportunities to improved parameterization
of the convective heat/mass transfer in environmental
models.
High resolution atmosphere
– ocean – water ecosystem modelling for coastal zones
Sergej Zilitinkevich, University of Helsinki, Finland
Coupled atmosphere-ocean-ecosystem modelling is a superior way of formalizing and testing our knowledge about coastal marine environment and solving the problem of how the rational management of aquatic resources should be organized. The quality of forecasting is determined by the adequate description of physical and ecological processes, adequate spatial resolution and high accuracy of numerical schemes. Simpler models can be used for educational purposes or at the preliminary estimation stage, whereas reliable water resources management requires the most realistic and complete three-dimensional modelling. This lecture presents the coupled model system developed at EMI and VitaMare in cooperation with numerous groups in Europe. The air pollution transport model HILATAR linked with the operational weather forecasting model HIRLAM are coupled on-line and off-line with the marine system model FRESCO to compose a user-friendly, operational forecasting system suitable for end-users. The coupled atmospheric-sea-ecosystem model suite FRESCO integrates the following sub-models.
Atmosphere
- weather forecasting model HIRLAM
- meso-scale non-hydrostatic model
- air pollution model HILATAR
- atmosphere-ocean interaction model
Hydrosphere
- meso-scale non-hydrostatic marine circulation model
- two-equation (k-w) turbulence model
- wind wave narrow directional approximation model
- marine ecosystem model
- suspended material transformation model
- oil-slick model
Author Rein Tamsalu
