Overview of Flight ER940528
The flight of May 28, 1994 was part of teh New Zealand based ASHOE mission (Airborne Southern Hemisphere Ozone Experiment). The pilot reported "moderate" CAT about near Auckand, about 1 hour after take-off from Christchruch while flying north.
Figure 1. Flight track for ER940528, showing the northbound portion where CAT was encountered at 76.4 and 76.7 ks (two red ovals). Mount Ruapehu, reaching 9177 feet, is indicated by a green square. Time ticks and "kilosecond" labels are shown at 1 ks intervals.
Isentrope Altitude Cross-Sections
The isentrope altitude cross-section, or IAC, is presented in the next figure.
Figure 2. Altitude of isentrope suraces, as determined by the MTP instrument. ER-2 altitude is shown as the red trace. The isentrope surfaces shown are 20 K apart. The 460 K surface is at 19.0 km at 79.5 ks.
In this IAC it is apparent that the isentrope surface nearest flight altitude is distorted in in altitude in the region 74 to 78 ks, which corresponds to flight across North Island. During the return flight leg there is a smaller distortion of the isentrope surface nearest the ER-2 in the region 95 to 97 ks, which is also over North Island (the southern part). Evidently a mountain wave was present over North Island, and it persisted for at least 6 hours.
Figure 3. Zoom of previous figure, showing the two CAT encounter locations in time and altitude. Isentrope surfaces are 5 K apart.
The CAT encounters occurred within the northbound passsage through the North Island mountain wave. Isentropes are steeply sloped before and after the two encounters. The CAT encounters occur at peaks in the isentrope displacement pattern.
Figure 4. An even larger zoom of CAT encounter region. Isentrope surfaces are 5 K apart.
A method was devised for "enhancing" CAT features with respect to the background of vertical accelerometer excursions that are present in the data that was used for this flight. Vertical accelerometer data were recorded at intervals of 0.417 seconds (i.e., 2.4 Hz). It's not worth describing in detail, since there are 3 stages of taking peak-to-peak excursions for various group sizes. The final stage of this procedure is to subtract 0.06 g-units from the data and multiply the result by 2.5. The use of 2.5 instead of 2.0 (as described on the ER911102 web page) is meant to overcome reduction of averaging effects during the 3 stages of peak-to-peak and averaging. I subjectively estimate that the accuracy for the resulting trace, called ACC, is 30%, or 0.05 g-units, whichever is greater.
The following trace of ACC shows the twin CAT encounters of this flight.
Figure 5. Vertical accelerometer peak-to-peak parameter ACC for a half-hour period centered on the CAT encounter.
The next figure shows the relationship of MTP's lapse rate with CAT encounters.
Figure 6. Lape rate, dT/dZp (green trace) and turbulence intensity, using a rescaling of undersampled measurements from a vertical accelerometer that is meant to estimate what a high sample rate accelrometer ercord would show (red trace).
CAT occurs within a region of relatively low static stability, dT/dZp = -1.7 [K/km], but is preceded by a brief high static stability, dT/dZp = +4.5 [K/km]. The picture of vertical compression leading CAT is not compelling in this data. The CAT must have been triggered by increasing VWS, or else if was generated at a neighboring altitude level and had affects at flight level.
The 10-second difference method, described on the web page ER911102, is used to derive RRi for this flight.
Figure 7. RRi and CAT intensity (inverted) for the 1.4 hour period centered on the CAT encounters. The 10-second difference method was used to calculate RRi. The 1-minute and 2-minute averages of RRi are shown with a red dotted and blue thin line trace, while an RC-averaged RRi is shown with a thick black trace. The1-minute 2-miute average traces are positioned in time at the center of the data used to derive them, while the RC-averaged trace is positioned in time to corresponds to a calculation of RRi based only on measurements available at the time of the plotted trace.
This figure shows one large enhanced RRi feature centered on the time of the CAT encounters. RRi reaches 4.0 and persists at this level before the first CAT encounter for 3.0 minutes of flight (43 km). The second CAT encounter is preceded by a large increase in RRi, reaching levels above 10 for the 1-minute and 2-minute averages.
If the CAT warning threshold is set at 2.5 (still to be established), and if they are to be based on the RC-average RRi trace, then warnings would have been issued at 75.1 ks (20 minutes before the most severe CAT), at 75.6 and 75.7 ks (about 11 minutes before) and finally at 76.17 ks (3.3 minutes before). The first warnings would have been intermittent, whereas the last one would have been persistent. I shall somewhat subjectively identify the time when the persistent warning begins as "the" time of warning for the first CAT encounter. Assuming the RC-averaged RRi is used for issuing warnings, the "warning time" for the first encounter is 3.3 minutes.
Flying "backwards" yields a warning time of approximately 7.3 minutes for the CAT event at 76.69 ks, assuming use of an appropriately calculated RC-average RRi.
As described on another web page, the issue of false alarms will be an important issue to address, but I prefer to do this at a later stage of this work. Flights identified as "smooth" by the pilots should serve as the best data source for evaluating false alarm rates.
Warning Time Summary
The following table summarizes the "warning times" for the CAT encounters of this flight.
WARNING TIMES FOR LEVEL FLIGHT CAT EVENTS
|Flight Direction||CAT Event||Warning Time|
|Forward||76.37 ks||3.3 minutes|
|Backward||76.69 ks||7.3 minutes|
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