Boundary layer clear-air echoes are routinely observed with sensitive, microwave, Doppler radars similar to the WSR-88D. Operational and research meteorologists are using these Doppler velocities to ...derive winds. The accuracy of the winds derived from clear-air Doppler velocities depends on the nature of the scatterers. This paper uses dual-wavelength and dual-polarization radars to examine the cause of these echoes and the use of Doppler velocities from the clear-air return to estimate winds. The origin of these echoes has been an ongoing controversy in radar meteorology. These echoes have been attributed to refractive-index gradient (Bragg scattering) and insects and birds (particulate scattering). These echoes are most commonly observed over land from spring through autumn. Seldom do they occur over large bodies of water. Widespread clear-air echoes have also been observed in winter when temperatures are above 10 degrees C. Radar reflectivity comparisons of clear-air echoes in Florida and Colorado were made at radar wavelengths of 3, 5, and 10 cm. These comparisons, when analyzed along with a theoretical backscattering model, indicate that the echoes result from both particulate and Bragg scattering with particulate scattering dominating in the well-mixed boundary layer. The return signal in this layer is highly horizontally polarized with differential reflectivity Z sub(D) sub(R) values of 5-10 dB. This asymmetry causes the backscattering cross section to be considerably larger than one for a spherical water droplet of equal mass. At X band and possibly even at C and S band the scattering enters the Mie region. It is concluded that insects are primarily responsible for the clear-air echo in the mixed boundary layer. At and above the top of the well-mixed boundary layer, Bragg scattering dominates and is frequently observed at S band. When insects and birds are not migrating, the Doppler velocities can be used to estimate horizontal winds in the boundary layer. Viewing angle comparisons of Z sub(D) sub(R) values were made to determine if migrations were occurring. Migrations were not observed in Florida and Colorado during summer daylight hours. Limited comparison of winds derived from Doppler radar with balloon-sounding winds showed good agreement. However, a more extensive study is recommended to determine the generality of this conclusion. Dual-Doppler analyses show that thin-line echoes are updraft regions. Comparison of these radar-derived vertical velocities with aircraft-measured vertical velocities showed a correlation coefficient of 0.79. In addition, the position of small-scale updraft maxima (1-2 km in diameter) along the sea-breeze front correspond to individual cumulus clouds. The good agreement between dual-Doppler-derived vertical motion fields and these other independent vertical velocity measurements provides evidence that the dual-Doppler-derived wind fields in the clear-air boundary layer are accurate and capable of providing details of the wind circulations associated with horizontal convective rolls and the sea breeze.
The detailed analysis of the three-dimensional structure of a dryline observed over the Oklahoma panhandle during the International H2O Project (IHOP_2002) on 11 June 2002 is presented. ...High-resolution observations obtained from the National Center for Atmospheric Research Electra Doppler Radar (ELDORA), S-band dual-polarization Doppler radar (S-Pol), water vapor differential absorption lidar (DIAL) Lidar pour l'Etude des Interactions Aerosols Nuages Dynamique Rayonnement et du Cycle de l'Eau (LEANDRE II; translated as Lidar for the Study of Aerosol-Cloud-Dynamics-Radiation Interactions and of the Water Cycle) as well as Learjet dropsondes are used to reveal the evolution of the dryline structure during late afternoon hours when the dryline was retreating to the northwest. The dryline reflectivity shows significant variability in the along-line direction. Dry air was observed to overrun the moist air in vertical cross sections similar to a density current. The updrafts associated with the dryline were 2-3 m s-1 and were able to initiate boundary-layer-based clouds along the dryline. The formation of this dryline was caused by high equivalent potential temperature air pushing northwestward toward a stationary front in the warm sector. Middle-level clouds with radar reflectivity greater than 18 dBZe near the dryline were detected by ELDORA. A roll boundary, which was associated with larger convergence and moisture content, was evident in the S-Pol data. It is found that the instability parameters most favorable for convection initiation were actually associated with the roll boundary, not the dryline. A storm was initiated near the roll boundary probably as a result of the combination of the favorable instability parameters and stronger upward forcing. It is noted that both the 11 June 2002 dryline and the roll boundary presented in this paper would not be identified if the special datasets from IHOP_2002 were not available. Although all model runs fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5), Meso Eta, and Rapid Update Cycle (RUC) suggested deep convection over the Oklahoma panhandle and several cloud lines were observed near the dryline, the dryline itself did not initiate any storms. The reasons why the dryline failed to produce any storm inside the IHOP_2002 intensive observation region are discussed. Both synoptic-scale and mesoscale conditions that were detrimental to convection initiation in this case are investigated in great detail.
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On 13 July 1986 a cold-air outflow from thunderstorms over Illinois and Missouri propagated through the MIST (Microburst and Severe Thunderstorm) network over northern Alabama. The study of this ...outflow is important since the gust front was solely responsible for the initiation of numerous convective cells. Previous studies have documented the initiation of convection due to colliding gust fronts. In addition, there was a pronounced mesoscale organization of the cells atop the outflow boundary. This was most likely due to a combination of Kelvin-Helmholtz (K-H) and internal gravity (IG) wave activity. In contrast to previous cases, the K-H wave crests were oriented nearly perpendicular to the gust front within the analysis area. The resulting intersections between the circulations associated with the K-H waves and the gust front produced favorable locations for the initiation of convection. Subsequently, the convective cells remained along the updraft side of the K-H wave circulations as they propagated back relative to the gust front. In addition, the gust front induced IG waves that were oriented parallel to the gust front. The enhanced upward motions associated with the IG waves resulted in a periodic arrangement of the convective cells along the updraft side of the K-H waves. The combined motion of the K-H and IG waves was consistent with the cell movement atop the cold-air outflow.
A detailed analysis of the structure of a double dryline observed over the Oklahoma panhandle during the first International H2O Project (IHOP_2002) convective initiation (CI) mission on 22 May 2002 ...is presented. A unique and unprecedented set of high temporal and spatial resolution measurements of water vapor mixing ratio, wind, and boundary layer structure parameters were acquired using the National Aeronautics and Space Administration (NASA) scanning Raman lidar (SRL), the Goddard Lidar Observatory for Winds (GLOW), and the Holographic Airborne Rotating Lidar Instrument Experiment (HARLIE), respectively. These measurements are combined with the vertical velocity measurements derived from the National Center for Atmospheric Research (NCAR) Multiple Antenna Profiler Radar (MAPR) and radar structure function from the high-resolution University of Massachusetts frequency-modulated continuous-wave (FMCW) radar to reveal the evolution and structure of the late afternoon double-dryline boundary layer. The eastern dryline advanced and then retreated over the Homestead profiling site in the Oklahoma panhandle, providing conditions ripe for a detailed observation of the small-scale variability within the boundary layer and the dryline. In situ aircraft data, dropsonde and radiosonde data, along with NCAR S-band dual-polarization Doppler radar (S-Pol) measurements, are also used to provide the larger-scale picture of the double-dryline environment. Moisture and temperature jumps of about 3 g kg-1 and 1-2 K, respectively, were observed across the eastern radar fine line (dryline), more than the moisture jumps (1-2 g kg-1) observed across the western radar fine line (secondary dryline). Most updraft plumes observed were located on the moist side of the eastern dryline with vertical velocities exceeding 3 m s-1 and variable horizontal widths of 2-5 km, although some were as wide as 7-8 km. These updrafts were up to 1.5 g kg-1 moister than the surrounding environment. Although models suggested deep convection over the Oklahoma panhandle and several cloud lines were observed near the dryline, the dryline itself did not initiate any storms over the intensive observation region (IOR). Possible reasons for this lack of convection are discussed. Strong capping inversion and moisture detrainment between the lifting condensation level and the level of free convection related to an overriding drier air, together with the relatively small near-surface moisture values (less than 10 g kg-1), were detrimental to CI in this case.
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NCAR–NOAA Lower-Tropospheric Water Vapor Workshop Weckwerth, Tammy M.; Wulfmeyer, Volker; Wakimoto, Roger M. ...
Bulletin of the American Meteorological Society,
11/1999, Volume:
80, Issue:
11
Journal Article
Peer reviewed
A workshop on remote sensing of lower-tropospheric water vapor, jointly sponsored by the National Center for Atmospheric Research (NCAR) and the National Oceanic and Atmospheric Administration, was ...held on 30 June and 1 July 1998 at NCAR in Boulder, Colorado. Scientists representing a variety of disciplines, from climate to weather forecasting, presented their needs for improved water vapor measurements. Engineers and scientists gave information on the current technology for water vapor measuring platforms and retrieval methods. The workshop concluded with an open forum at which the participants agreed upon a general recommendation supporting the use, improvement, and development of active and passive remote sensing techniques to obtain three-dimensional fields of water vapor and their evolution. In addition, several specific recommendations were adopted, including the organization of field campaigns for cross validation of measurement techniques, the focus of attention to improve water vapor measurements in and near clouds, and increased collaboration within the community to build, operate, and provide access to airborne and ground-based water vapor remote measuring systems.
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37.
Radar climatology of the COPS region WECKWERTH, Tammy M; WILSON, James W; HAGEN, Martin ...
Quarterly journal of the Royal Meteorological Society,
2011, Volume:
137
Journal Article
Peer reviewed
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40.
SUPPLEMENT Bluestein, Howard B.; Rauber, Robert M.; Burgess, Donald W. ...
Bulletin of the American Meteorological Society,
12/2014, Volume:
95, Issue:
12
Journal Article
Peer reviewed
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