The C-141 Severe CAT Encounter of 1982 December 8
Bruce L. Gary
1999 August 24
A severe encounter with Clear Air Turbulence, CAT, by NASA's C-141 was documented by the Microwave Temperature Profiler which shows that the CAT was associated with an inversion layer that underwent a thickness change at a location that was immediately upwind of the CAT. This inversion layer behavior is consistent with a conceptual model for CAT creation in which vertical compression causes isentropes and isotaches to be brought closer together, increasing dynamic instability (i.e., decreasing Richardson Number) sufficient to allow K-H wave growth, overturning,and CAT .Introduction
Ever since 1981 I've believed that CAT encounters were more likely during flight within inversion layers than outside them. This report describes what the NASA C-141 Kuiper Airborne Observatory flight crew describerd as the "most severe turbulence in a long time." I wasn't aboard the plane for this flight, but the MTP was operating in its "turnkey mode." Before data analysis I had expected to find that the encounter occurred near the tropopause, as this was the most common location for CAT encounters based on previous data (Gary, 1984). I was surprised to discover that it was well below the tropopasue, within the troposphere - but very close to in inversion layer. The behavior of the inversion layer was interesting, and supports the idea that CAT is triggered by a vertical compression of a layer with a concentration of vertical wind shear. This is another suspicion I had begun to believe in starting with my analysis of a 1979 CV-990 flight (Gary, 1981).
The C-141 was flying westbound approaching the Sierra Nevada mountain range on December 8, 1982 when it encountered Clear Air Turbulence which the flight crew described as "one of the most severe in a long time." The flight altitude was 41,000 feet, and winds were from the east. The flight track is shown in Fig. 1.
Figure 1. Flight track of the C-141. Times indicated are local.
MTP Temperature Field Analysis
The Microwave Temperature Profiler used in the C-141 measured T(z) every 20 seconds. The temperature profiles showed the presence of an inversion layer, IL, close to flight altitude throughout the entire CAT encounter sequence, as shown in Fig. Prior to and during the encounter the C-141 was flying near the top of the inversion layer. Above the IL the lapse rate was tropospheric, so the IL was embedded within the troposphere. Immediately upwind of the severe CAT region the IL was at a higher altitude, and it was thinner than at any other location. Since the wind was from the east, we must conclude that the distortion of the IL was caused by the passage of lower lying air over the Sierra Nevada Mountains. In other words, a mountain wave must have distorted the streamlines and in this case lifted the lower boundary of the IL more than the upper boundary, thus bringing the streamlines close together immediately downwind of the mountains.
Figure 2. Altitude cross-section along the flight track, showing the altitudes of an inversion layer near the aircraft during eastward flight into westward moving air.
Figure 3. Same as the previous figure except with an expanded altitude scale, showing in more detail the altitude of the inversion layer's upper and lower boundary.
I speculate that the underlying mountains are responsible for squeezing the IL at a location downwind of the last ridge line, which brought isotach surfaces closer together in that region. Possibly a "venturi" effect caused air to flow faster through a constricted IL, further increasing the magnitude of vertical wind shear within the IL. As described in Gary (1981), the vertical compression of an IL will cause a decrease of Richardson Number, Ri, under the assumption that isotaches and isentropes are squeezed together in the same manner. This could trigger CAT, provided Ri was decreased to below its critical value for Kelvin-Helmholtz wave growth long enough for growth to produce an overturning and subsequent CAT.
Unfortunately, I did not attempt to quantify the magnitude of vertical wind shear within the IL, so we do not know if Ri was indeed close to 1/4 when the IL was thinnest, just prior to CAT. This might have been accomplished if in situ winds were available from the INS, but that data is probably unavailable now. Radiosonde records of winds through the IL would be useful for such an analysis.
As I wrote in a 1983 report decribing this encounter, "The mountain lee wave disturbance that the IL experienced must have created the conditions for extraction of wind energy from the IL. This event warrants further analysis by a qualified meteorologist." To date, no meteorologist has studied this case. Nevertheless, I believe this encounter is consistent with the vertical compression model for CAT creation, and it joins a growing body of observational evidence that supports this conceptual model (as poorly worked out as it is).
Gary, B. L., 1981, "An Airborne Remote Sensor for the Avoidance of Clear Air Turbulence," AIAA 19th Aerospace Sciences Meeting, Jan. 12-15, 1981, Saint Louis, Missouri.
Gary, B. L., 1984, "Clear Air Turbulence Avoidance Using an Airborne Microwave Radiometer." AIAA 22nd Aerospace Sciences Meeting, January 9-12, 1984, Reno, Nevada.
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