Fall 1998
Special Seminar:
Testing GCM parameterization schemes: Sensitivity of the tropical
water-vapour profile to GCM vertical resolution.
Adrian Tompkins
Max Planck Institute for Meteorology
Hamburg, Germany
4:30pm, Thursday, Aug. 6, 1998
Note: This seminar will be in LSC 4258, Psychology Wing
Special Seminar:
Representing Bottom Boundary Layers
in Ocean General Circulation Models
Anand Gnanadesikan
Atmospheric and Oceanic Science Program
Princeton University
3:00pm, Thursday, Aug. 27, 1998
Please note special time.
Description:
The bottom boundary layer is a hard problem for ocean general circulation
models. Level coordinate models have trouble resolving the pressure
gradient and in maintaining dense water properties. Isopycnal layer
models have trouble getting the density contrast between the bottom layer
and the interior right. Sigma-coordinate models have trouble with
pressure gradient errors. This talk will explore a new approach in which
a piecewise-sigma coordinate is turned on in appropriate regions,
depending on the alongslope stratification of neutral density. It will
demonstrate that this approach greatly improves the representation of
bottom boundary layer physics. Impacts on a number of idealized problems
and on a global general circulation model are presented.
SPECIAL SEMINAR
Determining Shelf Circulation from Density Observations
Michael Dowd
Satlantic, Inc.
Thursday, Sept. 10, 1998
1:15 p.m.
LSC 4258 (Psychology Wing)
(NOTE Special time and place)
ABSTRACT:
This talk will investigate the estimation of coastal and continental
shelf circulation from scattered point observations of the density
field. This problem has a long history in oceanography and is
receiving increased attention due to advances in both numerical
ocean modelling and data assimilation methods. The purpose of this
talk is to motivate a dynamically consistent treatment of density and
velocity. We first discuss (and illustrate) the commonly used
'diagnostic' method for determining circulation from density.
Problem areas include the vorticity imbalance resulting from the
interaction of the prescribed density field and the bathymetry, and
the well-known barotropic indeterminacy (i.e. the 'level of no
motion' problem). These shortcomings may be alleviated by allowing
the density and velocity fields to co-evolve. Dynamically consistent
density and velocity is then obtained, but at the expense of
introducing significant complexity into both the dynamics and the
underlying statistical estimation problem. We examine such a joint
estimation procedure in the context of an idealized limited-area
shelf circulation model, and propose an approach which estimates
buoyancy fluxes across model open boundaries using density
observations in the model interior.
Fully-nonlinear Solitary Internal Waves
Kevin Lamb
Department of Applied Mathematics
University of Waterloo
4:30pm, Thursday, Sept. 10, 1998
Abstract: Nonlinear solitary-like internal waves are common features
in stratified coastal regions where they are generated by
tidal currents over topographic features. Observed waves are
often very large. Many features of these waves can be
understood using weakly-nonlinear wave theories however
these theories break down if the wave amplitude is too
large. In this talk I will review current knowledge of
fully-nonlinear solitary waves and make some comparisons
between the predictions of weakly-nonlinear theory and
fully nonlinear waves.
SPECIAL SEMINAR
Seasonal Circulation on the Western and Central Scotian Shelf
Charles Hannah
Coastal Oceanography
Bedford Institute of Oceanography
TUESDAY, Sept. 15, 1998
1:15 p.m.
LSC 4258 (Psychology Wing)
(NOTE Special day, time and place)
ABSTRACT.
Seasonal mean circulation fields for the western and central
Scotian Shelf have been estimated through a synthesis of
historical temperature and salinity observations, and a comprehensive
shelf circulation model using realistic geometry and tidal forcing.
The shelf-scale and bank-scale circulation will be
discussed with an emphasis on the role of geography; in particular the
proximity to major freshwater sources,
the location of deep-water connections to the deep ocean,
the distance from the tidally energetic Gulf of Maine, and
the topography.
SPECIAL SEMINAR
Predicting Subtidal Circulation over the Eastern Canadian Seaboard
Jinyu Sheng
Department of Physics and Physical Oceanography
Memorial University of Newfoundland
Thursday, Sept. 17, 1998
1:15 p.m.
LSC 4258 (Psychology Wing)
(NOTE Special time and place)
ABSTRACT.
It is a great challenge to make reliable predictions of subtidal
circulation over the eastern Canadian seaboard. In examining barotropic
shelf circulation, Sheng and Thompson developed a linear barotropic model
and applied it to the Scotian Shelf. We found that our model has
a significant predictive skill for wind-driven currents over the coastal
region with timescales of a few hours to a few days. The model
skill is further improved by assimilating oceanographic data
into the model. The Sheng-Thompson model however is less successful
for predicting shelf circulation with periods longer than a few weeks
during which the ocean baroclinicity plays a more important role.
We recently developed a three-dimensional primitive equation numerical
model known as CANDIE (CANadian version of the DIEcast model) and
applied it to the eastern seaboard including the Gulf of St. Lawrence,
Scotian Shelf and Gulf of Maine. Our immediate goal is to simulate the
multiple-year circulation of the region, allowing the density field to
evolve with the flow. We were encouraged to find the predicted
circulation pattern reproduces many of the observed features including
relatively strong coastal currents over the Gaspe the Scotian Shelf,
strong along-isobath shelf break currents and eddies in deep water
spun off the Gulf Stream.
Calibration and Validation of ERS-1 and ERS-2 scatterometer winds against
marine weather buoys
Yuri S. Geshelin
Department of Oceanography
Dalhousie University
and
Fred W. Dobson
Department of Fisheries and Oceans
Bedford Institute of Oceanography
4:30pm, Thursday, Sept. 24, 1998
Abstract. Interpretation of the data delivered by the European Remote-Sensing
Satellites (ERS-1 and ERS-2) constitutes a problem because of a lack of
understanding of the backscattering process at high wind speeds. Whereas
the relationship of radar normal cross-section to wind speed is well
known at wind speeds less than 20 m/s, the ERS retrieval algorithms are
not reliable at higher wind speeds. Another major problem in ERS
calibration is the lack of accurate marine wind data at high wind
speeds. We have some evidence that at winds greater than 20 m/s the
calibration formulae may be improved. Some relationships are found
between ERS values and concurrent buoy measurements. These relationships
are demonstrated to be lag-dependent and differ for ascending and
descending satellite tracks, which is accounted for by the specific
spatial orientation of storm tracks in the study area. We present the
method for assessing the noise level due to instrumentation error of the
satellite scatterometer.
At high wind speeds, one major problem with the buoys is inadequate and
unreliable measurements of wind speeds. Because the buoy is often
sheltered by the waves, it fails to measure wind speeds correctly.
Therefore, "sea truth" is no longer a valid calibration criterion. We
discuss the behaviour of buoys of different shapes and sensor heights at
rough sea states.
Forecast Error modelling in the 3D-Variational Data Assimilation System
at AES
Mark Buehner
Department of Oceanography
Dalhousie University
4:30pm, Thursday, Oct. 1, 1998
Abstract:
This talk will report on ongoing work to improve the 3-dimensional
variational data assimilation (3D-Var) system at AES. This system is used
in conjunction with atmospheric forecast models to produce routine weather
forecasts for Canada. The analysis system is used to correct the state
vector of the forecast model every 6 hours using all available
observations. To make optimal use of the observed data we need a good
estimate of the uncertainties (error statistics) associated with both the
observations and forecast model. This talk will focus on recent work to
improve the estimates of the error statistics associated with the forecast
model.
On Using Coupled Numerical Models to Improve Weather Forecasts
Harold Ritchie
Atmospheric Environment Service
Halifax/Montreal
4:30pm, Thursday, Oct. 8, 1998
This presentation will start with a brief update on the Atlantic
Environmental Prediction Research Initiative which has recently been
established in Halifax. It will then continue with a detailed example
based on a related study that is nearing completion, with the preparation
of an article entitled "On the Use of Coupled Atmospheric and Hydrologic
Models at Regional Scale" by Robert Benoit, Pierre Pellerin, Nick Kouwen,
Harold Ritchie, Paul Joe and Rick Soulis.
The purpose of this study is to present the capacities offered by coupled
atmospheric and hydrologic models as a new tool to validate and interpret
results produced by atmospheric models. The advantages offered by the
stream flow observations are different from those offered by conventional
precipitation observations. It is demonstrated that the dependence effect
between basins and sub-basins can be very useful, and that the
precipitation integrator effect of the larger basins facilitates the
evaluation of state-of-the-science atmospheric models by filtering out
some of the spatial and temporal variability that complicate the
point-by-point verifications that are more commonly used. The stream flow
permits a better estimate of the water amount fallen over a region.
Comparing the stream flow predicted by the coupled atmospheric-hydrologic
model versus the measured stream flow proves to be sufficiently sensitive
to clearly assess atmospheric model improvements resulting from increasing
the horizontal resolution and altering the treatment of precipitation
processes in the model.
A look at the "solibore" field on the New England continental shelf
Dr. Jim Lynch
Department of Applied Ocean Physics and Engineering
Woods Hole Oceanographic Institution
4:30pm, Thursday, Oct. 15, 1998
Atmospheric Aerosols In The Northern Polar Region
Leonard A. Barrie
Atmospheric Environment Service
Toronto
4:00pm, Thursday, Oct. 22, 1998
Room 5260, Psychology Wing
(Please note special time and place)
ABSTRACT.
Atmospheric aerosols play a key role in several major environmental issues:
climate change, , ozone depletion, pollution of ecosystems and human health.
The northern polar atmosphere is underlain by a biologically-active ocean
and surrounded by highly industrialized continents. In the cold half of the
year, it is particularly susceptible to pollution that is transported on
winds from Eurasia. This is superimposed on a natural background of sea salt
from northern oceans, soil dust from Asian deserts and marine biogenic
particles. Aerosol residence times are much longer in Arctic air
mass(several weeks) than in air further south(several days). The
concentration of aerosol mass is 10 to 20 times higher in winter than in
summer. The north has av much different light regime than further south. It
goes from 24-hours of sunlight in September to constant dark in December
and then back to full light in April. At polar sunrise in March and April,
dramatic changes in photochemically driven gas-to-particle conversion takes
place. An ozone hole occurs at the ground through chemistry that resembles
its stratospheric counterpart. However, instead of chlorine and bromine
originating from perfluorocarbons, they come from sea salt halides that are
chemically reduced by photochemical-heterogeneous reactions at the
atmosphere/Arctic ocean surface. Volatile organic hydrocarbons, that have
accumulated in the northen atmosphere during the dark, react in this
environment to produce organic particulate matter. Atmospheric mercury is
converted from a relatively inert gas to particulate mercury that is
deposited to marine ecosystems much faster. The relative influence of sea
salt, soil dust, marine biogenic and anthropogenic sources on northern
aerosols is discussed. The role of aerosols in climate, ozone depletion and
toxification of ecosystems is explored.
A framework for parameterizing transport by mesoscale eddies in
ocean circulation models
Richard Greatbatch
Department of Oceanography
Dalhousie University
4:30pm, Thursday, Oct. 29, 1998
Some estimates of the subtidal circulation in the Western Bank region
Dr. Gleb Panteleev
Department of Oceanography
Dalhousie University
4:30pm, Thursday, Nov. 5, 1998
Abstract:
Mean, density driven circulation in the Western Bank region were
investigated by different approches: dynamic height,
nonlinear diagnostic calculation and variational data assimilation
method. Some results and comparison with current meter data will be
presented.
Potential Vorticity Fluxes in a Shallow Water Model
Andrew Peterson
Department of Oceanography
Dalhousie University
4:30pm, Thursday, Nov. 12, 1998
Abstract: Recent work on new eddy parametrizations has focused upon
the role of eddies in the transport of water mass and tracers
either up the potential vorticity gradient or down the layer
thickness gradient. We will use eddy resolving 1 and 2 layer
models to test these hypothesis in a statistical sense.
3D numerical model of hydrodynamics and sediment transport
using HOS method and GAS grid system
Qimiao Lu
Center for Marine Vessel Development and Research
DalTech
4:30pm, Thursday, Nov. 19, 1998
Abstract:
A 3D numerical modeling of hydrodynamics and sediment transport is
developed by using a new numerical method, namely HOS method, and a
new vertical grid system, namely GAS grid system, to achieve efficient
and stable numerical environment. The HOS method is based on the
operator splitting method and combined with Eulerian-Lagrangian
method, FEM and FDM. The momentum equations and mass transport
equation are partitioned into three subsystems by the operator
splitting method according to physical and mathematical
significance. Each subsystem is discretized using the different
numerical method which can well represent the distinct physical
subsystem. The special grid arrangement for the method can fully fit
to complicated geometry to satisfy the boundary-fitted demands in
coastal engineering applications. The method is proven to be
unconditionally stable and efficient. To improve the numerical
accuracy and capture sharp variations in sediment transport
computations, a nonlinear vertical coordinate transformation, called
"Gradient Adaptive Sigma" (GAS) grid system, has been developed in the
model. GAS grid system transforms the varying water depths into unit
of one and converges the vertical grids at the places where the
vertical gradients of concentration are large. GAS grid system can
increase the numerical resolutions in vertical and capture sharp
variations of concentrations in transport modeling. Several numerical
tests and applications are carried out by the developed model. The
numerical results show that the developed model improve considerably
the numerical accuracy, stability and efficiency.
Title: TBA
AGU Rehearsal
Carolyn Smyth, Steve Henderson, and Phil MacAulay
Department of Oceanography
Dalhousie University
4:30pm, Thursday, Nov. 26, 1998
Simulation of Internal Tides: Experiments with
the Free-Surface CANDIE Ocean Model
Youyu Lu
Department of Oceanography
Dalhousie University
4:30pm, Thursday, Dec. 3, 1998
Abstract.
The CANDIE (Canadian version of Diecast) ocean model has recently
been modified to include a free surface.
In this talk I will present results from two experiments using
the CANDIE model. The first experiment deals with an idealized
situation in which (i) the flow field varies only in the vertical and
one horizontal direction (ii) water depth is constant in both the
deep and shallow water and with a linearly sloping shelf break
in between (iii) the density is initially uniform horizontally with
constant vertical gradient (iv) perturbations are so small that most
nonlinear terms can be dropped from the model. I will show that the
results from CANDIE are in good agreement with analytic solutions
obtained using the linear theory of Craig (1987). In the second
experiment we apply the fully nonlinear, three-dimensional model to
the Gulf of St. Lawrence using a summer density climatology to
initialize the model's density field. The boundary conditions are
obtained using an incremental data assimilation method in which a
linear shallow water model is used to minimize the difference between
observed and predicted sea surface elevations using the adjoint
method. The model generated co-amplitude and co-tidal charts agree
well with the assimilated tidal amplitudes and phases. More
encouragingly the predicted tidal currents are in good agreement with
an independent set of current measurements.
Climate of the Northwest Atlantic as seen from hydrographic data
Igor Yashayaev
Ocean Circulation Section
Ocean Sciences Division
Bedford Institute of Oceanography
4:30pm, Thursday, Dec. 10, 1998
Abstract.
Not long ago we deeply believed that
1) there is a quasi-equilibrium state of the ocean or a norm,
that can be used to reveal and trace any anomalous events
occuring in the marine environment,
2) we are able to construct such state by more or less sophisticated
averaging of existing observations.
Analyses of the most recent data show that we have to review this
concept and be more careful using
the "climatology" in both data analysis and models.
The upper waters on the either of the Subpolar Front have different
tendencies
and dominant scales of the interannual-to-interdecadal variability.
The Labrador Sea Water (LSW) has greatly changed both its properties
and distribution over time. It reached a maximum in temperature and
salinity in the late 60s - early 70s, while during the 90s it has
become colder, fresher, denser and deeper than at any time in this
century. The tendency changed around 1994 and now the LSW is steadily
getting warmer, saltier and lighter.
Between the early 70's and 90's North-East Atlantic Deep Water (NEADW)
and Denmark Strait Overflow (DSOW) also became colder and fresher over
a large portion of the North-West Atlantic.
The remnants of Antarctic Bottom Water (ABW) entering the subpolar
gyre have been getting colder in the past two decades, but the rate of
this change was less than that for DSOW, resulting in a change of
spatial gradients in the bottom layer.
The change in the the properties of the upper, intermediate and deep
waters throughout the North Atlantic resulted in changes of dynamic
heights and potential vorticity, those of a great importance for
understanding and modelling the ocean dynamics.
Capsize Analysis for Ships with Water Shipping on and off the Deck
C.C. Hsiung
Centre for Marine Vessel Development and Research
DalTech
4:30pm, Thursday, Dec. 17, 1998
Abstract: A prediction method of water shipping on and off the deck was
developed and the computed results were validated by model tests. The
amount of water trapped on the deck varies with time. The shallow water
wave equations are solved to obtain the force caused by water flow on
deck. The nonlinear equations of ship motion were solved in the time domain
to study the capsize of vessels with the varying mass of water flow on deck.