Davis Intercomparison Experiment

OPAL Ground-ozone Lidar Intercomparisons
with UC-Davis airborne UV ozone analyzer in Davis, California

An example of the intercomparison performed on July 15,1993.

OPAL on a private ranch 200 meters from the Yolo County Airport, close to Davis, California.

The Cessna-172 airplane of UC-Davis at the Yolo County Airport.

Summary:

The Davis Intercomparison Experiment marked the first long distance trip and first field campaign of the ETL ground-based ozone lidar OPAL (Ozone Profiling Atmospheric Lidar) in July 1993. The major objective was to test the remote sensing capability of OPAL and the accuracy of profiling ozone concentrations in a field environment.

During the intercomparison OPAL was staring vertically, while the airplane spiraled close to the lidar beam from the surface to 3 km. Each ascending or descending spiral took about a half hour. The intercomparison showed that the agreement between the airborne in situ instrument and the lidar was better than �10 ppb on the average (see figure above). Intercomparisons in the horizontal direction using an in situ ozone analyzer were also carried out in various locations in different experiments with good agreement (within a few ppb).

Principal Investigators:

Collaborator:

Field Crew:

Background:

This experiment was sponsored by California Air Resource Board (CARB). The lidar was completed in March 1993, and modifications to the seatainer housing the lidar were completed just few days before shipment. The objectives of the experiment were:

1. To verify the capability of the ETL ozone lidar for remotely sensing ozone distributions in the lower troposphere, and to evaluate the accuracy of the ozone lidar measurements.
2. To test the integrity of the lidar system and the mobile laboratory following long distance transport and to test its performance in a field environment.

The intercomparisons were performed between OPAL and the UC-Davis DASIBI UV ozone analyzer on board a Cessna 172 airplane, during the period of July 9 to July 23, 1993, near Davis, California. During the intercomparison OPAL was staring vertically, the airplane spiraled from the surface to 3 km around the lidar beam (but not centered at the lidar). The lidar and airplane data taken during the ascending and descending of the airplane were compared. Considering the differences in methodology between the lidar and the airplane, each ozone profile measured by the airplane in one spiral (about a half hour) is matched with one to four contemporaneous lidar-measured profiles to perform as accurate an intercomparison as possible. In each lidar-measured ozone profile only the one altitude section that matches the time of the airplane measurement is used. Thus, figures show discontinuities in the lidar-measured profiles. Examples of ten intercomparisons from July 15, July 19, and July 22 are shown in the figures.

Instruments:

• Ground-based ozone lidar OPAL
• TECO UV ozone analyzer (in the lidar)
• Airborne UC-Davis DASIBI UV ozone analyzer (Dr. John Carroll)
• Balloon radiosonde (CARB)

Details of the experiment and the results

We began the intercomparison experiment on July 14. Zbout 200 profiles were collected over seven weekdays. During each of those days, one to three aircraft flights were made; each flight consisted approximately of a 30-minute ascent and 30-minute descent. The flights took place at various times of the day between early or mid-morning and mid-afternoon. The lidar operated continuously during the flights, and usually gathered additional data before and after the flights.

Differences in methodology between the airborne in situ measurements and the lidar measurements

It is worth noting the methodological differences between the airborne in situ measurements and the lidar measurements before discussing the results of the intercomparison. The lidar was pointed vertically and operated at 2 Hz pulse rate. Each vertical ozone profile was an average over 7.5 min at a fixed horizontal location. Depending on wind speed which can vary with altitude, this 7.5-min average could be equivalent to an averaged profile over wind trajectories of path lengths ranging from 0 to 10 km. However, the airplane spiraled up and down with a vertical axis not centered over the lidar and projecting a large horizontal square, each side of which was much greater than the altitude range. The airborne DASIBI data are 10-second averages and correspond to averages over 0.3 to 0.5 km of the flight path, which was horizontally 0 to 8 km away from the lidar. In addition, the airborne measurements have an altitude resolution of 20-30 meters, but hardware problems caused the lidar measurements in Davis to have a much lower resolution (200-300 m below 1000 m and above 2000 m). Temporal variations of ozone can be significant as shown in both the airborne measurements and lidar measurements (see Figs. 6a and 6b). Horizontal variations may also be significant in certain circumstances, especially at higher altitudes. Consequently, temporal and spatial variation, range resolution, lidar errors and in situ inaccuracies are all included in the differences between the aircraft and lidar measurements.

Click on any figure to view the full-sized version.

Figure 1. 07:59-08:29, July 15, 1993	Figure 2. 08:49-09:00, July 15, 1993
Figure 3. 14:45-14:57, July 15, 1993	Figure 4. 14:59-15:16, July 15, 1993
Figure 5. 11:39-12:07, July 19, 1993	Figure 6. 12:12-12:35, July 19, 1993
Figure 7. 14:45-15:17, July 19, 1993	Figure 8. 15:23-15:55, July 19, 1993
Figure 9. 10:32-10:48, July 22, 1993	Figure 10. 10:53-11:20, July 22, 1993

Considering the above methodological differences, each ozone profile measured by the airplane in one spiral (about a half hour) is matched with one to four contemporaneous lidar-measured profiles to perform as accurate an intercomparison as possible. In each lidar-measured ozone profile only the one altitude section that matches the time of the airplane measurement is used. Thus, figures show discontinuities in the lidar-measured profiles.

Figures of intercomparison

Examples of ten intercomparisons from July 15, July 19, and July 22 are shown in Figs. 1-10. When the atmospheric conditions were stable and the ozone profiles exhibited small variations, the agreement between the lidar measurements and the airborne measurements were good (e. g., the morning to noon observations like Figs. 1, 2, and 5). When atmospheric ozone concentration varied rapidly (e. g., the afternoon observations in Fig. 7, 8, and 10), greater differences between the aircraft-measured and lidar-measured ozone profiles were observed. In the afternoon, convective activities might have caused cells or columns that led to greater horizontal variations. It is interesting to note that when ozone concentrations fluctuated in higher altitudes as measured by DASIBI, the lidar measured profiles go straight through the middle of the fluctuations like vertically averaged profiles. A reasonable explanation is that the lidar has a lower range resolution in the altitude range from 2500 to 3500 m. When the aircraft-measured profiles were smoothed with a resolution similar to the lidar's, the results were closer. In fact, a running average of the airplane profiles (the averaging interval was �20 points, equivalent to 500 to 600 m) at higher altitudes was carried out in Figs. 8 and 10. The smoothed airplane profiles have less fluctuations and are much closer to the lidar profiles.

Statistics

Statistical calculations for the intercomparisons were carried out for three altitude intervals (i.e., 0 to 1000 m, 1000 to 2000 m, and 2000 m to 3500 m). Twenty comparisons of the ozone profiles are included in the calculation for each layer as the differences in the ozone measurements (DO₃ = O_3lidar - O_3air) are calculated with a 50-m interval. Then the average value of DO₃, a_Do₃, defined as