PELTI Flight Report for RF02, 9 July 2000
UTC
15:01 Takeoff and start of sounding to 10,000'
Considerable haze. The PRIDE group reports optical depths of
0.2 to 0.3 due to dust. The lower mixed layer was pretty shallow,
perhaps less than 1000'. There was a gradual decrease in dew point
up to 6500' where there was a sharp dryout. We would choose to fly
a FT leg in the bottom of this dry layer.
15:15 At 10,000' and started descent to 7500'
15:19 Start of eastbound leg at 7500'
Several samplers were not started at the beginning of the leg, but
all were functioning by halfway through. (An extra pump installed to
achieve isokinetic flow for the LTI froze and threw the breakers that
were powering those devices.) The LTI total flow was about 92% of
isokinetic, based on the LFEs (the thermal mass flowmeters suggest
it was a bit closer to isokinetic), and it was kept laminar the whole
flight.
As the APS comparison above shows, the LTI consistently passed more
supermicron particles during this FT sample leg than the CAI and the
NASA solid diffuser. (The latter operated similarly to the CAI, but
its APS began to drift down in concentration later in the flight, so
its data is not shown for the next leg.) Some of this is no doubt due
to the enhancement of the largest particles by the curving streamlines
in the LTI, but much is no doubt due to the loss of particles in the
CAI and the solid diffuser. The DU group will do Fluent modeling to
compute the enhancement vs size in the LTI, while the UH group will
make lab measurements (monodisperse uranine particles from a VOAG) to
determine the size-dependent losses in the tubing behind the LTI and
the other inlets. The bump may be the combination of enhancements and
tubing losses, both of which are quantifiable.
16:19 End of 7500' leg; sounding to 100'
16:35 Start of 100' westbound leg
We flew this MBL leg at 220 knots IAS (rather than the usual 200
knots), because the attack angle is under 2 deg at 220 knots. We
had learned on the ferry from Miami to St. Croix (STX, our first-ever
tests at 1 Atm) that the turbulent wake from the forward CAI strut
impacts the LTI inlet at attack angles above about 2 deg. Flying
a bit faster levels the plane and allows the LTI to miss that wake.
This strut is a piece that will not be used on later projects if the
LTI makes the CAI dispensable.
The wind speeds on this low-level leg were between 7 and 9 m/s,
causing a few whitecaps to be visible. This meant that there was
measurable sea salt for us to use as a test aerosol. As the figure
below shows, the large-particle difference between the LTI and the
CAI was greater in the MBL than in the FT. This could be because dry
mineral particles have a significant probability of bouncing off the
inner inlet walls and being re-entrained into the flow, while sea salt
particles that impact on the walls never come off. Clarke and Howell
had inferred a similar behavior in the CAI by comparing CAINE-2
results with INDOEX dust-layer data that was constrained by Valero
and Bucholtz's AOD profile across the layer.
At both altitudes, however, it is clear that the particle
concentrations above 2-3 micrometers are definitely greater in the
LTI sample flow. We have noted this same behavior on all our ferry
flights as well.
17:15 Terminated inlet comparison but remained at 100'
To gauge the significance of the observed differences in APS data, we
ran all the APSs on the same sample flow, which happens to come from
the CAI. The APS on the NASA solid diffuser inlet is clearly low,
since it began losing sensitivity during the earlier MBL sample.
This may be due to a flow rate change that has not yet been diagnosed.
17:45 End of 100' leg
18:01 Touchdown at STX
Notes:
A comparison between the LTI APS and NCAR's wing-mounted FSSP-300 looks quite
promising, but is not yet vetted enough for broadcast. The comparison between
that 300 and the internal one, however, is not so clear-cut. The 300s will
be exchanged for the next flight (probably 11 July, since RAF crew duty limits
require a down day) to see if a calibration difference is responsible for the
curious results.
All filters and TAS extracts are being sent off for analysis, which may or
may not be completed prior to the end of the deployment. With the exception
of Clarke's FSSP-300, most equipment is functioning well. The total
nephelometers on all inlets gave sensible results on the common sample
comparison, but the submicron one from the LTI read an order of magnitude
high and seems to be malfunctioning. In general, the LTI neph records higher
scattering than the other two when behind their respective inlets.
Barry Huebert
St. Croix, Virgin Islands
9 July 2000