XII. A Large-Drop Icing Encounter Over the Park Range of Colorado

The 1994 crash of a commuter turboprop aircraft generated considerable interest in understanding supercooled large drop (SLD) conditions. SLD are those drops with a diameter exceeding 40 micrometers, and can be a significant hazard to the flight of some aircraft. The determination of the degree of the hazard posed to various aircraft is a difficult problem, requiring flight through these conditions with aircraft instrumented for measurement not only of atmospheric properties, but of the response of the aircraft to those properties. Few of these aircraft exist, however. If care is taken, analysis of the performance of other aircraft can be carried out and compared with these aircraft, as shown in this example. Information on a larger set of aircraft is needed to assess the extent of the hazard, and to guide forecasters in assessing the significance of SLD conditions.

M. Politovich and P. Lawson analyzed a Piper Cheyenne research aircraft encounter with SLD during the third Colorado Orographic Seeding Experiment (COSE), conducted over the Park Range in northern Colorado in winter 1981-82. Early on 5 January 1982, shallow orographic clouds formed over the Park Range, created by moist westerly flow. Figure 1 shows a profile view of the Cheyenne flight track during the SLD encounter, which occurred early during the flight. The aircraft flew between 4.3-4.7 km just below cloud top, from upwind to a position about 10 km downwind of the ridge line of the Park Range. Prior to 1436, droplet sizes were confined to diameters > 30 micrometers and LWC were > 0.05 g/m³. From 1436-1450 the drop spectra broadened dramatically. The mean droplet diameter based on Forward Scattering Spectrometer Probe (FSSP) measurements increased from 15 to 25 micrometers, the liquid water content increased to 0.1-0.3 g/m³ and drops to < 200 micrometers diameter were encountered. Graupel particles with diameters ~200-800 micrometers were also observed on 2D probe images.

The Cheyenne started a spiral ascent to 7 km immediately following the SLD encounter at 1451 UTC. This ascent was through a deeper cloud that entered the area at around 1445, containing primarily ice crystals and nearly no liquid water. It was likely responsible for the graupel observed with SLD, as ice crystals fell into the pre-existing lower, liquid cloud and accreted droplets to form the graupel particles. Thus additional ice accretion in the deeper cloud during the ascent was negligible.

The rate of ascent can be used to gauge the relative degradation in climb performance of the aircraft after the SLD encounter. The aircraft power settings were not recorded by the data acquisition system, however, a maximum allowable continuous climb rate was routinely executed during these spiral ascents. A similar spiral ascent not in icing conditions was analyzed to determine the un-iced climb performance. The rate of climb for the un-iced ascent was 2.2 m/s compared with 1.2 m/s on 5 January. Thus, climb capability after flight in these SLD conditions was reduced to about 45% of the un-iced value. Table 1 summarizes the icing conditions in this case, and compares it with another icing encounter by the University of Wyoming King Air on 26 February 1982.

Figure 2 shows the size distributions of liquid droplets for the two cases. Both spectra have broad distributions of droplet size. The King Air case had more liquid water than the Cheyenne case, with more of that liquid contained in "cloud-sized" droplets (diameters > ~40 micrometers). Graupel particles from the Cheyenne case were edited out by visual inspection for diameters 200 micrometers and greater in maximum dimension.

Using the relative climb rates as an indicator, the Cheyenne experienced about a 1 m/s or 45% decrease in climb performance after the SLD encounter. Calculations of climb loss for the King Air, in comparison, were 6.6 m/s or 66 to 94% of capability assuming a maximum continuous climb rate of 7-10 m/s 5. Using the severity index described by Politovich (1996) based upon liquid water content, droplet median volume diameter and temperature, the 26 February case falls into the "severe" category, and the 5 January case is "moderate."

The difference in apparent performance degradation of the Cheyenne compared with the King Air in similar microphysical conditions, but different temperatures, can be attributed to (at least) three factors:

1) The colder temperature of the 5 January Cheyenne encounter probably favors a more regular ice formation on the wing with less runback.

2) There was less liquid water to accrete onto the Cheyenne during similar time periods.

3) The Cheyenne and King Air have different wings and the King Air has a retrofitted spar strap. Thus, the King Air wing may be more adversely affected by freezing drizzle encounters than the Cheyenne. This factor is, of course, speculative, since there is no evidence at this time to directly support or refute this possibility.