Search Results

You are looking at 1 - 8 of 8 items for :

  • Author or Editor: John J. Carroll x
  • Refine by Access: All Content x
Clear All Modify Search
John J. Carroll

Abstract

The analysis of airborne Doppler lidar data taken by NASA near the top of the planetary boundary layer in the central valley of California is presented. These data include downward wan angles that intercept the ground. The maximum errors in the radial speeds, based on the apparent speed of the ground strikes, was found to have a magnitude of less than 2.0 m s−1. These errors appear to have a slowly varying systematic part and a random part. The tidal radial speeds were corrected for running averages of these errors resulting in a random residual error of less than 0.3 m s−1 in magnitude.

Data are considered usable for those range bins in which the return signal amplitude exceeds that of the average shot noise by two times the rms variation in the noise. These data were analyzed to produce wind vectors on a geographic grid divided into 1 kilometer square cells. The resulting wind field appears to be very realistic and contains features similar to those apparent in ground level wind observations. The vertical shear in the sea breeze is well resolved as is its apparent containment below well-defined inversions.

While the analysis was labor intensive and cumbersome at this early stage of system development, the final product is a heretofore unattainable high resolution depiction of air flow over a broad region. The analyzed wind fields appear to be very realistic and easy to interpret. The technique is accurate in the sense that wheel the flow is slowly varying relative to the time sale of the measurement technique, the accuracy of the system is comparable to more standard techniques but with much greater spatial resolution than possible with other techniques. Where the flow is nonstationary, the recovered wind fields show this as well, i.e., the variability of the wind is correctly indicated. The success of this experiment is due to the high precision of the modified inertial navigation unit used and the use of ground strikes to provide absolute verification of total system performance.

Full access
John J. Carroll

Abstract

Errors in airborne Doppler lidar measurements of the wind are evaluated using simulated wind fields and simulated lidar data derived from these wind fields. The primary motivation for this study is the evaluation of several sources of error in the measurement technique. Two types of errors are examined: those due to errors in aircraft velocity and those due to the displacement of the spatially varying wind field during the time between two intersecting looks at the sample volume.

It is found that small errors in the measured aircraft ground speed and track angle produce large errors in the recovered wind speed and direction. Errors in recovered wind speed and direction are strongly dependent on wind direction relative to the flight path. Errors due to nonsimultancous samples can also be large, and are strong functions of the range to the sample volume, wind direction, wind speed, and disturbance amplitude. Recovery of accurate wind fields in such cases requires that the lidar data be displaced to account for advection so that the intersections are defined by air parcels rather than fixed points in space.

The analysis is focused on a particular type of lidar system, but in fact, the results are applicable to other remote sensing systems, such as Doppler radars, that do not utilize simultaneous sampling of the same sample volume.

Full access
John J. Carroll
and
Ming Liu

Abstract

The authors have developed a relatively simple, first-order closure, Eulerian diffusion model, in which turbulent coherent structures of the convective boundary layer are explicitly included as periodic velocities imposed on a stationary and horizontally homogeneous wind field. The dimensions of the convective updrafts and downdrafts are assumed to be inversely proportional to the ratio of their respective vertical speeds and are constant with height, and the updraft and downdraft areas are constant in the horizontal. Sinusoidal vertical velocity variations are specified with amplitudes proportional to the mean vertical velocity profiles for skewed distributions described by Weil. The horizontal velocity components of the coherent structures are calculated using the continuity equation. Model simulations for conditions on the afternoon of Wangara day 33 reproduce the major features of the complicated plume dispersion behavior observed in the water tank experiments and the CONDORS experiments. The model produces results comparable to those obtained by complex large-eddy simulation models and random walk Lagrangian models, but is computationally much less demanding. Sensitivity tests are presented that show that the model is insensitive to physically realistic ranges of the modeling parameters.

Full access
Ming Liu
and
John J. Carroll

Abstract

The development of an air pollution transport model that uses an expanding terrain-following coordinate with high resolution in analytic form near the surface and a high-order accurate transport algorithm is described. The model is designed to be internally consistent in the application of numerical methods, computationally efficient, and suitable for pollutant dispersion studies in complex terrain. The application of the time-splitting Warming–Kutler–Lomax advection scheme is examined in both rotational and deformational flows for its conservation and stability properties. It is found that the combination of this scheme with a short-wave filter makes the integration mass conserving and dispersion free. The model is applied to some hypothetical cases that represent the typical phenomena occurring over mountains. The model proves to be capable of simulating realistic planetary boundary layer structure and stability variation, hydrostatic mountain waves, thermally induced mountain–valley winds, and passive scalar dispersion over sloped surfaces reproducing features observed in field experiments.

Full access
John J. Carroll
and
Ronald L. Baskett

Abstract

The results of a field study utilizing ground-based and aircraft measurements of meteorological parameters and several air pollutants are described for two summer periods in the vicinity of Yosemite National Park, California. These results are related to observed air quality and atmospheric circulation patterns in neighboring parts of the state and to transport by the local mountain-valley wind system. The conclusion is reached that maximum air quality degradation in the study area does not occur during persistent periods of large-scale stagnation, but occurs as the result of transport from area sources up to 200 km away by the typical extended sea breeze circulation which develops following such a period.

Full access
Luis R. Mendez-Nunez
and
John J. Carroll

Abstract

In a previous paper, the authors discussed the numerical properties of the MacCormack scheme, a finite-difference technique widely used in aerospace simulations. Here the authors report results of its application to the simulation, in two dimensions, of the development of a fully compressible buoyant bubble. The model uses the fully compressible Navier-Stokes equations applied to an inviscid, adiabatic atmosphere. It uses a nonstaggered grid. Both lateral and top boundary conditions are open and essentially reflection-first. The model produced reasonable solutions with no explicit numerical filtering. In regions with locally steep gradients, the MacCormack scheme produces numerical oscillations that locally distort the solution but do not lead to numerical unstability.

These results are compared with those of Droegemeier and of Carpenter et al., who show results using a filtered staggered leapfrog scheme. The fields computed by both schemes are very similar, with those, from the filtered leapfrog being smoother. The major difference is that the speed of propagation of the significant flow features is slower with the leapfrog scheme. The advantage of the MacCormack scheme is that it is numerically stable with no tuned filtering and gives its best results at Courant numbers four times larger than can be used with a leapfrog scheme. In long-term integrations in the presence of very steep gradients, numerically induced oscillations would require some degree of explicit filtering to control these numerical oscillations and improve the quality of the solution. The use of a second-order Fickian filter with the MacCormack scheme weakens the gradients.

Full access
Luis R. Mendez-Nunez
and
John J. Carroll

Abstract

The MacCormack scheme is a finite-difference scheme widely used in the aerospace simulations. It is a two-step algorithm, and contains a small amount of implicit numerical diffusion that makes it numerically stable without having to use any explicit filtering. It uses a nonstaggered grid. A detailed comparison with the leapfrog and Smolarkiewicz schemes is presented using the nonlinear advection equation and the Euler equations for a variety of conditions at different Courant numbers. Of the schemes tested, the unfiltered leapfrog is the least acceptable for the solution of nonlinear equations. Although it is numerically stable for linear problems, when used to solve nonlinear equations (without using any explicit filtering) it becomes numerically unstable or nonlinearly unstable. Furthermore, it introduces large phase errors, and produces better results with small Courant numbers. The MacCormack scheme is nonlinearly stable, produces modest amounts of numerical dispersion and diffusion, has no phase speed error, and works best at Courant numbers close to 1. When applied to nonlinear equations, the Smolarkiewicz scheme exhibits the least amount of numerical diffusion but more numerical dispersion than the MacCormack scheme. For stability it requires Courant numbers equal to or smaller than 0.5. In practical applications, we recommend the MacCormack scheme for the solution of the nonlinear equations, and either the Smolarkiewicz or the MacCormack scheme for equations involving conservation of a passive scalar.

Full access
Laura L. Zaremba
and
John J. Carroll

Abstract

This study utilized conditional sampling to identify three frequent wind regimes in the lower Sacramento Valley. The major flow features of the mean diurnal wind patterns in the southern Sacramento Valley and surrounding areas were analyzed for each wind regime. Afternoon wind directions at a pivotal observing site (Davis, CA) in the south-central part of the valley were used to classify the regimes as south wind (marine air intrusion), north wind (no marine air intrusion), and transitional wind days. In the summer of 1991, these occurred 72%, 14%, and 14% of the days, respectively. Daily data from 21 surface observing stations were segregated by wind regime, then averaged for quarters of the day to produce wind roses grouped by regime and time of day. These data were then plotted on a base map. The most frequent direction in each of these wind roses was used to construct streamlines for the area by quarter of the day for each regime. These analyses provide a climatology of the diurnal variation of the average wind flow for each of these frequent flow regimes, providing a wind climatology with greater spatial and temporal resolution than those in extant publications. These analyses are especially useful for evaluating transport patterns of air pollutants or contaminants.

Full access