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Brooks E. Martner

Abstract

Observations of mammatus-like cloud features associated with a convective rain shower were obtained using a vertically pointing 8-mm-wavelength Doppler radar. The radar's excellent sensitivity and resolution allowed even very weak, finescale features of the cloud to be resolved. The mammatus were located 3–5 km above ground along the underneath side of an anvil-like stratiform region of the storm cloud and preceded the arrival of precipitation. Both reflectivity and velocity exhibited strongly periodic patterns, with larger downward motions and larger reflectivities occurring together. The mammatus lobes were separated by about 90 s or approximately 1 km, horizontally. The mammatus features could be traced more than 500 m upward into the cloud echo interior where the amplitude of the vertical velocity oscillations was greatest. The measured vertical velocities of particle motions ranged from +0.5 to −3.0 m s−1. The observed reflectivity and velocity patterns suggest that although evaporation was acting to shape the mamma as they descended toward the echo bottom, other mechanisms may have been responsible for their initial formation in the interior region of the cloud echo.

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Roger M. Wakimoto and Brooks E. Martner

Abstract

An integrated analysis of photographic and Doppler radar observations of a tornadic storm during the Convection Initiation and Downburst Experiment (CINDE) is presented. High-resolution single-Doppler radar measurements are combined with cloud photogrammetry to reveal the detailed structural relationship of the hook echo and the misocyclone with visual features of the tornado. Vertical cross sections of dual-Doppler winds in the plane of the photographs were also examined to determine the complex motions within and surrounding the vortex. The tornado was found to be within a weak-echo hole of the hook echo. The hole progressed upward above cloud base as the tornado matured. An annulus of higher reflectivity that formed a book echo is hypothesized to have been composed of sparse but large raindrops. The airflow fields suggest that vortex breakdown and axial downdrafts were present near the ground at early stages shortly after the tornado became visible. Later, axial upward flow dominated at all levels until the collapse of the vortex.

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A. Shelby Frisch, Brad W. Orr, and Brooks E. Martner

Abstract

A single Doppler radar obtained detailed clear-air measurements of the development of a strong boundary-layer nocturnal jet in North Dakota during the summer of 1989. The evolution of the jet was monitored by the radar with a high degree of vertical and temporal resolution using a repetitive sequence of four different elevation scans. A new variation of the velocity-azimuth display (VAD) analysis technique provided vertical profiles of the mean wind components and several turbulence terms. Boundary-layer wind speeds began to increase in the late afternoon, well before sunset, as surface cooling began. Wind speeds accelerated faster after sunset and eventually produced a jet that exceeded 23 m s−1 at about 0.5 km AGL. The wind veered with height and time and followed the expected inertial oscillation pattern. Measured shear stresses, vertical fluxes of momentum, and velocity variances, which were initially large, decreased sharply after the surface began to cool. The directly measured vertical velocities were significantly downward during the late afternoon and upward at night.

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Brooks E. Martner, Paul J. Neiman, and Allen B. White

Abstract

A strong elevated temperature inversion in a landfalling winter storm in northern California produced two simultaneous melting layers with associated radar bright bands. The storm was observed with scanning and profiling radars. Serial radiosonde launches from the scanning radar site precisely documented the evolving temperature structure of the air mass that produced the double bright band. The radiosonde and radar observations, which were coincident in location and time, clearly illustrate the cause (two melting layers) and effect (two bright bands) of this unusual phenomenon. An automated algorithm for determining the melting-layer height from profiling radar data was tested on this situation. In its operational form, the algorithm detects only the lower melting layer, but in modified form it is capable of detecting both melting layers simultaneously.

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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.

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Brooks E. Martner, Jack B. Snider, Robert J. Zamora, Gregory P. Byrd, Thomas A. Niziol, and Paul I. Joe

Abstract

A destructive freezing-rain storm on 15 February 1990 was observed intensively with advanced ground-based remote sensors and conventional instruments by the Lake Ontario Winter Storms (LOWS) project in upstate New York. A deep layer of warm, moist, southwesterly flow overran a shallower layer of subfreezing, easterly flow ahead of a surface warm front. Precipitation at the surface changed from snowfall to ice pellets, to freezing rain, and, finally, to ordinary rain as an elevated layer of above-freezing air moved into the region and eventually extended to the ground. Measurements from a scanning Doppler radar, wind profilers, a microwave radiometer, and mobile rawinsondes provided detailed information on the storm's kinematic and thermodynamic structure and evolution, and allowed its basic microphysical structure to be inferred. The remote sensors detected signatures of the melting aloft that may be useful for improving detection and forecasts of freezing-rain hazards.

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