Advancing Tropical Cyclone Forecasts Using Aircraft Observations - Monitoring and Prediction of Tropical Cyclones in the Indian Ocean and Climate Change - page 175

for spray-induced evaporational cooling and adiabatic expansion. Black and
Holland (1995) and Wroe and Barnes (2003) confirmed these observations
and worked out the energetics within rainbands and near the eyewall. They
also confirmed the result of Gamache et al. (1993) that unsaturated convective
downdrafts near and under the eyewall may dry and evaporatively cool the air
near the surface.
4. Stepped Frequency Microwave Radiometer (SFMR)
Accurate specification of 1-min maximum surface wind speed and its location
and the distribution of surface gale (18 m s -1 ) and hurricane (33 m s -1 ) force
winds are essential for operational National Hurricane Centre (NHC) intensity
and structure forecasts, applications such as storm surge modelling (Jelesnianski
et al., 1992) and insured loss estimation models (Malmquist and Michaels,
2000). The maximum 1-minute surface wind speed is the official NHC intensity
estimate and determines to a large degree the extent of emergency preparations
and evacuations needed for each TC landfall.
A new suite of airborne remote-sensing instruments is available on the
NOAA P-3 aircraft for the purpose of measuring surface winds in and around
tropical cyclones. The major new instrument is the SFMR first tested in
Hurricane Allen in 1980, with the first real-time retrieval of winds on board
the aircraft in Hurricane Earl in 1985, and the first operational transmission of
winds to NHC in Hurricane Dennis in 1999 (Uhlhorn and Black, 2003). SFMR
measures brightness temperatures ( T B ) at six frequencies that are used to estimate
the emissivity of the ocean surface, derive estimates of the wind speed, wind
stress, rain rate, and to calculate two-way attenuation due to precipitation (e.g.,
Fig. 6). The SFMR infers sea surface wind speed from the increased T B due to
foam coverage, which is a function of the local energy input from the wind,
and perhaps the local wave environment.
Surface wind speeds in hurricanes derived from SFMR measurement of
microwave emissions from the wind-driven sea surface are well correlated
with in situ surface wind measurements by GPS dropwindsonde. SFMR wind
speeds are compared with independent measurements from Global Positioning
System (GPS) dropwindsondes from the 1998, 1999, and 2001 hurricane
seasons. SFMR yields wind speed measurements that are overall within ~4 m
s -1 rms of the dropwindsonde-estimated surface wind, and within ~5 m s -1 of
the direct 10-m wind speed measurement. Hence, the SFMR provides
independent estimates of surface winds at a horizontal resolution of ~10 s (1.5
km) along the flight track, suitable for correct location and estimation of
maximum sustained wind speeds, as well as high-resolution mapping of the
wind field. Currently the SFMR is the only reliable passive microwave
instrument for measuring surface winds >20 m s -1 .
The SFMR surface wind estimates are operationally available to the
hurricane specialists at NHC for use in forecast guidance, and in determining