A SPECIAL ANALYSIS OF SONEX MISSION DC-8 FLIGHT ON 1997.10.29
Bruce L. Gary

2002.01.01

On October 29, 1997, the DC-8 flew a SONEX Mission local south survey flight from the Azores.  It is said that this flight flew through a "cut-off low" pressure field feature.  The JPL Microwave Temperature Profiler, MTP, measured tropopause jumps at a latitude of ~33 degrees on both the southbound and northbound flight legs.  In situ ozone measurments (by PI Gregory) show the presence of stratospheric mixing ratios north of this tropopause jump region.  This constitutes strong evidence that a stratospheric intrusion was flown through that extended several km below the undisturbed tropopause.  This web page contains analyses of mostly MTP data which might be useful in the further study of this event.

Figure 1Ground track for DC971027.  Kilosecond ticks are shown for only the outbound (southward) portion of flight.

Figure 2IAC for DC971029.  Tropopause symbols are shown as small black triangles. The isentropes are 5 K apart, and the lowest blue trace corresponds to the 330 K isentrope. DC-8 flight altitude is shown by a thin black trace.

The steep isentrope slope at 62.7 ks is associated with a tropopause jump from 13.1 to 14.7 km. Since this was observed during level flight, with no altitude changes, it is very likely "real."  Additional support for the reality of this tropopause jump comes from the fact that the same tropopause behavior is seen at 44 ks, when the DC-8 was flying through the same region (in the opposite direction).  These two regions of interest are shown in zoom figures below.

Figure 3IAC versus latitude.  This is the same data as in the previous figure, but the X-axis is latitude instead of time, leading to a "folding" of all data.

In this figure it can be seen that the tropopause jumps both occur at the same latitude, 33.2 degrees North.  The isentropes in this region are steeply sloped for both the southbound and northboudn flight data.

Figure 4Zoom of IAC (two figures back) showing the 6-ks region during the northbound flight in which the tropopause jumped (at 62.65 ks).  (Disregard the "ER" in the title; it should be "DC.").

There's an appearance of a "separating of the tropopauses away from each other" at the jump location. Based on the steepening of the isentropes near the end of this flight leg, at 66.5 ks, it is reasonable to suspect that the tropopause would have jumped down if the flight leg had been extended.

Figure 5In this figure Outside Air Temperature, OAT, is plotted versus time for the same flight segment.

In this figure OAT rises steeply at the time of the tropopause jump.  This is essentially the same as saying that the tropopause jump occurs when the isentropes are sloped steeply, but this OAT rendition has greater temporal resolution since it is based on MMS 1 Hz data (Meteorology Measurement System).

Figure 6Wind speed versus time for the same flight segment.

Maximum wind is encountered just south (before) the tropopause jump time.  The wind speed is changing most rapidly at the time of the tropopause jump.

Figure 7. Ozone (green), air temperature (red) and DC-8 altitude (blue) are shown for the northbound tropopause jump (green arrow) region.

Ozone goes from tropospheric values (<100 ppbv) to stratospheric values at about 62.2 ks, just before the tropopause jumps.  Temperature correlates very well with ozone, which can be accounted for if the high ozone air has descended from the stratosphere. The adiabatic heating during the descent of stratospheric air above the tropopause at ~13.5 to the flight altitude of 11.9 km would be ~16 K.  MTP measures an air temperature at 13.5 km (at 62 ks) of 208.5 K, and adding 16 K yields a predicted temperature at 11.9 km of 224.5 K - assuming no mixing of stratospheric with tropospheric air.  Indeed, the air temperature within the region of high ozone is in agreement with this prediction, implying that no mixing has occured.  This constitutes evidence that the DC-8 flew into a stratospheric intrusion that extended at least 1.6 km below the undisturbed tropopause!

Earlier in the flight, on the southbound leg, the same region was sampled at 10.1 km.  It is possible to determine whether the stratospheric intrusion extended to this altitude by performing the same analysis of ozone and temperature data.  At 62.65 ks, during the northbound tropopause jump just analyzed, the DC-8 was at latitude/longitude coordinates of +33.22 and -31.74 degrees.  This same location was traversed on the southbound leg at 44.11 ks.

Figure 8Southbound flight data for the ground track region that produced a tropopause jump during the northbound flight.  Both horizontal and vertical scales are the same as for its Fig. 4 counterpart.  The northbound altitudes for the corresponding ground track locations are shown by a thick dashed blue line.  A green arrow shows where the northbound MTP data  shows a tropopause jump.

Since the southbound and northbound flight segments are in opposite directions (along the same ground track), the corresponding tropopause jump in this figure would be downward, at the green arrow location. Indeed, the southbound MTP tropopauses (the upper tropopause set) show a downward progression in the green arrow region.  Before the tropopause drop there are two tropopauses, one at about 9 km and the other at about 14.6 km.  After the green arrow region there is one tropopuase at about 13 km.

Figure 9. Ozone (green) and air temperature (red) for the southbound flight segment near the tropopause jump region.  The DC-8 altitude trace (blue) has been rescaled so that it fits on the graph; the highest altitude corresponds to 10.1 km).

Ozone changes correlate well with air temperature during the level portion of this southbound flight segment, indicating that the stratospheric intrusion reached as low as 10.1 km. The ozone mixing ratio at the tropopause jump region is about half its value at the corresponding location at 11.9 km (during the northbound traverse).  The temperature rise is also smaller, so apparently mixing of the intruding stratospheric air with ambient tropospheric air has occurred.  Ozone/N2O scatter diagrams can be used to establish the mixing fractions.  Unfortunately, Sachse's DACOM instrument failed to produce useful measurements of N2O throughout the northbound passage through the stratospheric intrusion.  DACOM did, however, measure CO throughout this period.  Although CO is not as well behaved as N2O for these purposes, it can usually be used to assign flight portions to either the troposphere or stratosphere, using CO > 37 [ppbv] to identify tropospheric flight.

Figure 10CO throughout the northbound passage through the stratospheric intrusion.

CO is tropospheric (higher than 37 ;ppbv]) until about 62.6 ks, when it plunges to stratospheric values.  This is a short time later than the time ozone indicates intrusion penetration (62.2 ks when ozone exceeds 100 [ppbv]).  Recovery begins to occur just prior to the descent, indicating that the north edge of the intrusion was being approached.  At 11.3 km the northern edge of the intrusion is probably encountered at 66.5 ks, which corresponds to a latitude of +38.9 degrees.  The south edge of the intrusion appears to be is somewhere between 62.2 and 62.6 ks, corresponding to a latitude of +32.4 to +33.1 degrees (at 11.9 km).

During the southbound passage through the stratospheric intrusion DACOM acquired some useful measurements of N2O.

Figure 11N2O throughout the southbound passage of the stratospheric intrusion. The dashed red line indicates how the measurements might have varied during the transition from stratospheric values (<312 [ppbv]) to tropospheric values.

When DACOM N2O data begins the DC-8 is already flying though stratospheric air, according to a N2O stratospheric/tropospheric threshold of 312 [ppbv].  Notice that stratospheric air is present throughout the altitude climb from 8.84 to 10.05 km at ~ 42.4 ks.  This assignment of air type is compatible with that based on the ozone data of Fig. 9, which calls for stratospheric air from 41.3 to 44.0 ks.

Based on all the tracer data presented so far, it can be said that the DC-8 passed through a stratospheric intrusion during the following times, altitudes and latitudes:

    41.3 ks      8.0 km    +38.1 deg.    Enter
    44.0 ks    10.1 km    +33.4 deg     Exit

    62.2 ks    11.9 km    +32.4 deg     Enter
    66.6 ks     8.3 km     +39.0 deg     Exit

These entry and exit locations can be plotted on an altitude cross-section along the ground track of the DC-8 (since the southbound and northbound ground tracks were the same for this region).

Figure 12Latitude/altitude cross-section with loci corresponding to stratospheric air indicated for the southbound flight (red) and northbound flight (green).  MTP-based tropopause altitudes are also shown.

The stratospheric intrusion appears to extend down to about 8 km, and is tilted down and to the north.

Figure 13Arrows (thick dashed black) go through local maxima in stratospheric air, and a boundary (thin black dotted) traces possible boundaries of the stratospheric intrusion. Maximum wind speed occurs at the "W" symbol, just outside the intrusion.

The suggested stratospheric intrusion direction and boundaries is not guided by theory, but is merely a simple interpretation of the measurements.  At the wind max location the U and V components were +34.4 and -44.3 [m/s], corresponding to a wind vector that pointed southeast.  This wind vector direction is "out of the paper/toward the reader."  The suggested intrusion is on the northern edge of the southeasterly moving jet core.  I don't know if this makes sense, theoretically, but that's my simple-minded interpretation of the measurements.

Figure 14Vertical wind during the northbound passage through the stratospheric intrusion.

The vertical component of wind, from the MMS instrument, shows a siignificant downward feature at 60.4 ks (29.2 degrees latitude) and an upward feature at 62.7 ks (33.3 degrees latitude).  These two features straddle the horizontal wind max time of 62.0 ks (32.1 degrees latitude).  Presumably, the DC-8 flew through the lower portion of the jet, in which case the jet core would have been offset horizontally - perhaps to the south of where it was encountered by the DC-8.  This pattern seems opposite of what is required by an intrusion that is downward at 34.1 degeres latitude (during the northbound pass at 11.9 km altitude).  Again, I have no theoretical insight into how a jet produces a stratospheric intrusion, but am merely presenting observations that call for competent theoretical interpretation.

DIAL lidar measurements of ozone mixing ratio above the DC-8 are available, and these should elucidate the structure of the stratospheric intrusion above the DC-8.
 
 
 

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This site opened:  October 26, 2001 Last Update:  January 25, 2002