CARG RESEARCH AIRCRAFT AND ASSOCIATED STUDIES
During the period 1969-2001, the Cloud and Aerosol Research Group (CARG) instrumented and utilized for its research three aircraft: a Douglas B-23, a Convair-130, and a Convair-580.
Douglas B-23 (1970-1984)
The CARG acquired a Douglas B-23 Dragon in September 1970. Only 38 B-23s were built by the Douglas Aircraft Company in the late 1930s. They were used during World War II for submarine patrol, training, and towing targets; by 1970, only a few were still flying. The B-23 that the CARG acquired had seen "action" only in war movies! It had been converted for civilian use by Howard Hughes (who was largely responsible for saving the B-23 fleet after WW II), and was later used by Standard Oil as an executive aircraft.
The B-23 is similar in appearance to the more familiar Douglas DC-3, but it has a narrower fuselage and a taller vertical stabilizer. It also has bigger engines, can fly higher (to ~9 km) and faster (cruise speed ~60 m s-1, stall speed ~27 m s-1), and is more rugged than the DC-3. These attributes, together with the capability for handling considerable icing, made it an excellent choice for cloud physics research.
Between 1970 and 1984 the CARG flew 3400 h on the B-23. The data collected on these flights provided the basis for some 80 scientific papers on subjects ranging from atmospheric aerosol and chemistry to cloud physics and mesoscale meteorology. A few of the highlights of this research are described briefly below.
a. The Cascade Project (1970-79)
The principal goals of this project were to study the structures of clouds associated with winter storms over the Cascade Mountains and to carry out exploratory experiments on how the distribution of snowfall across the Cascades might be modified by cloud seeding (Fraser et al. 1973; Hobbs et al. 1973; Hobbs and Radke 1973, 1975; Hobbs 1975a,b).
b. Wave clouds as an experimental laboratoryWave clouds form over hills and mountains. Air enters the upwind edge of a wave cloud and exits at the downwind edge. However, the cloud itself remains relatively stationary. In 1979 the CARG began to exploit this aspect of wave clouds to carry out some of the first studies of chemical reactions in clouds (Hegg and Hobbs 1981). Earlier Radke and Hobbs (1969 and Hegg et al. (1980) had noted that the concentrations of cloud condensation nuclei (CCN) active at a given supersaturation were often relatively high in evaporating clouds. We suggested that this might be due to the production of sulfate in cloud drops, which, when the drops evaporated, is deposited onto the original CCN on which the drop formed, thereby enhancing their activity as CCN.
We confirmed these ideas, using the B-23 aircraft, by providing measurements of sulfate production in wave clouds (Hegg and Hobbs 1982). Subsequently we extended our studies of cloud chemistry to other cloud types (Hegg et al. 1984; Hobbs 1986; Hegg and Hobbs 1986, 1988) and to rainbands (Hegg et al. 1986; Hegg et al. 1989).
c. The CYCLES Project (1973-86)
The Cascade Project was mainly concerned with cloud microphysical studies. By 1972 sufficient new meteorological observing facilities were becoming available to encourage the CARG to initiate a study of the mesoscale organization and structures of clouds and precipitation in winter cyclones in the Pacific Northwest. This project, known as CYCLES (for Cyclonic Extratropical Storms), made extensive use of the B-23 aircraft. Also the NCAR Doppler radars (which the CARG had earlier donated to NCAR) were first used in this project.
One of the main achievements of the CYCLES Project was the classification of description of precipitation bands in winter cyclones (Hobbs 1978; Hobbs et al. 1980, and subsequent papers in the series). Based on these field studies, the CARG developed numerical models for both the microphysics and chemistry of several types of precipitation bands (Rutledge and Hobbs 1983, 1984; Rutledge et al. 1986).
Another important accomplishment of the CYCLES Project was the demonstration that color display Doppler radars can, through simple "pattern recognition," provide a powerful means for identifying and tracking mesoscale features (Baynton et al. 1977).
The CYCLES Project, which revealed the importance of mesoscale structures and the viability of studying them, was the progenitor of many large cooperative field studies of the mesoscale structures of winter cyclones.
d. Volcanic emissions
The CAR group was one of the first to use aircraft for obtaining quantitative measurements of the emissions of particles and gases from volcanoes. The first such measurements were obtained in 1975 in the emissions from Mt. Baker, Washington (Radke et al. 1976). Thereafter, the CARG studied a variety of volcanoes from Alaska to the Antarctic (Hobbs et al. 1977; Stith et al. 1978; Hobbs et al. 1981; Radke 1981). The most extensive set of measurements was obtained in the 1980-81 emissions from Mt. St. Helens, Washington. On the day of themajor eruption of this volcano, measurements were obtained over a period of many hours of gaseous and particulate emissions (Hobbs et al. 1981; 1982).
In 1984, at 45 years of age, and after serving the CARG well for 14 years, the B-23 was retired. It now resides contentedly, restored to its original WW II configuration, in the McChord Air Museum at McChord Air Force Base, Tacoma, Washington.
The B-23 was replaced by a Convair C-131A in September 1984. This aircraft, which was built in 1954, was designed to carry up to 40 passengers for military medical evacuations and was nicknamed the "Samaritan." It is considerably larger than the B-23: 23 m long, 28-m wing span, and a ~20,000-kg takeoff weight (compared to ~14,500 kg for the B-23). It has a range of ~2200 km, a maximum altitude of ~7.6 km, and a cruise speed of ~82 m s-1.
Shortly after the CARG obtained the Convair-C131, the University of Washington purchased a hangar at Paine Field, Washington, to house the CARG's airborne research activities. After extensive renovations and modifications of the Convair at Paine Field, the aircraft was ready for research flying in April 1985.
In addition to instruments transferred from the Douglas B-23, a significant number of new instruments were added to the Convair C-131 to obtain detailed in-situ measurements of atmospheric aerosols, clouds, chemistry, and radiation. A 35-GHz cloud radar, a dual-wavelength lidar, and a multiwavelength scanning radiometer were also mounted on this aircraft.
Some of the uses to which the Convair C-131 was put are described briefly below.
a. Ice in clouds
Airborne studies with the Convair C-131 (Hobbs and Rangno 1985, 1990; Rangno and Hobbs 1991) showed that ice crystals often appear in both pristine form in high concentrations in narrow filaments near the tops of clouds. As these crystals grow and fall out through the cloud the filaments increase in width, appearing as virga orfallstreaks when they emerge from the base of the cloud.
During the course of these studies it was found that under some conditions the passage of an aircraft through a supercooled cloud can produce high concentrations of ice crystals (Rangno and Hobbs 1983, 1984).
b. Absorption of solar radiation by clouds
In collaboration with Dr. Michael King of NASA/Goddard, the CARG embarked on a study of the absorption of solar radiation by clouds. By flying deep within a cloud, and obtaining measurements of the angular distribution of scattered radiation (using the scanning radiometer), the absorption of radiation by the cloud (as indicated by the single scattering albedo and cloud asymmetry factor) can be derived (King 1981). Such measurements, made in clean, maritime, stratiform clouds, showed fairly good agreement with theoretical calculations based on Mie scattering by the cloud droplets (King et al. 1990).
c. Pollution in the Arctic
At times, pollution episodes in the Arctic (known as Arctic haze) can approach levels similar to those encountered in large cities. The primary source of the pollution is thought to be long-range transport from Europe, the USSR, and North America. As a consequence of the stability of the Arctic atmosphere, and the weakness of particle removal mechanisms, these pollutants tend to accumulate in the Arctic in shallow horizontal layers.
The CARG studied the nature and temporal and spatial occurrence of Arctic haze during airborne expeditions to the Arctic (Radke et al. 1984a,b, 1989; Bailey et al. 1984; Brock et al. 1989, 1990; Ishizaka et al. 1989; Radke and Hobbs 1989). Airborne in situ and lidar measurements were combined to derive the fluxes of sulfur in the particles in several Arctic haze events. They were found to be comparable to the fluxes in the plumes from large coal-fired power plants (~5 kg s5).
d. Emissions from biomass burning
The importance of biomass emissions in atmospheric chemistry and climate is now widely recognized. The CARG made some of the first measurements of the emissions of particles and gases from forest fires. We showed that forest fires are sources of both cloud condensation nuclei (CCN) and ice nuclei (Hobbs and Radke 1969; Hobbs and Locatelli 1969). We measured emission factors (i.e., mass of material released into the atmosphere per unit mass of material burnt) for a number of species from fires in North America, including ozone, ammonia, nitrogen compounds, and some chlorofluorocarbons (Radke et al. 1988); Hegg et al. 1987, 1988, 1990b).
Studies of emissions from biomass burning using the Convair C-131 culminated in theSCAR-B study in Brazil in 1995.
e. Sulfur chemistry in the marine atmosphere
Emissions of sulfur gases from the ocean provide the major source of sulfate in the marine atmosphere. The CARG obtained airborne measurements of the vertical profiles of dimethylsulfide (DMS), produced by oceanic phytoplankton activity, sulfur dioxide, sulfates, and CCN over the northeastern Pacific Ocean (Ferek et al. 1991). In the first airborne measurements of DMS in the Arctic atmosphere, we found the concentrations to increase from a few parts per trillion up to ~300 pptv (compared to maximum values of ~40 pptv off the Washington coast) over a two-week period in June 1990 as the sea ice melted (Ferek et al. 1992).
f. Aerosol-cloud interactions
As early as 1969 the CARG had started to explore heterogeneous chemical reactions in cloud droplets. Subsequent studies of aerosol-cloud interactions, using the Convair C-131, revealed some important new effects, which are described briefly below.
In studies of marine stratiform clouds, we found that the vertical profiles of Aitken nuclei (i.e., total particle concentrations) commonly exhibit peak values just above cloud tops (Hegg et al. 1990a). We attributed this to the (homogeneous) nucleation of new sulfate particles from SO2 and H2O in the presence of high relative humidities in the vicinity of the clouds. These Aitken nuclei are probably too small to serve as CCN at the supersaturations commonly achieved in stratiform clouds. However, numerical model calculations indicated that cloud-processing of these particles, which subjects them to heterogeneous chemical reactions, can increase their sizes so that they can serve as CCN in stratiform clouds (Hegg 1990).
We also measured large concentrations of Aitken nuclei within clouds (i.e., interstitial to cloud droplets) [Radke and Hobbs 1991; Hegg et al. 1991]. This was more difficult to understand in terms of homogeneous nucleation, since the cloud drops provide a large, efficient sink for gas-phase H2SO4. However, Hegg (1991) explained these results in terms of homogeneous nucleation in the presence of high concentrations of the OH radical in clouds, which are produced by enhanced radiation fluxes.
We observed that cumulus clouds are surrounded by "halos" of high humidity and aerosol concentrations that extended out to distances of several cloud radii beyond the visible boundaries of the clouds (Radke and Hobbs 1991; Perry and Hobbs 1994, 1996). These observations of the effects of clouds on aerosols and humidity have important implications for atmospheric chemistry and atmospheric radiation.
(g) Emissions from ships
Using the Convair C-131, the CARG obtained some of the first measurements off effluents from ships and their effects on cloud structures (Radke et al. 1989; King et al. 1993; Ferek et al. 1998). These studies culminated in the CARG's participation in theMAST field study, and a series of papers in a Special Issue of the J. Atmos. Sci. (15 August 2000 issue).
(h) Carbonaceous particles in the atmosphere
TheTARFOX field project on the U.S. East Coast, led to several papers by the CARG, which called attention to the importance of carbonaceous particles in aerosol radiative forcing (Hegg et al. 1997; Novakov et al. 1997; Hobbs 1998). Other results from TARFOX were published in two Special Issues of J. Geophys. Res. (January 27, 1999, and April 27, 2000).
In March 1997 the CARG acquired a Convair-580 aircraft. This is a similar sized aircraft to the Convair C-131, but had twin turboprop engines, and better range and height capabilities. As with previous CARG aircraft, the Convair-580 was instrumented for aerosol, trace gases, cloud microphysics and radiation measurements.
During the period 1998-2001 the Convair-580 played key roles in the following field projects:FIRE-ACE/SHEBA in the Arctic (1998), KWAJEX in the Marshall Islands (1999), SAFARI 2000 in Southern Africa (2000), IMPROVE-I and II in Washington State (2001), and CLAMS on the U.S. East Coast (2001). A common thread in all six of these field projects was the acquisition of in situ airborne measurements from the Convair-580 beneath remote sensing instruments aboard NASA satellites and the NASA high-flying ER-2 aircraft.
In January 2002, the CARG announced that, after 32 years of utilizing aircraft for atmospheric measurements, it would terminate this aspect of its research activities and concentrate its efforts on analyses and interpretation of the large quantity of field data acquired from its many airborne studies.
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