Armored T-28 Research Aircraft Facility

Examples of Research Results

The T-28 accomplished more than 900 storm penetrations in a series of research projects dating back to 1970. Those projects have involved investigations of convective storm processes in Alabama, Alberta, Colorado, Florida, Montana, New Mexico, Oklahoma, North Dakota, South Dakota, Texas and Switzerland. In most cases, the emphasis was on studies of hail development, for which the T-28 with its ability to penetrate storms containing hail up to more than 5 cm in diameter, was uniquely suited. The TRIP and CaPE projects in Florida had as a major focus the relationship between precipitation development and charge separation in convective clouds. The COHMEX project in Alabama (1986) was concerned with precipitation development, cloud electrical processes and the development of downbursts. Recent work in North Dakota involved investigations of transport and dispersion in convective clouds using gaseous tracer techniques. The TEXARC project in Texas focused on seeding effects on ice-phase development in towering cumulus growing into cumulonumbus clouds.

T-28 data have been employed in various studies of thunderstorm processes. In each case, the interpretation of the T-28 observations has been greatly facilitated by the availability of good aircraft tracks, supporting radar data (conventional, Doppler, and multiparameter), and observations from other research aircraft, as well as other comprehensive meteorological surface and upper air data. Much of this research has been undertaken jointly by scientists from the South Dakota School of Mines and Technology (SDSM&T) and other organizations, including NCAR, various universities, and federal and state agencies. Important contributions have been made to the resolution of major questions about the development of hail in thunderstorms. For example, it was established that there are no accumulations of high concentrations of supercooled raindrops, like those envisioned in the Soviet model of hail development, in Colorado or Swiss thunderstorms (Musil et al, 1973, 1976b; Sand, 1976; Knight et al, 1982; Waldvogel et al, 1987). Accumulations of this sort have been found in storms in the southeastern U.S., but rapid freezing and natural "beneficial competition" appear to prevent the development of large hailstones in most cases (Musil and Smith, 1989).

Mechanisms of hail development involving recirculation of ice particles (Musil et al, 1976a) or the transfer into the main storm of ice particles developed to embryo sizes in feeder cloud regions (Heymsfield and Musil, 1982; Heymsfield, 1983; Foote, 1984) have been established as important processes in the development of hail in at least some Colorado storms. Evidence was found in Oklahoma storms of mixed-phase precipitation processes with recirculation within the main storm likely being important (Heymsfield and Hjelmfelt, 1984). Analysis of T-28 data from SESAME 1979 and CCOPE (1981) showed that shedding of drops from graupel or hail undergoing wet growth or melting may produce enough supercooled raindrops in Oklahoma and Montana storms to account for the observed incidence of frozen-drop embryos (Heymsfield and Hjelmfelt, 1984; Rasmussen and Heymsfield, 1987).

The T-28 observations of the microphysical and updraft structure of high-reflectivity regions of thunderstorms have served to characterize the types and concentrations of particles in those regions, identify the types that may serve as hail embryos, and define the growth environment for those particles. They have revealed that supercooled cloud liquid water is often depleted by ice particle growth in the primary hail growth regions around the edges of the major storm updrafts as well as by entrainment (Musil et al., 1991). Updraft cores may be relatively undiluted in large High Plains thunderstorms; a study of a supercell storm investigated during CCOPE provides one example in which a huge updraft core (maximum updraft speed about 50 m/s) was relatively free of entrainment effects (Musil et al., 1986).

In 1987, 1989, and 1993, the T-28 was employed in studies of transport, dispersion, and precipitation initiation in developing cumulus. It was equipped with an SF6 analyzer in addition to its normal suite of microphysical instruments. Seeding agents and SF6 released into the base of cumuli tagged the inflow air. Upper cloud regions were then probed by the T-28 and other aircraft for evidence of the tracer gas and developing ice (Stith et al., 1990). Useful observations of untreated clouds also yielded new insight into natural ice initiation (Detwiler et al., 1994).

In 1986, and since 1989, the aircraft has carried electric field mills during penetrations of large storms. Results from a 1989 flight discussed in Detwiler et al. (1990) and Chang et al. (1995) show the presence of horizontally extensive charge accumulation regions sloping downward and downshear of relatively narrow updraft regions in which charge separation appears to be taking place. T-28 observations in a 1991 MCS stratiform region are being combined with simultaneous observations from balloon packages to extend the study of Stolzenburg et al. (1994) into the electrical structure of these stratiform regions.

T-28 microphysical and electrical observations obtained during the 1991 Convection and Precipitation/ Electricity (CaPE) experiment have been combined with observations from multi-parameter radar and from other aircraft to show that storm electrification in Florida thunderstorms proceeds rapidly after ice appears near the 6-7 km level via the freezing of raindrops in the upper updraft regions (Bringi et al., 1996; French et al., 1996, Ramachandran et al., 1996). Yuter and Houze (1995) used T-28 observations to verify Doppler wind fields in their study of convective structures on one day during CaPE.

Interpretation of multiparameter radar signatures has been improved through comparison with co-located T-28 microphysical observations in thunderstorm environments by Aydin and Walsh (1993), Smith et al (1995), Brandes et al. (1995), and Bringi et al. (1996).

These studies provide examples of the ways in which T-28 data have been used in investigations of cloud physics processes. In addition, coupl-ing of the aircraft data with radar and other related observations in a framework incorporating numerical cloud models (e.g., Kubesh et al., 1988; Huston et al., 1991) can further enhance the scientific value of the aircraft data.

Acknowledgements

The development of the T-28 system was supported over the years by the National Science Foundation (NSF) through a series of grants and a subcontract from NCAR as part of the National Hail Research Experiment. It last operated as a national facility under Cooperative Agreement No. ATM-9618569 between the NSF and the South Dakota School of Mines and Technology. Many persons too numerous to mention individually have contributed to the development and operation of the T-28, but the leadership of R. A. Schleusener in helping to get it all started deserves special mention.

References

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Aydin, K., T. M. Walsh and D. S. Zrnic, 1993: Analysis of the dual-polarization radar and T-28 aircraft measurements during an Oklahoma hail storm. Preprints, 26th International Conf. on Radar Meteorology, Norman, OK, 24-28 May 1993. Amer. Meteor. Soc., Boston. 540-542.

Brandes, E. A., J. Vivekanandan, J. D. Tuttle, and C. J. Kessinger, 1995: A study of thunderstorm microphysics with multiparameter radar and aircraft observations. Mon. Wea. Rev., 123, 3129-3143.

Bringi, V. N., K. Knupp, A. Detwiler, L. Liu, I. J. Caylor, and R. A. Black, 1997: Evolution of a Florida thunderstorm during the Convection and Precipitation/Electrification experiment: The Case of 9 August 1991. Mon. Wea. Rev., 125, 3121-2160.

Chang, W.-Y., A. G. Detwiler, M. R. Hjelmfelt and P. L. Smith, 1995: Radar and in situ microphysical observations in a High Plains squall li ne. Preprints, 27th Conf. on Radar Meteor., Vail, CO, 9-13 October 1995. Amer. Meteor. Soc., Boston. 559-561.

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