Geoffrey S. Kent

A comparison of the stratospheric aerosol background periods of 1979 and 1989–1991

A comparison of global stratospheric aerosol levels measured in 1979 by the Stratospheric Aerosol and Gas Experiment (SAGE) and in 1989–1991 by SAGE II is presented. These periods exhibit the lowest stratospheric aerosol levels in the era of modern measurements and are often referred to as background periods. We find that, depending on latitude, the 1-μm aerosol optical depth in 1989–1991 was 10 to 30% higher than that observed in 1979. We demonstrate that the latter period (prior to the June 1991 eruption of Mount Pinatubo) was characterized by an ongoing global recovery from the eruptions of El Chichon in 1982 and Nevado del Ruiz in 1985, with a further complication introduced by the February 1990 Kelut eruption. Therefore the differences between 1979 and 1989–1991 cannot be completely attributed to nonvolcanic sources.

A further study of the method for estimation of SAGE II opaque cloud occurrence

Information on vertical cloud distribution is important to atmospheric radiative calculation, general circulation modeling, and climate study. The method used for estimating the vertical structure of opaque cloud occurrence from the solar occultation observations obtained by the Stratospheric Aerosol and Gas Experiment (SAGE) II has been reviewed for further understanding of the nature of the derived cloud statistics. Most importantly, based on the SAGE II tropical observations (1985-1998), the present study illustrates that the derived opaque cloud occurrence at a given altitude is generally independent of the cloud occurrence at other altitudes, except for some anticorrelation between high-level (12.5 km) and low-level (1-3 km) clouds. This feature of the layer cloud frequency independence is also evident when regional data over the Pacific warm pool and the eastern Pacific are examined. The independent information of the layer cloud frequency is significant and makes it possible to use the derived vertical distribution of cloud occurrence to estimate the probability of multilayer clouds. The limitation is that it is difficult to determine how frequently the multilayer clouds are actually overlapping or how frequently thick cloud ( > 1 km) really occurs based on the SAGE II observations alone. A discussion of the SAGE II tropical opaque cloud occurrence in relation to the cloud climatology based on visual observations from surface stations and ships, the International Satellite Cloud Climatology Project data, and the cloud statistics using rawinsonde records is also provided.

Multiwavelength lidar observations of the decay phase of the stratospheric aerosol layer produced by the eruption of Mount Pinatubo in June 1991

A small three-wavelength (355-, 532-, and 1064-nm) lidar system at NASA Langley Research Center in Hampton, Virginia, has been used since 1992 to make measurements on stratospheric aerosols. The data have been processed to study the decay rate of the stratospheric aerosol layer formed after the eruption of Mount Pinatubo in 1991 and its modulation, the aerosol effective radius, and the column mass loading. The stratospheric aerosol decay curves show annual and biennial cycles as well as short-term changes. At 532 nm, the decay time constant was 302 days for the period from February 1992 to August 1994 and had increased to 645 days for the period from September 1994 to December 1997. By 1996 the integrated stratospheric aerosol backscatter had fallen to levels (7.7 x 10(-5) sr(-1) at 532 nm) close to those seen in 1979 and 1989-1991. This decreasing trend was still continuing in 1997, showing no evidence for any anthropogenic contribution to the stratospheric aerosol.

A model for identifying the aerosol‐only mode of SAGE II 1.02‐μm extinction coefficient data at altitudes below 6.5 km

A model is proposed for identifying the aerosol mode of the second Stratospheric Aerosol and Gas Experiment (SAGE II) 1.02-μm extinction coefficient measurements at altitudes below 6.5 km, which also contain cloud samples. This development allows one to extend the SAGE II satellite data analysis from the lower limit at 6.5 km of the SAGE II two-wavelength method into the lower troposphere. Thus the proposed model provides opportunities for fully utilizing the SAGE II tropospheric measurements important to the understanding of the global behavior of tropospheric aerosols, clouds, and ozone and related transports. The effectiveness of this model is examined by using the SAGE II two-wavelength technique at 6.5 km. Sample applications of the proposed model reveal encouraging results. To assess the quality of the aerosol 1.02-μm data, it is recommended that a comprehensive data comparison analysis be conducted by using tropospheric measurements from different instruments.

Scanning lidar with a coupled radar safety system

A small scanning three-wavelength lidar system at NASA Langley Research Center in Hampton, Virginia, has been used since 1992 to make atmospheric measurements on stratospheric and upper tropospheric aerosols and on the evolution of aircraft exhaust plumes. Many of these measurements have been made away from the zenith, and, to reduce the hazard to air traffic produced by the laser beam, a radar safety device has been installed. The radar application is original in that the radar beam is made collinear with the laser beam by use of a dichroic mirror that transmits the laser radiation and reflects the microwaves. This mirror is inserted into the outgoing optical path prior to the radiation from both the radar and the laser passing through the independent scanning unit. Tests of the complete system show that the lidar and radar beams remain collocated as they are scanned and that the radar can be used to inhibit the laser prior to an aircraft passing through the beam.

Properties of the 1979 SAM II Antarctic 1.0-μm extinction coefficients: Implications of dehydration and seasonal evolution of the Antarctic polar vortex

The present study investigates the 1.0-μm extinction coefficient measurements obtained in the Antarctic region in 1979 from the Stratospheric Aerosol Measurement (SAM) II, with particular focus on the background aerosol properties. Correlative meteorological information from the National Centers for Environmental Prediction is incorporated in this investigation. The results indicate that the data frequency distribution of the background aerosol extinction coefficient in the local summer and fall can be adequately modeled by using a single-mode normal distribution, and that a binormal distribution is needed for modeling the distribution in the local winter and spring because of the different characteristics of the aerosols inside and outside the polar vortex. In general, the vertical distribution of the aerosol mean extinction coefficient exhibits two regions of different seasonal variation. Above 16 km the extinction coefficient is the highest during the local summer, and the lowest during the local spring inside the polar vortex. Below 16 km the aerosol seasonal variation is more complex, but the winter enhancement of the aerosol extinction coefficient inside the Antarctic polar vortex is clearly evident. As the season changes from winter to spring, the results inside the Antarctic polar vortex also indicate a reduction in aerosol optical depth in the stratosphere, but no significant changes in the upper troposphere. The present study further indicates that the bottom of the winter polar vortex in Antarctica is located at an altitude as low as 8 to 9 km, which is about 4 to 5 km lower than the bottom of the Arctic polar vortex. This difference may be attributable to the different strengths of the winter polar vortex and the planetary wave activities between the two hemispheres. In summary, the properties of the Antarctic background aerosol are very consistent with the effect of polar stratospheric clouds on the aerosol vertical distribution through their formation, sedimention, and evaporation, and with the seasonal evolution of the polar vortex. Finally, the result of the present study provides valuable opportunities for fully utilizing the multiyear SAM II tropospheric and stratospheric measurements to investigate the aerosol climatology and long-term variations in the Arctic and Antarctic regions.

Aircraft exhaust particle measurement with multiple ground-based lidar systems

Recently NASA Langley Research Centers (LaRC) Aerosol Research Branch conducted an aircraft exhaust particle experiment involving tow ground based lidar systems and NASAs B737-100, T39 and OV10 aircraft. The experiment took place at LaRC in February and March of 1996. During flight, exhaust particles exiting the two wing-mounted engines of the B737 become quickly entrained into the aircrafts wingtip vortices. The LaRC lidar systems were used to measure the distribution and optical properties of these exhaust particles as the B737 overflew the lidar facility. Two lidar systems, located in a common facility, were utilized for this experiment. One system was a fixed zenith- viewing lidar with a 48-inch receiver and a 2J transmitter, and the second was a scanning lidar with a 14-inch receiver and a 600 mJ transmitter. Two measurement geometries were employed for the experiment. In the first geometry, the B737 flew upwind of the lidar facility and perpendicular to the ambient wind. The second design had the aircraft fly directly over the facility, and parallel to the ambient wind.Under either scenario data were acquired at 20 and 30 Hertz, by the fixed zenith and scanning system respectively, as the ambient wind carried the vortex pair across the field of view of the lidars. The two supporting aircraft were used to collect in-situ particle data and to measure atmospheric turbulence, respectively. In this paper all aspects of the experiment will be discussed including the lidar systems, the geometry of the experiment, and the aircraft used. Also, selected data obtained during the experiment will be presented.

SAGE II long-term measurements of stratospheric and upper tropospheric aerosols

The Stratospheric Aerosol and Gas Experiment (SAGE) II solar occultation instrument has been making measurements on stratospheric aerosols and gases continually since October 1984. Observations from the SAGE II instrument provide a valuable long-term data set for study of the aerosol in the stratosphere and aerosol and cloud in the upper troposphere. The period of observation covers the decay phase of material injected by the El Chichon volcanic eruption in 1982, the years 1988 - 1990 when stratospheric aerosol levels approached background levels, and the period after the eruption of Mount Pinatubo in 1991. The Mount Pinatubo eruption caused the largest perturbation in stratospheric aerosol loading in this century, with effects on stratospheric dynamics and chemistry. The SAGE II data sequence shows the global dispersion of aerosols following the Mount Pinatubo eruption, as well as the changes occurring in stratospheric aerosol mass and surface area. The downward transfer of stratospheric aerosols into the upper troposphere following the earlier eruption of El Chichon is clearly visible. Estimates have been made of the amount of volcanic material lying in the upper troposphere and the way in which this varies with latitude and season.

SAGE II studies of lofted aerosol over Asian deserts and other regions of the Northern Hemisphere

Three methods of analyzing Stratospheric Aerosol and Gas Experiment (SAGE) II tropospheric aerosol extinction data are described and intercompared in terms of global maps and vertical contour plots of the extinction coefficient, or its equivalent. The first method, which has been in use for several years, is found to be biased toward smaller aerosols (effective radius < about 0.25 μm), while the second more recently developed method characterizes the distribution of larger aerosols (effective radius > 0.25 μm). The third method which, unlike the first two methods, is capable of producing an altitude resolved aerosol climatology down to about 1 km above the earths surface, requires an assumption about the amount of cloud contamination in the data set. Given the correctness of this assumption, the method is able to derive the total extinction due to both large and small aerosols. Aerosol climatologies produced by all three methods are shown and intercompared, with particular emphasis on the lofting of dust from Asian and other Northern Hemisphere deserts and its subsequent advection over the western Pacific Ocean.