Assessing Hurricane Intensity Using Satellites - Satellite-based Applications on Climate Change - page 151

continues to be improved (Olander and Velden 2007 ). Automated center finding
algorithms are also under development (Wimmers and Velden 2010 ).
10.3 Satellite Microwave Intensity Estimation Techniques
In parallel with applications of visible and IR imagery such as the Dvorak technique,
microwave observations from LEO satellites provided important observations of
tropical cyclones in the 1970s and 1980s (Kidder et al. 2000 ). Microwave data
have two main advantages over visible and IR images: (1) microwave radiation
penetrates clouds; (2) Microwave radiation is sensitive to a wide variety of geophys-
ical parameters, such as temperature, water vapor, cloud liquid water, cloud ice water,
rain, and surface wind speed.
Microwave sensors can be divided into the two basic categories of imagers and
sounders. Similar to IR and visible imagery, microwave imagery provides informa-
tion about atmospheric and cloud properties due to the interaction with the upwelling
radiation. Sounders measure microwave radiation in a range of frequencies centered
about an atmospheric absorption band to provide vertical profiles of atmospheric
moisture. TC intensity estimation techniques from microwave sounders have gener-
ally been more successful than those from microwave imagery. These are described
first, followed by attempts to utilize microwave imagery.
10.3.1 Microwave Sounder Applications
The first operational microwave soundings were obtained from the microwave
sounding unit (MSU) on TIROS-N, beginning in 1978, after successful demonstra-
tion on NIMBUS-5 earlier that decade. Shortly after this data became available,
techniques to estimate TC intensity began to emerge (e.g., Kidder et al. 1978 ). An
advantage of sounding methods compared to the Dvorak technique is that they have
a firmer physical foundation. The minimum surface pressure near the center of a
tropical cyclone is directly related to the vertical atmospheric temperature profile
above that point through the hydrostatic equation. The minimum sea-level pressure
has a strong relationship with the maximum surface wind through the horizontal
momentum equations. To a reasonable level of approximation, these equations are
diagnostic. For example, in the case of steady circular flow that occurs in strong
tropical cyclones above the boundary layer, the wind and pressure field are related
through the gradient wind equation.
A rather severe limitation of the early TC estimation techniques from the MSU
was the 150-km footprint size of the measurements. This is much larger than the
scale of the tropical cyclone eye, so the very warm temperatures in the eye cannot
be resolved. This situation improved considerably beginning in 1998, when the
advanced microwave sounding unit (AMSU) with its improved resolution began