Abstracts
Speaker: Richard
Dowden, Emeritus Professor of Physics
Institution: University of Otago, New Zealand
Title: Global
Lightning Location by VLF TOGA
When: Monday,
September 15 2008 at 4pm.
Where: MP 408
Abstract:
Radio location
of lightning, and lightning hazard prediction, over some developed countries
is obtained by a high density of ground stations. Thus the US National
Lightning Detection Network (NLDN) uses 100 ground stations to cover the 48
contiguous states (or about 10 million square km), with a corresponding
ground station spacing of about 300 km. Cloud-to-ground lightning strokes are
located by both the time of arrival (TOA) of the radio impulse and direction
of its arrival using crossed loops (magnetic direction finding, or MDF). Both
methods require line-of-sight (ground wave) propagation at light speed,
avoiding all ionospheric reflections (sky waves). This is achieved by
ignoring all but the first few microseconds of the radio impulse; thus
excluding the sky wave which arrives later.
Such a
system provides high accuracy (~1km), but to be commercial it needs wealthy
customers to meet the costs of setup and operation. In our case we were
initially engaged in lightning research in a sparsely populated area not covered
by any location network, so we set up our own based on VLF propagation in the
Earth-Ionosphere Wave Guide (EIWG).
Our system
is similar in many ways to the global Omega Navigation System, now superseded
by GPS. The essential difference being that the lightning strokes are the
transmitters to be located, while the receiving stations are already known.
The Omega system consisted of eight transmitters optimally placed around the
world, each transmitting a set of eight frequencies in turn. To get your location
the receiver measured the phases of all the VLF transmissions. The lightning
impulse (officially named sferic by the International Union of Radio Science)
is already very wide-band (ULF to optical), so we measure the rate of change
of phase with respect to frequency: returning the time of arrival of the
sferic in seconds. The pulse (sferic) propagates at the waveguide group
velocity, so we call this the Time Of Group Arrival (TOGA).
Our system
works (as did Omega) because VLF propagation in the EIWG is very stable and
long range. It is also cheap to set up and run. In fact (once fully
established) we intend to make it free for non-commercial use. The location
accuracy is never worse than 10 km RMS.
Speaker: Meiyun Lin
Institution: University
Wisconsin-Madison
Title: Regional
Atmospheric Chemistry in Asia
When: Wednesday,
September 17 2008 at 4pm.
Where: MP 408
Abstract:
Atmospheric
chemistry in Asia is strongly affected by
local emissions, monsoon circulation, regional transport, and global pollution
inflow. This work employs global and regional models, satellite observations,
and ground-based data to distinguish the mechanisms controlling acid
deposition, nitrate and sulfate aerosols, and tropospheric ozone (O3)
throughout Asia. In evaluating tropospheric
NO2 column, we found a more than 50% discrepancy between the
Community Multiscale Air Quality (CMAQ) model calculated values and the GOME
(Global Ozone Monitoring Experiment) satellite retrieval over industrial area
of eastern China
in March and December. The pronounced discrepancy is not only due to the
uncertainty in the bottom-up emission inventory used in the model but also to
the reliability of the GOME retrieval scheme. By using boundary conditions
from the global Model for Ozone and Related Tracers (MOZART), the full
chemistry CMAQ regional model estimated surprisingly high levels (10-40%) of
sulfur imported to Asia from other parts of
the world. Our most recent work examines the seasonal and diurnal variability
of O3 in the boundary layer over Asia using the regional CMAQ model and
recently available observational data including six sites in China and nine sites in Japan. We find that the seasonal
cycle of simulated O3 in the boundary layer does not exhibit the
observed sharp low-O3 drop in July and August that is evident in Beijing and at two mountain sites in central east China.
In addition to model inability in simulating cloud activities and convective
mixing as suggested by previous studies, our analysis suggest that the
systematic overprediction of summertime O3 over central east China
and central Japan are also attributed to the uncertainties in photochemical
production of O3, inter-annual variability of monsoonal intrusion
of low-O3 marine air masses during East Asian summer monsoon, and
overprediction of nighttime O3 in urban and rural areas. Building
on these past studies, we are now working with the Weather Research and
Forecasting Model with Chemistry (WRF-Chem) and the Community Atmosphere
Model with Chemistry (CAM-Chem) to examine how the regional processes such as
surface driven convection, frontal activity, and land-sea breezes can affect
the intercontinental transport of O3, carbon monoxide, aerosols,
and other climate- and heath-relevant pollutants.
Speaker: Edwin Schneider
Institution: COLA / George Mason University
Title: The role of
weather noise in climate variability
When: Monday,
September 22 2008 at 4pm.
Where: MP 408
Abstract:
A method
has been developed to test the null hypothesis that low frequency surface
climate variability in a coupled GCM is the response to weather noise
forcing. The method also provides opportunities to better understand the
mechanisms of simulated and observed climate variability. The method has two
parts:
1)
"Weather noise" surface fluxes are defined as the residual, after
the feedbacks from the boundary evolution are removed from the total surface
fluxes by AMIP-like AGCM ensemble simulations.
2) An
interactive ensemble (IE) CGCM, in which a single OGCM is coupled to the mean
of an ensemble of the AGCMs is then forced by this weather noise.
The IE CGCM
will reproduce the observed SST evolution and the null hypothesis will be
satisfied, except for errors in the model, the surface fluxes, and the ocean
initial state, as long as there is no internal ocean “weather
noise” or coupled instabilities. External forcing must also be taken
into account. The method is applied both to understanding low frequency
variability in CGCM simulations and to diagnosing the mechanisms of observed
low frequency variability. Using the NCEP reanalysis surface fluxes and a
state of the art coupled GCM, the observed North
Atlantic low frequency SST evolution for the latter half of the
20th century can be simulated reasonably well, indicating that it is weather
noise forced. Local heat flux weather noise forcing plays a major role. Ocean
dynamics also has important effects on the SST evolution.
Speaker: Gloria Manney
Institution: Jet
Propulsion Laboratory, California Institute of Technology
Title: Trace Gas Evolution
in Recent Satellite Datasets: Relationships to the Stratospheric Polar Night
Jet, Upper Tropospheric Jets and the Tropopause
When: Monday,
September 29 2008 at 4pm.
Where: MP 408
Abstract:
A method
has been developed for categorizing the location and characteristics of upper
tropospheric jets and of the lower reaches of the stratospheric polar night
jet that extends into the upper troposphere/ lower stratosphere (UTLS). This
method is applied to define the position, width and dynamical characteristics
(e.g., windspeed/direction, potential vorticity, temperature, static
stability, etc) of the primary subtropical jet (STJ) core, as well as similar
information on multiple jets in the extratropical (ET) UTLS. The polar night
jet in the UTLS is characterized at each level by its position, width, and
dynamical characteristics. Jet characteristics during quasi-isentropic
stratosphere-troposphere exchange (STE) events and seasonal evolution of jet
structure are investigated in Goddard Earth Observing System Version 5
(GEOS-5) and other meteorological analyses. Satellite trace gas measurements
from several current instruments, including the Aura Microwave Limb Sounder
(MLS), Atmospheric Chemistry Experiment-Fourier Transform Spectrometer
(ACE-FTS) and the Aura High Resolution Dynamics Limb Sounder (HIRDLS), are
studied to further our understanding of the seasonal evolution ET-STE events
in relation to the STJ and the tropopause.
Speaker: Qinbin Li
Institution: University of California, Los
Angeles
Title: What can we
learn about upper tropospheric CO2?
When: Monday,
October 20 2008 at 4pm.
Where: MP 408
Abstract:
It is generally assumed that upper
tropospheric CO2 exhibits little variability. We present clear evidence to
the contrary. Our analysis centers on CO2 mixing ratios from Atmospheric
Infrared Sounder (AIRS) retrievals. The AIRS middle to upper tropospheric
(~500-300 hPa) CO2 data show unequivocally large spatial and temporal
variability in the northern mid-latitudes in summer. For example, there is a
2-3 ppmv longitudinal gradient in the CO2 concentrations across North America in July. Aircraft in situ observations
from the COBRA and ICARTT campaigns support such large variability. In
addition to the surface sources/uptakes, the distributions of AIRS CO2 are
modulated by large-scale circulation such as the mid-latitude jet streams in
both hemispheres and by synoptic weather systems including the warm conveyor
belt. How well can chemistry and transport models(CTMs) simulate upper
tropospheric CO2 including its seasonal cycle and trend? Is there a
semi-annual cycle in upper tropospheric CO2, and if so, what are the
controlling processes? These questions and the impact of stratospheric sudden
warming on polar upper tropospheric CO2 will also be addressed in this talk.
Speaker: Tim Palmer
Institution: European
Centre for Medium-Range Weather Forecasts
Title: Seamless
prediction of weather and climate
When: Wednesday,
October 22 2008 at 4pm.
Where: MP 408
Abstract:
"Seamless
Prediction" has become a bit of a buzz-phrase in the last year or so,
and it probably means different things to different people. Here I use the
phrase to invoke the objective of bringing insights and constraints from
numerical weather prediction into the climate-change arena. I will offer
three examples of the potential of seamless prediction. The first is where
6-hour weather forecasts have been used to constrain significantly the
probability distributions of climate sensitivity (to doubling of CO2).
The second is the use of diagnostics on the reliability of seasonal forecast
multi-model ensembles to calibrate regional predictions of precipitation
climate change. The third is the use of stochastic parametrisations in
numerical weather prediction models as the basis of next-generation
Probabilistic Earth Systems Models.
Speaker: Michael Gausa
and Kolbjorn Dahle
Institution: Andoya Rocket Range, Norway
Title: The ALOMAR
Observatory and Andoya
Rocket Range
When: Thursday,
October 23 2008 at 11am.
Where: MP 606
Abstract:
ALOMAR, the Arctic Lidar Observatory
for Middle Atmosphere Research is a research facility initiated for a special
height region. Since the introduction one and a half decade ago, ALOMAR has
developed into a modern and international recognized research facility.
ALOMAR is part of the Andøya
Rocket Range,
the northernmost launch site for scientific sounding rockets and balloons.
ALOMAR hosts a variety of remote sensing instruments operated by groups from
the US, Canada, Japan and five European
countries, and has several times received funding from the EU
commission’s excellence programmes for infrastructures. ALOMAR’s
location at high latitude (69N) gives rise to topical scientific questions in
all atmospheric layers. The present talk will introduce some of these topics
and show how the participating international scientists gain through the
synergy generated by the set of instruments located at ALOMAR. Synergy is not
only gained through co-location of different instruments, investigating in
the same height region; co-operation across the atmospheric layers can
achieve snap shots of physical atmosphere properties from ground to the
ionosphere. The proximity to the Rocket
Range and the local
airport (3.2km runway) offers unique opportunities for large scale ground
based, air- and rocket-borne atmosphere studies.
Speaker: Prashant D.
Sardeshmukh
Institution: CIRES, University of Colorado
Title: An
interpretation of non-Gaussian Statistics in Geophysical Data
When: Monday,
October 27 2008 at 2pm.
Where: MP 408
Abstract:
The statistics of climate variables
are often found to be non-Gaussian, and this fact is often attributed to
non-linear dynamics. Nonetheless, we find that many features of the
non-Gaussian statistics can also be reconciled with linear dynamics that are
stochastically perturbed. A mixture of stochastic noises whose amplitudes are
either system state-independent ("additive noise") or linearly
state-dependent ("multiplicative noise") can produce not only
systematic but also skewed non-Gaussian probability distributions if the
additive and multiplicative noises are correlated. Furthermore, one expects
such correlations from first principles. A generic stochastically generated
skewed (SGS) distribution can be analytically derived from the Fokker-Planck
equation for a single-component system. In addition to skewness, all such SGS
distributions have power-law tails, and a striking property that the kurtosis
K is always greater than 1.5 times the square of the skew S. Remarkably, this
K-S inequality is found to be satisfied by circulation variables even in the
observed multi-component climate system. This can be understood in terms of a
principle of "Diagonal Dominance" in the equations for the higher statistical
moments of mutli-component linear systems.
To clarify the nature of the
stochastic noises (turbulent adiabatic versus diabatic fluctuations)
responsible for the observed non-Gaussian statistics of atmospheric flows at
the jet stream level, a long 1200-winter simulation of the northern winter
climate is generated using an adiabatic atmospheric general circulation model
forced only with the observed long-term mean diabatic forcing as a constant
forcing. Despite the complete neglect of diabatic variation, the model
reproduces the observed K-S relationships, and also the spatial patterns of
the skewness and kurtosis of the daily vorticity variations at the jet stream
level. This suggests that the stochastic generators of these higher moments
of vorticity are mostly associated with local adiabatic turbulent fluxes. The
model also simulates fifth moments that are approximately 10 times the
skewness, and probability densities with power-law tails, as predicted by the
remarkably simple linear theory.
Speaker: Anne Thompson
Institution: Penn. State
Title: Insights
into the Tropical Troposphere and Tropopause Layer from Ten Years of SHADOZ
(Southern Hemisphere Additional Ozonesondes) - 1998-2007
When: Thursday,
November 13 2008 at 2pm.
Where: MP 408
Speaker: Laura Pan
Institution: NCAR
Title: Concept of
the tropopause revisited
When: Monday,
January 6 2009 at 4pm.
Where: MP 606
The tropopause is an important
physical boundary in the atmosphere and a
much watched climate change
indicator. The behavior of the tropopause as a
transport boundary between the
stratosphere and the troposphere has been an
ongoing research topic. This talk
will focus on the extratropical
tropopause. The history and the concept of the 3
existing models of the
extratropical tropopause, i.e., the
thermal, the dynamical tropopause and
the extratropical transition layer,
ExTL, will be reviewed. These models
are examined, using new aircraft and
satellite observations, for how well
they represent the discontinuities,
or the lack of, in the air mass. The
investigations reveal the
relationship of the tropopause definitions and the
transport pathways between the
stratosphere and troposphere. In particular,
these issues will be discussed using
recent observations on NCAR research
aircraft HIAPER during the
Stratosphere-Troposphere Analyses of Regional
Transport experiment in 2008
(START08).
Speaker: Isla Simpson
Institution: Imperial College
Title: Solar
influence on tropospheric circulation via the stratosphere
When: Monday,
January 12 2009 at 4pm.
Where: MP 606
There is increasing evidence that
changing solar activity over the 11-year
solar cycle influences the Earth's
climate. However, as yet the mechanisms
involved remain uncertain. With the
now widely accepted view that there is
a two way dynamical coupling between
the stratosphere and troposphere, a
possible explanation for the
tropospheric response to solar activity is
through a dynamical response to the
larger heating that occurs in the
stratosphere.
A weakening and poleward shift of the
mid-latitude jets along with a
weakening and expansion of the Hadley
cells and a poleward shift of the
Ferrell cells is found at solar
maximum compared to solar minimum in the
observations. This is accompanied by
a banded increase in temperature in
the mid-latitudes. Previous results
using a simplified general circulation
model (GCM) have demonstrated that a
similar response to that seen over the
solar cycle can be produced by simply
heating the lower stratosphere in the
equatorial region.
Spin-up ensemble experiments using
this simplified GCM have now been used to
further investigate the mechanisms by
which altered stratospheric heating
can produce tropospheric circulation
changes. Results suggest that changes
in eddy momentum flux are important
in producing the response.
Investigations using the
quasi-geostrophic index of refraction have shown
that the change in vertical
temperature gradient around the tropopause is
primarily what influences eddy
propagation to initially trigger the
response. Furthermore, there is a
feedback with zonal wind changes in the
troposphere influencing eddy
propagation there.
Speaker: Phil Austin
Institution: University of British Columbia
Title: Aerosol
indirect effects: Hot or Cold?
When: Wednesday,
January 28, 2009 at 3pm.
Where: MP 606
Abstract:
The impact of aerosols on clouds and
climate is often represented by two pathways that assume: i) increasing
aerosol concentrations reduce cloud droplet size and increase cloud
reflectivity (the Twomey effect) while ii) reducing collision-coalescence and
decreasing precipitation (the lifetime effect). Both of these effects are
presumed to exert a negative radiative forcing on the atmosphere, with an
average impact in global climate models of more than -1 W/m^2.
Modelling and observational work by
many investigators has added a wide variety of additional cloud-aerosol
interactions to this pair, including aerosol-induced changes to boundary
layer stability and large-scale boundary layer circulations that in turn
effect cloud liquid water path and rain rate, introducing significant new
uncertainties to the calculation of aerosol indirect forcing. I will present
a new technique to retrieve cloud droplet size, cloud thickness and their
uncertainties from nighttime satellite imagery, and show how it might be used
to constrain some of these cloud/aerosol interactions.
Speaker: John Worden
Institution: Jet
Propulsion Laboratory
Title: Insights
into the water cycle using satellite measurements of water vapor isotopes
from Aura TES
When: Monday,
February 2, 2009 at 4pm.
Where: MP 606
Abstract:
Water Isotopes are useful
measurements for estimating the sources and processes controlling the
hydrological balance in the atmosphere and on or in the earth because lighter
isotopes preferentially evaporate, heavier isotopes preferentially condense,
and the isotopic composition of the primary water source, the ocean, is well
known. In this talk I discuss the remote sensing of tropospheric
concentrations of water vapor and its isotopes using new space based infrared
measurements from the Tropospheric Emission Spectrometer. I will then discuss
some recent applications of these measurements such as estimating the balance
of moist processes controlling hydrology over tropical regions and estimating
evaporation and condensation strengths.
Speaker: Tianjun Zhou
Institution: Chinese
Academy of Sciences
Title:
Understanding and Attributing East Asian Climate Change
When: Monday,
February 23, 2009 at 4pm.
Where: MP 606
Abstract:
Associated with the regime shift of
global climate system around the late 1970s, East Asian climate has
experienced an inter-decadal transition. Summer precipitation has increased
in central eastern China along the middle and lower reaches of Yangtze River
valley, whereas it has decreased in North China. In the mean time, against
the surface warming elsewhere, a surface cooling has been observed over
central eastern China. There are controversies in the mechanisms responsible
for the observed change. Recent work of the authors' group demonstrates the
dominance of large scale circulation change on the regional scale East Asian
climate change. This presentation will summarize the authors’
progresses in understanding and attributing East Asian climate change. A
significant interdecadal cooling with vivid seasonality and three-dimensional
structure is revealed in the upper troposphere and lower stratosphere over
East Asia. The seasonally-dependent upper tropospheric cooling leads to a
clear seasonality of interdecadal changes in the atmospheric general
circulation and precipitation against their normal seasonal cycle over East
Asia. Possible causes for the robust interdecadal change are discussed. Some
model results supporting the proposed mechanism will be presented, including
a set of Indo - Pacific Ocean forcing experiments, global and tropical sea
surface temperature forcing experiment, CLIVAR C20C experiments and WCRP
CMIP3 experiments.
Speaker: Gang Chen
Institution: Massachusetts
Institute of Technology
Title: Quantifying
the Eddy Feedback and the Persistence of Zonal Index in an Idealized
Atmospheric Model
When: Monday,
March 16, 2009 at 11am.
Where: MP 408
Abstract:
An idealized atmospheric model is
used to examine what determines the strength of the eddy feedback and the
persistence of zonal index. The strength of the surface frictional damping on
the zonal index is varied, and an external zonal momentum forcing is included
to compensate for the momentum change associated with the friction change so
as to keep the climatological jet latitude and shape.
The model can generate a nearly
identical climatology and leading mode of the zonal mean zonal wind for
different frictional damping rates, except when the jet undergoes a regime
transition. As the surface friction is increased, the strength of eddy
feedback is enhanced, but the zonal index becomes less persistent. A simple
feedback model suggests that the e-folding decorrelation time scale of zonal
index can be determined by the frictional damping rate and the strength of
eddy feedback. The strength of eddy feedback is found to be related to the
instantaneous vertical wind shears near the surface controlled by the
frictional damping. Accordingly, the lagged regression with respect to the
zonal index shows that the eddy heat flux responds roughly linearly to the
magnitude of surface meridional temperature gradient. Furthermore, the
climate response to an external zonal torque is found to be proportional to the
decorrelation time scale, although our simple prediction overestimates the
climate response by a factor of two.
Speaker: Ron Stewart
Institution: University of
Manitoba
Title:
Precipitation in our Canadian winter
When: Monday,
March 30, 2009 at 11am.
Where: MP 606
Abstract:
The issue of precipitation continues
to be one of the key issues for both weather and climate. A great deal of
progress has been made in understanding its formation although the emphasis
has been on rain and hail and, to a lesser extent, snow. What about the other
types of precipitation that we experience in Canada such as wet snow,
freezing rain and ice pellets? These precipitation types bring misery each
year to all parts of the country but little research has actually focussed on
them. This presentation will discuss
some of the fundamental aspects of this precipitation including its
occurrence, morphology, formation and organization within winter storms. Even
though progress is being made, we are still a long way from understanding all
the features of this fascinating precipitation.
Speaker: Beatriz
Monge-Sanz
Institution: University of
Leeds
Title: Data
assimilation winds in the SLIMCAT chemistry-transport model: Stratospheric
transport evaluation
When: Wednesday,
April 1, 2009 at 4pm.
Where: MP 606
Abstract:
The accuracy of the stratospheric transport provided by
meteorological analyses is crucial for off-line chemistry transport models
(CTMs) to produce realistic distributionsof tracers. However, stratospheric
CTMs driven by most existing (re)analyses are known to produce too young
age-of-air distributions, i.e. they overestimate the stratospheric
circulation,and produce excessive subtropical mixing.
In this study the TOMCAT/SLIMCAT CTM has been used to
evaluate different stratospheric (re)analyses from the European Centre for
Medium-Range Weather Forecasts (ECMWF), including experimental datasets from
the ERA-Interim preparatory phase, to assess how the newer analyses compare
with previous ones (e.g. ERA-40 or U.K. Met Office). This work has revealed
CTMs as powerful tools for evaluating meteorological datasets, and
information from our investigations fed into the final set-up for the
ERA-Interim reanalysis production.
Various transport diagnostics have been calculated to
assess the Brewer-Dobson circulation and mixing processes in the different
datasets. Problems detected in the past for ERA-40 have been greatly overcome
in the recent ECMWF reanalysis tests and ERA-Interim, which provide realistic
age-of-air in the lower stratosphere, and better overall agreement with
observations. The use of there more realistic stratospheric (re)analyses
result into more accurate tracers distributions and trends being obtained by
the CTM.
Causes for the improvements in the (re)analyses have also
been explored, revealing theroles that some aspects of the assimilation
technique play in the description of stratospheric transport. The effect of
different frequencies for the winds update in the CTM is also discussed.
Results show there is a threshold to the analysis frequency CTMs can use to
correctly simulate stratospheric transport. In particular, the use of 12
hourly winds results in very unrealistic transport in the stratosphere. Some
transport deficiencies attributed in the past to the use of ECMWF DAS winds
in CTMs appear to be due to the use of unsuitable too long update
frequencies. The effect of different (re)analysis read-in frequencies is
further explored with reanalysis data from the Canadian Middle Atmosphere
Model (CMAM), which enable a clean comparison of trajectories obtained with
3-hourly and 6-hourly atmospheric data.
Speaker: Adam Sobel
Institution: Columbia
University
Title: Atmospheric
Water Vapor: Some Studies of Mechanisms that Control It
When: Monday,
April 13, 2009 at 4:10pm.
Where: MP 606
Abstract:
I will present two studies with numerical models which
address two different aspects of the physics controlling atmospheric water
vapor. In the first, we disable all evaporation of condensate in an
atmospheric GCM while holding the circulation fixed to that of a control
simulation. The difference between the resulting humidity distribution and
that of the control case can be viewed as giving us an upper bound (according
to this model) on the importance of cloud and precipitation microphysics for
atmospheric water vapor. In the second, we focus on the role of large-scale
advection and condensation, using "tracers of last saturation" in
an offline transport model driven by reanalysis data. These tracers allow us
to diagnose the pathways by which air with low relative humidity is produced
by tracing it back to the locations at which it was last saturated. We find
that the dry air at lower and midlevels in the subtropics was to a large
extent last saturated in the regions of the midlatitude jets, rather than in
the outflow of the tropical deep convective regions, indicating that
baroclinic eddies rather than the Hadley circulation are most directly
responsible for the dryness of the subtropics.
Speaker: Isaac Held
Institution: NOAA
Geophysical Fluid Dynamics Laboratory, Princeton, USA
Title: Some
variations on the theme of climate sensitivity
When: Wednesday,
April 15, 2009 at 11:10am.
Where: MP 606
Abstract:
Several aspects of climate feedback analysis and the use of
simple models to analyze observational constraints on climate sensitivity
will be discussed.
Speaker: Seok-Woo Son
Institution: McGill
University
Title: Southern
Hemisphere Westerly Jets in the Future Climate: Interpretation with a Simple AGCM
and Results of Chemistry-Climate Models.
When: Monday,
April 20, 2009 at 4:10pm.
Where: MP 606
Abstract:
The spatial structure of the Southern Annular Mode (SAM)
shows a remarkable resemblance to that of the recent and future circulation
changes in the southern extratropics. By integrating a series of simple dry
AGCM, we examine to what extent the annular-mode-like internal variability
projects onto the climate change. A wide range of parameter study shows that
climate change is not necessarily projected onto the annular mode. It
suggests that the projection of Southern Hemisphere climate change onto the
SAM could be a coincidence.
The future trend in Southern Hemisphere westerlies are
further examined with SPARC/Chemistry-Climate Model Validation (CCMVal) and
IPCC/AR4 model output. It is found that, owing to the disappearance of the
ozone hole in the 21st century, the CCMVal models predict deceleration of
westerlies in southern high latitudes, or negative trend of the SAM index, in
stark contrast to the IPCC/AR4 models. This result suggests that
stratospheric ozone should be more carefully taken into account in the
climate model integrations.
Speaker: Stephen
Griffiths
Institution: University of
Leeds
Title: Global
Modeling of Ocean Tides
When: Thursday,
July 30, 2009 at 3:30 pm.
Where: MP 606
Abstract: A
longstanding challenge in oceanography is the production of accurate global
maps of the amplitude and phase of the ocean tides, from simple
hydrodynamical equations with appropriate astronomical forcing and global
topography. Over the last forty years, such prognostic models have improved
in accuracy, partly due to increases in computing power, but also as the role
of supposedly secondary physical processes have been recognised. Here, the
development of a new prognostic global tidal model is described. Special
emphasis is placed upon accounting for an internal-tide drag in a physically
consistent way, which is achieved via explicit modeling of small-scale internal
waves. Having demonstrated the integrity of this model by comparison with
data-constrained tidal models, we use it to predict the form of the tides
during the last Ice Age, when the ocean depth and coastal configuration were
quite different to those of today.
Speaker: Ralph Lehmann
Institution: Alfred
Wegener Institute for Polar and Marine Research
Title: Two
applications of chemical modelling: Calculation of ascent rates in the
tropical lower stratosphere and the automatic determination of catalytic
ozone production and destruction cycles
When: Friday, July
31, 2009 at 12:00 pm.
Where: MP 606
Abstract: The
presentation gives examples for the gain of information arising from the
combination of observations and a chemical model (A) and the application of
an additional algorithm to better understand the complex interactions in the
chemical model (and also in the atmosphere) (B):
A) Mean ascent rates of air in the
tropical lower stratosphere are
calculated from the combined
information on vertical gradients and temporalvariations of ozone mixing
ratios, supplied by ozonesondes, and the rate ofchemical ozone production,
calculated by a chemical model. This providesestimates independent from
conventional methods, that are based on thecalculation of radiative heating
rates or the analysis of the verticaltransport of chemically inert tracers
(e.g., the water vapour "taperecorder"). The dependence of the
ascent rates on the phase of theQuasi-Biennial Oscillation (QBO) is
investigated in more detail.
B) An algorithm for the automatic
determination of reaction pathways, e.g.catalytic cycles, in complex chemical
reaction systems is presented.The algorithm was applied to analyse output
from a chemical box model thathad been initialised by a three-dimensional
chemistry-climate model. It wasused to determine catalytic production and
destruction cycles of ozone inthe stratosphere and mesosphere. The dominant
ozone destruction cycles atdifferent altitudes in the atmosphere belong to
the following families: HOx(lower stratosphere: tropics, mid latitudes), ClOx
and BrOx (lowerstratosphere: polar spring), NOx and ClOx (upper
stratosphere), HOx(mesosphere).
Speaker: Hiro Yamazaki
Institution: University of
Oxford
Title: Paleo-Data
Constraints on Ensemble Climate Predictions
When: Friday, July
31, 2009 at 3:30 pm.
Where: MP 606
Abstract: We use a
simple energy balance model and discuss a way to improve probabilistic
climate predictions by employing paleoclimate data for the last millennium.
We will also discuss ClimatePrediction.net, an internet-wide distributed
computing framework that allows large-ensemble GCM experiments to be
performed.
Speaker: Colin Price
Institution: Tel Aviv
University
Title: Will a Drier
Climate Result in More Lightning Activity?
When: Tuesday,
August 4, 2009 at 12:00 pm.
Where: MP 606
Abstract: With recent
projections of a warmer climate in the future, one of the key questions is
related to the impact of global warming on thunderstorms, and severe weather.
Will lightning activity increase in a warmer world? Since the majority of
global lightning activity occurs in the tropics, changes in future global
lightning activity will depend on changes in the tropical climate. The latest
IPCC (2007) projections show a partial drying out of the tropical landmasses
as the global climate gets warmer. This is caused by both changes in rainfall
patterns, but also due to increases in evapo-transpiration. We would expect a
drier climate to produce fewer thunderstorms, and less lightning. However,
experimental and modeling studies have shown that as tropical regions dry in
the present climate, they experience greater lightning activity. This paradox
may be explained by noting that while drier climate conditions result in
fewer thunderstorms and less rainfall, the thunderstorms that do occur are
more explosive, resulting in more lightning activity.