TARFOX Field Program

Cordinators: P. Russell, P. Hobbs, L. Stowe

The overall goal of the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) is to reduce uncertainties in the effects of aerosols on climate by determining the direct radiative impacts, as well as the chemical, physical, and optical properties, of the aerosols carried over the western Atlantic Ocean from the United States. Subsidiary objectives of TARFOX are to:

An important component of the closure studies is tests and improvements of algorithms that retrieve aerosol properties and effects from satellite and aircraft radiometers. The resulting validated algorithms will permit extensions of the TARFOX results to other times and locations that have aerosol properties similar to those of the TARFOX Intensive Field Period (IFP).

The TARFOX IFP was conducted July 10-31, 1996. It included coordinated measurements from four satellites (GOES-8, NOAA-14, ERS-2, LANDSAT), four aircraft (ER-2, C-130, C-131, and a modified Cessna), land sites, and ships. A variety of aerosol conditions was sampled, ranging from relatively clean behind frontal passages to moderately polluted with aerosol optical depths exceeding 0.5 at mid-visible wavelengths. The latter conditions included separate incidents of enhancements caused primarily by anthropogenic sources and another incident of enhancement apparently influenced by recent fog processing. Spatial gradients of aerosol optical thickness were sampled to aid in isolating aerosol effects from other radiative effects and to more tightly constrain closure tests, including those of satellite retrievals.

Coordination of the four TARFOX aircraft was greatly aided by (1) near-realtime imagery of aerosols, clouds, and fog from the GOES-8 satellite and the AVHRR radiometer on the NOAA-14 satellite, (2) special 24- and 48-hour forecasts of the areas most likely to be cloud-free during TARFOX flights, and (3) forward and backward trajectory calculations at several altitudes. Aircraft flights were coordinated with overflights of the NOAA-14, ERS-2, and LANDSAT satellites, as well as with other times and locations of interest, including lidar and radiometer measurements on Wallops Island and radiometer measurements at other East Coast sites, on two cruise ships, and Bermuda. Realtime vertical profiles of backscatter from the LASE lidar on the ER-2 were used to direct the other aircraft to altitudes of aerosol layers for intensive sampling, as well as to document cloud layers and cloud-free regions. The MODIS Airborne Simulator (MAS) on the ER-2 provided multispectral images for post-mission analyses of ocean reflectance, aerosol optical depth and particle size information, and cloud properties (including cirrus correction of aerosol products). ER-2 cameras also documented cloud presence in study scenes.

The University of Washington's integrated airborne system aboard the C-131A was used extensively to measure aerosol precursor and other gases, CN and CCN, aerosol composition and size distribution, total scattering, backscattering and absorption coefficients, graphitic and organic carbon, aerosol hygroscopic growth factors, and a variety of cloud/fog properties using in situ sensors and samplers, as well as optical depth spectra, backscatter profiles, aerosol and cloud absorption and scattering, and surface reflectivity using a sunphotometer, lidar, and scanning radiometer. These airborne measurements were obtained during a total of 72 C-131A research hours of flying. The measurements included vertical and horizontal profiles on twelve occasions beneath NOAA-14 satellite overpasses, on two occasion beneath an ERS-2 satellite overpass, and on one occasion beneath a LANDSAT overpass. Seven sets of vertical profile measurements were obtained beneath the ER-2 (some with satellite overpasses), and seven vertical profiles above two sunphotometers and the Raman lidar at Wallops Island.

The United Kingdom Meteorological Research Flight C-130 measured radiances and hemispheric fluxes in selected wavelength bands from the ultraviolet through the infrared, total scatter, backscatter and absorption coefficients, aerosol and cloud particle size spectra, and aerosol chemical composition. The C-130 made several flights in good conditions for shortwave radiative flux measurements. Each flight included horizontal legs at typically ten altitudes with vertical profiles between the legs. The measurement periods coincided with overpasses of NOAA-14 and three were closely coordinated with the C-131. Profiles were also flown over the Wallops Island surface instruments.

The Pelican (a modified Cessna) of the Center for Interdisciplinary Remotely Piloted Aircraft Studies measured aerosol size spectra, and spectral radiances and radiative fluxes in a variety of spectra bandwidths. The Pelican made measurements at numerous altitudes up through the top of the main haze layer. Six flights were flown under NOAA-14 overpasses and one flight was flown under an ERS-2 overpass. The Pelican flew in close coordination with the C-131A on three occasions during the final six flights. The first measurements with a new Ames tracking sunphotometer aboard the Pelican were collected during the final several flights of the Pelican in TARFOX.

At Wallops Island a Raman lidar measured vertical profiles of aerosol extinction and backscatter, and a variety of sun and sky radiometers measured optical depths and sky radiances used to derive scattering phase functions and aerosol size distributions. Two sunphotometers measured multiwavelength optical depth. Sun and sky radiometers were also operated at several other East Coast sites and Bermuda, as well as on cruise ships transiting between Bermuda and New York. Numerous aircraft flight legs and vertical profiles were flown over the surface sites to permit detailed intercomparisons. Other flight segments were devoted to intercomparisons among aircraft sensors.

The figure shows an illustrative composite of data acquired from satellite, aircraft, and surface sensors on one day of the TARFOX IFP.

The combined surface, air, and space data sets obtained in TARFOX will permit a wide variety of closure analyses. Specifically, in-situ measurements of aerosol light-scattering, absorption and forward/backscatter ratios at a given height will be used to derive the basic quantities needed to determine the effects of aerosols on solar radiation, namely, extinction, single-scattering albedo, and the asymmetry factor. "Internal closure" will be assessed by comparing the quantities thus derived with those deduced from the simultaneous in-situ measurements of aerosol size distribution and chemical composition. Analyses will seek to apportion the measured extinction by chemical species and hence (although more problematically) by source. The main tool for this analysis will be multiple linear regression. At each altitude in the vertical profiles of aerosol properties, the measured aerosol scattering and absorption coefficients will be regressed onto the co-measured mass concentrations of the various chemical components of the aerosol. This will yield chemically specific aerosol mass scattering efficiencies, and hence the relative contribution of each chemical component to the light scattering at that altitude. Weighted vertical integration of the "scattering budgets" will yield a "chemical budget".

"External column closure" will be assessed by comparing aerosol extinction computed from the in situ measurements on the C-131A with those derived from the airborne sunphotometers, satellite radiometers, and ER-2 imaging spectrometers. Another aspect of column closure will be comparisons of aerosol radiative forcing, or radiative flux changes, determined by: (1) airborne flux radiometer measurements, (2) satellite flux retrievals from radiance measurements, and (3) flux calculations from (a) in-situ measured aerosol scattering, absorption and asymmetry factors, (b) the same properties derived from aerosol size distribution and composition measurements, and (c) sunphotometer- and satellite-derived optical depths with ancillary single-scattering albedos and asymmetry factors. The sensitivity of radiative forcing to changes in aerosol optical thickness will be derived from the detailed in situ measurements and compared to the empirical sensitivity obtained by regressing satellite-derived radiative forcing vs. satellite-derived optical depth.

Such closure analyses will yield critically needed assessments and should reduce the uncertainties in derived values of anthropogenic aerosol radiative forcing. The closure analyses that use satellite optical depth and flux results will provide tests and, where necessary, improvements of satellite retrieval algorithms. The resulting validated algorithms will permit extensions of the TARFOX results beyond the TARFOX period and to other locations dominated by similar aerosols (e.g., the European Atlantic coast). Lessons learned and remaining questions will provide a useful background for ACE-2, which is designed to study both direct and indirect effects of both the European industrial aerosol plume and mineral dust from Africa.

Data examples from a TARFOX aerosol closure experiment, July 17, 1996.

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Last updated 11-07-96

Responsible NASA Official: Dr. Philip Russell

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