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R. J. Zamora, M. A. Shapiro, and C. A. Doswell III

Abstract

Wind fields derived from a network of three VHF Doppler radars are used to calculate the mean kinematic properties of the wind field over Colorado and an area-averaged geostrophic and ageostrophic wind. A numerical technique that is equivalent to the line integral method is used to compute the kinematic quantities. Details of this technique, termed the linear vector point function method (LVPF) are discussed. The behavior of the vorticity, divergence, deformation, geostrophic wind and ageostrophic wind are examined for two case studies when the synoptic scale weather patterns over Colorado are dominated by moderately intense upper-level troughs and jet streams. We find that the computed quantities of divergence, absolute vorticity, deformation, geostrophic and ageostrophic wind are modified by the passage of the weather systems in a manner consistent with our present understanding of upper-level dynamics. In addition, temporal variations in the kinematic properties, geostrophic wind and ageostrophic wind are revealed that are beyond the resolution of the existing rawinsonde network.

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R. J. Zamora, B. L. Weber, and D. C. Welsh

Abstract

The effects of spatial, combined spatial and temporal sampling errors, and wind measurement errors on profiler-derived divergence estimates computed using the linear vector point function method are examined. Analysis indicates that divergence errors are minimized when the ratio between the spacing of the profilers and the sampled wavelength (Δx/Lx) is between 0.15 and 0.24 and the ratio between the profiler sampling time to the timescale of the weather system (Δt/T) is less than 0.055.

When Δx/Lx ≤ 0.24, synoptic-scale divergence smaller than ±1.0 × 10−5 s−1 cannot be measured, because the error in the profiler wind estimates is larger than the horizontal velocity gradients. The expected errors in divergence calculations given typical profiler spatial and temporal sampling strategies are examined.

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E. E. Gossard, J. E. Gaynor, R. J. Zamora, and W. D. Neff

Abstract

A study of the finestructure within elevated stable atmospheric layers is described. The observational program consisted of measurements made with fast-response turbulence sensors on a carriage traversing a 300 m tower and comparison of the carriage data with data from acoustic and radar echo sounders. Some supporting observations using a free balloon-borne sensor of the temperature structure parameter are also shown. The layers studied were found to be composed of sheets and layers in temperature, humidity and wind reminiscent of the sheet and layer structures often reported in lakes, estuaries and the oceans. Finestructure in the profiles of temperature and humidity are very highly correlated within elevated stable layers. The sheets are generally accompanied by thin zones of very large temperature and humidity structure parameter, apparently the result of Kelvin-Helmholtz instability, that account for the strong returns from these zones recorded by short wavelength radar and acoustic sounders. The distributions of turbulence properties through the layered structures are described, and some implications for models are discussed. A quite general ratio of sheet-to-layer thickness is proposed toward which the process of step formation proceeds. Measured profiles of short term averages of wT′ show thin zones of apparently strong upward flux imbedded within generally stable regions of weak downward flux. These layers of positive flux are associated with thin superadiabatic zones in the temperature profile and suggest a much more complicated process of heat and momentum transport within stable, elevated regions than process suggested by classical turbulence theory.

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F. M. Ralph, T. Coleman, P. J. Neiman, R. J. Zamora, and M. D. Dettinger

Abstract

This study is motivated by diverse needs for better forecasts of extreme precipitation and floods. It is enabled by unique hourly observations collected over six years near California’s Russian River and by recent advances in the science of atmospheric rivers (ARs). This study fills key gaps limiting the prediction of ARs and, especially, their impacts by quantifying the duration of AR conditions and the role of duration in modulating hydrometeorological impacts. Precursor soil moisture conditions and their relationship to streamflow are also shown. On the basis of 91 well-observed events during 2004–10, the study shows that the passage of ARs over a coastal site lasted 20 h on average and that 12% of the AR events exceeded 30 h. Differences in storm-total water vapor transport directed up the mountain slope contribute 74% of the variance in storm-total rainfall across the events and 61% of the variance in storm-total runoff volume. ARs with double the composite mean duration produced nearly 6 times greater peak streamflow and more than 7 times the storm-total runoff volume. When precursor soil moisture was less than 20%, even heavy rainfall did not lead to significant streamflow. Predicting which AR events are likely to produce extreme impacts on precipitation and runoff requires accurate prediction of AR duration at landfall and observations of precursor soil moisture conditions.

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J. M. Wilczak, D. E. Wolfe, R. J. Zamora, B. Stankov, and T. W. Christian

Abstract

On 2 July 1987 a nonmesocyclone tornado was observed in northeastern Colorado during the Convection Initiation and Downburst Experiment (CINDE). This tornado, reaching FI–F2 intensity, developed under a rapidly growing convective cell, without a preceding supercell or midlevel mesocyclone being present.

The pretornado environment on 2 July is described, including observations from a triangle of wind profilers, a dense surface mesonet array, and a special balloon sounding network. Important features contributing to tornado generation include the passage of a 700-mb short-wave trough; the formation of an ∼70-km diameter, terrain-induced mesoscale vortex (the Denver Cyclone) and its associated baroclinic zone; the presence of a stationary low-level convergence boundary; and the presence of low-level azimuthal sheer maxima (misovortices) along the boundary.

Vorticity budget terms are calculated in the lowest 2 km AGL using a multiple-Doppler radar analysis. These terms and their spatial distributions are compared with observations of mesocyclone-associated supercell tornadoes. Results show that vorticity associated with the 2 July nonsupercell tornado was generated in a more complicated manner than that proposed by previous nonsupercell tornadogenesis theory. In particular, tilting of baroclinically generated streamwise horizontal vorticity into the vertical was important for the formation of low-level rotation, in a manner similar to that previously proposed for supercell tornadic storms.

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Shekhar Gupta, R. T. McNider, Michael Trainer, Robert J. Zamora, Kevin Knupp, and M. P. Singh

Abstract

Theoretical plume growth rates depend upon the atmospheric spatial energy spectrum. Current grid-based numerical models generally resolve large-scale (synoptic) energy, while planetary boundary layer turbulence is parameterized. Energy at intermediate scales is often neglected. In this study, boundary layer radar profilers are used to examine the temporal energy spectrum, which can provide information about the atmospheric structure affecting plume growth rates. A boundary layer model (BLM) into which the radar information has been assimilated is used to drive a Lagrangian particle model (LPM) that is subsequently employed to examine plume growth rates. Profiler and aircraft data taken during the 1995 Southern Oxidants Study in Nashville, Tennessee, are used in the model study for assimilation and evaluation. The results show that the BLM without assimilation significantly underestimates the strength of the diurnal–inertial spectral peak, which in turn causes an underestimate of plume spread. Comparison with measures of plume width from aircraft data also shows that assimilation of radar information greatly improves plume spread rates predicted by the LPM.

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David B. Parsons, Melvyn A. Shapiro, R. Michael Hardesty, Robert J. Zamora, and Janet M. Intrieri

Abstract

During spring and early summer, a surface confluence zone, often referred to as the dryline, forms in the midwestern United States between continental and maritime air masses. The dewpoint temperature across the dryline can vary in excess of 18°C in a distance of less than 10 km. The movement of the dryline varies diurnally with boundary layer growth over sloping terrain leading to an eastward apparent propagation of the dryline during the day and a westward advection or retrogression during the evening. In this study, we examine the finescale structure of a retrogressing, dryline using data taken by a Doppler lidar, a dual-channel radiometer, and serial rawinsonde ascents. While many previous studies were unable to accurately measure the vertical motions in the vicinity of the dryline, our lidar measurements suggest that the convergence at the dryline is intense with maximum vertical motions of ∼5 m s−1. The winds obtained from the Doppler lidar Measurements were combined with the equations of motion to derive perturbation fields of pressure and virtual potential temperature θv. Our observations indicate that the circulations associated with this retrogressing dryline were dominated by hot, dry air riding over a westward moving denser, moist flow in a manner similar to a density current. Gravity waves were observed above the dryline interface. Previous observational and numerical studies have shown that differential heating across the dryline may sometimes enhance regional pressure gradients and thus impact dryline movement. We propose that this regional gradient in surface heating in the presence of a confluent flow results in observed intense wind shifts and large horizontal gradients in θv across the dryline. The local gradient in θv influences the movement and flow characteristics of the dryline interface. This study is one of the most complete and novel uses of Doppler lidar to date.

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Paul J. Neiman, M. A. Shapiro, R. Michael Hardesty, B. Boba Stankov, Rhidian T. Lawrence, Robert J. Zamora, and Tamara Hampel

Abstract

The NOAA/WPL pulsed coherent Doppler lidar was used during the Texas Frontal Experiment in 1985 to study mesoscale preconvective atmospheric conditions. On 22 April 1985, the Doppler lidar, in conjunction with serial rawinsonde ascents and National Weather Service rawinsonde ascents, observed atmospheric features such as middle-tropospheric frontal and vertical wind shear layers and the planetary boundary layer. The lidar showed unique evidence of the downward transport of strong winds from an elevated vertical speed shear (frontal) layer into the planetary boundary layer. The lidar provided further evidence of atmospheric processes such as clear-air turbulence within frontal layers, and dry convection turbulence within the superadiabatic planetary boundary layer. As a result, high-technology remote sensing instruments such as the Doppler lidar show considerable promise for future studies of small-scale weather systems in a nonprecipitating atmosphere.

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