Assessing Hurricane Intensity Using Satellites - Satellite-based Applications on Climate Change - page 152

116 118 120 122 124 126 128 130 116 118 120 122 124 126 128 130

116 118 120 122 124 126 128 130 116 118 120 122 124 126 128 130

SATELLITE: NOAA SATELLITE: NOAA 14

SENSOR: MSU Channel 3 (55GHz)
DATE/TIME: 13OCT98 1836UTC
MAX TEMP: − 40.9C

− 15

SENSOR: AMSU Channel 7 (55GHz)
DATE/TIME: 13OCT98 2326UTC
MAX TEMP: − 39.1C

−

Fig. 10.4 An illustration of the improvement in spatial resolution of the AMSU over the MSU for
Typhoon Zeb from the western North Pacific (From Kidder et al. 2000 )

observing tropical cyclones. A comparison of the resolution of the MSU and AMSU
is shown in Fig. 10.4 for Typhoon Zeb in the western North Pacific. The horizontal
footprint of AMSU-A is about 50 km near nadir.
Observations and high-resolution modeling studies of tropical cyclones show
that the warm core is a maximum near the storm center and the scale of the warm
core increases with height due to the tendency for outward sloping eyewalls. Also,
the strongest warm anomaly relative to that outside of the storm is a maximum in
the middle and upper troposphere (e.g., Hawkins and Imbembo 1976 ; Stern and
Nolan 2012 ). Based on this observed structure, two approaches have been taken to
estimate TC intensity from AMSU. In the first approach, brightness temperatures
from AMSU channels that sense the upper troposphere are used directly to estimate
the warm core (Spencer and Braswell 2001 ; Brueske and Velden 2003 ). The
characteristics of the warm core are then related to the TC intensity. In the second
approach, temperature retrieval algorithms are applied using all of the AMSU-A
channels to provide a three-dimensional temperature structure. Using an upper
boundary condition from a large-scale analysis, the hydrostatic equation is
integrated downward to provide the pressure field at each vertical level. The
pressure gradient can then be calculated, and the wind field is determined from an
appropriate approximation of the horizontal momentum equations. This method
was applied by Demuth et al. ( 2004 ), assuming radially symmetric temperature and
wind fields relative to the storm center, so that the gradient wind equation could be
used. Figure 10.5 shows an example of the temperature anomaly and wind speed
estimated by this technique. This figure shows that this retrieval method can also be
used to provide information about the horizontal and vertical structure of a storm.