M. Patrick McCormick

Methodology for error analysis and simulation of lidar aerosol measurements.

We present a methodology for objective and automated determination of the uncertainty in aerosol measurements made by lidar. The methodology is based on standard error-propagation procedures, a large data base on atmospheric behavior, and considerable experience in processing lidar data. It yields algebraic expressions for probable error as a function of the atmospheric, background lighting, and lidar parameters. This error includes contributions from (1) lidar signal; (2) molecular density; (3) atmospheric transmission; and (4) lidar calibration. The validity of the algebraic error expressions is tested by performing simulated measurements and analyses, in which random errors of appropriate size are injected at appropriate steps. As an example, the methodology is applied to a new airborne lidar system used for measurements of the stratospheric aerosol. It is shown that for stratospheric measurements below about 25 km, molecular density uncertainties are the dominant source of error for wavelengths shorter than about 1.1 microm during nonvolcanic conditions. Because the influence of molecular scattering (relative to particulate scattering) decreases with increasing wavelength, stratospheric measurements with a Nd:YAG lidar can thus be more accurate than those made with a ruby lidar, provided that a suitable detector is used.

Stratospheric ozone profile and total ozone trends derived from the SAGE I and SAGE II data

SAGE I/II ozone data from the period 1979–1991 have been used to derive global trends in both stratospheric column ozone and as a function of altitude. A statistical model containing quasibiennial, seasonal, and semiannual oscillations, a linear component, and a first-order autoregressive noise process was fit to the time series of SAGE I/II monthly zonal mean data. The linear trend in column ozone above 17 km altitude, averaged between 65°S and 65°N, is −0.30 ± 0.19%/year or −3.6% over the time period February 1979 through April 1991. The data further show that the column trend above 17 km is nearly zero in the tropics and increases towards the high latitudes with values of −0.6%/year at 60°S and −0.35%/year at 60°N. Both these results are in agreement with the recent TOMS results. Furthermore, the profile trend analyses show the column ozone losses are occurring below 25 km, with most of the loss coming from the region between 17 and 20 km. Negative trend values on the order of −2%/year are found at 17 km in mid-latitudes.

A method for estimating vertical distribution of the SAGE II opaque cloud frequency

A method is developed to infer the vertical distribution of the occurrence frequency of clouds that are opaque to the Stratospheric Aerosol and Gas Experiment (SAGE) II instrument. An application of the method to the 1986 SAGE II observations is included in this paper. The 1986 SAGE II results are compared with the 1952-1981 cloud climatology of Warren et al. (1986, 1988)

Stratospheric Aerosol and Gas Experiment II and ROCOZ-A ozone profiles at Natal, Brazil: A basis for comparison with other satellite instruments

We present the results of comparisons of satellite measurements of ozone from the Stratospheric Aerosol and Gas Experiment II (SAGE II) with in situ measurements from ROCOZ-A and electrochemical concentration cell (ECC) ozonesondes at Natal, Brazil (5.9°S, 35.2°W), during the southern hemisphere autumn of 1985. Since data were collected over a region of low ozone variability, comparisons were made of the mean values of 14 SAGE II profiles with the mean values of 7 ROCOZ-A and 7 ECC profiles, rather than of a more limited set of paired comparisons. The basic comparison presented here is ozone number density versus geometric altitude, the fundamental ozone measurement from SAGE II. Over the altitude region from 20 to 52 km SAGE II ozone densities averaged 0.4% lower than ROCOZ-A. The average of the absolute values of the ozone density differences for the two instruments was 2.4% over these altitudes. Owing in large part to the number of profiles in the data sets, the 95% confidence limits for the ozone density differences in these comparisons averaged 3.5% from 20 to 52 km, a significant improvement over previous results. In terms of ozone mixing ratio versus geometric altitude from 20 to 52 km, SAGE II values had a difference of 3.4% from ROCOZ-A (SAGE II higher), an average absolute difference of 3.8%, and an average of 4.7% for the 95% confidence limits. Differences between the ozone density and mixing ratio results are due to the auxiliary temperature and pressure values for the satellite and in situ instruments. The effects of pressure differences on the vertical positioning of the ozone profiles are not important for the altitude-based comparisons of SAGE II and ROCOZ-A. However, they become an important consideration in the comparisons of SAGE II with pressure-based ozone measurements from other satellite instruments. The composite sets of ozone, temperature, and pressure values presented here form an excellent basis for comparisons with other satellite ozone measurements.

The Flight of the Lidar In-Space Technology Experiment (LITE)

For 11 days during September 1994, the lidar community achieved its first spaceborne lidar flight orbiting Earth to study our atmosphere. LITE aboard Shuttle worked flawlessly measuring backscatter at three wavelengths from clouds, aerosols and the Earth’s surface. A summary of the LITE mission is presented.

Error analysis of DIAL measurements of ozone by a Shuttle excimer lidar

An error analysis of DIAL (differential absorption lidar) measurements of stratospheric ozone from the Space Shuttle is discussed. A transmitter system consisting of a KrF excimer laser pumping gas cells of H2 or D2 producing output wavelengths in the near UV is shown to be useful for the measurement of ozone in a 15-50-km altitude range.

A Conceptual Design Study For The Eos Lidar Atmospheric Sounder And Altimeter Facility

As part of the Space Station program, NASA is collaborating with the European and Japanese space agencies to develop an unmanned, polar orbiting Earth observing system (Eos) to begin operation in the mid 1990 s Eos will provide, global. measurements with active and passive remote sensors having greater resolution and accuracy than those currently in use. One of the proposed Eos facility instruments, the Lidar Atmospheric Sounder and Altimeter (LASA), is an active remote sensor that offers the possibility of measurements such as the global vertical distributions of aerosols, cloud top heights, atmospheric trace gasses such as water vapor and ozone, and atmospheric temperature and pressure; and the height of the planetary boundary layer (PBL). LASA employs the principles of optical radar (lidar), differential absorption 1 i da r (DIAL), and laser altimetry to provide these measurements with unprecedented resolution. This paper describes the conceptual design of LASA and also describes the conceptual design of one of the experiments proposed for the LASA facility the Eos Atmospheric Global Lidar Experiment (EAGLE).

Major Results from Sage II

The SAGE II (Stratospheric Aerosol and Gas Experiment II) sensor was launched into a 57° inclination orbit aboard the Earth Radiation Budget Satellite (ERBS) in October 1984. During each sunrise and sunset encountered by the orbiting spacecraft, the instrument (Mauldin et al., 1985) uses the solar occupation technique to measure attenuated solar radiation through the Earth’s limb in seven channels centered at wavelengths ranging from 0.385 to 1.02 μm. The exo-atmospheric solar irradiance is also measured in each channel during each event for use as a reference in determining limb transmittances. The transmittance measurements are inverted using the “onion-peeling” approach (Chu et al., 1989) to yield 1-km vertical resolution profiles of aerosol extinction (at 0.385, 0.453, 0.525, and 1.02 μm), ozone, nitrogen dioxide, and water vapor. The focus of the measurements is on the lower and middle stratosphere, although retrieved aerosol, water vapor, and ozone profiles often extend well into the troposphere under non-volcanic and cloud-free conditions. SAGE II was preceded into orbit by sister instruments SAM II (Stratospheric Aerosol Measurement II), which has been measuring 1.0-μm aerosol extinction in the polar regions since 1978, and SAGE I, which provided near global measurements of aerosol extinction (at 0.45 and 1.0 μm), ozone, and nitrogen dioxide from 1979–1981 (McCormick et al., 1979).