| August 16 |
Timescales of the NAO and Annular Modes: Tropospheric Feedbacks and Stratospheric Coupling |
Ed Gerber, Columbia University |
The North Atlantic Oscillation (NAO) and Annular Modes are the dominant patterns of intraseasonal variability in the extratropical atmosphere. I will present work with a hierarchy of dry, primitive equation atmospheric GCMs to isolate the dynamics governing the temporal structure of these patterns, with a focus on the influence of stationary planetary waves. In the first, a model driven with the Held and Suarez (1994, BAMS) forcing, we investigate an eddy-mean flow feedback within the troposphere that slows movement of the extratropical jet, so extending the persistence of the annular modes. The addition of realistic topography and land-sea contrast (approximated by a large scale thermal perturbation) breaks the
strong feedback found in simulations with zonally uniform forcing. In the second, we focus on the influence of the stratosphere on the tropospheric annular modes. We begin with the model of Polvani and Kushner (2002, GRL), but add topography to generate a planetary wave flux into the stratosphere -- and hence a more variable polar vortex. We find that the mean structure and variability of the vortex in the model is very sensitive to the amplitude of the topography, and Northern Hemisphere-like variability, with the correct frequency of warming events, occurs only for a relatively narrow range of topographic heights. In these simulations we find an active coupling that connects the strength of the polar vortex to the latitude of the tropospheric jets, as found on climatological timescales by Polvani and Kushner. |
| August 23 |
No colloquium - All college faculty meeting |
| August 30 |
Understanding Ocean General Circulation using the CSU Spin Tank |
A. Butler, L. Ciasto, T. Ellis, L. Van Roekel, and T. Ito |
We use the CSU spin tank as an illustrative tool to discuss underlying fluid dynamics for the large-scale ocean circulation, in particular, wind-driven gyres and abyssal circulation. Through the group study course, a group of graduate students designed, built and tested new laboratory experiments highlighting the role of surface wind stress, interior mass sources and sphericity of the rotating planet (beta effect). We will demonstrate and discuss how these mechanisms work together to produce the observed features of ocean general circulation in the laboratory scale. |
| September 6 |
No colloquium - New student reception and department picnic |
| September 13 |
Some Aspects of the Inner Core Structure of Hurricanes |
Wayne Schubert, CSU Atmospheric Science |
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| September 20 |
Modeling and Predicting Three Asian Monsoon Systems |
Thomas Chase, CU Department of Geography |
Monsoons are hydro-climatological circulations of enormous importance to regional water supplies and agriculture and also to the global circulation of the atmosphere and therefore global precipitation patterns. I compare the effects of landcover change and land surface
hydrology on summer monsoonal circulation in three distinct Asian monsoon regions, India, Northeast Asia and Southeast Asia from general circulation modeling experiments and in statistical prediction models based on observations. We find that the correct land surface hydrology is vital to simulating regional monsoons as well as the global climate and can have impacts on global land surface temperature of similar magnitude to that observed over the past century. Additionally, the predictability of east Asian monsoon rainfall is nearly doubled when pre-monsoon season vegetation characteristics are included in a forecast model over using sea surface tempertaures alone. Both results highlight the importance of the land surface in simulating and predicting both regional
and global climate. |
September 26
(Wednesday) |
Cloud Radiative Feedbacks and Their Role |
Sandrine Bony-Lena, LMD/IPSL, France |
Cloud radiative feedbacks are critical for predicting the global climate response to anthropogenic radiative forcings. They are critical also for understanding and simulating other aspects of the climate system, including the intra-seasonal variability of the tropical atmosphere. The role of cloud-radiative feedbacks in the climate system, as well as key uncertainties, will be discussed and illustrated based on observations, idealized modelling studies, and results from GCMs participating in the 4th Assessment Report of the IPCC. Different projects or initiatives
aiming at improving our understanding and our evaluation of cloud processes in large-scale models will be discussed. |
| October 4 |
Aerosol Influences on Clouds and Precipitation |
Bill Cotton, CSU Atmospheric Science |
In this talk I will overview the influences of aerosols on clouds and precipitation. Material used for this talk will be derived from our own work using RAMS and as well as a summary of other investigators contributions. I will begin with an overview of how aerosol indirect effects are treated in RAMS. Then I will discuss how aerosols influence boundary layer stratocumuli and cumuli, wintertime orographic clouds, deep convective clouds, and hurricanes. The theme of the talk is that once aerosols alter the precipitation process the cloud responses are highly nonlinear making the representation of those processes very challenging and in many cases unpredictable. |
| October 11 |
No colloquium - Faculty retreat |
October 17
(Wednesday) |
Dynamical Seasonal Prediction: Model
Fidelity vs. Predictability |
Jagadish Shukla, Distinguished University Professor at George Mason University and President of the Institute of Global Environment and Society |
|
| October 25 |
Can Aquaplanets Predict A GCM's Climate Sensitivity? |
Brian Medeiros |
Clouds remain the largest source of uncertainty in projections of future climate and estimates of climate sensitivity using general circulation models (GCMs). We use two current generation atmospheric GCMs that have very different climate sensitivity to show that a simplified framework -- the aquaplanet -- can replicate the sensitivity of more realistic configurations. In both Earth-like and aquaplanet configurations, tropical low clouds play a leading role in determining the GCM's climate sensitivity. The low cloud response in the GCMs is opposite in sign, and appears to arise from differences in the models' parameterized physics. These results suggest that the idealized aquaplanet configuration is an appropriate laboratory for investigations of cloud effects in GCMs, and is an attractive option because of its relative simplicity. Ongoing work makes use of the aquaplanet configuration to better characterize the tropical cloud response in GCMs. |
| November 1 |
Enhanced CO2 outgassing in the Southern Ocean from a positive phase of the Southern Annular Mode |
Nikki Lovenduski, CSU Atmospheric Science |
An ocean model is used to investigate the interannual variability in the flux of CO2 between the atmosphere and the Southern Ocean, with particular emphasis on the role of the Southern Annular Mode (SAM). Approximately half of the air-sea CO2 flux variability is coherent
with the SAM. The positive phase of the SAM is found to be associated with anomalous outgassing of CO2 nearly everywhere in the Southern Ocean. Budget analyses indicated that the primary cause of the outgassing is enhanced surface pCO2, caused by elevated dissolved inorganic carbon (DIC) concentration. These anomalies in DIC are primarily a result of the circulation changes associated with the southward shift and strengthening of the zonal winds during positive phases of the SAM. If the current secular trend in the SAM persists, the CO2 flux anomalies from the Southern Ocean could have a substantial positive feedback on the global carbon cycle. |
| November 8 |
Changes in the Arctic Ice Cover: Greenland Ice Sheet and Surrounding Oceans |
Konrad Steffen, CIRES |
Air temperatures on the Greenland ice sheet have increased by 4 deg. C since 1991. The ice sheet melt area increased by 30% for the western part between 1979-2006. The increasing trend in the total area of melting bare ice is unmistakable at 13% per year, significant
at a probability of 0.99. Hence, the bare ice region, the wet snow region, and the equilibrium line altitude have moved further inland and resulting in increased melt water flux towards the coast. Increase in ice velocity in the ablation region and the concurrent increase in melt water suggests that water penetrates to great depth through moulins and cracks, lubricating the bottom of the ice sheet. New insight was gained of subsurface hydrologic channels and cavities using new instrumentation and a video system during the melt peak in August 2007. These new results will be discussed in view of the rapid increase in melt area and mass loss of the Greenland ice sheet due to increasing air temperatures. |
| November 15 |
Contrasting climate effects of fire in boreal and tropical regions
|
Jim Randerson, University of California Irvine |
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| November 22 |
No colloquium - Thanksgiving break |
| November 29 |
Surface Chemistry and Physics: Implications for Terrestrial Polar
Science and Planetary Science |
Christopher Boxe, Jet Propulsion Laboratory |
Field measurements have only recently unearthed the discovery that the terrestrial polar snowpack is one of the most photochemically active environments on Earth. Due to the strong feedback between the sunlit snowpack and the air over it, such chemistry in the terrestrial polar regions is now recognized to strongly influence the overlying atmospheric boundary layer and potentially the free troposphere due
to mixing. It is becoming more accepted that iodine chemistry, intimately linked to NOx chemistry, plays a major role in controlling ozone concentrations in the overlying boundary layer. In addition, iodine forms aerosols and contributes to the transformation of elemental mercury to its reactive form. Considering that polar tropospheric
ozone exhibits a large radiative forcing, iodine forms aerosols, and reactive mercury is detrimental to the polar
ecosystems, it is imperative to understand the source of iodine and NOx. An overview covering laboratory results, which elucidates the properties of ice as a reaction medium, and provides the framework (i.e., the surface chemistry and physics) for the creation of the first multi-phase model for the polar regions, CON-AIR (Condensed Phase to Air
Transfer Model) will be presented. This model accounts for the measured iodine and NOx in the terrestrial polar boundary layer. These model simulations have implications for: 1) being available as a tool for the atmospheric science community to perform additional modeling, laboratory, and field studies relevant to the dynamic interplay of the polar boundary layer and the snowpack; 2) the influence of heterogeneous surface photochemistry on overlying gas phase chemistry of planetary bodies with tenuous atmospheres; and 3) carrying out laboratory and modeling investigations relevant to planetary surfaces (e.g., Martian, Enceladus, and interstellar clouds) to study the possible formation and chemistry of life forming molecules. |
| December 6 |
Dynamic and Thermodynamic Controls on Tropical Cyclone Intensity Change |
Mark DeMaria, CIRA |
A simplified dynamical system is developed that can predict many aspects of tropical cyclone intensity change. The system is based on a logistic growth equation for the time evolution of the maximum sustained surface wind, which is a first-order differential equation that constrains the solution to lie between zero and the maximum potential intensity (MPI). The MPI is estimated from thermodynamic soundings along the track of the storm. Once the MPI is specified the primary free parameter of the model is the growth rate, which is estimated from the vertical wind shear and a parameter that measures the convective instability. The convective instability parameter is determined from an entraining parcel model, which utilizes the thermodynamic formulation of Ooyama (1990). The convective instability and vertical shear provide a two-parameter phase space which can be used to classify the behavior of tropical cyclone intensity changes. This simple dynamical system can also be used for real time intensity forecasting and to evaluate the impact of thermodynamic soundings from satellite-based retrievals. |
Please contact Dave Randall [randall (at) atmos.colostate.edu] or Scott Denning [denning (at) atmos.colostate.edu] for more information.
