RICO (clouds & climate processes)
101 Circles each approximately 60 km in diameter and 30 minutes in duration were flown with the C130 during RICO. This means that approximately 50 hours of flight time, spanning a distance equal to half the Earth's equatorial circumference were devoted to circular flight patterns. Why? Circular flight patterns, long advocated by D. Lenschow (right), were used extensively during DYCOMS-II, where they were shown to be optimal for estimating a variety of state parameters, including divergence on the mesoscale. During RICO they were used to provide a standard reference for the state of the lower troposphere across flight days. NSF/NCAR C130 HerculesDonald H. Lenschow (the father of the circles)

Description of flight strategy:

  • Circles were flown in a Lagrangian sense, so that the aircraft heading changed linearly in time.
  • Circles were flown in two sets, one at the beginning of the flight, another at the end.
  • Each set consisted of circles at three levels: (i) Free tropospheric (FT) circles were flown near the freezing level at about 5000 m; (ii) sub-cloud (SC) circles were flown about 100 m below the estimated lowest cloud bases, which placed the aircraft at about 400 m above the surface; (iii) Surface (SF) circles were flown at the lowest safe flight level, typically about 85 m above the surface.
  • The orientation of the circles between sets was reversed. For instance if the FT circle at the start of the flight was flown in a clockwise sense it was flown in a counter-clockwise sense (heading decreasing) in the second (end of flight) set.
  • RF01 only included the first set of circles.
  • No circles were flown during RF02.
  • Only the SC1 and FT1 circles were flown on the student flight (RF16).
Analysi of a C130 Flight Circle

Times: (flights.mdat) This ASCII file contains six, 19 element, arrays, called SF1, SC1, FT1, SF2, SC2, and FT2 respectively. These arrays contain the offset time (in seconds from the start of the data record) corresponding to the start of a circle. For flights in which a particular circle was not flown a missing value -999 is entered. Start times were selected by manually examining the aircraft heading, roll, pitch, and GPS altitude data to pick the best 1800s interval (to within 5-10 s) over which to define a circle. An example of the result of such a procedure is shown in the figure on the left, where the selected interval is delineated by the red dashed line. The ncl code used to create this plot is here: (find_legs.ncl).

In some cases the circles appear to terminate a few seconds shy of 1800s, which means that the end points of each circle should be examined critically if velocity data (which is sensitive to sharp changes in pitch and roll) is to be used along a circle.