4 Discoveries About Tropical Cyclones Provided by Microwave Remote Sensing
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simultaneously on three satellites. In the section on scatterometry below we discuss
the merging of the wind speed data from microwave radiometers with the vector
wind data derived from scatterometers.
4.2.2 TRMM Rain Radar and Radiometers
The advent of a rain radar in space on TRMM, even though it had only a 215 km
wide swath, allowed calibration of the estimates of precipitation over the ocean
obtained from the microwave radiometers. The TRMM rain radar was calibrated
against coastal radars, which in turn have been calibrated with arrays of rain gauges
in their vicinity, so this has been a great step forward. TRMM is also in a lower
non-sun-synchronous Earth orbit only viewing 45 ◦ on either side of the equator,
thereby crossing the swaths of the microwave radiometers. It also carries its own
overlapping radiometer, the TRMM Microwave Imager (TMI). Figure 4.6 below
(in the section on SAR), illustrates the results of the precipitation algorithm from
TRMM data in conjunction with a SAR image. These high quality precipitation
estimates are valuable for interpretation of signals from other spaceborne sensors,
such as the SAR.
A wonderful application of TMI estimates of precipitation due to TC’s in all
ocean basins was provided by Lonfat et al. (2004). They produced a climatology of
the rainfall in the path of tropical cyclones at various stages of development (categories 1–5 on the Saffir-Simpson scale) and for various radii of maximum wind.
The quantitative aspect of precipitation derived from the TRMM satellite’s rain algorithm allowed this climatology to be developed. It can be used to predict the amount
of rainfall due to land-falling hurricanes and be an aid in forecasting flooding.
4.2.3 Scatterometry and TCs
A scatterometer is an active system that measures wind vectors by the varied return
from the rough sea surface, when viewing the same pixel on the sea surface at different incidence angles (see Robinson, 2004, for details). Upwind, crosswind and
downwind directions give different diffuse backscatter from the gravity-capillary
waves. SEASAT carried a 3-stick Ku-band radar looking off to one side, a scatterometer named SASS. Many years were needed to fully develop algorithms and
resolve the ambiguities that are due to noise in both the surface wave field and
the electronics. It took 15 years until the European Space Agency (ESA) launched
a C-band scatterometer of similar 3-stick design on the European Remote Sensing
Satellite 1 (ERS-1), in August 1991. C-band penetrates clouds and precipitation better, but is less sensitive at low wind speeds than the Ku-band. The ERS-1 was quite
successful and was followed by ERS-2 in 1996.
NASA launched NSCAT, the NASA scatterometer on the Advanced Earth
Observation Satellite (ADEOS) a Japanese satellite in 1995. This satellite and a
later version, ADEOS 2 (2002) were short-lived. In 1999, the US launched a single
instrument SEAWINDS on the QuikSCAT satellite to compensate for the loss of
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simultaneously on three satellites. In the section on scatterometry below we discuss
the merging of the wind speed data from microwave radiometers with the vector
wind data derived from scatterometers.
4.2.2 TRMM Rain Radar and Radiometers
The advent of a rain radar in space on TRMM, even though it had only a 215 km
wide swath, allowed calibration of the estimates of precipitation over the ocean
obtained from the microwave radiometers. The TRMM rain radar was calibrated
against coastal radars, which in turn have been calibrated with arrays of rain gauges
in their vicinity, so this has been a great step forward. TRMM is also in a lower
non-sun-synchronous Earth orbit only viewing 45 ◦ on either side of the equator,
thereby crossing the swaths of the microwave radiometers. It also carries its own
overlapping radiometer, the TRMM Microwave Imager (TMI). Figure 4.6 below
(in the section on SAR), illustrates the results of the precipitation algorithm from
TRMM data in conjunction with a SAR image. These high quality precipitation
estimates are valuable for interpretation of signals from other spaceborne sensors,
such as the SAR.
A wonderful application of TMI estimates of precipitation due to TC’s in all
ocean basins was provided by Lonfat et al. (2004). They produced a climatology of
the rainfall in the path of tropical cyclones at various stages of development (categories 1–5 on the Saffir-Simpson scale) and for various radii of maximum wind.
The quantitative aspect of precipitation derived from the TRMM satellite’s rain algorithm allowed this climatology to be developed. It can be used to predict the amount
of rainfall due to land-falling hurricanes and be an aid in forecasting flooding.
4.2.3 Scatterometry and TCs
A scatterometer is an active system that measures wind vectors by the varied return
from the rough sea surface, when viewing the same pixel on the sea surface at different incidence angles (see Robinson, 2004, for details). Upwind, crosswind and
downwind directions give different diffuse backscatter from the gravity-capillary
waves. SEASAT carried a 3-stick Ku-band radar looking off to one side, a scatterometer named SASS. Many years were needed to fully develop algorithms and
resolve the ambiguities that are due to noise in both the surface wave field and
the electronics. It took 15 years until the European Space Agency (ESA) launched
a C-band scatterometer of similar 3-stick design on the European Remote Sensing
Satellite 1 (ERS-1), in August 1991. C-band penetrates clouds and precipitation better, but is less sensitive at low wind speeds than the Ku-band. The ERS-1 was quite
successful and was followed by ERS-2 in 1996.
NASA launched NSCAT, the NASA scatterometer on the Advanced Earth
Observation Satellite (ADEOS) a Japanese satellite in 1995. This satellite and a
later version, ADEOS 2 (2002) were short-lived. In 1999, the US launched a single
instrument SEAWINDS on the QuikSCAT satellite to compensate for the loss of