The T-28 armored research aircraft was developed beginning in 1968 with a grant from the National Science Foundation (NSF) to the South Dakota School of Mines and Technology (SDSMT). Paul MacCready, then head of Meteorology Research, Inc., directed the modification of the stock military trainer, with much of the engineering work performed by his associate, Robin Williamson, of Williamson Aircraft Co. Both were involved in the 1960s in pioneering airborne studies of convective clouds, cloud seeding, and atmospheric electricity, research areas in which the SDSMT was also active. Modifications to the aircraft for protection from hail during storm penetrations included shielding critical exposed engine components, a new canopy design incorporating bulletproof plexiglass, and armoring of leading edges. An anti-icing system for the propeller facilitated operations in heavy icing environments. Electrical discharge wicks on the trailing edges minimized radio communication interference due to precipitation static, and protected the aircraft and its instrumentation from damage due to buildup of static charge. The Institute of Atmospheric Sciences (IAS) at SDSMT operated the modified T-28 from the time it entered service in 1970 through 2003. During these years, the hardened single-engine aircraft participated in regional, national, and international convective storm research projects, supporting field programs by providing unique in situ observations in regions of convective storms that could not be safely penetrated using any other powered aircraft. These observations are now being made available in an online archive for continued use in both research and education.
Initially, the T-28 armored aircraft was supported on a project-by-project basis. It was a key platform for in situ observations of convective storm microphysics in the National Hail Research Experiment (NHRE) from 1972 to 1976. Following NHRE, the aircraft participated in a series of convective storm projects in the High Plains and southeastern United States, Canada, and Switzerland. Beginning in 1986, it was operated as a national facility under a cooperative agreement between SDSMT and NSF.
The aircraft was outfitted with a suite of state-of-the- art instrumentation for microphysical observation. It was capable of accurate measurements of all species of hydrometeors from a few micrometers to several centimeters in size. On a typical mission, the aircraft penetrated storms containing strong updrafts, moderate-to-severe turbulence, and nearly adiabatic concentrations of supercooled liquid water. On some missions, it encountered hail several centimeters in diameter. In addition to using its observations to understand precipitation evolution in these storms, radar scientists were able to use the observations to learn to interpret polarimetric radar observations of rain, snow, graupel, and hail. Beginning in 1989, electric field mills were added to the T-28 to support atmospheric electrical studies. Researchers used the observations of in situ electric fields to map the electrical structure of thunderstorms. Microphysical and electrical observations were an important component of several large field programs, including the Cooperative Oklahoma Profiler Studies (COPS) and Convection and Precipitation/Electrification (CaPE) experiment, both in 1991; the Severe Thunderstorm Electrification and Precipitation Study (STEPS) in 2000; and the Thunderstorm Electrification and Lightning Experiment (TELEX) in 2003 (see Table 1). For several projects, part of the payload included various types of gasanalyzing instrumentation. Sulfur hexafluoride was continuously monitored to trace storm circulations in the North Dakota Thunderstorm Project (NDTP) in 1989 and the North Dakota Tracer Experiment (NDTE) in 1993. To study the lightning production of oxides of nitrogen, the instrumentation continuously monitored these compounds and their subsequent transport in thunderstorms in 2002 and 2003.
DEVELOPMENT OF THE ARCHIVE.
During its 34 years of operation, the T-28 aircraft participated in more than two dozen major national and international convective storm field projects, and also many smaller, more focused studies. Instrumentation and data acquisition and processing improved over this time. Unfortunately, changes in data acquisition and storage technology resulted in the loss of the original digitally recorded data for projects prior to the 1990s. However, the IAS retained digital data from the early 1990s through 2003. To organize and make these unique and extensive in situ data from convective storms easily available, the SDSMT and NCAR's Earth Observing Laboratory (EOL), with joint sponsorship from NSF, collaborated to create a comprehensive archive of the T-28 observations (including supporting datasets such as project radar data). The archive is available through a website located at EOL: www.eol.ucar.edu/projects/t28.
CONTENTS OF THE ARCHIVE.
Figure 1 shows an image of the home page of the T-28 archive. This page links to a history and extensive documentation of the T-28 aircraft, descriptions of T-28 instrumentation, a list of projects in which the T-28 participated, photographs, access to publications ranging from internal reports to peer-reviewed publications (many older paper documents have been digitized), related relevant links, and access to the original digital data through the EOL Data Management System. Much of the digital data have been packaged into modern NetCDF-3 (version 3) formatted files compatible with a wide range of current data-analysis software, including software available from EOL and Unidata, as well as many commercial packages. Hydrometeor image data are available in their native formats. Along with the aircraft data, supporting radar data either from research project radars or WSR-88D radars covering the project region are available from the archive website. Additional supporting data available at the website include flight videos, pilot notes, and meteorological observations. EOL can update the archive with additional project information from other organizations and investigators involved in these projects as it is located.
Visualization tools are built into the website so a user can view data online and identify flights and flight segments of interest. The tools allow online generation of custom plots of altitude, temperature, vertical winds, electric fields, and other parameters of interest over a specified time period during a flight. Figure 2 shows a portion of a plot generated with this package. NetCDF-3 data files can be downloaded for more detailed analysis. Hydrometeor image data files and software applications for their offline analysis also can be downloaded. Online tutorial material is available to guide users with interpretation of the data.
USE OF THE ARCHIVE.
The data were generally obtained with custom-built or limited-production instrumentation. Documentation is available on the website to inform the nonspecialist about the instrumentation and to help interpret the data. However, much of the data can be understood and interpreted qualitatively by someone with a general meteorological background.
SDSMT and NCAR designed the T-28 data archive website to be easily accessible to users with a range of backgrounds, from upper-division undergraduate students in atmospheric and cloud physics classes to researchers with interest in detailed meteorological, microphysical, and electrical observations in convective storms. The goal is to support continued use of the data for both research and education. The archive includes data from projects ranging in meteorological regimes from Florida to the central and northern Great Plains. For researchers, the data will be useful in storm process and microphysical climatological studies. For students, the archive is a source of easily accessible original observations in convective storms that can be used to learn about basic storm processes. Instructors may find the observations useful in the context of class or homework exercises in which students learn about interpretation of in situ data as well as about cloud processes.
A data archive containing unique airborne in situ observations of meteorological, microphysical, and electrical characteristics of convective storms is introduced (www.eol.ucar.edu/projects/t28). The T-28 aircraft archive contains digital data from field projects spanning the years 1991–2003 conducted in a range of meteorological regimes. Supporting field data are also in the archive. Finally, an extensive collection of literature ranging from project reports to published analyses is available to provide background on the archived data and a window into highlights from T-28 projects prior to 1991 for which digital data are no longer available.
The T-28 aircraft operated through most its years of service with support from NSF. Stable funding through a cooperative agreement since 1986 is responsible for the survival of the digital data and its documentation that is the core of the archive. In addition, field project support was provided by a variety of sources, including NSF, NOAA, NASA, and the Federal Aviation Administration. NSF funded the development of this data archive. All of this support is most gratefully acknowledged.