Solar Physics | Year: 2014
We use a publicly available numerical wave-propagation simulation of Hartlep et al. (Solar Phys. 268, 321, 2011) to test the ability of helioseismic holography to detect signatures of a compact, fully submerged, 5 % sound-speed perturbation placed at a depth of 50 Mm within a solar model. We find that helioseismic holography employed in a nominal "lateral-vantage" or "deep-focus" geometry employing quadrants of an annular pupil can detect and characterize the perturbation. A number of tests of the methodology, including the use of a plane-parallel approximation, the definition of travel-time shifts, the use of different phase-speed filters, and changes to the pupils, are also performed. It is found that travel-time shifts made using Gabor-wavelet fitting are essentially identical to those derived from the phase of the Fourier transform of the cross-covariance functions. The errors in travel-time shifts caused by the plane-parallel approximation can be minimized to less than a second for the depths and fields of view considered here. Based on the measured strength of the mean travel-time signal of the perturbation, no substantial improvement in sensitivity is produced by varying the analysis procedure from the nominal methodology in conformance with expectations. The measured travel-time shifts are essentially unchanged by varying the profile of the phase-speed filter or omitting the filter entirely. The method remains maximally sensitive when applied with pupils that are wide quadrants, as opposed to narrower quadrants or with pupils composed of smaller arcs. We discuss the significance of these results for the recent controversy regarding suspected pre-emergence signatures of active regions. © 2012 Springer Science+Business Media Dordrecht.
Milliff R.F.,NWRA |
Bonazzi A.,Operational Oceanography Group |
Wikle C.K.,University of Missouri |
Pinardi N.,Operational Oceanography Group |
Berliner L.M.,Ohio State University
Quarterly Journal of the Royal Meteorological Society | Year: 2011
A Bayesian hierarchical model (BHM) is developed to estimate surface vector wind (SVW) fields and associated uncertainties over the Mediterranean Sea. The BHM-SVW incorporates data-stage inputs from analyses and forecasts of the European Centre for Medium-Range Weather Forecasts (ECMWF) and SVW retrievals from the QuikSCAT data record. The process-model stage of the BHM-SVW is based on a Rayleigh friction equation model for surface winds. Dynamical interpretations of posterior distributions of the BHM-SVW parameters are discussed. Ten realizations from the posterior distribution of the BHM-SVW are used to force the data-assimilation step of an experimental ensemble ocean forecast system for the Mediterranean Sea in order to create a set of ensemble initial conditions. The sequential data-assimilation method of the Mediterranean forecast system (MFS) is adapted to the ensemble implementation. Analyses of sample ensemble initial conditions for a single data-assimilation period in MFS are presented to demonstrate the multivariate impact of the BHM-SVW ensemble generation methodology. Ensemble initial-condition spread is quantified by computing standard deviations of ocean state variable fields over the ten ensemble members. The methodological findings in this article are of two kinds. From the perspective of statistical modelling, the process-model development is more closely related to physical balances than in previous work with models for the SVW. From the ocean forecast perspective, the generation of ocean ensemble initial conditions via BHM is shown to be practical for operational implementation in an ensemble ocean forecast system. Phenomenologically, ensemble spread generated via BHM-SVW occurs on ocean mesoscale time- and space-scales, in close association with strong synoptic-scale wind-forcing events. A companion article describes the impacts of the BHM-SVW ensemble method on the ocean forecast in comparisons with more traditional ensemble methods. Copyright © 2011 Royal Meteorological Society.
Journal of Physics: Conference Series | Year: 2011
Local helioseismology is a set of methods that are used to study wave propagation and infer physical conditions in the solar interior. Sunspots are a particularly challenging target for local helioseismology. In this review, I will show that some new methods (magnetoconvection simulations and numerical wave propagation simulations) lead to shallow sunspot models that are apparently inconsistent with traditional inferences from local helioseismology. In addition, I will show that inferences for the depth structure of moat flows are not in general agreement either. © Published under licence by IOP Publishing Ltd.
Journal of Geophysical Research: Space Physics | Year: 2013
We derive the analytic, linear, f-plane compressible solutions to local, interval, 3-D horizontal and vertical body forces, and heat/coolings in an isothermal, unsheared, and nondissipative atmosphere. These force/heat/coolings oscillate at the frequency, and turn on and off smoothly over a finite interval in time. The solutions include a mean response, gravity waves (GWs), and acoustic waves (AWs). The excited waves span a large range of horizontal/vertical scales and frequencies ω. We find that the compressible solutions are important for GWs with vertical wavelengths |λz|>(1to2)×πH if the depth of the force/heat/cooling is greater than the density scale height H. We calculate the primary GWs excited by a deep convective plume, ray trace them into the thermosphere, and calculate the body force/heat/coolings which result where the GWs dissipate. We find that the force/heat/cooling amplitudes are up to ∼40% smaller using the compressible (as compared to the Boussinesq) GW spectra. For a typical plume, the force/heat/coolings are deeper than H and have maximum amplitudes of ∼0.2 to 0.6 m/s2and ∼0.06 to 0.15 K/s for solar maximum to minimum, respectively. The heat/cooling consists of dipoles at z∼150-200 km and a heating at z∼240-260 km. We find that the compressible solutions are necessary for calculating the secondary GWs excited by these thermospheric force/heat/coolings. ©2013. American Geophysical Union. All Rights Reserved.
Nicolls M.J.,SRI International |
Vadas S.L.,NWRA |
Meriwether J.W.,Clemson University |
Conde M.G.,University of Alaska Fairbanks |
Hampton D.,University of Alaska Fairbanks
Journal of Geophysical Research: Space Physics | Year: 2012
In a companion paper, we derived the high-frequency, compressible, dissipative polarization relations for gravity waves (GWs) propagating in the thermosphere. In this paper, we apply the results to nighttime thermospheric observations of a GW over Alaska on 9-10 January 2010. Using a vertically-pointed Fabry-Perot interferometer (FPI) at Poker Flat that measured vertical wind perturbations (w') and two FPIs that measured the line-of-sight (LOS) velocities in four common volumes, we inferred a GW ground-based period ∼32.7 ± 0.3 min, horizontal wavelength λH=1094 ± 408 km, horizontal ground-based phase speed cH∼560 ± 210m/s, and propagation azimuth θ∼33.5 ± 15.8° east-of-north. We compared the phase shifts and amplitude ratios of this GW with that predicted by the GW dissipative polarization relations derived in the companion paper, enabled by the ability of the FPIs to measure fundamental GW parameters (wind and temperature perturbations). We find that GWs with λH∼700-1100 km, λz∼-500 to -350 km, θ∼ 15 to 50, and cH∼350-560m/s agree with the observations if the primary contribution to the 630-nm emission was near the upper portion of that layer. The source of GW was likely thermospheric given the large intrinsic phase speed of the wave. Possible sources are discussed, the most likely of which are related to the onset of auroral activity near the time that the wave was initially observed. Copyright 2012 by the American Geophysical Union.