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Thomas Matejka and Diana L. Bartels

Abstract

Eight methods of calculating vertical air velocity in a column are compared. Each method requires some or all of the following data: horizontal divergence, vertical precipitation velocity, hydrometeor terminal fall speed, and vertical air velocity boundary conditions. Some or all of these quantities are commonly deduced or specified by a researcher during the analysis of Doppler radar data. The responses of the methods to different magnitudes and behaviors of errors in the input data are examined with a Monte Carlo method. The experiments are conducted with both random and systematic errors. Two idealized kinds of systematic errors are considered: bias error (a constant error through the column) and trend error (an error of constant magnitude that changes sign at the midpoint of the mass of the column). The performances of the methods are mapped over error space. A researcher, knowing approximately the characteristics of the errors of a particular set of Doppler radar data, can use the results of these experiments to select a method based on how well it performs in the presence of random errors and how well its performance holds up as bias and trend errors increase.

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Richard H. Johnson and Diana L. Bartels

Abstract

The vertical structure of a midtropospheric mesovortex that developed during the decay of a midlatitude mesoscale convective system over Kansas and Oklahoma on 23–24 June 1985 is documented. Surface, rawinsonde, wind profiler, and dual-Doppler data are used to define its structure.

As has been observed in other midlatitude and tropical cases, the mesovortex occurred within the trailing stratiform precipitation region of a squall-line system. It was associated with a preexisting synoptic-scale short-wave trough, and as the circulation developed, it deformed the back edge of the stratiform precipitation region into a hooklike pattern. The ∼100–200-km mesovortex was confined to the midtroposphere (3–8 km), with a maximum amplitude just above the 0°C level. The vortex axis sloped toward the northeast, but its orientation changed hour by hour over the 2-h period of dual-Doppler coverage. Overall, the mesovortex was warm core, although its thermal structure was complex and apparently significantly influenced at the analysis time by a descending rear-inflow jet entering the rear portion of the stratiform region. The warmest anomaly was found at low levels (near 850 mb) with a shallow cool anomaly in the midtroposphere near the 0°C level and a weak warm anomaly aloft. At 500 mb the warmest air was shifted to the north of the vortex center, where the atmosphere was also relatively dry, while at 400 mb (near the top of the mesovortex) the warmest air coincided with the vortex center.

Although the mesovortex was sampled for only a brief portion of its lifetime, the observations suggest a close coupling between synoptic, mesoscale, and even microscale (cloud) processes in its formation. A vorticity budget based on the sounding data during the decaying stage of the storm indicates that convergence production of vorticity associated with a mesoscale updraft-downdraft couplet in the stratiform precipitation region was a critical factor in intensifying the circulation at that time. Vorticity production by tilting played a minor role during this period due to relatively weak environmental wind shear. The weak shear, however, likely contributed to the longevity of the mesovortex.

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Diana L. Bartels and Robert A. Maddox

Abstract

Despite the large number of convective systems that occur over the central United States every year, there are typically only a few well-defined, midlevel vortices apparent in satellite imagery after the overlying anvil debris from some convective complexes has dissipated or advected away. A climatology of mesoscale convectively generated vortex (MCV) events for 1981-1988 is presented and the synoptic setting in which the circulation becomes apparent is discussed. Proximity sounding data from numerous cases are used to examine features of the kinematic and thermodynamic setting of MCVs at various lifelycle stages defined by satellite imagery. Features ofthe large-scale environment that appear conducive to the formation and longevity of MCVS include weak flow, weak vertical shear, weak background relative vorticity, and intense horizontal and vertical moisture gradients. The rapid mesovortex generation observed can be explained by the stretching term of the vorticity equation.

Most MCVs emerge from MCC-type (i.e., circular) systems, but of the cases noted (24 events over the central United States between 1981-1988) only half originated in systems that met Maddox's stringent MCC size and duration criteria. Furthermore, since several MCVs emerged from small and relatively short-lived convective systems, the background synoptic environment, in addition to the magnitude of latent heating, may provide important controlling factors in determining which MCSs will lead to documentable MCVS. The majority of MCVs (i.e., 80%) were first observed at latitudes south of 40°N. Since many convective syftms occur at latitudes north of 40°N, the paucity of MCVs in northern latitudes is not the result of a lack of convective systems.

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James D. Doyle, Melvyn A. Shapiro, Qingfang Jiang, and Diana L. Bartels

Abstract

A large-amplitude mountain wave generated by strong southwesterly flow over southern Greenland was observed during the Fronts and Atlantic Storm-Track Experiment (FASTEX) on 29 January 1997 by the NOAA G-IV research aircraft. Dropwindsondes deployed every 50 km and flight level data depict a vertically propagating large-amplitude wave with deep convectively unstable layers, potential temperature perturbations of 25 K that deformed the tropopause and lower stratosphere, and a vertical velocity maximum of nearly 10 m s−1 in the stratosphere. The wave breaking was associated with a large vertical flux of horizontal momentum and dominated by quasi-isotropic turbulence. The Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) nonhydrostatic model with four-nested grid meshes with a minimum resolution of 1.7 km accurately simulates the amplitude, location, and timing of the mountain wave and turbulent breakdown. Finescale low-velocity plumes that resemble wakelike structures emanate from highly dissipative turbulent regions of wave breaking in the lower stratosphere. Idealized adiabatic three-dimensional simulations suggest that steep terrain slopes increase the effective Rossby number of the relatively wide Greenland plateau, decrease the sensitivity of the wave characteristics to rotation, and ultimately increase the tendency for wave breaking. Linear theory and idealized simulations indicate that diabatic cooling within the boundary layer above the Greenland ice sheet augments the effective mountain height and increases the wave amplitude and potential for wave breaking for relatively wide obstacles such as Greenland.

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Paul J. Neiman, F. Martin Ralph, Allen B. White, David D. Parrish, John S. Holloway, and Diana L. Bartels

Abstract

Experimental observations from coastal and island wind profilers, aircraft, and other sensors deployed during the California Land-falling Jets Experiment of 1997/98 and the Pacific Land-falling Jets Experiment of 2000/01–2003/04 were combined with observations from operational networks to document the regular occurrence and characteristic structure of shallow (∼400–500 m deep), cold airstreams flowing westward through California’s Petaluma Gap from the Central Valley to the coast during the winter months. The Petaluma Gap, which is the only major air shed outlet from the Central Valley, is ∼35–50 km wide and has walls extending, at most, a modest 600–900 m above the valley floor. Based on this geometry, together with winter meteorological conditions typical of the region (e.g., cold air pooled in the Central Valley and approaching extratropical cyclones), this gap is predisposed to generating westward-directed ageostrophic flows driven by along-gap pressure differences. Two case studies and a five-winter composite analysis of 62 gap-flow cases are presented here to show that flows through the Petaluma Gap significantly impact local distributions of wind, temperature, precipitation, and atmospheric pollutants. These gap flows preferentially occur in pre-cold-frontal conditions, largely because sea level pressure decreases westward along the gap in a stably stratified atmosphere in advance of approaching cold-frontal pressure troughs. Airstreams exiting the Petaluma Gap are only several hundred meters deep and characterized by relatively cold, easterly flow capped by a layer of enhanced static stability and directional vertical wind shear. Airborne air-chemistry observations collected offshore by the NOAA P-3 aircraft illustrate the fact that gap-flow events can transport pollutants from inland to the coast, and that they can contribute to coastally blocked airstreams. The strongest gap-flow cases occur when comparatively deep midtropospheric troughs approach the coast, while the weak cases are tied to anticyclonic conditions aloft. Low-level cold-frontal pressure troughs approaching the coast are stronger and possess a greater along-gap pressure gradient for the strong gap-flow cases. These synoptic characteristics are dynamically consistent with coastal wind profiler observations of stronger low-level gap flow and winds aloft, and greater rainfall, during the strong gap-flow events. However, gap flow, on average, inhibits rainfall at the coast.

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Edward I. Tollerud, Fernando Caracena, Steven E. Koch, Brian D. Jamison, R. Michael Hardesty, Brandi J. McCarty, Christoph Kiemle, Randall S. Collander, Diana L. Bartels, Steven Albers, Brent Shaw, Daniel L. Birkenheuer, and W. Alan Brewer

Abstract

Previous studies of the low-level jet (LLJ) over the central Great Plains of the United States have been unable to determine the role that mesoscale and smaller circulations play in the transport of moisture. To address this issue, two aircraft missions during the International H2O Project (IHOP_2002) were designed to observe closely a well-developed LLJ over the Great Plains (primarily Oklahoma and Kansas) with multiple observation platforms. In addition to standard operational platforms (most important, radiosondes and profilers) to provide the large-scale setting, dropsondes released from the aircraft at 55-km intervals and a pair of onboard lidar instruments—High Resolution Doppler Lidar (HRDL) for wind and differential absorption lidar (DIAL) for moisture—observed the moisture transport in the LLJ at greater resolution. Using these observations, the authors describe the multiscalar structure of the LLJ and then focus attention on the bulk properties and effects of scales of motion by computing moisture fluxes through cross sections that bracket the LLJ. From these computations, the Reynolds averages within the cross sections can be computed. This allow an estimate to be made of the bulk effect of integrated estimates of the contribution of small-scale (mesoscale to convective scale) circulations to the overall transport. The performance of the Weather Research and Forecasting (WRF) Model in forecasting the intensity and evolution of the LLJ for this case is briefly examined.

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