NorthWest Research Assoc

Boulder City, Colorado, United States

NorthWest Research Assoc

Boulder City, Colorado, United States
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Muschinski A.,NorthWest Research Assoc
Journal of the Optical Society of America A: Optics and Image Science, and Vision | Year: 2016

It is a standard assumption in the theory of optical propagation through the turbulent atmosphere that the refractive-index fluctuations n1(x) are statistically isotropic. It is well known, however, that n1(x) in the free atmosphere and in the nocturnal boundary layer is often strongly anisotropic, even at very small scales. Here we present and discuss a model atmosphere characterized by randomly undulating, non-turbulent and nonoverturning, quasi-horizontal refractive-index interfaces, or "sheets." We assume n1(x)= ν[z - h(x;y)], where μ(z) is a random function that has a 1D spectrum V (κz), and where h(x;y) is a vertical displacement that varies randomly as a function of the horizontal coordinates(x)and y. We derive a closed-form expression for the 3D spectrum φ(κ) and show that the horizontal 1D spectra have the same power law as V (κz) if the structure function of h(x;y) is quadratic. Moreover, we evaluate the scintillation index σ2/I for a plane wave propagating horizontally through the undulating sheets, and we compare σ2/I predicted for undulating sheets with Tatarskii's classical predictions of σ2I for fully developed, isotropic turbulence. For Phillips-type sheets, where V (κz) ( κ-2 z, in the diffraction limit we find σ2/I ∝ k (where k= 2π/λ is the optical wavenumber), which is only slightly different from Tatarskii's famous k7/6 law for propagation through fully developed, Obukhov-Corrsin-type, isotropic turbulence where π(κ) ∝ κ-11/3. Our model predicts that σ2/I is inversely proportional to the sheet tilt angle standard deviation √ hθ2/x p i, regardless of whether or not diffraction plays a role and regardless of the value of the power-law exponent of V (?z). © 2016 Optical Society of America.

Wright C.J.,University of Bath | Hindley N.P.,University of Bath | Hindley N.P.,University of Leeds | Hoffmann L.,Jülich Research Center | And 2 more authors.
Atmospheric Chemistry and Physics | Year: 2017

Gravity waves (GWs) transport momentum and energy in the atmosphere, exerting a profound influence on the global circulation. Accurately measuring them is thus vital both for understanding the atmosphere and for developing the next generation of weather forecasting and climate prediction models. However, it has proven very difficult to measure the full set of GW parameters from satellite measurements, which are the only suitable observations with global coverage. This is particularly critical at latitudes close to 60° S, where climate models significantly under-represent wave momentum fluxes. Here, we present a novel fully 3-D method for detecting and characterising GWs in the stratosphere. This method is based around a 3-D Stockwell transform, and can be applied retrospectively to existing observed data. This is the first scientific use of this spectral analysis technique. We apply our method to high-resolution 3-D atmospheric temperature data from AIRS/Aqua over the altitude range 20-60 km. Our method allows us to determine a wide range of parameters for each wave detected. These include amplitude, propagation direction, horizontal/vertical wavelength, height/direction-resolved momentum fluxes (MFs), and phase and group velocity vectors. The latter three have not previously been measured from an individual satellite instrument. We demonstrate this method over the region around the Southern Andes and Antarctic Peninsula, the largest known sources of GW MFs near the 60° S belt. Our analyses reveal the presence of strongly intermittent highly directionally focused GWs with very high momentum fluxes (∼80-100 mPa or more at 30 km altitude). These waves are closely associated with the mountains rather than the open ocean of the Drake Passage. Measured fluxes are directed orthogonal to both mountain ranges, consistent with an orographic source mechanism, and are largest in winter. Further, our measurements of wave group velocity vectors show clear observational evidence that these waves are strongly focused into the polar night wind jet, and thus may contribute significantly to the "missing momentum" at these latitudes. These results demonstrate the capabilities of our new method, which provides a powerful tool for delivering the observations required for the next generation of weather and climate models. © Author(s) 2017.

Muschinski A.,NorthWest Research Assoc | De Bruyn Kops S.M.,University of Massachusetts Amherst
Journal of the Optical Society of America A: Optics and Image Science, and Vision | Year: 2015

For almost four decades, Hill's "Model 4" [J. Fluid Mech. 88, 541 (1978)] has played a central role in research and technology of optical turbulence. Based on Batchelor's generalized Obukhov-Corrsin theory of scalar turbulence, Hill's model predicts the dimensionless function h(κl0; Pr) that appears in Tatarskii's well-known equation for the 3D refractive-index spectrum in the case of homogeneous and isotropic turbulence, Ψ n(κ)=0.033Cn 2κ -11/3h(κl0,Pr). Here we investigate Hill's model by comparing numerical solutions of Hill's differential equation with scalar spectra estimated from direct numerical simulation (DNS) output data. Our DNS solves the Navier-Stokes equation for the 3D velocity field and the transport equation for the scalar field on a numerical grid containing 40963 grid points. Two independent DNS runs are analyzed: one with the Prandtl number Pr = 0.7 and a second run with Pr = 1.0. We find very good agreement between h(κl0, Pr) estimated from the DNS output data and h(κl0, Pr) predicted by the Hill model. We find that the height of the Hill bump is 1.79 Pr1/3, implying that there is no bump if Pr < 0.17. Both the DNS and the Hill model predict that the viscous-diffusive "tail" of h(κl0, Pr) is exponential, not Gaussian. © 2015 Optical Society of America.

Manney G.L.,NorthWest Research Assoc | Manney G.L.,New Mexico Institute of Mining and Technology | Lawrence Z.D.,New Mexico Institute of Mining and Technology | Santee M.L.,Jet Propulsion Laboratory | And 3 more authors.
Atmospheric Chemistry and Physics | Year: 2015

A sudden stratospheric warming (SSW) in early January 2013 caused the Arctic polar vortex to split and temperatures to rapidly rise above the threshold for chlorine activation. However, ozone in the lower stratospheric polar vortex from late December 2012 through early February 2013 reached the lowest values on record for that time of year. Analysis of Aura Microwave Limb Sounder (MLS) trace gas measurements and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) polar stratospheric cloud (PSC) data shows that exceptional chemical ozone loss early in the 2012/13 Arctic winter resulted from a unique combination of meteorological conditions associated with the early-January 2013 SSW: unusually low temperatures in December 2012, offspring vortices within which air remained well isolated for nearly 1 month after the vortex split, and greater-than-usual vortex sunlight exposure throughout December 2012 and January 2013. Conditions in the two offspring vortices differed substantially, with the one overlying Canada having lower temperatures, lower nitric acid (HNO3), lower hydrogen chloride, more sunlight exposure/higher ClO in late January, and a later onset of chlorine deactivation than the one overlying Siberia. MLS HNO3 and CALIPSO data indicate that PSC activity in December 2012 was more extensive and persistent than at that time in any other Arctic winter in the past decade. Chlorine monoxide (ClO, measured by MLS) rose earlier than previously observed and was the largest on record through mid-January 2013. Enhanced vortex ClO persisted until mid-February despite the cessation of PSC activity when the SSW started. Vortex HNO3 remained depressed after PSCs had disappeared; passive transport calculations indicate vortex-averaged denitrification of about 4 parts per billion by volume. The estimated vortex-averaged chemical ozone loss, ∼ 0.7-0.8 parts per million by volume near 500 K (∼21 km), was the largest December/January loss in the MLS record from 2004/05 to 2014/15. © Author(s) 2015. CC Attribution 3.0 License.

Kim J.-E.,University of Colorado at Boulder | Kim J.-E.,NorthWest Research Assoc | Alexander M.J.,NorthWest Research Assoc
Geophysical Research Letters | Year: 2015

Cold point tropopause temperature is a key regulator of cirrus clouds and stratospheric water vapor, which have significant impacts on the Earth's radiation budget and climate. Using tropical radiosonde observations, we show that waves in the tropical tropopause layer lower cold point temperature by 1.6 K on average relative to the seasonal mean. Furthermore, wave activity in the tropical tropopause layer has not been constant over the last 2.5 decades, altering the magnitude of the wave impacts on cold point temperature at a decadal scale. The change in the direct wave impact is partially (~20-30%) responsible for the sudden decrease in cold point temperature and stratospheric water vapor at the end of 2000, which has not been fully explained by changes in the Brewer-Dobson circulation. We further show that these wave impacts are not well represented in reanalysis data. © 2015. American Geophysical Union. All Rights Reserved.

Lawrence Z.D.,New Mexico Institute of Mining and Technology | Manney G.L.,New Mexico Institute of Mining and Technology | Manney G.L.,NorthWest Research Assoc | Minschwaner K.,New Mexico Institute of Mining and Technology | And 2 more authors.
Atmospheric Chemistry and Physics | Year: 2015

We present a comprehensive comparison of polar processing diagnostics derived from the National Aeronautics and Space Administration (NASA) Modern Era Retrospective-analysis for Research and Applications (MERRA) and the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Reanalysis (ERA-Interim). We use diagnostics that focus on meteorological conditions related to stratospheric chemical ozone loss based on temperatures, polar vortex dynamics, and air parcel trajectories to evaluate the effects these reanalyses might have on polar processing studies. Our results show that the agreement between MERRA and ERA-Interim changes significantly over the 34 years from 1979 to 2013 in both hemispheres and in many cases improves. By comparing our diagnostics during five time periods when an increasing number of higher-quality observations were brought into these reanalyses, we show how changes in the data assimilation systems (DAS) of MERRA and ERA-Interim affected their meteorological data. Many of our stratospheric temperature diagnostics show a convergence toward significantly better agreement, in both hemispheres, after 2001 when Aqua and GOES (Geostationary Operational Environmental Satellite) radiances were introduced into the DAS. Other diagnostics, such as the winter mean volume of air with temperatures below polar stratospheric cloud formation thresholds (VPSC) and some diagnostics of polar vortex size and strength, do not show improved agreement between the two reanalyses in recent years when data inputs into the DAS were more comprehensive. The polar processing diagnostics calculated from MERRA and ERA-Interim agree much better than those calculated from earlier reanalysis data sets. We still, however, see fairly large differences in many of the diagnostics in years prior to 2002, raising the possibility that the choice of one reanalysis over another could significantly influence the results of polar processing studies. After 2002, we see overall good agreement among the diagnostics, which demonstrates that the ERA-Interim and MERRA reanalyses are equally appropriate choices for polar processing studies of recent Arctic and Antarctic winters. © Author(s) 2015. CC Attribution 3.0 License.

Naylor J.,NorthWest Research Assoc | Naylor J.,Purdue University | Schecter D.A.,NorthWest Research Assoc
Journal of Advances in Modeling Earth Systems | Year: 2015

This paper examines the nature and consequences of inner core instabilities in several intense tropical cyclones (TCs) simulated with a cloud-resolving numerical model. The initial wave growth leading to polygonal eyewalls and mesovortices in each TC is shown to closely resemble that found in a dry nonconvective vortex with the same primary circulation. Such agreement is reasonable partly because the bulk of the cloudy eyewall updraft is outside the vorticity ring in which the instability occurs. An energetic analysis of the TC instability verifies that the symmetric secondary circulation contributes relatively little to the early growth of eddy kinetic energy. On a longer time scale, isentropic potential vorticity mixing triggered by the instability irreversibly reduces the maximum wind speed of the dry vortex. In the more realistically simulated TC, moist convection modulates the mixing and regenerates the broken vorticity ring. Eventually, the maximum wind speed of the TC and the local parameters that determine its theoretical magnitude return to within a few percent of their preinstability values. It is found that the time scale for wind speed restoration is sensitive to the surface drag coefficient. The long-term effects of inner core instability on precipitation are briefly addressed. Key Points Moist convection has little impact on initial asymmetric wave growth Main source term of asymmetric KE involves radial shear of primary circulation Moist convection reduces intensity loss otherwise caused by instabilities © 2014. The Authors.

Fontenla J.M.,NorthWest Research Assoc | Stancil P.C.,University of Georgia | Landi E.,University of Michigan
Astrophysical Journal | Year: 2015

The previous calculations of the Solar Spectral Irradiance (SSI) by the Solar Radiation Physical Modeling, version 2 system, are updated in this work by including new molecular photodissociation cross-sections of important species, and many more levels and lines in its treatment of non-LTE radiative transfer. The current calculations including the new molecular photodissociation opacities produce a reduced over-ionizaton of heavy elements in the lower chromosphere and solve the problems with prior studies of the UV SSI in the wavelength range 160-400 nm and now reproduce the available observations with much greater accuracy. Calculations and observations of the near-UV at 0.1 nm resolution and higher are compared. The current set of physical models includes four quiet-Sun and five active-region components, from which radiance is computed for ten observing angles. These radiances are combined with images of the solar disk to obtain the SSI and Total Solar Irradiance and their variations. The computed SSI is compared with measurements from space at several nm resolution and agreement is found within the accuracy level of these measurements. An important result is that the near-UV SSI increase with solar activity is significant for the photodissociation of ozone in the terrestrial atmosphere because a number of highly variable upper chromospheric lines overlap the ozone Hartley band. © 2015. The American Astronomical Society. All rights reserved..

Braun D.C.,NorthWest Research Assoc
Astrophysical Journal | Year: 2016

We use helioseismic holography to study the association of shallow flows with solar flare activity in about 250 large sunspot groups observed between 2010 and 2014 with the Helioseismic and Magnetic Imager on the Solar Dynamics Observatory. Four basic flow parameters: horizontal speed, horizontal component of divergence, vertical component of vorticity, and a vertical kinetic helicity proxy, are mapped for each active region (AR) during its passage across the solar disk. Flow indices are derived representing the mean and standard deviation of these parameters over magnetic masks and compared with contemporary measures of flare X-ray flux. A correlation exists for several of the flow indices, especially those based on the speed and the standard deviation of all flow parameters. However, their correlation with X-ray flux is similar to that observed with the mean unsigned magnetic flux density over the same masks. The temporal variation of the flow indices are studied, and a superposed epoch analysis with respect to the occurrence to 70 M and X-class flares is made. While flows evolve with the passage of the ARs across the disk, no discernible precursors or other temporal changes specifically associated with flares are detected. © 2016. The American Astronomical Society. All rights reserved..

Masci F.,Italian National Institute of Geophysics and Volcanology | Thomas J.N.,NorthWest Research Assoc | Thomas J.N.,DigiPen Institute of Technology | Thomas J.N.,University of Washington
Radio Science | Year: 2015

The scientific literature includes many reports of ionospheric phenomena that are retrospectively identified prior to seismic events. These disturbances of the Earth's ionosphere are considered to be possible precursors of the impending earthquakes. However, a causal relationship between ionospheric phenomena and earthquakes has never been definitively demonstrated, and attempts at identifying precursory effects in the ionosphere have been called into question by several studies. Among the candidate indicators of ionospheric precursors there is the Spatial Scintillation Index (SSI) proposed by Pulinets et al. (2007). The usefulness of this index in the search for precursory effects of earthquakes has been criticized by Thomas et al. (2012) and Masci (2013). In a recent report, Pulinets and Davidenko (2014) attempt to briefly respond to the remarks of these researchers. Here we cast doubt that Pulinets and Davidenko (2014) have shown that SSI is a reliable indicator of precursory effects of earthquakes in the ionosphere. © 2015. American Geophysical Union. All Rights Reserved.

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