Search Results

You are looking at 1 - 2 of 2 items for :

  • Author or Editor: John J. Carroll x
  • Journal of Atmospheric and Oceanic Technology 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