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EARL S. JOHNSTON

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Bruce W. Johnston, John D. Marwitz, and Richard E. Carbone

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A method for the analysis of single-Doppler radar data called band-velocity-processing (BVP) is presented. The BVP method was designed to exploit the two-dimensional nature of banded precipitation systems with crossband length scales of 5–50 km. The BVP method offers improved horizontal (crossband) resolution over existing volume scan methods (VAD/VVP), plus the ability to extract band-perpendicular vertical cross sections from a single volume scan. The BVP analysis yields a more complete kinematic structure than RHI analyses, providing estimates for both horizontal wind components, both horizontal divergence components, the shearing deformation, and the vertical shear vector. The bias and variance errors for the BVP method are shown to be acceptably small.

The application of the BVP method to a narrow cold frontal rainband reveals several important kinematic features. The mesoscale structure of the low-level jet ahead of the narrow cold frontal rainband is examined. The band relative flow field exhibits a closed helical circulation cell at the front of the band.

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D. M. Rodgers, K. W. Howard, and E. C. Johnston

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An important class of convective weather system, the mesoscale convective complex (MCC), presents many challenges and problems to both the research and operational communities. In addition, thew very large and long-lived thunderstorm systems have a significant social and economic impact resulting from associated severe weather phenomena and widespread beneficial rain. Enhanced infrared satellite images were surveyed to document MCCs which occurred over the United States during 1982. Thirty-seven convective mesosystems were identified that displayed satellite-observable characteristics which satisfied the MCC criteria described by Maddox. Details of the life cycles of the 37 cases are given and several specific cases are discussed. Current and proposed future research will focus on what are perceived to be key questions surrounding these important weather systems. This annual summary is offered as a starting point for scientists interested in pursuing studies of mesoscale convective weather systems.

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Peter T. May, A. R. Jameson, Thomas D. Keenan, Paul E. Johnston, and Chris Lucas

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An experiment combining wind profiler and polarimetric radar analyses of intense, but shallow, tropical thunderstorms has been performed. These storms are important as they are very common along many tropical coasts and islands and are sometimes the precursors to large intense multicellular storms such as occur over the Tiwi Islands north of Darwin, Australia. All the storms sampled had a similar structure, with intense updrafts on the periphery of the cells producing significant-sized hail and downdrafts in the storm center. The hail concentrations are relatively small, but have a large effect on the radar reflectivity and polarimetric measurands because of the size (10–20 mm). It is this hail melting that produces characteristic Z DR columns in the polarimetric radar data.

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Robert Schafer, Susan K. Avery, Kenneth S. Gage, Paul E. Johnston, and D. A. Carter

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A method is presented that increases the detectability of weak clear-air signals by averaging Doppler spectra from coplanar wind profiler beams. The method, called coplanar spectral averaging (CSA), is applied to both simulated wind profiler spectra and to 1 yr of archived spectra from a UHF profiler at Christmas Island (1 October 1999–30 September 2000). A collocated 50-MHz wind profiler provides a truth for evaluating the CSA technique.

In the absence of precipitation, it was found that CSA, when combined with a fuzzy logic quality control, increases the height coverage of the 1-hourly averaged UHF profiler winds by over 600 m (two range gates). CSA also increased the number of good wind estimates at each observation range by about 10%–25% over the standard consensus method.

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Peter T. May, A. R. Jameson, Thomas D. Keenan, and Paul E. Johnston

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This paper describes the results of an experiment that combines the data from a 5-cm-wavelength polarimetric radar and multiple-frequency wind profilers to examine the polarimetric signatures associated with the microphysical structure of several relatively shallow thunderstorms and also to examine quantitative rainfall measurements made with the polarimetric radar. These shallow storms produce considerable amounts of centimeter-sized hail. The presence and size of this hail are deduced from the wind profiler data. The melting hail particles produce a distinctive polarimetric signature with large values of differential reflectivity Z DR and suppressed values of the correlation coefficient between the signals at horizontal and vertical polarization. Comparisons between the mass-weighted mean drop diameter and differential reflectivity have been performed and show reasonable agreement with theoretical expectations, although the observed Z DR are somewhat smaller than expected. This may be associated with the theoretical assumption of the Pruppacher–Beard oblateness relationship even though there is evidence to suggest that real raindrops may be less oblate on average in convective rain. Quantitative polarimetric rainfall estimators have been compared with rainfall rates derived from the profiler drop size distribution retrievals and show reasonably good agreement when reflectivity values are matched.

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Robert Schafer, Peter T. May, Thomas D. Keenan, Kendal McGuffie, Warner L. Ecklund, Paul E. Johnston, and Kenneth S. Gage

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Data collected during the Maritime Continent Thunderstorm Experiment (MCTEX) (10 November–10 December 1995) have been used to analyze boundary layer development and circulations over two almost flat, tropical islands. The two adjacent islands have a combined length of about 170 km from east to west and 70 km from north to south. Intense thunderstorms formed over these islands every day of the field campaign. The boundary layer depth, temperature, and circulation over the island have been measured over the full diurnal cycle using a multiple radar analysis combined with surface and radiosonde measurements. On average, the island boundary layer depth reaches 1.5 km by early to midafternoon coinciding with the development of the deep convection. Thus, the island boundary layer is significantly deeper than the typical tropical oceanic boundary layer. In the midafternoon, thunderstorm outflows and their associated cold pool stabilize the lower boundary layer, suppressing late convection. This is followed by a period of partial boundary layer recovery for 1–2 h. After sunset, cooling leads to a deepening ground-based inversion below a residual mixed layer. Near the island center, the residual mixed layer of island-modified air is replaced by air of oceanic origin by about 2300 LST (local standard time) that then persists until sunrise the next day. The advection of boundary layer air of oceanic origin over the islands every evening resets the boundary layer development cycle. It is shown that much of the variation in the diurnal temperature profile is a result of thunderstorm activity, radiative processes, and the advection of island and oceanic boundary layer air.

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Luc Rainville, T. M. Shaun Johnston, Glenn S. Carter, Mark A. Merrifield, Robert Pinkel, Peter F. Worcester, and Brian D. Dushaw

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Most of the M 2 internal tide energy generated at the Hawaiian Ridge radiates away in modes 1 and 2, but direct observation of these propagating waves is complicated by the complexity of the bathymetry at the generation region and by the presence of interference patterns. Observations from satellite altimetry, a tomographic array, and the R/P FLIP taken during the Farfield Program of the Hawaiian Ocean Mixing Experiment (HOME) are found to be in good agreement with the output of a high-resolution primitive equation model, simulating the generation and propagation of internal tides. The model shows that different modes are generated with different amplitudes along complex topography. Multiple sources produce internal tides that sum constructively and destructively as they propagate. The major generation sites can be identified using a simplified 2D idealized knife-edge ridge model. Four line sources located on the Hawaiian Ridge reproduce the interference pattern of sea surface height and energy flux density fields from the numerical model for modes 1 and 2. Waves from multiple sources and their interference pattern have to be taken into account to correctly interpret in situ observations and satellite altimetry.

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K. S. Gage, J. R. Mcafee, W. L. Ecklund, D. A. Carter, C. R. Williams, P. E. Johnston, and A. C. Riddle

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After a decade of development, VHF wind profilers are being used for atmospheric research at several locations in the tropical Pacific. A prototype 50-MHz wind profiler was installed on Christmas Island in 1985 and has operated continuously since March 1986 to monitor tropical wind fields in the altitude range 1.8–1 8 km. This paper presents an overview of the Christmas Island wind profiler and reviews its performance. A survey of sample wind observations and a brief climatology of the observed winds are included.

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J. M. Warnock, T. E. VanZandt, W. L. Clark, S. J. Franke, H. S. Kim, G. D. Nastrom, and P. E. Johnston

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An experimental field campaign to measure synoptic-scale vertical velocities was conducted from 5 to 11 January 1991 in the Urbana-Champaign, Illinois, region, which is in very flat terrain far from mountains. Both the Flatland and the Urbana wind-profiling radars, which are separated by 23.1 km, participated in the campaign. Meteorological sounding balloons were also launched from the Flatland Observatory site. In this study, lime averages are compared of the vertical wind velocity measured directly by both radars in order to help verify the capability of wind-profiling radars to measure synoptic-scale vertical velocities. This comparison, of course, also provides an opportunity to evaluate the performance of both radars.

The variance of the vertical velocity observed by the Flatland radar has been previously shown to be dominated by short-period fluctuations with most of the variance occurring at periods less than 6 h. Also, since March 1987 when the Flatland radar began operating nearly continuously, the vertical velocity measurements showed a nearly constant downward mean value of several centimeters per second in the troposphere. After bandpass filtering, the time-series measurements of vertical velocity to obtain 6-b and 1-day means, the filtered signal is compared to similar measurements made by the newly constructed Urbana radar. Both the 6-b and 1-day time averages of vertical velocity measured by the radars displayed large variations in time and height. Variations of 1.0–1.5 cm s−1 occurred frequently, which are considerably larger than the expected measurement error. Good to excellent agreement is generally found in the shape of height profiles measured by the two radars. These results suggest that wind-profiling radars located in very flat terrain are capable of measuring synoptic-scale vertical velocity profiles with useful precision.

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