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L. J. Pratt, William Johns, Stephen P. Murray, and Katsurou Katsumata

Abstract

Acoustic Doppler Current Profiler velocity measurements in the Bab al Mandab during the period June 1995–March 1996 are used to assess the hydraulic character of the exchange flow. The strait is 150 km long and contains two distinct geometrical choke points: the Hanish sill and Perim narrows. The authors use a three-layer approximation of the monthly mean velocity and density structure at the sill and narrows to calculate the phase speeds of the first and second internal, long gravity waves. The first (second) mode is generally characterized by in-phase (out-of-phase) motions of the two interfaces. The calculations take cross-strait topographic variations into consideration by using a piecewise linear representation of the actual bottom. The resulting phase speeds are used to determine whether the flow is subcritical, supercritical, or critical with respect to the first and second modes. Subcritical (supercritical) means that the two wave pairs corresponding to a given mode propagate in opposite (the same) directions, whereas “critical” means that one or both members of a pair has zero phase speed. Critical or supercritical conditions are indications of hydraulic control and imply that signal propagation through the strait associated with a particular mode can occur in only one direction, or perhaps not at all.

During the summer months, the velocity profiles indicate a “three-layer” structure, with surface water outflowing from the Red Sea, intermediate water inflowing from the Gulf of Aden, and Red Sea Water outflowing at the bottom. During this time period, the flow is found to be subcritical with respect to both internal modes, although tides and mesoscale disturbances are potentially strong enough to temporarily create critical or supercritical conditions, particularly with respect to the second internal mode at the narrows. During the winter and transitional months the velocity has a two-layer character with inflowing surface water and outflowing Red Sea Water. However, the outflowing Red Sea Water can further be partitioned into an intermediate layer originating from the Red Sea thermocline and a deeper, homogeneous layer originating from below the thermocline. A subtle three-layer character therefore exists and the three-layer model is configured accordingly. Surprisingly, the monthly mean narrows flow during this time period is found to be substantially subcritical with respect to the first baroclinic mode. At the Hanish sill the flow is marginally to moderately subcritical with respect to the first mode, suggesting the possibility of proximity to a section of critical control. It is possible that friction may be strong enough to shift the control section to the south of the sill. With respect to the second mode, the flow at both the Hanish sill and Perim narrows are found to be very close to the critical speed with respect to the second internal mode, suggesting hydraulic control. The wave whose propagation is arrested is one attempting to move from the Gulf of Aden into the Red Sea. The vertical structure of this wave suggests a role in determining how much Upper Red Sea Deep Water is able to pass through the Bab al Mandab and into the Gulf of Aden. The strength of tides and mesoscale disturbances in the strait suggest that this upstream influence may be intermittent.

Estimation of internal Rossby radii of deformation for the first and second internal modes indicates that rotation (which is neglected in our wave speed calculations) is only moderately weak. Nevertheless, the errors in calculated propagation speeds due to the neglect of rotation are estimated to be quite small.

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Larry J. Pratt, Heather E. Deese, Stephen P. Murray, and William Johns

Abstract

The continuous dynamical modes of the exchange flow in the Bab al Mandab are computed in an attempt to assess the hydraulic character of the flow at the sill. First, an extended version of the Taylor–Goldstein equation for long waves that accounts for cross-channel topographic variations, is developed. A series of calculations using idealized background velocity U(z) and buoyancy frequency N(z) are presented to illustrate the effects of simple topographic cross sections on the internal modes and their speeds. Next, hydrographic and direct velocity measurements from April to November 1996 using moored CTDs and a bottom-mounted ADCP are utilized to construct monthly mean vertical profiles of N 2(z) and at the U(z) sill. An analytical approximation of the true topography across the strait is also constructed. The observed monthly mean profiles are then used to solve for the phase speeds of the first and second internal modes. Additional calculations are carried out using a selection of “instantaneous” (2-h average) profiles measured during extremes of the semidiurnal tide. The results are compared with a three-layer analysis of data from the previous year.

Many of the authors’ conclusions follow from an intriguing observation concerning the long-wave phase speeds. Specifically, it was nearly always observed that the calculated speeds c −1 and c 1 of the two waves belonging to the first internal mode obey c −1 < U min < U max < c 1, where U min and U max are the minimum and maximum of the velocity profile. An immediate consequence is that neither wave has a critical level. For monthly mean profiles, each of which have U min < 0 < U max, the flow is therefore subcritical (the phase speeds of the two waves have opposite signs). For instantaneous profiles this relationship continues to hold, although the velocity profile can be unidirectional. Thus the flow can be critical (c −1 = 0 and/or c 1 = 0) or even supercritical (c −1 and c 1 have the same sign) with respect to the first mode. Similar findings follow for the second baroclinic mode phase speeds (c −2 and c 2). The authors conclude that hydraulically critical flow is an intermittent feature, influenced to a great extent by the tides. It is noted that the phase speed pairs for each mode lie very close to U min and U max. As suggested by the analysis of idealized profiles, this behavior is characteristic of flows that are marginally stable, perhaps as a result of prior mixing. This suggestion is supported by Richardson number (Ri) profiles calculated from the monthly mean and instantaneous data. Middepth values of Ri were frequently found to be O(1) and sometimes <1/4, a result consistent with the presence of mixing over portions of the water column.

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Xubin Zeng, Steve Ackerman, Robert D. Ferraro, Tsengdar J. Lee, John J. Murray, Steven Pawson, Carolyn Reynolds, and Joao Teixeira
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John R. Mecikalski, Wayne F. Feltz, John J. Murray, David B. Johnson, Kristopher M. Bedka, Sarah T. Bedka, Anthony J. Wimmers, Michael Pavolonis, Todd A. Berendes, Julie Haggerty, Pat Minnis, Ben Bernstein, and Earle Williams

Advanced Satellite Aviation Weather Products (ASAP) was jointly initiated by the NASA Applied Sciences Program and the NASA Aviation Safety and Security Program in 2002. The initiative provides a valuable bridge for transitioning new and existing satellite information and products into Federal Aviation Administration (FAA) Aviation Weather Research Program (AWRP) efforts to increase the safety and efficiency of project addresses hazards such as convective weather, turbulence (clear air and cloud induced), icing, and volcanic ash, and is particularly applicable in extending the monitoring of weather over data-sparse areas, such as the oceans and other observationally remote locations.

ASAP research is conducted by scientists from NASA, the FAA AWRP's Product Development Teams (PDT), NOAA, and the academic research community. In this paper we provide a summary of activities since the inception of ASAP that emphasize the use of current-generation satellite technologies toward observing and mitigating specified aviation hazards. A brief overview of future ASAP goals is also provided in light of the next generation of satellite sensors (e.g., hyperspectral; high spatial resolution) to become operational in the 2007–18 time frame.

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Dian J. Seidel, Franz H. Berger, Howard J. Diamond, John Dykema, David Goodrich, Franz Immler, William Murray, Thomas Peterson, Douglas Sisterson, Michael Sommer, Peter Thorne, Holger Vomel, and Junhong Wang

While the global upper-air observing network has provided useful observations for operational weather forecasting for decades, its measurements lack the accuracy and long-term continuity needed for understanding climate change. Consequently, the scientific community faces uncertainty on key climate issues, such as the nature of temperature trends in the troposphere and stratosphere; the climatology, radiative effects, and hydrological role of water vapor in the upper troposphere and stratosphere; and the vertical profile of changes in atmospheric ozone, aerosols, and other trace constituents. Radiosonde data provide adequate vertical resolution to address these issues, but they have questionable accuracy and time-varying biases due to changing instrumentation and techniques. Although satellite systems provide global coverage, their vertical resolution is sometimes inadequate and they require independent reference observations for sensor and data product validation, and for merging observations from different platforms into homogeneous climate records. To address these shortcomings, and to ensure that future climate records will be more useful than the records to date, the Global Climate Observing System (GCOS) program is initiating a GCOS Reference Upper-Air Network (GRUAN) to provide high-quality observations using specialized radiosondes and complementary remote sensing profiling instrumentation that can be used for validation. This paper outlines the scientific rationale for GRUAN, its role in the Global Earth Observation System of Systems, network requirements and likely instrumentation, management structure, current status, and future plans. It also illustrates the value of prototype reference upper-air observations in constructing climate records and their potential contribution to the Global Space-Based Inter-Calibration System. We invite constructive feedback on the GRUAN concept and the engagement of the scientific community.

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Randall M. Dole, J. Ryan Spackman, Matthew Newman, Gilbert P. Compo, Catherine A. Smith, Leslie M. Hartten, Joseph J. Barsugli, Robert S. Webb, Martin P. Hoerling, Robert Cifelli, Klaus Wolter, Christopher D. Barnet, Maria Gehne, Ronald Gelaro, George N. Kiladis, Scott Abbott, Elena Akish, John Albers, John M. Brown, Christopher J. Cox, Lisa Darby, Gijs de Boer, Barbara DeLuisi, Juliana Dias, Jason Dunion, Jon Eischeid, Christopher Fairall, Antonia Gambacorta, Brian K. Gorton, Andrew Hoell, Janet Intrieri, Darren Jackson, Paul E. Johnston, Richard Lataitis, Kelly M. Mahoney, Katherine McCaffrey, H. Alex McColl, Michael J. Mueller, Donald Murray, Paul J. Neiman, William Otto, Ola Persson, Xiao-Wei Quan, Imtiaz Rangwala, Andrea J. Ray, David Reynolds, Emily Riley Dellaripa, Karen Rosenlof, Naoko Sakaeda, Prashant D. Sardeshmukh, Laura C. Slivinski, Lesley Smith, Amy Solomon, Dustin Swales, Stefan Tulich, Allen White, Gary Wick, Matthew G. Winterkorn, Daniel E. Wolfe, and Robert Zamora

Abstract

Forecasts by mid-2015 for a strong El Niño during winter 2015/16 presented an exceptional scientific opportunity to accelerate advances in understanding and predictions of an extreme climate event and its impacts while the event was ongoing. Seizing this opportunity, the National Oceanic and Atmospheric Administration (NOAA) initiated an El Niño Rapid Response (ENRR), conducting the first field campaign to obtain intensive atmospheric observations over the tropical Pacific during El Niño.

The overarching ENRR goal was to determine the atmospheric response to El Niño and the implications for predicting extratropical storms and U.S. West Coast rainfall. The field campaign observations extended from the central tropical Pacific to the West Coast, with a primary focus on the initial tropical atmospheric response that links El Niño to its global impacts. NOAA deployed its Gulfstream-IV (G-IV) aircraft to obtain observations around organized tropical convection and poleward convective outflow near the heart of El Niño. Additional tropical Pacific observations were obtained by radiosondes launched from Kiritimati , Kiribati, and the NOAA ship Ronald H. Brown, and in the eastern North Pacific by the National Aeronautics and Space Administration (NASA) Global Hawk unmanned aerial system. These observations were all transmitted in real time for use in operational prediction models. An X-band radar installed in Santa Clara, California, helped characterize precipitation distributions. This suite supported an end-to-end capability extending from tropical Pacific processes to West Coast impacts. The ENRR observations were used during the event in operational predictions. They now provide an unprecedented dataset for further research to improve understanding and predictions of El Niño and its impacts.

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