Session B33: Focus Session: The Physics of Climate I

A Test for Periodic and Quasi-Periodic Fluctuations in Past Climate Change Data

In this work the temperature fluctuations for a number of proxy data sets were analyzed to test for periodic and quasi-periodic fluctuations in climate changes in the past. The data sets analyzed indicate temperature functions which could be modeled using amplitude and frequency modulated sinusoidal waves. Data for the past 2000 years were tested and they show select periods of 11 years, 100 years, 300 years and 600 years. Longer term data (million years) indicate periods of 21000 and 41000 years as predicted by Milankovitch.   

Nonlinear/Non-Gaussian Data Assimilation

Data and models, with their inherent uncertainties and errors, are blended within a Bayesian framework with the aim of improving estimates of dynamic processes. This process, called {\it data assimilation}, is said to be responsible for significantly better weather/climate forecasts. Nonlinear/non-Gaussian processes, however, pose special conceptual and computational challenges. In the context of generic transport problems of importance in climate and weather a strategy which I have been investigated involves adding physically based constraints, leading to smaller but higher quality ensembles with which to produce estimates. I will describe some of the tradeoffs and their implications on filtering and forecasting.   

The Role of Radiation in Organizing Tropical Convection

Convective organization regulates the radiation emitted to space, and therefore is important for the global heat budget. Organized convection--regions of intense convection surrounded by large cloud-free regions--permit more longwave radiation to escape and therefore may constitute a net cooling effect, while more scattered convection promotes greenhouse warming. Models which simulate the spontaneous organization of deep tropical convection--self-aggregation--suggest that radiative cooling in response to water vapor content is essential for convection to spontaneously organize. Multiple equilibria--steady states which maintain persistent precipitating convection or are completely dry--in small domains with weak temperature gradients (WTG) are analogous to dry and moist regions in larger scale simulations of convective self aggregation. We explore the role of radiative cooling in multiple equilibria. Interactive radiative cooling suppresses convection in the dry state and it permits multiple equilibria over a larger parameter range. However, multiple equilibria still exist with fixed radiative cooling. This suggests that while interactive radiation is conducive for organizing convection, it is not essential. This study elucidates radiation's role in convective organization.   

Stratified shear flow instability: Application to oceanic overflows

The Earth's thermohaline circulation provides major oceanic transport of heat and salinity and is an important determining factor in the climate of nearby land areas. We address the stability of overflow currents of heavier water moving into a region of less heavy quiescent fluid using experimental measurement of wall bounded stratified boundary currents under controlled laboratory conditions. In these currents, the stratification acts to stabilize the flow whereas the shear associated with the moving current produces turbulent kinetic energy and has the potential for destabilizing the flow. Our experimental measurements using particle-image velocimetry and laser-induced fluorescence allow the simultaneous acquisition of velocity and density fields, respectively. Rather than using traditional time-averaged statistics, we consider the stability of unperturbed sections of the interface and use a measure of the overturning or mixing called the Thorpe length. We present evidence for universal behavior in the normalized Thorpe length probability distribution and the general properties of the system under increasingly stable conditions. We relate these properties to realistic circumstances in the ocean.   

Eliminating Major Tornadoes in Tornado Alley

In my recent paper, I propose that major tornadoes in Tornado Alley can be eliminated by building east-west ranged walls, 300 meter high and 50 meter wide. The work has received much attention, but some meteorologists are against the idea, claiming that the major tornadoes in Tornado Alley are not related to the collisions between northbound warm air flow and southbound cold air flow because supercells are not at the collision front. In this talk, we will show that wind tunnel experiments and airplane wing tip vortices clearly demonstrate that vortices produced by air mass collisions are usually not at the collision front because of the extremely volatile condition over there; they are either near the ends or at side of the collision fronts. When the warm and moist wind collides with the cold wind violently in Tornado Alley, similarly, the supercell storms cannot be right at the collision fronts, but are near the ends or at sides of the collision fronts. While only a small portion of vortices in the warm air side may have a chance to develop into tornadoes, the major tornadoes in Tornado Alley indeed start from the air mass clashes. If we can weaken such violent air mass collisions, we will eliminate the major tornadoes in Tornado Alley.   

Random Focusing of Tsunami Waves

When waves propagate through a weakly scattering, correlated random medium, the consecutive effects of small focusing events give rise to the phenomenon called branched flow, producing patterns of high intensity fluctuations. As tsunamis are deflected by underwater structures in the depth profile of the ocean floor, we investigate how it affects tsunami propagation and derive the typical length scale on which the highest waves are to be expected. We show that as a consequence of this effect the inaccuracies in the current knowledge of the ocean floor topography can prevent reliable tsunami forecasts on medium to large length scales.   

Energy Dissipation when Internal Wave Beams Reflect from a Slope

Internal wave reflection from a uniform sloping boundary is often analyzed using linear or a weakly nonlinear inviscid theory\footnote[1]{T. Dauxois and W.R. Young, J. Fluid Mech. {\bf390}, 271-295 (1999)}. Under these assumptions for a linearly stratified fluid, Thorpe\footnote[2]{S. A. Thorpe, J. Fluid Mech., {\bf178}, 279-302 (1987)} and Tabaei et al.\footnote[3]{A. Tabaei, T. R. Akylas and K. G. Lamb, J. Fluid Mech. {\bf526}, 217-243 (2005)} derived predictions for the boundary angle where second harmonic generation should be most intense. We previously conducted experiments and simulations that found the angle that maximizes second harmonic generation is given instead by an empirical geometric relationship between the wave beam and boundary angles\footnote[4]{B. E. Rodenborn, D. Kiefer, H. P. Zhang, and H. L. Swinney. Phys. Fluids, 23(2), 2011.}. In the previous study, we used integrated kinetic energy as a measure of beam intensity. We compare these results with a method using energy flux. We also study the energy flux into and out of a surface above the reflection region $E_{\rm out}/E_{\rm in}$ and find high rates of energy dissipation $O$(90\%). The rates remain high even for weakly nonlinear wave beams and with the viscosity reduced by an order of magnitude.   

Lamination in Atmospheric Ozone: A Diagnostic for Tracer Transport Mechanisms

An understanding of ozone variability in the upper troposphere (from $\sim$5 km altitude up to the tropopause level) is critical to assessing the radiative forcing of climate by ozone, and for evaluating the impact of transport on regional air quality. Part of this variability arises in fine-layered ($\sim$0.2 to $\sim$2 km) structures seen in vertical profile measurements of ozone. These laminae are also generally limited on horizontal scales (10's to 100's of km), leading to spatial ozone variability observed on quasi-horizontal coordinate surfaces. Given the relatively long photochemical time constants for ozone in the upper troposphere, most of the observed variability arises from transport rather than photochemistry. There are a wide range of dynamical processes that can generate ozone laminae in the upper troposphere, such as gravity and Rossby waves, convective lofting and detrainment of either high or low ozone amounts from the boundary layer, and intrusions of air masses with high ozone concentrations from the stratosphere. Here, we examine the range of observed laminae characteristics and describe methods for tracing the origins of tropospheric ozone laminae.