478
K.B. Katsaros
occur from September through April and occasionally develop in May and June. The horizontal
scale of these lows is of the order of a few hundred kilometers, and they have a vertical depth
of 1-5 km. Rather strong winds and snowfalls are associated with these phenomena, which
can develop in only a few hours (Lystad, 1986). One major difficulty in studying these lows is
that they develop in regions with very few meteorological stations. Therefore they sometimes
go undetected until they affect coastal regions. In addition, the mechanisms involved in the
formation, maintenance and decay of polar lows are still being discussed, i.e. low-level baroclinicity, convective forcing (CISK), or both, thermal instability, even barotropic instability (e.g.
Rasmussen, 1979; Businger and Reed, 1989; Emanuel and Rotunno, 1989). A very complete description of polar lows may be obtained by research aircraft observations as shown by Shapiro et
al. (1987) and Douglas et al. (1991). Data from polar-orbiting satellites could be of importance
for monitoring and short-range forecasting since in polar regions, frequent sampling of any mesoscale weather system can be obtained due to overlapping swaths. In addition to using visible
and infrared images from the Advanced Very High Resolution Radiometer (AVHRR) on NOAA
satellites, the study by Claud et al. (1995) describes observations from three instruments. The
Special Sensor Microwave/Imager (SSM/I) provides estimates of surface wind speed, scattering
by precipitation size ice particles, integrated cloud liquid water, and water vapor contents over
the ocean. The Geosat radar altimeter provides estimations of surface wind speed, significant
wave height and swell height. The TIROS-N Operational Vertical Sounder (TOVS) provides
temperature and humidity profiles in the atmosphere, as well as cloud parameters. Figure 17.8
illustrates the results obtainable with the SSM/I for a Polar Low that occurred off Norway on
February 15, 1989.
17.7 Tropical Cyclones
These most violent of the cyclonic storms develop exclusively over oceanic areas. Their detection
and monitoring, therefore, benefit especially from satellite observations. Since the precipitation is extreme and these storms are associated with widely spread cirrus cloud shields, the
microwaves ability to penetrate clouds provides advantages over monitoring these storms in the
visible and infrared (e.g. Quilfen et al., 1993 and 1994; Katsaros et al., 1993 and 1995). ERS-l
scatterometer and SSM/I observations have been found extremely useful in studying tropical
cyclones.
17.7.1 Scatterometer and SAR observations
An example of the surface wind fields obtained by the ERS-l scatterometer is seen in Figure
17.9. The pattern of convergence lines in the rear of the storm has been observed in numerous
tropical cyclones since the launch of ERS-l, but a statistical and physical analysis has not yet
been carried out to determine how frequent these features are, and whether they are directly
correlated to the rain bands. The SSM/I and ERS-l satellite sensors do not view such storms
frequently enough to determine physical relationships.
The 500 km swath width and the small size of tropical cyclones (typically of radius less than 500
km), prevent frequent and dependable observations of these storms by the ERS-l scatterometer.
However, many such storms have been observed by the ERS-l and an example is shown in
Figure 17.9. Analysis of the directional wave field using the SAR in wave mode, shows rapid
variations of the swell in space, and the persistence of swell, which can be traveling with or
even perpendicular to the strong winds near the tropical cyclone center (Katsaros et al., 1995
and Biraud, 1995).
K.B. Katsaros
occur from September through April and occasionally develop in May and June. The horizontal
scale of these lows is of the order of a few hundred kilometers, and they have a vertical depth
of 1-5 km. Rather strong winds and snowfalls are associated with these phenomena, which
can develop in only a few hours (Lystad, 1986). One major difficulty in studying these lows is
that they develop in regions with very few meteorological stations. Therefore they sometimes
go undetected until they affect coastal regions. In addition, the mechanisms involved in the
formation, maintenance and decay of polar lows are still being discussed, i.e. low-level baroclinicity, convective forcing (CISK), or both, thermal instability, even barotropic instability (e.g.
Rasmussen, 1979; Businger and Reed, 1989; Emanuel and Rotunno, 1989). A very complete description of polar lows may be obtained by research aircraft observations as shown by Shapiro et
al. (1987) and Douglas et al. (1991). Data from polar-orbiting satellites could be of importance
for monitoring and short-range forecasting since in polar regions, frequent sampling of any mesoscale weather system can be obtained due to overlapping swaths. In addition to using visible
and infrared images from the Advanced Very High Resolution Radiometer (AVHRR) on NOAA
satellites, the study by Claud et al. (1995) describes observations from three instruments. The
Special Sensor Microwave/Imager (SSM/I) provides estimates of surface wind speed, scattering
by precipitation size ice particles, integrated cloud liquid water, and water vapor contents over
the ocean. The Geosat radar altimeter provides estimations of surface wind speed, significant
wave height and swell height. The TIROS-N Operational Vertical Sounder (TOVS) provides
temperature and humidity profiles in the atmosphere, as well as cloud parameters. Figure 17.8
illustrates the results obtainable with the SSM/I for a Polar Low that occurred off Norway on
February 15, 1989.
17.7 Tropical Cyclones
These most violent of the cyclonic storms develop exclusively over oceanic areas. Their detection
and monitoring, therefore, benefit especially from satellite observations. Since the precipitation is extreme and these storms are associated with widely spread cirrus cloud shields, the
microwaves ability to penetrate clouds provides advantages over monitoring these storms in the
visible and infrared (e.g. Quilfen et al., 1993 and 1994; Katsaros et al., 1993 and 1995). ERS-l
scatterometer and SSM/I observations have been found extremely useful in studying tropical
cyclones.
17.7.1 Scatterometer and SAR observations
An example of the surface wind fields obtained by the ERS-l scatterometer is seen in Figure
17.9. The pattern of convergence lines in the rear of the storm has been observed in numerous
tropical cyclones since the launch of ERS-l, but a statistical and physical analysis has not yet
been carried out to determine how frequent these features are, and whether they are directly
correlated to the rain bands. The SSM/I and ERS-l satellite sensors do not view such storms
frequently enough to determine physical relationships.
The 500 km swath width and the small size of tropical cyclones (typically of radius less than 500
km), prevent frequent and dependable observations of these storms by the ERS-l scatterometer.
However, many such storms have been observed by the ERS-l and an example is shown in
Figure 17.9. Analysis of the directional wave field using the SAR in wave mode, shows rapid
variations of the swell in space, and the persistence of swell, which can be traveling with or
even perpendicular to the strong winds near the tropical cyclone center (Katsaros et al., 1995
and Biraud, 1995).