NASA DC-8 AIRCRAFT MEASUREMENTS
(this material prepared prior to the aircraft campaign - useful for historical perspective and general strategy)
Example of one of the NASA DC-8 possible flight tracks. For more examples scroll down to the end of the page.
A research aircraft provides a mobile platform to measure moving features
and to concentrate observations in the desired location. Although the
DC-8 has many measurement capabilities this proposal will primarily
depend on measurements from the GPS dropwindsonde system and the flight
level standard meteorological observations.
General measuring strategy
The NASA DC-8 aircraft will be flown into and around African waves on
successive days to describe the amplification process in unprecedented
detail. A minimum of three flights would be needed for each wave, though
up to 5 flights could be carried out to measure a developing system from
the Cape Verde Islands. The flights would be near maximum aircraft
duration, since they would be designed to measure both the synoptic-scale
environment of the depression and mesoscale details near the circulation center.
The first portion of each flight would be that of deploying DWS's, since
the exact circulation center of an over-land African wave would probably
be difficult to locate from satellite imagery and the sparse sounding data.
DWS observations would be made at approximately 200km separation along the
aircraft track. These observations would be analyzed in flight to determine
the position of the lower-tropospheric circulation center location, or
the region of maximum vorticity if no circulation center was evident.
Using this information, a flight pattern at a specific lower-mid
tropospheric level (between 3-4 km above sea level) would be designed
and carried out. Since there is considerable vertical shear over the
region, this flight level should be the same for each flight, so that
comparisons can easily be made from day to day and wave to wave.
There are a number of ways to describe intensification of a tropical
wave into a depression or tropical storm. The change in the absolute
vorticity, or the potential vorticity, is probably the best indicator
of the intensification process. However, the measurement of vorticity
is often scale dependent, with mesoscale measurements (such as those
made by a research aircraft) often revealing much higher values than
those resolved by synoptic scale observations. Thus it is a challenge
from the observing perspective to measure vorticity accurately.
Perhaps the most common measurement of intensification is that of
minimum surface pressure. However, surface pressure is affected not
only by local, but planetary scale pressure changes, and by the
semidiurnal and diurnal tides. In the weak wave to storm transition
phase that will be studied here, surface pressure may not be an
accurate indicator of intensity. The temperature anomaly of the eye
with respect to the surrounding tropical environment is another quantity
(essentially similar to surface pressure anomaly) that can be used and
is easy to measure with an aircraft, but this generally is applicable
only to systems with a developed eye. In a baroclinic African wave,
with warmest air far to the north, such a local maxima in temperature
may not exist in the early stages of amplification.
Given the availability of a research aircraft, we believe that direct
measurement of vorticity is the best indicator of intensification.
The other measures, related to the temperature and pressure fields,
will also be obtained through the aircraft and DWS measurements.
Determining the relative intensities of AEW's: routine observations in support of the DC-8 activity
The DC-8 aircraft observation program is assumed to last approximately
one month, during which 5 to 10 well-defined wave passages across
west Africa might be expected according to climatology. The DC-8
will be able to describe aspects of perhaps 2-3 of these waves,
given anticipated flight resources (though actual available aircraft
flight hours are unknown to the PI). In order to compare the intensity
of the aircraft-sampled waves with other AEW's that occur during the
field program it will be necessary to have an independent estimate of
the wave strength as the AEW's pass a certain longitude.
The upper air conventional observations from radiosondes and the
proposed pilot balloon sites are intended to provide the additional
information to assess the relative intensities of the AEW's.
This is major advantage of coupling both programs.
Example of the NASA DC-8 possible flight tracks designed to follow
the propagation of a relative vorticity maxima.