Mission Summary for 24 August, 1999

Michael Biggerstaff, Aircraft Mission Coordinator (TRMM OPS)

Anthony Guillory, DC-8 Chief Scientist

Anthony Tony Grainger, Citation Chief Scientist

Peter Hobbs, Convair Chief Scientist

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This is the only mission of the day.

All times below are UTC

Triple aircraft mission: DC-8, Citation, and Convair

DC-8 t/o 0255, land 0612

Citation t/o 0308, land 0611

Convair t/o 0202, land 0635

TRMM OVERPASS: 0540, good TMI coverage, some PR coverage to the extreme northeast quadrant.

We'll could call this mission the "take what you can mission". The aircraft were alerted for an afternoon flight to coincide with a TRMM overpass. It was hoped that an area of active convection to the east would help trigger activity within quantitative radar range of Kwajalein (within 150 km). Similiar to yesterday, activity split into two main regions, one well north and the other well south of the KWAJEX nominal operational area. Unlike yesterday, virtually nothing filled the substantial gap between these two hot spots.

Weather support prior to the take-off decision was limited. GOES satellite coverage was unavailable due to scheduled maintenance. The KMR radar was also off-line for an attempt at a sphere calibration. It was a noble effort. But the high winds kept the tethered sphere too low to the surface for the radars to detect. Indeed, the 20-25 knot winds were strong enough to cause one ballon to burst which led to the sphere falling to the ocean before being raised again. Given the upcoming aircraft mission, the sphere calibration was abandoned.

After regaining radar support it became apparent that little activity was in the vicinty. The DC-8 needed to perform calibration runs for the AMPR since the insulation around the calibration loads had been modified. These runs required severe clear, which was relatively easy to accomodate. The Convair, which departed first, was directed to work the "small cumulus/precipitation initiation" experiment. This generally requires the aircraft to make multiple penetrations through developing cumulus before radar echo occurs. It is inherently a visual, self-directed activity. The Convair performed numerous penetrations of developing and precipitating cumulus during the first 2.5 hours of the mission.

The Citation was directed toward the only decent radar echo within 150 km range. It was a dissipating small cluster of cells just north of Meck. The large separation between the Convair and the Citation also allowed the dismissal of the lower "no fly" zone which gave the Convair additional flexibility to extend their penetrations above the 18K ft level if necessary. Unfortunately, the clouds seldom exceeded this height. As the initial cell the Citation was directed to further dissipated, it was decided to send the Citation to the southern "hot spot". Several convective cells were ~250 km away from Kwajalein but provided returns in excess of 30 dBZ at that range. This is a strong indication of intense convection with a well defined mixed-phase precipitation layer. Indeed, a TIROS satellite image at 0415 showed vigorous convection in this area.

After passing the point for the strongest cell to the Citation, taking into account the time required for the Citation to fly there and advection/propagation of the precipitation, the DC-8 concluded the runs needed for calibration of the AMPR and headed to the same area. Together, the Citation and DC-8 surveyed the precipitation region and set up coordinated passes sampling the most intense of the convective cells. The DC-8 performed multiple bow-ties at 41000 feet and the Citation repeatedly penetrated the convection at different altitudes. For the most part, the region worked was about 220 km from Kwajalein. Data from the AMPR and ARMR showed moderately heavy rain with strong ice scattering signatures. From our radar perspective, this appeared to be the early dissipating stage of the intense convective cell. The Citation reported reflectivity maxmima of about 40 dBZ from their radar.

Communication between the ground-based and airborne scientists was minimal at the range being worked. But the DC-8 ground controller had continuous ability to pass information to the DC-8 which then relayed it to the appropriate aircraft. This coordination was crucial to the success of the mission and it was performed extremely well. As the time of the TRMM overpass neared, the Convair was directed toward the region being worked by the other aircraft. To maximize the potential of a three aircraft mission over precipitation during the TRMM overpass, the Citation and DC-8 were directed toward the west to what had been a fairly active area with evidence of multiple cell redevelopment. Unfortuately the main cell in that region dissipated without triggering new convection as the aircraft approached. By the time of the satellite overpass, only debris remained. After making two passes through the debris, the aircraft returned to base.

The short summary of this mission is that the Convair worked mostly small cumulus that produced warm rain. Echo tops did not extend above the freezing level. The Citation and DC-8 worked a moderately strong convective zone at about 220 km from Kwajalein. Coordination was handled by the aircraft and several good passes were made. This effectively extends the range at which the aircraft can work, though we still desire quantitative radar coverage whenever possible. The TRMM overpass coincided with passes through virga out of the anvil which had been produced by a deep convective cloud.

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