Woodard M.,NorthWest Research Associates, Inc.
Solar Physics | Year: 2014
The accuracy of helioseismic measurement is limited by the stochastic nature of solar oscillations. In this article I use a Gaussian statistical model of the global seismic wave field of the Sun to investigate the noise limitations of direct-modeling analysis of convection-zone-scale flows. The theoretical analysis of noise is based on hypothetical data that cover the entire photosphere, including the portions invisible from the Earth. Noise estimates are derived for measurements of the flow-dependent couplings of global-oscillation modes and for combinations of coupling measurements that isolate vector-spherical-harmonic components of the flow velocity. For current helioseismic observations, which sample only a fraction of the photosphere, the inferred detection limits are best regarded as optimistic limits. The flow-velocity fields considered in this work are assumed to be decomposable into vector-spherical-harmonic functions of degree less than five. The problem of measuring the general velocity field is shown to be similar enough to the well-studied problem of measuring differential rotation to permit rough estimates of flow-detection thresholds to be gleaned from past helioseismic analysis. I estimate that, with existing and anticipated helioseismic datasets, large-scale flow-velocity amplitudes of a few tens of ms-1 should be detectable near the base of the convection zone. © 2013 Springer Science+Business Media Dordrecht. Source
Andreas E.L.,NorthWest Research Associates, Inc.
Quarterly Journal of the Royal Meteorological Society | Year: 2011
A bulk flux algorithm predicts the turbulent surface fluxes of momentum and sensible and latent heat from mean measured or modelled meteorological variables. The bulk flux algorithm resulting from data collected over winter sea ice during SHEBA, the experiment to study the Surface Heat Budget of the Arctic Ocean, failed, however, in its first trial to predict the turbulent momentum flux over sea ice in the Antarctic. This result suggests that the main parameter for predicting the momentum flux, the aerodynamics roughness length z0, does not respond just to the friction velocity, as in the SHEBA algorithm, but is closely related to the physical roughness of snow-covered sea ice and may need to be site-specific. I investigate this idea with simultaneous measurements of z0 and the physical roughness of the surface, ξ, at Ice Station Weddell. The metric ξ derives from surveys of surface elevation and is related to but always less than the standard deviation in surface elevation. On combining the z0-ξ pairs from Ice Station Weddell with similar data obtained over Arctic sea ice, I show that the Arctic and Antarctic z0-ξ data lie along a continuum such that measuring ξ could provide a means for estimating a site-specific z0 for any global sea ice surface. Backscatter data from satellite-borne synthetic aperture radar might provide a remotely sensed estimate of ξ. © 2011 Royal Meteorological Society. Source
Kim J.-E.,University of Colorado at Boulder |
Joan Alexander M.,NorthWest Research Associates, Inc.
Journal of Climate | Year: 2013
Tropical precipitation characteristics are investigated using the Tropical Rainfall Measuring Mission (TRMM) 3-hourly estimates, and the result is compared with five reanalyses including the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim), Modern Era Retrospective Analysis for Research and Applications (MERRA), National Centers for Environmental Prediction (NCEP)-National Center for Atmospheric Research (NCAR) reanalysis (NCEP1), NCEP-U.S. Department of Energy (DOE) reanalysis (NCEP2), and NCEP-Climate Forecast System Reanalysis (CFSR). Precipitation characteristics are evaluated in terms of the mean, convectively coupled equatorial wave activity, frequency characteristics, diurnal cycle, and seasonality of regional precipitation variability associated with submonthly scale waves. Generally the latest reanalyses such as ERA-Interim, MERRA, and CFSR show better performances than NCEP1 and NCEP2. However, all the reanalyses are still different from observations. Besides the positive mean bias in the reanalyses, a spectral analysis revealed that the reanalyses have overreddened spectra with persistent rainfall. MERRA has the most persistent rainfall, and CFSR appears to have the most realistic variability. The diurnal cycle in NCEP1 is extremely exaggerated relative to TRMM. The low-frequency waves with the period longer than 3 days are relatively well represented in ERAInterim, MERRA, and CFSR, but all the reanalyses have significant deficiencies in representing convectively coupled equatorial waves and variability in the high-frequency range. © 2013 American Meteorological Society. Source
Gizon L.,Max Planck Institute for Solar System Research |
Birch A.C.,NorthWest Research Associates, Inc. |
Spruit H.C.,Max Planck Institute for Astrophysics
Annual Review of Astronomy and Astrophysics | Year: 2010
The Sun supports a rich spectrum of internal waves that are continuously excited by turbulent convection. The Global Oscillation Network Group (GONG) network and the SOHO/MDI (Solar and Heliospheric Observatory/Michelson Doppler Imager) space instrument provide an exceptional database of spatially resolved observations of solar oscillations, covering more than an entire sunspot cycle (11 years). Local helioseismology is a set of tools for probing the solar interior in three dimensions using measurements of wave travel times and local mode frequencies. Local helioseismology has discovered (a) near-surface vector flows associated with convection, (b) 250 m s-1 subsurface horizontal outflows around sunspots, (c) ̃50 m s-1 extended horizontal flows around active regions (converging near the surface and diverging below), (d ) the effect of the Coriolis force on convective flows and active region flows, (e) the subsurface signature of the 15m s-1 poleward meridional flow, ( f ) a ±5 m s-1 time-varying depth-dependent component of the meridional circulation around the mean latitude of activity, and ( g) magnetic activity on the farside of the Sun. © 2010 by Annual Reviews. Source
Agency: NSF | Branch: Standard Grant | Program: | Phase: PHYSICAL & DYNAMIC METEOROLOGY | Award Amount: 603.14K | Year: 2016
This study seeks to improve our understanding of the interaction of weak wind flow in the lowest layer of the atmosphere and generation of turbulence in gently sloping terrain. An improved understanding of weak-wind boundary layers could potentially lead to more accurate modeling of the formation of cold pools, damaging frost (including impact to local vineyards), fog formation, and dispersion of contaminants.
This study seeks to investigate weak-wind boundary layers using detailed spatial-temporal measurements. These boundary layers demonstrate non-stationarity and the turbulence in them is generally in a non-equilibrium state with the mean flow. Previous studies of such boundary layers were mostly limited to temporal data at fixed locations, and therefore could not take into account hysteresis when investigating scaling properties. This study analyzes detailed spatial-temporal data to form a more complete picture of boundary layer structures. The evolution of the structures and the impact of the sub-mesoscale motions will be investigated.