Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session HG: GFD: Atmospheric Flows II |
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Chair: Georgios Matheou, Jet Propulsion Laboratory/NASA Room: Long Beach Convention Center 103B |
Monday, November 22, 2010 10:30AM - 10:43AM |
HG.00001: Microscale Measurement in the Atmospheric Boundary Layer: Collapse of Turbulence Harindra Fernando, Eliezer Kit, Ann Dallman During cooling of the Earth's surface in the evening, day-time convection subsides due to cut-off of its energy sources and a stable density stratified layer develops near the ground. In complex terrain, this evening transition from the convective boundary layer to the stable boundary layer is associated with low wind speeds, and hence low shear production of turbulence. Often the wind direction is also variable during the evening transition, and hence the use of probes such as hotwires/films for the measurements of microscale turbulent quantities needs special handling as they require the winds to have a specific alignment with the probe. To circumvent this problem, a combo of co-located sonic and hot-film anemometers, with the former measuring mean winds and aligning the latter in the appropriate wind direction via an automated platform, was successfully designed and implemented. A novel calibration procedure for the probes was also developed. It was found that the evening transition in complex terrain is associated with a sudden collapse of turbulence spectra across the entire spectrum. Observations taken in multiple locations show that the collapse is a complex phenomenon, sometimes showing layering with low rms vertical velocities and in other times showing higher vertical velocities perhaps due to instabilities and billowing. [Preview Abstract] |
Monday, November 22, 2010 10:43AM - 10:56AM |
HG.00002: Jupiter's Zonal Winds: Are They Bands of Homogenized Potential Vorticity and Do They Form a Monotonic Staircase? Philip Marcus, Sushil Shetty It has been hypothesized that the potential vorticity (PV) in Jupiter's atmosphere is mixed in a manner that is analogous to the Phillips effect in the ocean. When the upper ocean is mixed, the salt density distribution changes from a smoothly increasing function of depth to a nearly monotonic {\it staircase} with regions of nearly uniform salt density separated from each other by sharp interfaces where the density gradient is large. It is hypothesized that the profile of PV in Jupiter's east-west zonal winds (visible stripes) is a staircase, decreasing from north to south. Measurements of the Jovian zonal velocity are sufficiently precise to determine vorticity, but the PV also depends on unknown parameters that cannot be observed directly. Therefore, the distribution of PV cannot be tested directly. By using new high-precision observations of Jupiter, we have solved numerically the {\it inverse problem} between the latitudes of $9^{\circ}$S and $39^{\circ}$S and found the PV (and its uncertainties) that best fits the observations. Although we find that the PV distribution is approximately piecewise-constant, the zonal PV is not monotonic. We show that this non-monotonicity is necessary to make the Great Red Spot nearly round (aspect ratio of 1.6), and that without the non-monotonicity, the Red Spot would be highly elongated in the east-west direction and probably unstable. [Preview Abstract] |
Monday, November 22, 2010 10:56AM - 11:09AM |
HG.00003: ABSTRACT WITHDRAWN |
Monday, November 22, 2010 11:09AM - 11:22AM |
HG.00004: Lagrangian coherent structures and transport in hurricanes Doug Lipinski, Kamran Mohseni Hurricane intensity forecasting remains one of the most difficult challenges in weather research. At present, there is a lack of understanding with regards to the appropriate oceanic boundary conditions for the hurricane and the corresponding energy and moisture transport. In this talk, the Lagrangian coherent structures (LCS) present in a numerical simulation of hurricane Rita (2005) are identified. The LCS reveal the low level atmospheric transport in this hurricane and provide insight into the conditions which may strengthen or weaken the hurricane. This information may be used to better focus future research efforts in this area by illuminating the key mechanisms for transport in the low level atmosphere of a hurricane. [Preview Abstract] |
Monday, November 22, 2010 11:22AM - 11:35AM |
HG.00005: Stable stratification in turbulent Ekman layers Oscar Flores, James Riley, Nicholas Malaya, Robert Moser In order to study the day to night transition in the atmospheric boundary layer we perform DNS of turbulent Ekman layers whose height ($h$) is prescribed by an overlying inversion. We will present results from several simulations, with friction Reynolds numbers up to $Re_\tau = u_\tau h/\nu = 2800$ depending on the domain size. In all cases, a quasi-steady state is reached with an adiabatic boundary condition at the ground. Then a constant negative heat flux is applied at the ground, to mimic the radiative cooling of the ground during clear sky nights. The results indicate that the buffer region locally collapses when $L u_\tau/\nu < 100$, where $L$ is the Monin- Obukov lengthscale. In the outer region of the flow, eddies with sizes larger than $L$ are damped by the stratification in times of the order of their eddy turn-over time, even if at those times the mean temperature profile is relatively shallow. These results are consistent with Monin's self-similar theory and with both experimental and field observations. Funded by ARO Grant No. W911NF-08-1-0155 and NSF Grant No. OCI- 0749209. [Preview Abstract] |
Monday, November 22, 2010 11:35AM - 11:48AM |
HG.00006: Stability of a stratified boundary layer flow when shear and stratification are not aligned Julien Candelier, Stephane Le Dizes, Christophe Millet The inviscid stability properties of a boundary layer flow with a tanh velocity profile in a stably stratified fluid with a constant Brunt-V\"ais\"al\"a frequency is examined when the direction of the shear is inclined with an angle $\alpha$ with respect to the vertical direction of stratification. We show that for all Froude numbers there exists a critical angle $\alpha_c$ above which the boundary layer becomes inviscidly unstable. The characteristics of the unstable modes are analysed. For small Froude numbers, unstable modes are shown to be 3D radiative modes with a internal wave structure that extends to infinity. For large Froude numbers, the modes are localized in the boundary layer and their frequency and growth rate are proportional to the Brunt-V\"ais\"al\"a frequency. Non-Boussinesq and compressibility effects on the stability properties are also considered. The results are discussed in the context of atmospheric applications. [Preview Abstract] |
Monday, November 22, 2010 11:48AM - 12:01PM |
HG.00007: Internal waves crossing an interface John McHugh Internal waves in continuously stratified flow in two layers is considered. The interface between layers is defined by a jump in the Brunt-Vaisala frequency, assumed constant in each layer. The density profile is chosen to be continuous across the interface, and the flow is assumed to be Boussinesq. The waves are periodic in the horizontal but modulated in the vertical. A weakly nonlinear approach produces three amplitude equations with cubic nonlinearity, one for incident, reflected, and transmitted wave packets. The results show that a wave-induced mean flow is strongest near the interface and underneath it. Furthermore, this mean flow is discontinuous, and has an oscillatory component. These results provide a likely scenario for higher levels of atmospheric turbulence near Earth's tropopause and other similar interfaces. [Preview Abstract] |
Monday, November 22, 2010 12:01PM - 12:14PM |
HG.00008: Synchronization of modulated travelling baroclinic waves in thermally driven rotating annulus flows Alfonso A. Castrejon Pita, Peter L. Read Synchronization in a fluid dynamical analogue of atmospheric circulation is studied experimentally by investigating the dynamics of a pair of rotating annulus systems, coupled in real time via their thermal boundary conditions, in both periodic and chaotic regimes. The combined effects of differential heating in the horizontal and background rotation leads to the formation of a zonally-symmetric baroclinic jet flow that may become unstable under some conditions to travelling baroclinic waves which may be steady or modulated in amplitude, and a range of more complex spatiotemporal flows. Synchronization tools such as phase analysis and frequency locking are used to study the resulting dynamics of the coupled system and, depending upon the coupling configuration (master-slave or bidirectional), coupling strength and parameter mismatch, demonstrate various degrees of synchronization including partial/imperfect phase and complete phase synchronization (at various frequency ratios). These results suggest the possible importance of synchronization in natural climate variability since the studied coupled system forms a direct analogue of coupled weather systems in different locations in the atmosphere on seasonal and intraseasonal timescales. [Preview Abstract] |
Monday, November 22, 2010 12:14PM - 12:27PM |
HG.00009: Radiative instability in stratified rotating flows Stephane Le Dizes, Xavier Riedinger, Patrice Meunier We present new theoretical and experimental works which demonstrate the existence of an instability associated with the emission of internal waves in rotating flows when they are stably stratified along their rotation axis. A comprehensive stability diagram is obtained for both a potential flow and a Keplerian flow defined by their angular velocity $\Omega_P(r)=1/r2$ and $\Omega_K(r)=1/r^{3/2}$ respectively in an infinite domain ($r\in (1,\infty)$) as functions of the Rossby number (background rotation) and the Froude number (strength of the stratification along the rotation axis). Both flows are shown to become unstable in centrifugally stable regimes thanks to the stratification. Experimental evidence is also provided for the potential flow around a rotating cylinder. Measurements of the characteristic wavelength and frequency are compared to the theory and a good agreement is demonstrated. The same instability is shown to be active in compressible or shallow-water flows where acoustic waves or surface waves can be emitted. [Preview Abstract] |
Monday, November 22, 2010 12:27PM - 12:40PM |
HG.00010: Temporal behavior and vortex topology of topographic wave-breaking Olivier Eiff, Nicolas Boulanger, Karine Leroux, Alexandre Paci At low Froude numbers or strong stratification, the internal waves generated by flow over an obstacle or mountain will overturn and break. Surprisingly litte is known, however, of the dynamics of the wave breaking itself. Afanasyev and Peltier (1998) investigated the wave breaking region via LES and Eiff et al. (2005) via PIV measurements, but both presumed a statistically stationary wave-breaking process after the initial wave overturning. Here, we propose to take a closer look at this assumption by closely analyzing the spatio-temporal structure of internal wave breaking region and the surrounding flow. The analysis is based on Hovm\"{o}ller diagrams and spatial correlations obtained from 2D-PIV measurements of flows generated in uniform stratified flow over 2D and quasi-2D obstacles in salt-stratified hydraulic channels at different Reynolds numbers ranging from laminar to turbulent. The results reveal low frequency variations throughout the flow field, in and outside the wave-breaking region. This characteristic frequency can be related to be due to a sequence of growth and decay of wave-breaking. Finally, new 3D-3C PIV measurements at high Reynolds numbers reveal a first glimpse of the 3D vortex topology. [Preview Abstract] |
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