Airborne aerosol lidar profiles in the California Central Valley

This paper will describe the observed lidar profiles obtained during five flights over the California Central Valley using an aerosol lidar on the NASA DC-8 aircraft. The resulting vertical lidar profiles were plotted on a topographic relief map of the Central Valley to give the geographic context of the aerosol distributions. Also back trajectories were plotted to understand the air parcel temporal history. All this data was then displayed on a three dimensional web based projection of the Central Valley that could be manipulated to observe different computer generated projections, giving a complete contextual representation of the lidar data.

The aerosol lidar system produces 42 mJ pluses at 1064-nm and 56-J at 532-nm at 20-Hz repetition rate. The receiver used a 28-cm diameter telescope and avalanche photodetector to detect the 1064-nm return and a microchannel plate detector to detect the 532-nm return. Only data from the 1064-nm channel will be discussed here. All data was averaged for 2 seconds on a 14-bit waveform digitizer then saved on the data acquisition computer.

In order to better study the 3-D (three-dimensional) placement of the lidar flight profiles with respect to land topography and atmospheric backward trajectories, an interactive computer visualization technique was developed. The data, underlying geography, and the backward trajectories were all transformed into 3-D graphics representations for simultaneous display within a standard Web browser. Within a 3-D latitude-longitude-altitude coordinate system, the user can interactively view a full-color lidar profile displayed over a 3-D high resolution land map, and atmospheric backward trajectories can be observed intersecting the lidar profile and passing over the land masses. Data from multiple flights can also be displayed simultaneously.

To produce this 3-D visualization, custom software was developed to encode all the graphics into a standard format known as VRML97, also referred to as VRML2.0, which is a version of the Virtual Reality Modeling Language. A free plug-in, such as CosmoPlayer or Cortona, is used within the Web browser to view the result on most desktop PCs. The common technique of texture mapping, by which a raster image is aligned and mapped onto a set of 3-D polygons, is used extensively in this implementation because it is an accelerated operation available on most graphics cards. Texture mapping is a very efficient approach to visualize very high resolution image-based data applied to true 3-D surfaces, such as lidar profiles and topographical maps.

Figures will show the topography of the California Central Valley and the lidar flights through the valley. The valley is bounded by the Sierra Nevada Mountains to the east, the Tehachapi Mountains to the south and the Coast Range to the west. The winds in the valley, as noted from the backward trajectories at ground, 1 km and 4 km, flow from north to south pushing the aerosols into the lower valley. Most flights flew over Bakersfield, CA before landing at Edwards AFB, CA. The source region for the aerosols seems to be located near Bakersfield, with little aerosols being transported from other regions into the valley. Near Bakersfield there are at least five oil refineries as well as other sources of aerosols. The prevailing winds force these aerosols southward toward the surrounding mountains where they are confined.