Cloud Rings

Historical Record of Ozone in the Troposphere over the Tropical Pacific:

Measurements from 1978-79 Winter Monsoon Experiment Elena A. Deviatova 1, Anthony C. Delany 2, Russell R. Dickerson 1

1. University of Maryland, College Park, 2. National Center for Atmospheric Research, Boulder, Colorado.

Overview

Acknowledgements

Results

Ozone Profile (500 m Running Average)

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Ozone is a critical trace constituent of the atmosphere and an important indicator of changing global pollution levels. Transport of Asian emissions to the Pacific Ocean and consequentially eastward towards North America is becoming a growing concern. In tropical Asia, ozone monitoring has been rare and irregular until recent decades. Analysis of unpublished aircraft ozone measurements from the 1978-1979 Winter Monsoon Experiment (WMONEX) in Malaysia will be of great interest to the global scientific community. Our objective is to determine the accuracy and precision of this historic data set in order to provide a benchmark against which recent or future measurements can be compared. Our hypothesis is that analysis of WMONEX data will reveal an ozone mixing ratio increase on the order of 5-10 parts per billion by volume in tropical Asia in the past 2 decades.

WMONEX data

Fig. 1. a) WMONEX spatial coverage (minutes within 1° x 1° grid). The bold black line depicts the December 29, 1978 flight when several cloud rings were penetrated. b) Ozone mixing ratio (ppbv) averaged over 5.5 – 6.5 km cruising altitude.

WMONEX data remained in the original GENPRO-I format for decades and not been published before this study. With advice and guidance from several scientists at the National Center for Atmospheric Research (NCAR), the data were converted from the archaic GENPRO-I format into netCDF format using NCAR’s Xanadu software.

During WMONEX missions, ozone was sampled directly from the ventilation system of the NCAR Electra aircraft, supplied by the engine compressors; passage through the compressors resulted in a small loss of ozone. On other aircraft, comparisons of ozone measured directly in ambient air to air that has passed through engine compression systems typically show that about 5 % of the ozone is destroyed by surface deposition. For our work we assume that the actual ambient ozone mixing ratio was 5 % greater than that indicated, and estimate the ± 2σ uncertainty in ozone due to this correction to be 5 %, i.e., the actual loss is between 0% and 10% with 95% confidence.

Direct observations of ozone loss in an equivalent engine are necessary, in order for the WMONEX ozone mixing ratios to be representative of ozone levels in 1978-1979 over the South China Sea. This type of experiment is planned for future missions of NOAA P-3, the closest aircraft in comparison to NCAR Electra still in operation.

Fig. 2. Average Ozone Profile

We would like to acknowledge the Research Aviation Facility of the National Center for Atmospheric Research supported by the National Science Foundation. Many thanks to Ron Ruth and Al Cooper for assisting with the WMONEX archived data. We extend our gratitude to the U.S. Environmental Protection Agency and University of Maryland for fellowship support of E.A.D.

Cloud Rings

Cloud rings are characterized by a circular structure with cumulus ring walls and clear area in the center. These doughnut-shaped clouds occur in tropical latitudes above oceanic surface, in regions with abundant warm, moist air supply for convective growth.

Cloud rings were visually identified during the December 29, 1978 flight by observers on the aircraft. During this flight, the aircraft flew at a cruising altitude of 6 km from Kuala Lumpur to Kota Kinabalu in East Malaysia, crossing the South China Sea. A Dasibi ultraviolet absorption sensor monitored ozone mixing ratios during the flight (Figure 3). Although the average background ozone for this flight was around 25 ppb, significant signal increases were noted throughout the flight. Ozone mixing ratios of 40-50 ppb are rarely observed in the tropical, marine boundary layer.

Fig. 3. Ozone mixing ratio time series with respect to distance and local time. Distance is measured as traversed flight path from Kuala Lumpur. Flight start time 8:41 LST and end time 16:48 LST.

Fig. 4. (a) Blow up of ozone mixing ratios observed in a cloud ring penetrated as the aircraft descended into Kota Kinabalu. (b) Same as (a) but for downward shortwave radiation; the instrument automatically zeros every 600 seconds.

The last peak in ozone time series was analyzed using shortwave irradiance data, measured during the flight (Figure 4b). Reductions in downward shortwave irradiance indicate clouds above the aircraft. The last ozone local maximum of the flight corresponds to a local maximum in the solar irradiance plot.

Simultaneous ozone and irradiance peaks can be interpreted as a clear sky region with subsiding upper-tropospheric air. The irradiance minima and ozone mixing ratio minima on both sides of this clear region correspond to convective clouds, transporting low-ozone marine air upward.

WMONEX observations are consistent with the hypothesis that convective cloud rings result from collapsed thunderstorms, and demonstrate how the subsidence within the cloud ring’s clear center transports upper tropospheric air into the lower levels where ozone’s lifetime is shorter.

WMONEX campaign included flights over the Pacific Ocean and South China Sea between November 17, 1978 and January 9, 1979 (Figure 1a). About 60% of all observations are between 5.5 km and 6.5 km cruising altitude. The tropopause height was about 19 km during WMONEX flights. Vertically averaged ozone over the South China Sea is shown in Figure 1b and average ozone profile is shown in Figure 2.

Electrochemical concentration cell (ECC) ozonesonde observations made at American Samoa, South Pacific (14S, 170W) and Watukosek, Java (7.5S, 112.7E), for 5000 m to 8000 m was 32 to 34 ppb respectively (Komhyr 1989; Thompson et al. 2003).

Direct comparison is difficult due to fluctuation of tropical tropospheric ozone with Madden-Julian Oscillation and El Niño Southern Oscillation.

Future Work

Data from existing ozonesonde stations (e.g. Java, Hong Kong) and recent field campaigns (e.g. PEM-WEST A) will be used for comparison with WMONEX ozone data.

Following considerations will be made in the comparison:

• Position of the Intertropical Convergence Zone (ITCZ)• Phase of the El Niño Southern Oscillation and Madden-Julian Oscillation.• Seasonal variation of ozone

References

Komhyr, W.D., et al., 1989: The latitudinal distribution of ozone to 35 km altitude from ECC ozonesonde observations, 1985-1987, Ozone in the Atmosphere, ed. R.D. Bojkov and P. Fabian, Proceedings of Quadrennial Ozone Symposium 1988 and Tropospheric Ozone Workshop, pp. 147-150.

Thompson, A. M., et al., 2003: Southern Hemisphere Additional Ozonesondes (SHADOZ) 1998-2000 tropical ozone climatology, 1, Comparison with Total Ozone Mapping Spectrometer (TOMS) and ground-based measurements, Journal of Geophysical Research, 108 (D2), 8238.