ARMORED T-28 RESEARCH AIRCRAFT FACILITY

1. Development and History
2. Aircraft Description
3. Instrumentation Description
3.1 State Variables
3.2 Kinematic Measurements
3.3 Hydrometeor Measurements
3.4 Electric Field Measurements
3.5 Navigation and Performance Variables
3.6 User-supplied Instrumentation
4. Data Acquisition System
5. Outline of Operational Procedures
5.1 Flight Operations
5.2 Ground Operations
6. Examples of Research Results
7. Acknowledgments
8. References
9. Return

T-28

 

1. DEVELOPMENT AND HISTORY

The original concept of a meteorological research aircraft capable of penetrating hailstorms was developed and promoted by Paul MacCready beginning in the late 1950's. At the time, this was widely regarded as not feasible. The hail researchers who had been developing a hail suppression program in the Soviet Union considered storm penetrations by an aircraft as too dangerous. They did not send research or seeding aircraft into hailstorms. Instead, they relied on rockets and artillery shells to deliver their seeding materials into what they thought would be appropriate locations in the storms. Within the American research community, the mainstream thought was similar to that of the Soviets; namely, that penetrations of hailstorms by manned aircraft were too dangerous.

The idea of developing a storm penetration aircraft persisted, however, and began to approach reality following Project Hailswath in 1966, when the National Science Foundation provided funds to support a detailed investigation. MacCready commissioned an associate, Robin Williamson, to do the aircraft feasibility study.

Williamson considered all aircraft in the military and civilian fleet of the Vietnam war era. All aspects of aircraft operation including survivability, maintainability, costs, and performance were analyzed. Strictly on the basis of performance characteristics, he concluded that the best aircraft for hailstorm penetrations would be a Douglas "Dauntless" dive bomber, a World War II era combat aircraft. His second choice was a T-28 military trainer, developed in the late 1940's as a high-performance, prop-driven, pre-jet trainer. When costs and maintainability were factored in, however, the T-28 was judged to be the overall top choice.

Some of the prime deciding factors in Williamson's study were:

1) Aircraft equipped with reciprocating engines were better suited to severe-storm penetrations than those equipped with jet engines, being less susceptible to hail and ice damage.
  
2) Twin-engine aircraft were not superior to single-engine aircraft. Williamson's philosophy was that if the environment is so severe that one engine goes out, the second will soon be lost, too.
  
3) No existing airframe was designed to survive impacts of more than 3/4 inch hail, so any aircraft chosen would require additional armoring.  The lower speed of prop-driven aircraft meant that thinner metal
      plating would be required on the leading edges of the aircraft to survive hail impacts.  The added armor that would be required on faster jet aircraft countered the advantage of their higher-performance powerplants.
  
4) De-icing capabilities were not considered critical because very high supercooled liquid water concentrations were expected and it would be impossible to de-ice completely anyway.  Williamson was more concerned with wind shear and turbulence than icing.

5) While the Dauntless had more power and a stronger airframe (it could stand 12-g accelerations), the T-28 was adequate for penetrating the anticipated turbulence and strong shear. 
6) At the time, T-28's were much more plentiful than Dauntless dive bombers, and T-28 parts were readily available in the supply pipeline.

Using the results of Williamson's study, MacCready successfully presented his idea to the National Science Foundation (NSF) in 1967. MacCready's company, Meteorology Research, Incorporated, under contract to the Institute of Atmospheric Sciences' NSF-funded Hailstorm Models Project, acquired and registered a T-28. Williamson Aircraft Co. (headed by Robin Williamson) contracted to outfit the aircraft for hailstorm penetrations. This work began in 1968 and was carried on through 1969. The basic modifications to the aircraft are discussed in Section 2 and in Sand and Schleusener (1974).

After the modifications were complete, the T-28 was capable of performing hailstorm penetrations to altitudes up to about 25,000 ft (7.6 km) and was able to withstand impacts of hailstones up to 7.5 cm in diameter at 100 m/s relative speed with minimal damage. Some meteorological instrumentation was installed and the aircraft made some test flights during the summer of 1969 at Rapid City, South Dakota, and Flagstaff, Arizona, for the primary purpose of determining its capacity for carrying heavy loads of structural ice. It was found that the aircraft could handle up to an inch of ice with only a relatively small increase in speed to maintain controllability. Since the T-28 was to be used in summer thunderstorms where a layer of warm air would be present between icing zones and the ground, icing was not felt to be a major problem.

At this time, ownership of the T-28 passed to the Institute of Atmospheric Sciences at the South Dakota School of Mines and Technology. The Institute's Director, Dr. Richard Schleusener, was a leading figure in the U.S. hail research effort at that time and had actively supported MacCready's concept of an armored aircraft for hailstorm penetrations.

In 1969, a military technical order (TO) revealed that the aircraft needed strengthening of the wing main spar to keep it airworthy. This order came as a result of extensive military experience with T-28 aircraft, including some wing failures during high-g aerobatic maneuvers. The T-28 was therefore sent to the Naval Air Rework Facility at Pensacola, Florida, for wing spar strengthening.

The T-28 returned to active duty in time to make a few cloud penetrations prior to the end of the 1970 hail season in Colorado. These operations with the Joint Hail Research Project proved the viability of the T-28 as a thunderstorm research platform. Fifteen cloud penetrations into increasingly larger storms were made to build confidence in the system.

Engine problems and other experiences in 1971 led to the conclusion that the T-28 required a significant change in the engine and an updating of the basic aircraft. As a result, in 1972 the aircraft was sent back to the Naval Air Rework Facility at Pensacola, Florida, to be upgraded with extensive airframe updating and installation of a more state-of-the-art engine (R-1820-86A) and a stronger propeller. These changes increased engine power from 1200 to 1425 horsepower. After the Naval Air Rework Facility accomplished these changes, the T-28 was in outstanding mechanical condition. It made nearly 200 research flights from 1972 through 1976 in support of the National Hail Research Experiment (NHRE) without further modifications. Then, structural strengthening in the tail section was accomplished by the Naval Air Rework Facility in 1977 in response to another TO.

Two more successful field seasons, involving thunderstorm studies in Colorado, Florida, and Oklahoma, followed in 1978 and 1979 before a mishap on a slippery runway necessitated replacement of the engine and propeller in 1980. The replacement engine was identical to the damaged one. However, the replacement propeller was a shortened model used for carrier-based aircraft; this resulted in an estimated 15% increase in rate of fuel consumption with a corresponding decrease of ~15 min in flight duration but no significant loss in other performance characteristics.

The T-28 participated in summer field programs almost every year from 1972 through 2003. Overhauls of the last engine in 1981 and 1982 left the T-28 in the late 1990's with a low-time engine (600 hours) having a life expectancy equivalent to nearly another decade of field seasons. Another long-blade propeller was located in 1987 and was installed on the aircraft to return its endurance to the former levels.

 


2. AIRCRAFT DESCRIPTION

The T-28 was modified extensively from its original configuration. The leading edges of the wings and tail surfaces were covered with 2.29 mm (0.090 inch) 2024T4 heat-treated aluminum sheets formed to fit and bonded to the existing wing and tail surfaces. The tops of the wings were covered with 0.81 mm (0.032 inch) sheets of the same material. The leading edges of the cowling were covered with an additional sheet of fitted 3.18 mm (0.125 inch) aluminum. This armor plating adds about 318 kg (700 lb) to the aircraft weight.

The carburetor was protected from ingestion of large hailstones by the addition of a metal grate to the air intake to break up the hailstones  entry. A similar device was installed over the oil cooler intake to prevent damage to the relatively fragile oil radiator.

The canopy also required substantial modification since the standard Plexiglas bubble canopy was much too weak to withstand encounters with large hail. The windshield was replaced with flat sheets of 1.91 cm (0.75 inch) stretched acrylic and the side panels were made of flat sections of 1.52 cm (0.60 inch) stretched acrylic. The windshield and the leading-edge armor were tested to withstand 7.6-cm diameter hail at penetration velocities by firing ice balls from a specially-built hail "cannon" at test sections of the aircraft.

The engine installed in 1972 required a new scheme for protecting it from hail damage since the cylinders and front of the engine were of a slightly different design than the engine originally installed on the aircraft. The push-rod housings and the ignition harness were protected with sections of electrical conduit formed to fit the respective areas. The propeller governor was protected with a shield of aluminum. The areas now most susceptible to damage on the entire aircraft were the baffles between the cylinders and the cooling fins on the cylinders themselves.

The T-28 was equipped with alcohol for anti-icing of the propeller and the carburetor to permit the engine to develop full power in icing conditions. Since the aircraft was somewhat overpowered, it was able to carry a substantial load of ice on the airframe if the engine developed full power.

The T-28 airframe was occasionally struck by lightning so lightning rods were placed on the aircraft extremities to attempt to reduce the lightning damage. The propeller was occasionally struck but usually sustained no physical damage other than small burned spots on its trailing edges. Significant burn marks on some of the airframe trailing edges resulted from lightning strikes in Oklahoma during SESAME '79 (Musil and Prodan, 1980). During subsequent field seasons, there were additional lightning strikes to the propeller and discharge pits on the trailing edges.

Since the distinct possibility existed that the entire electrical system could be disabled by a lightning strike or an unintentional overload, precautions were taken to minimize damage under such conditions. The aircraft has a 300 A 28 VDC generator used to power the sensing, recording, and de-icing equipment. To protect the system from damage due to equipment failures, double circuit breakers were placed on all high-power equipment and the switches were made easily accessible to the pilot. The pilot could thus turn off any or all scientific equipment in the event of an electrical problem. Since a primary flight attitude reference panel was not considered adequate for continued flight in the severe environment of a thunderstorm, the T-28 was equipped with dual artificial horizons, one electrical and the other vacuum driven, to enable continued safe flight in the event of a complete electrical failure.

The breathing oxygen system was completely overhauled and updated by the Naval Air Rework Facility during the extensive rework in 1972. This system provided safe, reliable operation to well above the T-28 service ceiling.


3. INSTRUMENTATION DESCRIPTION

The basic requirement of the instrumentation system was that it must reliably measure and record the selected quantities with minimum attention during flight, since the aircraft was flown solo. Limited capability for monitoring the data system in flight was available. There was also a system to telemeter some data to the ground for examination. The data system was designed to be remotely controlled from the front cockpit with a minimum requirement for in-flight checks. Johnson and Smith (1980) provide a basic description of much of the T-28 instrumentation system, although the replacement of the data acquisition system in 1989 and other more recent equipment additions rendered some of this description obsolete.

The data acquisition system is a single-board "personal computer". It was normally activated on the ground prior to each research flight, although it could be started and restarted in flight. The pilot could monitor certain aspects of data system operation in flight through a programmable CRT display. During flight, he had displays of time, and liquid water concentration from the Johnson-Williams device, and on occasion monitored special user-supplied equipment.


3.1 State Variables

Because of the importance of the static pressure and temperature measurements, these variables were measured redundantly. The static pressure instrument, the Rosemount 1301-A-4-B, has a basic accuracy of 0.1% and response time of a few tens of milliseconds. Two of these units were carried on the aircraft. The resolution of the 16-bit analog-to-digital converter in the data recording system is 0.0000152, so the realized resolution of incremental pressure measurements with the Rosemount instrument was about 0.0015 kPa.

Reliable temperature measurements outside the clouds were obtained from both a Rosemount and an NCAR reverse-flow sensor (Rodi and Spyers-Duran, 1972). In-cloud temperature could not be measured as reliably because of wetting of the sensing elements (Heymsfield et al. , 1979; Lawson and Cooper, 1990). The NCAR reverse-flow thermometer has a basic accuracy similar to that of the Rosemount device (+- 0.5 d C) but a slower response, with the Rosemount time constant being ~1 s and the reverse-flow time constant ~3 s. Usually the NCAR device was relied upon for in-cloud measurements because of its superior wetting resistance, but it was not always possible to provide reliable in-cloud temperature measurements under all conditions, particularly at above-freezing temperatures (Lawson and Cooper, 1990).

The T-28 system did not include a humidity sensor. The emphasis of research involving T-28 observations was usually on the interior characteristics of storms, and a robust, generally suitable instrument for measuring humidity in clouds was not found.


3.2 Kinematic Measurements

Horizontal winds can be obtained by subtracting aircraft movement relative to the air, based on measured heading and true airspeed, from the aircraft ground track obtained from an on-board Global Positioning system (GPS). Winds obtained in this way are not comparable in accuracy with winds obtained by INS-equipped aircraft, but can yield a semi-quantitative picture of storm circulations.

Updraft measurements can be derived from aircraft rate-of-climb values obtained by differentiating the static pressure (altitude) values or from direct rate-of-climb measurements made with a Ball variometer. The altitude differentiation approach is normally used to get the rate-of-climb values. The variometer is not used as the primary sensor because it has a limited range (+- 30 m/s) and becomes nonlinear near the ends of this range. It also has an inaccuracy near the zero point which results in a reading of -3 m/s when the actual value is between +- 3 m/s. If the rate-of-climb is outside the +- 3 m/s range and within the +- 30 m/s limit, the instrument functions properly and the data can be used for backup purposes.

The simplest approach to updraft calculation is an expansion of the concept used by Auer and Sand (1966); some of the aircraft-induced vertical motions are removed during post-flight processing of the data by correcting the rate-of-climb values for the effects of airspeed and engine power variations. An additional term based on energy conservation considerations (Dye and Toutenhoofd, 1973; Cooper, 1978) can be applied to correct further for pilot-induced effects. In this manner, the larger-scale updrafts can be measured with an estimated accuracy of better than +- 3 m/s or 10%, whichever is larger. Small-scale motions (i.e., gusts) with horizontal dimensions smaller than 0.5 km or so are below the sensitivity of this relatively simple system.

An improved method of calculating updraft speeds from the T-28 measurements has been developed by Kopp (1985) based on earlier work by Lenschow (1976). It uses the aircraft equations of motion to obtain a better correction for the aircraft-induced motions. The required measurements are static pressure, dynamic pressure, and pitch angle. Estimated accuracy is +- 3 m/s and spatial resolution is ~0.5 km.

Redundancy is an important feature of the T-28 instrumentation system. Only a few storms a year can be studied in detail and the data are sufficiently important to warrant the maintenance of back-up instrumentation. For determinations of vertical air motion, for example, the variometer (rate-of-climb indicator) output is used in the event of a malfunction of the Rosemount pressure sensors. If both pressure instruments and the variometer should fail, the accelerometer data could be integrated to determine rate of climb.


3.3 Hydrometeor Measurements

One unique feature of the T-28 instrumentation system was its ability to measure the numbers and sizes of hydrometeors over almost the entire size spectrum present within a storm. The particles may have ranged from cloud droplets a few micrometers in diameter to hailstones several centimeters in diameter. Various sensors covered different portions of the size range in an overlapping fashion. Somewhat comparable measurements were obtained for each subrange from two different sensors, again affording a useful degree of redundancy.

The sensors applicable to each particle size category are listed below. The values in parentheses indicate the approximate sampling volume per unit distance along the flight path for each instrument; the sampling volume per unit time can be estimated by multiplying by the nominal T-28 true airspeed of 0.1 km/s.

 1) Cloud droplets, up to about 30 um in diameter: 
	J-W cloud liquid water concentration sensor; Particle Measuring Systems Forward Scattering Spectrometer Probe (FSSP) (3.E-4 m**3/km).

  2) Intermediate or "embryo" size particles, 30 to more than 1000 um: 
	Particle Measuring Systems (PMS) two-dimensional optical array spectrometer (0.1 m**3/km); particle camera (up to 2.6 m**3/km).

  3) Raindrops, graupel, and snowflakes, from about 1 mm up to 5 mm or larger: 
	Continuous hydrometeor sampler (foil impactor; 1.4 m**3/km); particle camera.

  4) Hailstones, from 4 mm to more than 5 cm: 
	Hail spectrometer (100 m**3/km); foil impactor.

The sampling volumes tend to increase for instruments designed to sample larger particles to compensate for the smaller concentrations of such particles. The particle camera and hail spectrometer could not be carried simultaneously because both required the same mounting points under the left wing of the aircraft.

The above allocation of instruments to particle size categories is arbitrary to some extent. For example, the two-dimensional probe (2D-C) provides partial images of particles considerably larger than 1000 mm, while the particle camera can photograph centimeter-size hailstones. However, the instrument sampling volumes imposed serious limitations on the representativeness of the data. It was also generally recognized that all of the available instruments were deficient in the 50-150 mm size range.

Our data system accepted data from a PMS 2D-P probe (covering the size range from ~200 mm to ~6.4 mm) and the T-28 did, in fact, carry a 2D-P on many projects. However, it could carry only one PMS imaging probe at a time. Normally no 2D-P was available for use on the T-28, but one could sometimes be borrowed on a project-by-project basis.

A variety of computer techniques were developed to process the two-dimensional image data to determine particle sizes and crystal habits. A preliminary capability to automate the processing of foil impactor data has been developed but additional work is needed to make this routine. Information about particle size distributions can be obtained from the PMS probes, the particle camera, the foil impactor, and the hail spectrometer. Particle phases (ice or water) can be determined unequivocally from the particle camera data and frequently can be identified from the foil and PMS two-dimensional images as well. (Attempts to identify phases from the foil impactor data occasionally can be suspect, as shown by Knight et al., 1977.) Particles larger than 5 mm, which are measured mainly by the foil impactor and hail spectrometer, are normally assumed to be ice because raindrops of these sizes break up very quickly in nature due to dynamic instabilities.

The hailstone spectrometer, developed at the South Dakota School of Mines and Technology, operated on a "shadowgraph" principle similar to that employed in the PMS probes. It used 128 phototransistors spaced at 0.9 mm intervals in a linear array to count, size, and image hailstones as they passed through a planar beam of laser light perpendicular to the flight path. Shadows smaller than about 4.5 mm were not counted, and the data were usually analyzed with the assumption that all particles larger than this were hail.

A device has been developed by NCAR scientists to capture hailstone samples inside the thunderstorm. Frozen particles are decelerated and captured in a chilled receptacle for later analysis in the laboratory. This device was available for use with the T-28.


3.4 Electric Field Measurements

The T-28 carried five cylindrical rotating-shutter electric field mills, located at: (1) the upper rear canopy facing upward; (2) the lower fuselage baggage-bay door facing downward; (3) and (4) the wing tips facing outward; and (5) the outboard hail spectrometer pylon, facing downward.

Experience and in-flight intercomparisons with other aircraft using the five mill locations have shown that reliable estimates of the electric field components in the vertical and transverse directions could be obtained in clear air and in the presence of light precipitation. It was also possible to derive an estimate of charge on the aircraft using field mill readings or instrumented discharge wicks on the fuselage. More work is required to provide reliable interpretations of observations obtained during penetrations of severe storm interiors when the aircraft becomes highly charged (see, e.g., Jones, 1990).


3.5 Navigation and Performance Variables

The aircraft carried a GPS navigation system as well as a VOR/DME system including two DME's. There was no on-board radar. The aircraft navigation equipment was used by the pilot to arrive at the desired initial point for a cloud penetration, but instructions relating to penetration headings were transmitted from colleagues with access to ground radar. The equipment on board was not considered sufficient for precise navigation in regions of mature storms where heavy precipitation zones and strong up- and downdrafts had sharp boundaries which didn't always correspond to visible features. Real-time tracking on the ground coordinated with a state-of-the-art meteorological radar display, with aircraft position data based on the GPS system, FAA surveillance radar, or other precision ground-based radar or radio-direction finding systems, was therefore required for operations in mature storms. Telemetry of the position data from the T-28 to the ground was available to assist in this process (see Sec. 4). The position data were also recorded on the aircraft data system for use in later analyses.

A gyro-stabilized platform and accelerometer system was available to provide aircraft pitch and roll data as well as vertical accelerations. No angle-of-attack or yaw data are available with the present system configuration.

Dynamic pressure (indicated airspeed) and aircraft heading data were recorded routinely, the former redundantly. A real-time true airspeed computer supplied data to timers in instruments requiring this synchronization, such as the PMS imaging probe or the particle camera. Post-hoc true airspeed calculations were made to determine the exact sampling volumes of other instruments. As indicated in Sec. 3.2, the rate-of-climb data serve mainly a backup function.


3.6 User-supplied Instrumentation

The facility was able to accommodate user-supplied instrumentation. Space in the rear cockpit was available, as well as various hard points and pylons on each wing. The aircraft normally flew near its maximum allowed gross weight and could only carry an additional load of 70 kg (about 150 lbs); however, further capacity to carry user-supplied instrumentation could be made available if some of the standard instrumentation was removed. About 500 W of 28V DC and 700 VA of 115 VAC (400 Hz) power were available above the requirements of the standard instrumentation and other aircraft systems. The instrument operating environment was unheated, unpressurized, and subject to significant levels of shock and vibration.

The T-28 carried more than a dozen different precipitation, cloud, and aerosol particle samplers over the years, in addition to the suite of instrumentation described above. In some instances the T-28 carried dedicated data acquisition systems associated with this equipment. It also carried an SF6 analyzer during three field campaigns in which tracer techniques were used to study cloud circulations and precipitation development.


4. DATA ACQUISITION SYSTEM

The last data acquisition system on the T-28 was installed in 1989. Its core was an IBM PC-AT-compatible industrial-grade microcomputer. It had 32 analog input channels and used 16-bit analog-to-digital converters. There are also interfaces to accept digital data from the IAS hail spectrometer, one PMS FSSP or 1D probe, and one PMS 2D imaging probe. Data were stored on a 40-MB streaming tape cartridge. Most data were sampled once per second, but some variables (e.g., electric fields) were sampled at rates up to 20 per second. The pilot could enter event codes and had the ability to reboot the data system in flight should a failure occur. He also had a display screen in the cockpit to allow him to monitor critical data.

A small audio stereo recorder was carried to record pilot comments on one track and the sounds of hailstone impacts on the windscreen of the aircraft on the other track. The volume swept out by the windscreen is about the same as the sampling volume of the hail spectrometer, so two somewhat comparable modes of hail detection were available. A filter was inserted in the hail impact channel to reduce the masking effects of the engine noise. This recorder proved to be extremely valuable in providing supporting qualitative data for the other instruments. It also allowed the pilot to concentrate on flying the aircraft and make subjective observations without the burden of taking notes during the penetrations.

An air-to-ground telemetry system, a Data Radio G5S1SSSG system including a 2 W transmitter operating on 418 Mhz, a receiver, and a PC for display of telemetered data on the ground, was carried on the T-28. It  transmitted digital data to the ground at 4800 baud using a network protocol that allowed several telemetry systems to share the same frequency. The standard frequency was one also used by the NCAR "NATS" telemetry system. Other frequencies could also be used, if necessary, at additional expense for factory modification.

Capability existed for quick-look data reduction on the ground after a flight. Data was able to be listed or plotted within roughly two hours of landing.


5. OUTLINE OF OPERATIONAL PROCEDURES

5.1 Flight Operations

The general objective of most T-28 projects was to obtain data within and in the immediate vicinity of thunderstorms and hailstorms. The main emphasis of the research has been on the study of mature hailstorms. A typical scientific objective of the mature storm studies was to locate and characterize the precipitation growth regions for different types of storms. This involved determining the hydrometeor types and size distributions in various parts of the storm at various stages of storm development. A combination of aircraft, radar, and other data was also used to estimate precipitation growth trajectories.

For these investigations, a variety of flight patterns for penetrating mature storms evolved over the years. The basic procedure was that a qualified meteorologist with access to a quantitative weather radar system and real-time aircraft flight track data was in charge of vectoring the aircraft into desired areas of a storm. The vector selected normally permits penetration of a high radar reflectivity zone as well as an updraft region at aircraft altitude. Penetration of adjacent feeder clouds or other regions of the storm was important in some cases. A penetration was normally made at a constant heading until clear air was encountered or the T-28 was well clear of any radar echoes. The aircraft then reversed course in preparation for another penetration.

A limit was normally imposed on the maximum radar reflectivity factor permitted for penetration. This reflectivity limit was based on coordinate analysis of the experience from past penetrations along with radar reflectivity data and hailstones collected at the ground. The normal limit was 55 dBz along the penetration path at or above the altitude of penetration. This criterion was selected to permit the aircraft to encounter hail, but to avoid hail of destructively large sizes which would tend to damage the instruments and thus prevent the collection of the desired data. The maximum hailstone size encountered using this limit was normally about 2.5 cm, but larger particles were sometimes found.

A normal flight consisted of from three to six storm penetrations, although more than 10 have been made on some flights. This was limited mainly by the fuel supply (about 2 hrs with reserves). In the early years, the normal operational procedure in hail research programs was to begin penetration at 7.3 or 6.7 km MSL (24,000 or 22,000 ft MSL) and proceed downward at 0.6 km (2,000 ft) intervals on successive penetrations until 4.9 km (16,000 ft) was reached. This routine was interrupted on occasions when airframe ice built up to a point where the pilot considered another penetration unwise. Further penetrations were then delayed until a descent was made to melt the accumulated ice.

The pattern involving successive penetrations at progressively lower altitudes used in the early studies (e.g., Sand and Schleusener, 1974) was abandoned because of the difficulty of sorting out temporal from spatial (vertical) differences found in the data. More recent projects generally involved making repeated penetrations at a single level for each storm, usually in the temperature range from -5.0 to -15.0C where ice processes are believed to become operative in the storms. This indicates the evolution of each storm at the chosen level, and by studying other similar storms at different levels inferences can be drawn about the vertical structures. When additional observations are available from other aircraft penetrating the clouds at other altitudes, or operating below cloud base, information about the vertical structure is more readily obtained. Scientific questions about coalescence and recirculation processes, and unique radar signatures associated with melting precipitation, have suggested greater emphasis on storm penetrations near the 0.0C level, and some recent projects have included penetrations near that level.

The T-28 typically operated in projects involving other aircraft and often became involved in simultaneous penetrations of the same storm by two or more aircraft. This type of study has proven highly successful. Due to the workload in the T-28 cockpit, overall coordination of multiple aircraft missions had to be carried out from the ground or from another aircraft with sufficient crew.

The problems of greatest concern during penetrations were the intense turbulence encountered in some storms and the occasional occurrence of extremely rapid ice accumulation on the airframe (sometimes at an observed rate of 2.5 cm/min). The latter had the effect of rapidly increasing the weight and changing the aerodynamics of the T-28. When the air intakes to the carburetor and oil cooler were constricted by ice accumulation in such icing situations, a loss of power and high oil temperatures also resulted. The carburetor intake is protected from large hail ingestion by the grate described earlier, and this grate provides a surface area for heavy ice accumulation in regions of the cloud with high supercooled liquid water concentrations. Limited airflow to the carburetor was partially restored with the addition of carburetor heat; this procedure used air from inside the cowling, but could not restore full engine power. Even so, the engine occasionally stopped running due to excessive ingestion of ice and water. Fortunately, engine restarts were accomplished and there was no loss of data continuity in such cases.

It took about 20 min to climb to, or descend from, the normal T-28 operating altitudes (~18,000 ft MSL), leaving typically 80 min on-station time. This is less than the lifetimes of many thunderstorms, so judicious timing of the initiation of T-28 flights was important. Data tapes couldn't be changed in flight, but total data storage capacity was ~40 MB on the most recent data system. Typically, only ~20 MB was used even on the longest flights, so this was not a limiting factor.


5.2 Ground Operations

Because of the adverse conditions in which the aircraft operated and the comparatively abusive treatment that resulted, high quality maintenance was imperative. This was provided by the employment of a mechanic whose primary responsibility was the meticulous care of the T-28. Normal maintenance included a very detailed inspection before and after each research flight by both the pilot and the mechanic. The engine was checked at regular intervals to detect any possible damage from rapid heating and cooling during penetrations. An oil sample was subjected to spectrometric analysis for metals at regular intervals to check for any abnormal internal wear. Only knowledgeable and qualified personnel were allowed to work around the aircraft and all work was checked and double checked. All modifications on this restricted-category aircraft were approved by the Federal Aviation Administration. The mechanic also conducted a regular program of progressive inspections to comply with FAA mandates concerning airworthiness.

For field operations, the T-28 facility required hangar space with sufficient power (about 20 A at 110 V) to run the aircraft systems (using our own 28 V power supply). The height clearance and floor space required was approximately 14 ft x 40 ft x 40 ft. Space required for tools and materials for in-field maintenance was an additional 20 ft x 20 ft. Separate space was generally required for the quick-look data reduction activities.

Minimum runway length was 4,000 ft. Other requirements were one hundred octane aviation gasoline, deicing alcohol, engine oil, and breathing oxygen.

The aircraft's VHR radios covered the band from 118.0 to 135.975 MHz. A dedicated project frequency for use between a meteorologist on the ground and the aircraft in flight, in addition to the frequencies normally used for conversations with FAA Air Traffic Controllers, was also required for T-28 flights.


6. 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.


7. ACKNOWLEDGMENTS

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.


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