| January 18 |
Postponed to February 8 |
Peter Thornton, NCAR |
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| January 25 |
The chemistry of atmospheric particle formation in Cache Valley, UT, and implications for other rapidly urbanizing areas in the west |
Phil Silva, Utah State |
Logan (Cache County), Utah frequently experiences very high PM2.5 mass loadings during wintertime inversions. During a severe inversion during 2004, 17 exceedances of the 24-hour PM2.5 standard (then 65 mg m-3) were observed with 10 days showing mass concentrations above 100 mg m-3. Using an aerosol mass spectrometer, PM2.5 concentrations were found to be dominated by ammonium nitrate, accounting for over 50% of the non-refractory aerosol matter throughout the study and 80% on the highest pollution days. Another 15-20% of the particulate matter was composed of organic carbon which appears from several distinct sources, including woodsmoke, fossil fuel combustion, and secondary processes. The high particle concentration loadings in Utah's Cache Valley result from a combination of unfavorable meteorology dominated by a severe cold-temperature inversion, a mix of agricultural and urban emission sources, and a confined geographical area. Since the initial 2004 study, research has shown much lower concentrations of particulate matter indoors during acute inversions and that agricultural emissions may contribute significantly to the organic carbon fraction of PM2.5. As an area undergoing rapid urbanization but still retaining a strong agricultural-based economy, is the Cache Valley a harbinger of future air pollution problems for other rural areas of the interior West that experience harsh wintertime meteorology or is it just a unique situation? |
| February 1 |
Measuring atmospheric CO2 from space: The NASA Orbiting Carbon Observatory Mission |
Dave Crisp, NASA Jet Propulsion Laboratory |
The NASA Orbiting Carbon Observatory (OCO) will make space-based measurements of atmospheric CO2 with the precision, resolution, and coverage needed to characterize CO2 sources and sinks on regional scales and quantify their vari ability over the seasonal cycle. This Earth System Science Pathfinder (ESSP) mission will be launched in late 2008 and will fly in a 705 km altitude, 1:26 PM sun-synchronous polar orbit that provides near-global coverage of the sunlit hemisphere with a 16-day ground track repeat cycle. OCO carries a single instrument that incorporates 3 high resolution grating spectrometers that will make boresighted measurements of reflected sunlight in near-infrared CO2 and molecular oxygen (O2) bands. These measurements will be combined to provide spatially resolved estimates of the column-averaged CO2 dry air mole fraction, XCO2. The instrument collects 12 to 24 XCO2 soundings/second over the sunlit portion of the orbit, yielding 200 to 400 soundings per degree of latitude, or 7 to 14 million soundings every 16 days. Thick clouds and aerosols will reduce the number of soundings available for XCO2 retrievals by 80-90%, but the remaining data is expected to yield XCO2 estimates with accuracies of ~0.3 to 0.5% (1 to 2 ppm) on regional scales every month.
A comprehensive ground-based validation network is being implemented to ensure that measurements meet these requirements. These measurements are expected to improve our understanding of the nature and processes that regulate atmospheric CO2 enabling more reliable forecasts of CO2 buildup and its impact on climate change. |
| February 8 |
Effects of carbon-nitrogen cycle coupling on climate-carbon cycle dynamics |
Peter Thornton, NCAR |
Coupling between the biogeochemical cycles of carbon and nitrogen in terrestrial ecosystems places important constraints on biophysical and biogeochemical dynamics across time scales from seasons to millenia. Starting with an introduction to some of the fundamental aspects of C-N coupling, this presentation will explore the consequences for climate-carbon cycle feedbacks of introducing a fully prognostic treatment of terrestrial carbon and nitrogen cycles into the land component of the NCAR Community Climate System Model (CCSM). Results from offline, partly coupled, and fully coupled simulations are compared. Taken together, these results show that C-N coupling has multiple significant impacts on climate-carbon cycle feedback mechanisms. Interactions with land cover change and other biogeochemical cycles are also discussed. |
| February 15 |
Exploring and exploiting the coupling of sulfate aerosol with gas-phase precursors using the adjoint of a global chemical transport model |
Daven Henze, Cal Tech |
We present the adjoint of the global chemical transport model GEOS-Chem, focusing on the chemical and thermodynamic relationships between sulfate-ammonium-nitrate aerosols and their gas-phase precursors. Explicit inclusion of the processes that govern secondary formation of inorganic aerosol is shown to afford efficient calculation of model sensitivities and allows us to assess the dependence of sulfate and nitrate aerosol concentrations on emissions of SOx, NOx, and NH3 on a global scale. The adjoint model is used to assimilate speciated aerosol measurements from surface monitoring stations. Additionally, the potential for remotely sensed gas-phase measurements of aerosol precursors to further constrain model simulations is explored. |
| February 22 |
Why it is so hard to explain the glacial pCO2? |
Taka Ito, CSU Department of Atmospheric Science |
Polar ice cores show the intriguing fact that atmospheric partial pressure of CO2 was lower by approximately 100 ppm during the ice ages relative to the preindustrial condition. A number of hypotheses have been put forward for the cause of the lowering of atmospheric pCO2, however, the community has not yet identified the principal mechanism for this major perturbation in the global carbon cycle. In this presentation, I will first review some of the previous hypotheses, and then focus our discussion on the ocean's biological pump, one of the leading candidates, using a new theory that the impact of biological carbon sequestration can be effectively quantified using the concept of "preformed" nutrient. This new theory
helps to bring conceptual model and GCMs into an excellent agreement, but fails to explain the glacial pCO2 with stronger biological pump
in consistency with all the existing geochemical evidences. Finally I discuss the implication of this result for the glacial-interglacial climate change and the future research strategies. |
| March 1 |
A comparison of convection and convective transport in single-column and cloud-resolving models |
Leslie Ott, NASA GSFC |
Parameterized convection and convective transport of chemical constituents in NASA's GEOS-5 GCM are examined using a single column version of GEOS-5. Convective events observed during field campaigns were simulated using both cloud-resolving models (CRMs) and the
single-column model (SCM) and the results compared to airborne observations of species concentrations. In addition, cloud properties simulated using CRMs and the SCM are compared. The sensitivity of the parameterized convection in the SCM to small temperature and moisture perturbations is investigated, as is the
sensitivity to parameters used in the model's convective scheme. |
| March 8 |
Ocean acidification |
Ken Caldeira, Carnegie Institution |
Most of the carbon dioxide released to the atmosphere from the burning of coal, oil, and gas is ultimately absorbed by the ocean. When carbon dioxide reacts with seawater, it forms carbonic acid. In high enough concentrations, this acid can corrode the shells and skeletons of marine organisms. The current trend of increasing carbon dioxide emissions may produce chemical conditions that are challenging to many types of marine organisms, including corals, with unknown consequences for marine ecosystems globally. This talk will describe some of what is known about the chemistry of ocean acidification, its environmental consequences, and the scale of actions that would need to be taken to avoid the risk of environmental catastrophe in our oceans. |
| March 15 |
No colloquium - Spring break |
| March 22 |
GFD experiments in climate and paleoclimate |
John Marshall, MIT |
Processes controlling the total meridional transport of energy and its partition between the atmosphere and ocean are discussed in the context of both observations and a hierarchy of models. In particular we experiment with a coupled atmosphere, ocean and ice
model of an aqua-planet which is entirely covered by ocean but in the absence of land. A mean state is found in which ice caps reach down to 55 degrees of latitude and both the atmosphere and ocean comprise eastward and westward flowing zonal jets. Remarkably, the partition of energy transport between the atmosphere and ocean is much the same as the present climate, with the ocean dominating in the tropics and the atmosphere in middle to high latitudes.
By introducing meridional barriers in to the ocean we go on to study the role of geometrical constraints on the ability of the ocean to transport heat
from equator to pole and mediate the latitude of the ice edge. The role of openings in the meridional barrier - representing Drake Passage and an equatorial
passage - is discussed in the context of paleoclimate. |
| April 2 |
Tropical Circulations in a Hierarchy of Atmospheric Models |
Dargan Frierson, University of Chicago |
We investigate the large scale dynamics of the tropical atmosphere using a variety of atmospheric models, from the simple to the comprehensive. The primary model we use is an idealized moist GCM, with simplified physical parameterizations that isolate the effect of latent heat release on the general circulation. This model fills a largely unexplored piece of a model hierarchy, which allows for both clean tests of simple
theories, and comparisons with full GCM's. We use the model to investigate the dynamics of the Hadley circulation, ITCZ, and convectively coupled waves,
comparing with atmosphere-only and fully coupled general circulation models. Our experiments with convection schemes classify the ways in which convection
can influence the ITCZ and Hadley circulation, suggesting a mechanism of deep tropical control on the Hadley cell. The experiments with convectively coupled equatorial waves proposes and explains a simple way to slow down and intensify equatorial Kelvin waves and other tropical variability, which is tested in a full GCM. |
| April 5 |
Monsoons as regime transitions of a Hadley cell in an idealized aquaplanet GCM |
Simona Bordoni, UCLA |
In simulations with an idealized aquaplanet GCM, the tropical mean meridional circulation undergoes rapid regime transitions in the course of a seasonal cycle. A transition in early summer is characterized by strengthening and broadening of the cross-equatorial
winter Hadley cell, rapid relocation into the subtropics and intensification of precipitation, and reversals in the upper-level and lower-level winds. A reverse transition occurs in late summer. The simulated transitions resemble the onset and end of Earth's monsoons. Consistent with similar dry simulations, the rapid rearrangements of the circulation mark shifts in the leading balance of the vertically integrated zonal momentum equation, from regimes in which the
momentum flux divergence by the large-scale baroclinic eddies dominates near the center of the Hadley cell, to regimes in which the eddy momentum flux divergence is negligible and the mean momentum flux dominates. The summertime convergence zones form in the subtropics just equatorward of the lower-level moist static energy maximum, which is colocated with the boundary between the winter and summer cell. These results show that land-sea contrasts are not essential to reproduce monsoon-like transitions and raise the question as to the role that feedbacks between the large-scale baroclinic eddies and the
mean flow, identified as fundamental driving mechanisms in the simulated transitions of the Hadley cell, play in Earth's monsoons. |
| April 16 |
The thermal structure of the extratropical tropopause |
Thomas Birner, University of Toronto |
The tropopause region is both affected by, and itself affects, a wide range of atmospheric processes encompassing radiation, dynamics on a variety of scales, chemistry and microphysics. The region therefore represents one of the most sensitive atmospheric regions in the climate system. In this talk, a novel analysis of the thermal structure of the extratropical tropopause is presented. A climatology in tropopause-based
coordinates obtained from high-vertical resolution radiosoundings reveals the ubiquitous existence of a tropopause inversion layer (TIL) in the extratropics. Temperature strongly increases with altitude in this TIL in contrast to the isothermal behaviour in, e.g., the U.S. standard atmosphere for midlatitudes. Associated with this temperature increase is a strong local maximum in static stability just above the tropopause, with decreasing values towards the lower stratosphere. CMAM - the Canadian Middle Atmosphere Model, a comprehensive chemistry climate model - exhibits a TIL. However, a much weaker TIL exists in the
data assimilation version of CMAM. It is hypothesized that this discrepancy is mainly due to data assimilation acting to smooth the sharp vertical curvature in temperature at the tropopause. In support of this hypothesis, NCEP Reanalysis data only shows a weak hint of a TIL. ERA40 Reanalysis data exhibits a somewhat more realistic TIL, partly due to its higher horizontal and vertical resolution. In the last part of this talk an outlook on possible causes of the TIL is given. It will be shown that the strength of the TIL is linked to the strength of the stratospheric residual circulation. |
| April 19 |
Figuring out why your model is wrong |
Vince Larson, University of Wisconsin - Milwaukee |
It is often easy to see when an atmospheric model disagrees with data. It is usually much harder to locate the ultimate sources of model error. It is particularly difficult to diagnose errors in a model's structure, that is, errors in the functional form of the model
equations. One technique that may help is parameter estimation, that is, the optimization of model parameter values. Typically, parameter estimation is usedsolely to improve the fit between a model and observational data. In the process, however, parameter estimation may cover up structural model errors.
In a quite opposite application, parameter estimation may be used to uncover the ways in which a model is wrong. The basic idea is to separately optimize model parameters to two different data sets, and then identify parameter values that differ between the two optimizations. When no single value of a particular parameter fits both datasets, then there must exist a related structural error. The parameter estimation method that we use may be interpreted
as an ensemble Bayesian technique. It produces an entire multi-variate distribution of parameter values. It may prove useful for a wide range of computationally inexpensive models. We apply the method to a model of boundary layer clouds, uncover the presence of a structural model error, revise the model structure, and obtain improved results. |
| April 26 |
The Madden-Julian oscillation in a general circulation model and observations |
Eric Maloney, Oregon State University |
The Madden-Julian oscillation (MJO) of the tropical atmosphere is characterized by complex interactions between convection and the large-scale circulation. General circulation models (GCMs) have a notoriously difficult time in simulating the MJO. A GCM that produces
a realistic MJO in certain key measures is analyzed. MJO events in the model show behavior consistent with a discharge-recharge mechanism, in which a buildup
of moist static energy precedes convection, and a discharge occurs during and after convection. Latent heat flux and horizontal advection of moist static
energy are the two largest terms in the vertically-integrated moist static energy budget. The magnitude and phase relationship (relative to precipitation) of model latent heat flux variability is consistent with that derived from satellite and buoy observations. A sensitivity study shows that MJO convection in the model responds strongly when latent heat flux variations are suppressed. Somewhat unexpectedly, MJO-related horizontal advection of moist static energy is dominated by eddy fluxes of humidity. These results suggest that variations in eddy activity on synoptic timescales play a key role in regulating
model MJO convection. Strong variations of eddy kinetic energy occur during observed MJO events. |
| May 3 |
Reserved for faculty interview |
Please contact Dave Randall [randall (at) atmos.colostate.edu] or Scott Denning [denning (at) atmos.colostate.edu] for more information.
