David M. Tratt

Asian dust events of April 1998

On April 15 and 19, 1998, two intense dust storms were generated over the Gobi desert by springtime low-pressure systems descending from the northwest. The windblown dust was detected and its evolution followed by its yellow color on SeaWiFS satellite images, routine surface-based monitoring, and through serendipitous observations. The April 15 dust cloud was recirculating, and it was removed by a precipitating weather system over east Asia. The April 19 dust cloud crossed the Pacific Ocean in 5 days, subsided to the surface along the mountain ranges between British Columbia and California, and impacted severely the optical and the concentration environments of the region. In east Asia the dust clouds increased the albedo over the cloudless ocean and land by up to 10–20%, but it reduced the near-UV cloud reflectance, causing a yellow coloration of all surfaces. The yellow colored backscattering by the dust eludes a plausible explanation using simple Mie theory with constant refractive index. Over the West Coast the dust layer has increased the spectrally uniform optical depth to about 0.4, reduced the direct solar radiation by 30–40%, doubled the diffuse radiation, and caused a whitish discoloration of the blue sky. On April 29 the average excess surface-level dust aerosol concentration over the valleys of the West Coast was about 20–50 μg/m3 with local peaks >100 μg/m3. The dust mass mean diameter was 2–3 μm, and the dust chemical fingerprints were evident throughout the West Coast and extended to Minnesota. The April 1998 dust event has impacted the surface aerosol concentration 2–4 times more than any other dust event since 1988. The dust events were observed and interpreted by an ad hoc international web-based virtual community. It would be useful to set up a community-supported web-based infrastructure to monitor the global aerosol pattern for such extreme aerosol events, to alert and to inform the interested communities, and to facilitate collaborative analysis for improved air quality and disaster management.

April 1998 Asian dust event: A southern California perspective

In late April 1998 an extreme Asian dust episode reached the U.S. western seaboard. This event was observed by several in situ and remote sensing atmospheric measurement stations. Dramatic reductions in boundary layer visibility were recorded and the resultant peak backscatter coefficients exceeded prevailing upper tropospheric background conditions by at least 2 orders of magnitude. An analysis of this event is given using lidar vertical backscatter profilometry, concurrent Sun photometer opacity data, and transport modeling. At San Nicolas Island the measured and modeled aerosol optical thickness at 500 nm increased dramatically from 0.15 on April 25 to 0.52 on April 26-27. Volume size distribution on April 27 exhibited a prominent coarse mode at 1-2 μm radius, and single-scattering albedo was observed to increase from 0.90 in the blue to 0.93 in the near infrared. Concurrent lidar observations tracked the evolution of the plume vertical structure, which consisted of up to three well-defined layers distributed throughout the free troposphere.

Differential laser absorption spectrometry for global profiling of tropospheric carbon dioxide: selection of optimum sounding frequencies for high-precision measurements

We discuss the spectroscopic requirements for a laser absorption spectrometer (LAS) approach to high-precision carbon dioxide (CO2) measurements in the troposphere. Global-scale, high-precision CO2 measurements are highly desirable in an effort to improve understanding and quantification of the CO2 sources and sinks and their impact on global climate. We present differential absorption sounding characteristics for selected LAS transmitter laser wavelengths, emphasizing the effects of atmospheric temperature profile uncertainties. Candidate wavelengths for lower-troposphere measurements are identified in the CO2 bands centered near 1.57, 1.60, and 2.06 microm.

Airborne CO 2 coherent lidar for measurements of atmospheric aerosol and cloud backscatter

An airborne CO(2) coherent lidar has been developed and flown on over 30 flights of the NASA DC-8 research aircraft to obtain aerosol and cloud backscatter and extinction data at a wavelength near 9µm. Designed to operate in either zenith- or nadir-directed modes, the lidar can be used to measure vertical profiles of backscatter throughout the vertical extent of the troposphere and the lower stratosphere. Backscatter measurements in absolute units are obtained through a hard-target calibration methodology. The use of coherent detection results in high sensitivity and narrow field of view, the latter property greatly reducing multiple-scattering effects. Aerosol backscatter profile intercomparisons with other airborne and ground-based CO(2) lidars were conducted during instrument checkout flights over the NASA Ames Research Center before extended depolyment over the Pacific Ocean. Selected results from data taken during the flights over the Pacific Ocean are presented, emphasizing intercom arisons with backscatter profile data obtained at 1.06 µm with a NASA Goddard Space Flight Center Nd:YAG lidar on the same flights.

The Multi-center Airborne Coherent Atmospheric Wind Sensor

Abstract In 1992 the atmospheric lidar remote sensing groups of the National Aeronautics and Space Administration Marshall Space Flight Center, the National Oceanic and Atmospheric Administration/Environmental Technology Laboratory (NOAA/ETL), and the Jet Propulsion Laboratory began a joint collaboration to develop an airborne high-energy Doppler laser radar (lidar) system for atmospheric research and satellite validation and simulation studies. The result is the Multi-center Airborne Coherent Atmospheric Wind Sensor (MACAWS), which has the capability to remotely sense the distribution of wind and absolute aerosol backscatter in three-dimensional volumes in the troposphere and lower stratosphere. A factor critical to the programmatic feasibility and technical success of this collaboration has been the utilization of existing components and expertise that were developed for previous atmospheric research by the respective institutions. For example, the laser transmitter is that of the mobile ground-based Do...

Lidar In-space Technology Experiment measurements of sea surface directional reflectance and the link to surface wind speed

The dependence of sea surface directional reflectance on surface wind stress suggests a method for deriving surface wind speed from space-based lidar measurements of sea surface backscatter. In particular, lidar measurements in the nadir angle range from 10 degrees to 30 degrees appear to be most sensitive to surface wind-speed variability in the regime below 10 m/s. The Lidar In-space Technology Experiment (LITE) shuttle lidar mission of September 1994 provided a unique opportunity to measure directional backscatter at selected locations by use of the landmark track maneuver and to measure fixed-angle backscatter from the ocean surfaces on a global scale. During the landmark track maneuver the shuttle orbiter orientation and roll axis are adjusted continuously to maintain the lidar footprint at a fixed location for a duration of ~1 min. Several data sets were converted to calibrated reflectance units and compared with a surface reflectance model to deduce surface wind speeds. Comparisons were made with ERS-1 scatterometer data and surface measurements.

Evidence of seasonally dependent stratosphere-troposphere exchange and purging of lower stratospheric aerosol from a multiyear lidar data set

Tropospheric and lower stratospheric aerosol backscatter data obtained from a calibrated backscatter lidar at Pasadena, California (34 deg N latitude) over the 1984-1993 period clearly indicate tightly coupled aerosol optical properties in the upper troposphere and lower stratosphere in the winter and early spring, due to the active midlatitude stratospheric-tropospheric (ST) exchange processes occurring at this time of year. Lidar data indicate that during pre-Pinaturbo background conditions, the subsequent purging of the aerosol in the upper troposphere caused a significant reduction in the aerosol content throughout the 8 - 18 km altitude region in the early spring period. The post-Pinatubo evidence of intense exchange in the winter and early spring is a significant increase in the upper tropospheric aerosol content, such that the backscatter levels reach values nearly equivalent to the enhanced backscatter levels existing in the lower stratosphere. The calculated stratospheric mass extrusion rate is consistent with a 45-day lifetime of lower stratospheric aerosol during this part of the year, which implies that midlatitude ST exchange is a significant sink for stratospheric aerosol.

CO 2 DIAL measurements of water vapor

CO(2) lidars have heretofore been used to measure water vapor concentrations primarily using the 10R(20) line at 10.247 microm, which has a strong overlap with a water vapor absorption line. This paper discusses the use of that line as well as other CO(2) laser lines for which the absorption coefficients are weaker. The literature on measurement of water vapor absorption coefficients using CO(2) lasers is reviewed, and the results from four laboratories are shown to be generally consistent with each other after they are normalized to the same partial pressure, temperature, and ethylene absorption coefficient for the 1P(14) CO(2) laser line; however, the agreement with the Air Force Geophysics Laboratorys HITRAN and FASCOD 2 spectral data tapes is not good either for the water vapor absorption lines or for the water vapor continuum. Demonstration measurements of atmospheric water vapor have been conducted using the Mobile Atmospheric Pollutant Mapping System, a dual CO(2) lidar system using heterodyne detection. Results are discussed for measurements using three sets of laser line pairs covering a wide range of water vapor partial pressures.

Recent climatological trends in atmospheric aerosol backscatter derived from the Jet Propulsion Laboratory multiyear backscatter profile database

An update is provided on the Jet Propulsion Laboratory aerosol backscatter climatology database, with emphasis on the impact of the June 1991 eruption of Mt. Pinatubo. The data set is acquired at thermal infrared wavelengths with a range-gated coherent CO(2) lidar system, which has been in regular operation since 1984. A number of analyses have been carried out to assess long-term trends in the tropospheric and lower stratospheric aerosol backscatter, as observed from the lidar site at Pasadena, California.

Airborne lidar observations of tropospheric aerosols during the Global Backscatter Experiment (GLOBE) Pacific circumnavigation missions of 1989 and 1990

Tropospheric and lower stratospheric aerosol backscatter profiles were obtained with an airborne backscatter lidar at 9.25-μm wavelength during the NASA Global Backscatter Experiment (GLOBE) airborne field campaigns in November 1989 and May/June 1990. The range of latitudes extended from 70°N to 62°S over the Pacific Ocean basin. The data provide evidence that the tropics are an effective sink for aerosol particles in the lidar-active size range, most likely through efficient wet deposition processes. A reduction of planetary boundary layer (PBL) thickness and aerosol mass density was observed within the tropical regions of enhanced cumulus convection. PBL thickness maxima were consistently observed in the southern hemisphere at subtropical and high latitudes. The downward transport of volcanic aerosol from the February 1990 Kelut eruption into the upper troposphere was observed in the southern hemisphere during the May/June 1990 period. A springtime enhancement of aerosol in the middle troposphere, due to convection and transport of surface material, was observed in both hemispheres, although the seasonal enhancement was much more dramatic in the northern hemisphere due to the influence of the Asian continental source. Above the PBL the observed springtime enhancement was in the form of extensive layers, with the high-altitude layers reaching the midlatitude tropopause near 140°E longitude.