PELTI Flight Report for RF08, 21 July 2000

   The objective of this flight was to assess the ambient/LTI relationship,
with more emphasis on dust than sea salt.  We flew four (4) one-hour legs,
each with TAS and FSSP measurements to evaluate the inside/outside difference.
Nuclepore filters were used in TAS for the three legs in FT dust.  This flight
may have had the highest dust concentrations of any, since it was at the start
of a major dust event.


 UTC
14:04:15 to
14:26:30   Takeoff, South, Sounding to 5300 m
           Dust maxima at 1970 - 2120 m, 2730 m, and 4240 m

14:31:10 to
14:41:30   Common-inlet cals at 4100 m

14:42 to
15:42   Dust Leg 1, Nuclepore in TAS, Laminar 200 lpm, 4100 m

14:44:33 to
15:42 (?)  APS Sampling

15:13:30   Flew out of dust, descent to 3790 m, 3 and 7 micrometer peaks vary
15:26:30   Flew out of dust, descent to 3480 m, 7 micrometer peak only
15:31:30   Flew out of dust, descent to 3180 m and then 2270 m; broad peak
15:36   Level at 2270 m in heavy dust, 3-4 micrometer peak only
 
15:43 to
15:53   Sounding to 1200 m and back to 2120 m, turned East

15:53 to
16:53   Dust Leg 2, Nuclepore in TAS, Laminar 200 lpm, 2120 m

15:54:20 to
16:53      APS Sampling

15:54 to
16:30   7 micrometer peak 2-3x larger than 2-3 micrometer peak;
        Visible even in NASA & CAI

16:35 to
16:50   7 micrometer peak disappeared, and a large 2-3 micrometer peak appeared

16:55:50 to
17:08:30   Sounding from 910 m to 3030 m, turned North

17:07:27 to
17:12:25   APS and Nephs zeroed on filtered air

17:08:30 to
17:53:52   Dust Leg 3, Nuclepore in TAS, 2570 m, RH 25%

17:12:59 to
17:28      7 micrometer mode dominates over smaller mode

17:28:12 to
17:38:18   7 micrometer mode fades leaving 3 micrometer mode

17:42   Flew out of dust; clear sky above and below; small 2 micrometer mode

about 17:30 to
17:48         Displays froze; Data system rebooted

17:53:20 to
18:00:20   Sounding to 30 m and Turned to West

18:00:20 to
19:01:45   (Salt-less) MBL Leg, at 30 m, winds 5 m/s, no whitecaps
           7 micrometer mode always larger than 2-3 micrometer mode;
           little seasalt

19:04:52 to
19:30 (?)  Common-inlet calibrations

19:18   Data display froze again

19:40   Landed at STX


Notes:

Some data was lost due to RAF data system problems.  Fortunately, however, the
LTI flows, APS, and neph data were recorded on other computers.  During data
system problems, we left filter flows unchanged, so that we could interpolate
to fill any missing sections. 

The CAI APS was an order of magnitude below the others at 18:18, but the
common-inlet calibrations showed it was working very similarly to the other
two.  Under some conditions the CAI performed far worse than either the LTI
or the NASA inlet.


Commentary:

   The variation of dust distributions on these legs was surprising.  Each
filter from the dust legs will be averaging over several quite different
domains.  At times the 7 micrometer peak was virtually the only one, while at
other times the big peak disappeared completely in favor of a 2-3 micrometer
one.  The facts that 1) the two peaks varied independently and 2) the larger
one sometimes even showed up behind the NASA inlet and the CAI (in dry regions
where the mineral particles could bounce off the inlet walls) both argue that
the 7 micrometer mode is a real one, and is not the result of enhancement by
the LTI.  This does not say that this peak is not enhanced nor that the large
end of it is not truncated by losses in tubing.  It does, however, indicate
that the ubiquity of the peak on other flights is not an artifact of the
LTI.  Presumably the SEM analyses of TAS filters and cones will tell us
how accurately the LTI air represented the two ambient peaks on each leg.
   We found it hard to stay in the dust as we flew south, because the layer
was clearly sloping downwards toward the south.  Layers often slope; it is
very hard to sample such a layer without the aid of an on-board lidar that
can profile scattering in the vicinity of the plane.  Since most lidars have
a significant dead zone near their own altitude, the ability to scan the lidar
over 180 degrees would be of tremendous value in these situations: by scanning
out to the side (or front) of the plane, you could tell when the highest
dust density is sloping above or beneath the flight altitude and adjust the
sampling altitude accordingly.  It is important that the ATD development of
a radar controller for SABLE (to shut the beam off when an aircraft is within
the non-eye-safe distance) proceed so that this capability will be available
during ACE-Asia.
   In spite of the data system problems, this flight was a very good test of
the LTI in dust.  The wide variety of distributions we encountered confirmed
that we were, in fact, sampling two or more size modes, and that the LTI was
more successful at getting those modes into the aircraft than the other two
inlets.  Under some dry conditions (but not all), the solid diffuser (NASA)
inlet passed nearly as many particles as the LTI.

Barry Huebert
1 Aug 2000