Search Results

You are looking at 1 - 10 of 23 items for

  • Author or Editor: David A. Brooks x
  • Refine by Access: All Content x
Clear All Modify Search
David A. Brooks

Abstract

A quasi-linear prognostic numerical model has been used to study the three-dimensional baroclinic circulation in the Gulf of Maine region, with a focus on the buoyancy-driven circulation in the inner basins and near the coast where the influence of relatively fresh water from rivers and the Scotian shelf is important. The model uses a horizontal grid with 6-km resolution and a terrain-following vertical coordinate with ten levels to represent the bathymetry and stratification. Several model experiments explore the influences of fluxes from the principal rivers, tidal mixing, and shelf water inflow, beginning with a case with no initial horizontal density gradients and ending with cases initialized by temperature and salinity data from cruises in June of 1982 and 1983, augmented by climatological data. With composite initialization, the model develops a circulation containing realistic elements, including an anticlockwise gyre in the interior of the gulf and a prominent southwestward coastal current. For the 1982 case, characterized by relatively low river runoff, outflow from the Penobscot River guides a branch of the eastern Maine coastal current offshore in a thermohaline front 10–20 km east of Penobscot Bay; a similar dislocation is associated with the Kennebec and Androscoggin Rivers farther to the west. For the comparatively wet spring of 1983, the coastal current continues with less interruption from the Bay of Fundy to Cape Cod and consequently contributes less to the development of the interior circulation.

Full access
David A. Brooks

Abstract

The eastern United States continental margin is naturally divided by Cape Hatteras into the Middle and South Atlantic Bights. The margin depth profile is relatively uniform throughout the Middle Atlantic Bight, but in the South Atlantic Bight it bifurcates into inner and outer slope regions. Coastal tide gage records indicate that long period (1–2 weeks) sea level oscillations propagate southward as continental shelf waves in both Bights, thereby providing a coupling mechanism between the Bights. However, short-period (several days) oscillations appear to be confined to the south Atlantic Bight, and may result from backscattering of long-wave energy by the variable topography and the Gulf Stream. The coastal sea level short-period phase data are not easily attributable to monochromatic propagating waves; rather, it appears that wave group properties may lead to a more consistent explanation of the phases. Cross-shelf and longshelf wind stress components were both strongly coupled to sea level fluctuations for long periods; short-period motions were more closely associated with nonstatic atmospheric pressure fluctuations.

Full access
David A. Brooks

Abstract

Coastal sea level fluctuations in Onslow Bay are selectively coupled with local atmospheric forcing variables. The coupling is strongest in period bands of 2.5–3.5 and ≳10 days, which span absolute zero group speed, barotropic continental shelf wave periods. The phase of the sea level disturbances propagated upstream from Beaufort to Wilmington, N. C., as expected for stable discrete shelf waves, consistent with earlier results by Mysak and Hamon (1969). The “barometric function”, when corrected for coherent wind stresses, indicated selective, super barometric atmospheric-pressure-to-sea-level coupling in the zero group speed period bands. Stochastic models of atmospheric cold front wind stress and wind stress curl fields were found to selectively force barotropic shelf wave responses near zero group speed periods and wavelengths. A strong Onslow Bay response to the model cold front occurred near the second harmonic forcing frequency, the wind stress curl contributed importantly to the shape and the amplitude of the response spectrum. The results suggest that shelf waves forced by the atmosphere contribute to the Gulf Stream meander field off North Carolina.

Full access
David A. Brooks

Abstract

In August 1980, Hurricane Allen passed over a moored array of instruments recording current, temperature and conductivity in the western Gulf of Mexico. An alongshore surge occurred during the storm passage, with the horizontal current speed reaching 91 cm s−1 in the thermocline (200 m) and diminishing to 15 cm s−1 32 m above the bottom (732 m). A wake of near-inertial frequency internal waves commenced after the storm passed the array. The alongshore current oscillation reached a maximum range of 50 cm s−1 within 3 days and decayed thereafter with a time scale of about 5 days. The current oscillations were clockwise-polarized and slightly elliptical, with a period of 22–23 h or about 85% of the local inertial period. Near-uniform upwelling of ±20 m occurred in the thermocline region (200–300 m) during the most intense part of the wake. Depth-leading phases of the horizontal current and temperature oscillations indicated downward radiation of wake energy. The wake oscillations were highly coherent over the vertical (500 m) and horizontal (100 km) scales of the array. The oscillations had a vertical scale much greater than the thermocline thickness (∼150 m) and several times the ocean depth at the array site. Bemuse of the large vertical scale, downward radiation of wake kinetic energy was sufficient to account for the energy depletion rate in the thermocline, which suggests that dissipation was relatively unimportant during the early stage of wake decay.

Full access
David A. Brooks

Abstract

During long deployment periods, it is desirable to communicate remotely with moored or drifting instruments. In addition to providing access to the data set as it is collected, a telemetry capability provides insurance against undetected instrument failure and the associated risk of jeopardized scientific objectives. As part of an ongoing study in the Gulf of Maine, an experimental VHF transhorizon telemetry system was recently deployed with a current meter mooring. The refractive properties of the marine boundary layer made it possible to transmit data with about 70% reliability over a 100 km distance, or about ten times the optical-horizon distance for the antenna heights used. During weather anomalies, large signal-strength enhancements occurred, and these could easily be exploited to increase the data-transfer reliability.

Full access
David A. Brooks and Melbourne G. Briscoe

Abstract

Experiences with two prototype telemetry systems developed for potential use with moored or drifting ocean instruments are described. The systems transfer data and commands between remote and base stations using direct high-frequency (HF) ionospheric radio propagation (shortwave radio) without intervening relay stations or satellites. The strategy exploits recent developments in digital packet-switching technology, which is readily available and can be inexpensively applied to oceanic problems. The potential advantages of packet methods over satellite methods include low cost, autonomy, two-way exchanges with the remote stations, and typical data rates of 1–10 bit s−1 averaged over several days. Coverage is effectively global but intermittent. Disadvantages mostly result from the interference and skip zones that characterize HF propagation. The tests described here took place near the time of a sunspot minimum; the utility of HF packet telemetry will be greatly increased when the sunspot cycle is near its maximum.

Full access
David A. Brooks and Christopher N. K. Mooers

Abstract

The dispersion characteristics of stable, discrete, barotropic, continental shelf wave (CSW) modes propagating in a barotropic boundary current are strongly modified by the dynamical effects of nonuniform horizontal shear. For example, the CSW's propagate cum sole with no mean current, but their direction of propagation can be reversed by an opposing uniform mean current. In contrast, an opposing sheared mean current increases the tendency for cum sole propagation relative to an opposing uniform mean current, and produces a high-wavenumber cutoff, at least for modes higher than the first. If the sheared mean flow vanishes somewhere, the discrete CSW modes all propagate cum sole once again. For the mean current profiles considered, the high-frequency cutoff is lowered in the nonuniform shear case compared to the zero current case.

In a simple geometry motivated by the Florida Current and Florida Straits, southward CSW propagation can occur, in opposition to the Current, primarily because the cyclonic shear of the Current is similar in magnitude to the local Coriolis parameter. The short-period cutoff (zero group speed) for the first mode CSW is about 12 days; this CSW has a wavelength of about 190 km, corresponding to a southward phase speed of about 17 cm s−1. Within the limitations of the model, the results indicate that the Florida Straits–Florida Current system can accumulate energy at time scales of 10–14 days, corresponding to those of atmospheric cold front forcing.

Full access
David A. Brooks and John M. Bane Jr.

Abstract

Gulf Stream fluctuations observed over the 200 and 400 m isobaths off Onslow Bay, North Carolina have a prominent weekly time scale. The principal fluctuations observed during the 4-month winter experiment are consistent with Webster's (1961a) description of downstream propagating, skewed, lateral meanders of the Gulf Stream over the upper continental slope. The subtidal velocity fluctuations were highly coherent over the vertical extent (∼120 m) and over the horizontal extent (64 km) of our array. The implied downstream propagation speed was ∼30 km day−1 for the weekly period meanders. Concurrent satellite images of a sea surface temperature (SST) meander pattern indicate that subsurface temperature, salinity, velocity and relative-vorticity maxima occurred as meander crests (shoreward SST-front excursions) passed over the experiment site. The meandering currents were not coherent with nearby wind stress or coastal sea level fluctuations. Eddy-flux estimates indicate energy conversion from the fluctuations to the mean Stream.

Full access
Kenneth P. Moran, Brooks E. Martner, M. J. Post, Robert A. Kropfli, David C. Welsh, and Kevin B. Widener

A new millimeter-wave cloud radar (MMCR) has been designed to provide detailed, long-term observations of nonprecipitating and weakly precipitating clouds at Cloud and Radiation Testbed (CART) sites of the Department of Energy's Atmospheric Radiation Measurement (ARM) program. Scientific requirements included excellent sensitivity and vertical resolution to detect weak and thin multiple layers of ice and liquid water clouds over the sites and long-term, unattended operations in remote locales. In response to these requirements, the innovative radar design features a vertically pointing, single-polarization, Doppler system operating at 35 GHz (Ka band). It uses a low-peak-power transmitter for long-term reliability and high-gain antenna and pulse-compressed waveforms to maximize sensitivity and resolution. The radar uses the same kind of signal processor as that used in commercial wind profilers. The first MMCR began operations at the CART in northern Oklahoma in late 1996 and has operated continuously there for thousands of hours. It routinely provides remarkably detailed images of the ever-changing cloud structure and kinematics over this densely instrumented site. Examples of the data are presented. The radar measurements will greatly improve quantitative documentation of cloud conditions over the CART sites and will bolster ARM research to understand how clouds impact climate through their effects on radiative transfer. Millimeter-wave radars such as the MMCR also have potential applications in the fields of aviation weather, weather modification, and basic cloud physics research.

Full access
Paul J. Neiman, Gary A. Wick, F. Martin Ralph, Brooks E. Martner, Allen B. White, and David E. Kingsmill

Abstract

An objective algorithm presented in White et al. was applied to vertically pointing S-band (S-PROF) radar data recorded at four sites in northern California and western Oregon during four winters to assess the geographic, interannual, and synoptic variability of stratiform nonbrightband (NBB) rain in landfalling winter storms. NBB rain typically fell in a shallow layer residing beneath the melting level (<∼3.5 km MSL), whereas rainfall possessing a brightband (BB) was usually associated with deeper echoes (>∼6 km MSL). The shallow NBB echo tops often resided beneath the coverage of the operational Weather Surveillance Radar-1988 Doppler (WSR-88D) scanning radars yet were still capable of producing flooding rains.

NBB rain contributed significantly to the total winter-season rainfall at each of the four geographically distinct sites (i.e., 18%–35% of the winter-season rain totals). In addition, the rainfall observed at the coastal mountain site near Cazadero, California (CZD), during each of four winters was composed of a significant percentage of NBB rain (18%–50%); substantial NBB rainfall occurred regardless of the phase of the El Niño–Southern Oscillation (which ranged from strong El Niño to moderate La Niña conditions). Clearly, NBB rain occurs more widely and commonly in California and Oregon than can be inferred from the single-winter, single-site study of White et al.

Composite NCEP–NCAR reanalysis maps and Geostationary Operational Environment Satellite (GOES) cloud-top temperature data were examined to evaluate the synoptic conditions that characterize periods of NBB precipitation observed at CZD and how they differ from periods with bright bands. The composites indicate that both rain types were tied generally to landfalling polar-cold-frontal systems. However, synoptic conditions favoring BB rain exhibited notable distinctions from those characterizing NBB periods. This included key differences in the position of the composite 300-mb jet stream and underlying cold front with respect to CZD, as well as notable differences in the intensity of the 500-mb shortwave trough offshore of CZD. The suite of BB composites exhibited dynamically consistent synoptic-scale characteristics that yielded stronger and deeper ascent over CZD than for the typically shallower NBB rain, consistent with the GOES satellite composites that showed 20-K warmer (2.3-km shallower) cloud tops for NBB rain. Composite soundings for both rain types possessed low-level potential instability, but the NBB sounding was warmer and moister with stronger low-level upslope flow, thus implying that orographically forced rainfall is enhanced during NBB conditions.

Full access