Advancing Tropical Cyclone Forecasts Using Aircraft Observations - Monitoring and Prediction of Tropical Cyclones in the Indian Ocean and Climate Change - page 172

These data also became the standard for surface wind estimates in TCs. To
take advantage of these developments and to insure that high-quality
measurements could be received in real-time by NHC, AOML/HRD scientists
and programmers developed on-board quality control software for the
dropsondes and studied the most efficient targeting of them (Aberson, 2003).
3.2 Expendable Ocean Probes
Expendable oceanographic instruments used in TC research can be divided
into the following three categories: profilers, drifters and floats . Snapshots of
the three-dimensional upper-ocean thermal, salinity and current structure under
a TC are measured with airborne expendable bathythermographs (AXBT),
current profilers (AXCP), and conductivity-temperature-depth sensors
(AXCTD) deployed one of the NOAA aircraft. Each instrument deploys a
sensor that descends from a surface float at a rate of 4-5 m s -1 and measures the
water temperature, conductivity, and current induced electric fields between
two horizontally separated sensors spaced about 5 cm apart (Sanford et al.,
1982). The electromagnetic current measuring technique acquires observations
of the baroclinic currents at intervals of 0.3 m relative to an unknown reference
velocity. Flight patterns are designed (Jaimes and Shay, 2009) to sample
mesoscale ocean features such as warm and cold core eddies that a TC may
move over (e.g., Fig. 4).
Figure 5 shows drawings of some drifters and floats, and a schematic of
their operation. Drifters aim to follow the ocean current at 15-m depth while
measuring both near-surface atmospheric and upper-ocean properties. A small
surface float supports a much larger drogue centered at 15-m depth. The large
drogue causes the drifter to nearly follow the horizontal water motion at
approximately 15 m. A transmitter in the surface drifter sends data to the Argos
satellite system. The same signals are used to track the drifter. The standard
drifter measurements are position and near-surface temperature. Minimet
drifters are also designed to estimate wind speed using the sound level at 8
kHz (Nystuen and Selsor, 1997) and wind direction using a vane on the surface
float. Evaluation of the accuracy of this approach at hurricane wind speeds is
still under way. ADOS drifters additionally measure the temperature profile to
100-m depth.
The three varieties of floats are shown in Fig. 5. All floats operate by
mechanically changing their volume, and thus their density, in order to control
their depth. By making themselves light, they can profile to the surface, thereby
extending an antenna out of the water, enabling them to obtain a GPS fix and
relay data to and receive instructions from their shore-based operators. The
EM-APEX floats (Fig. 5, green lines) operated as profilers, continuously cycling
while measuring temperature, salinity, and velocity. Profiles are extended from
the surface to 200 m with profiles to 500 m every half inertial period. During
the storm, the top of the profiles terminated at 50 m. The Lagrangian floats
(D'Asaro, 2003) profiled only before and after the storm (Fig. 5, black line).