Search Results

You are looking at 1 - 10 of 182 items for

  • Author or Editor: J. A. Smith x
  • Refine by Access: All Content x
Clear All Modify Search
R. Pinkel and J. A. Smith

Abstract

Repeat-sequence coding is a robust method for improving the precision of velocity estimates from incoherent Doppler sounders. The method involves transmitting a number of repeats of a broadband “subcode.” The Doppler shift is estimated from the complex autocovariance of the return, evaluated at a time lag, equal to the subcode duration. The repeat-sequence code is an extension of the simple pulse-train concept developed in the early days of radar. By transmitting codes, rather than discrete pulses, the average transmitted power is increased. A model is developed here to predict performance enhancement for specified codes. The model is based on the sample error of the covariance estimates. It explicitly accounts for the lag used. Root-mean-square precision is enhanced roughly in proportion to the square root of the time-bandwidth product of the subcode. Coded pulse technology has been implemented on a variety of Doppler sonar systems at Scripps Institution of Oceanography and used in both open-ocean (volume-scattering) and shallow-water (surface-scattering) applications. Field measurements of sonar precision roughly agree with predictions of the model, although with some increase in error.

Full access
Randy A. Peppier and Phillip J. Smith

Abstract

Comparisons are made between second-order and fourth-order finite differencing using three grid resolutions in order to analyze the truncation errors in geostrophic wind and relative vorticity estimates. Horizontal wave dimensions are also varied to study sensitivity to different scales of motion. Exact calculations derived from analytic equations describing geostrophic wind and relative vorticity provide the standard against which the finite-difference estimates are compared.

Both wind speed and relative vorticity truncation errors maximized in regions where speed and vorticity values were greatest. These errors were, in general, smaller for longer wavelengths, finer grid resolution and fourth-order differencing. Although second-order differencing produced fields that were numerically less than observed and yielded errors which decreased with reduced grid interval, fourth-order differencing departed from this anticipated behavior. Rather, when the grid interval was reduced to the point that the wavelength was ten times or more greater than the grid interval, wind speeds and vorticities estimated by the fourth-order scheme were overestimated and were accompanied by increased truncation errors at finer grids.

Full access
Harry J. Cooper and Eric A. Smith

The local meteorological events leading up to the launch of the space shuttle Atlantis on 2 August 1991 were captured in full-resolution GOES visible data being archived for the Convection and Precipitation/Electrification Experiment. The postponement of the launch on 1 August, and the successful lift-off on the following day provide a good example of the important role played by nowcasting and short-term forecasting at Cape Canaveral. In this brief article, we discuss the local weather conditions prior to, during, and after the launch and demonstrate the importance of short-term forecasting capabilities around the cape during launch operations.

Full access
R. A. Peppler and P. J. Smith

Abstract

No abstract available.

Full access
J. A. Smith, A. A. Bradley, and M. L. Baeck

Abstract

The space–time structure of extreme storm rainfall in the southern plains is examined through empirical analyses of radar and rain gauge data. Two networks of rain gauge data are utilized: a collection of more than 200 rain gauges in the southern plains from the National Weather Service (NWS) climatological network and a network of 168 rain gauges from the Southern Great Plains Research Watershed covering an area of approximately 3000 km2. The NWS climate network is useful for analysis of extreme rainstorms that cover relatively large areas, whereas the experimental network is useful for analysis of heavy isolated rainstorms that have been described as affecting areas less than 1000 km2 and having 1–4-h rainfall totals exceeding 75 mm. Analyses of rain gauge data from the two networks are used to characterize seasonality and spatial scale of extreme rainstorms. It is shown that there are pronounced spring and fall peaks in extreme rainstorm occurrence and that these peaks depend on the spatial scale of observations The space–time structure of extreme storms is examined through analyses of radar and rain gauge data for four storms that represent distinctive modes of organization of extreme rainstorms in the southern plains. These analyses are used to characterize and contrast the space–time structure of linear and chaotic convective systems, weak and strong dynamic forcing events, and spring and fall events. Analyses of these four events are augmented by climatological analyses of extreme rainstorms from the two rain gauge networks. The analyses presented in this paper are motivated by problems of engineering hydrometeorology and flood hazard assessment.

Full access
L. J. Fritschen, L. K. Balick, and J. A. Smith

Abstract

The interpretation of an infrared nighttime imagery over a forested valley visualizes cold air drainage with the warmer tops of trees protruding near the center of the valley and whole crowns protruding on higher ridges. This interpretation is supported by detailed measurements of needle and air temperatures. Thermal IR imagery may be used to investigate air circulation patterns in complex terrain.

Full access
Harry J. Cooper, Eric A. Smith, and J. David Martsolf

Abstract

Observations taken by two surface radiation and energy budget stations deployed in the University of Florida/Institute for Food and Agricultural Service experimental citrus orchard in Gainesville, Florida, have been analyzed to identify the effects of sprayer irrigation on thermal stability and circulation processes within the orchard during three 1992 winter freeze episodes. Lapse rates of temperature observed from a micrometeorological tower near the center of the orchard were also recorded during periods of irrigation for incorporation into the analysis. Comparisons of the near-surface temperature lapse rates observed with the two energy budget stations show consistency between the two sites and with the tower-based lapse rates taken over a vertical layer from 1.5 to 15 m above ground level. A theoretical framework was developed that demonstrates that turbulent-scale processes originating within the canopy, driven by latent heat release associated with condensation and freezing processes from water vapor and liquid water released from sprayer nozzles, can destabilize lapse rates and promote warm air mixing above the orchard canopy. The orchard data were then analyzed in the context of the theory for evidence of local overturning and displacement of surface-layer air, with warmer air from aloft driven by locally buoyant plumes generated by water vapor injected into the orchard during the irrigation periods. It was found that surface-layer lapse rates were lower during irrigation periods than under similar conditions when irrigation was not occurring, indicating a greater degree of vertical mixing of surface-layer air with air from above treetops, as a result of local convective overturning induced by the condensation heating of water vapor released at the nozzles of the sprinklers. This provides an additional explanation to the well-accepted heat of fusion release effect, of how undertree irrigation of a citrus orchard during a freeze period helps protect crops against frost damage.

Full access
E. M. Poulter, M. J. Smith, and J. A. McGregor

Abstract

A ground-based frequency-modulated continuous wave microwave radar has been developed and applied to the measurement of ocean surface velocities. Full Doppler spectra are available, and both range and lime variations of the surface drifts can be recorded with resolutions down to 2.5 m and 0.3 s, respectively, over range extents of up to 1000 m. The ability of the radar to provide information over a wide range of sea conditions is demonstrated. These range from the identification of 5-cm scattering waves and their damping by a surface slick, to the propagation characteristics of long (∼200 m) ocean waves in deep and shallow water. The longwave propagation is in accord with linear wave theory. The relationship of the backscattered power at low grazing angles to the wave features is also demonstrated. The consequences of measuring radial velocity components at low grazing angles with a narrow beam antenna are considered. In this situation, wave height spectra and the principal wave direction are well determined in a unimodal sea. However, this radar configuration will overestimate the angular spreading in highly directional seas. In the nearshore zone, velocity measurements can be used to quantify the evolution of shoaling waves and wave breaking.

Full access
S. J. Murray, A. D. Smith, and J. C. Phillips

Abstract

A modified flash flood severity assessment is presented, based on scoring a set of factors according to their potential for generating extreme catchment-scale flooding. Improvements are made to the index through incorporation of parameter uncertainties, managing data absence, and clearer graphical communication. The motive for proposing these changes is to better inform flood managers during the development of a flash flood that may require an emergency response. This modified decision-support system is demonstrated for the Boscastle flood of 2004 and other historical floods in the United Kingdom. For Boscastle, the extreme nature of the flood is underestimated, which is likely to be due to the lack of sophistication in weighting flood parameters. However, the proposed amendments are able to rapidly reflect the reliability of a catchment severity rating, which may further enhance this technique as a decision-support tool alongside radar observations of localized storms.

Full access
E. J. Smith, J. A. Warburton, K. J. Heffernan, and W. J. Thompson

Abstract

The performance of a silver iodide smoke generator, mounted on an aircraft, was measured in flight. The ice-nucleus output was 1014 per gram of silver iodide active at −15C and 1012 at −10C. Considerable variations in the burner configuration and the solution which was burnt had little effect on the performance.

Full access