Assessing Hurricane Intensity Using Satellites - Satellite-based Applications on Climate Change - page 147

The question of whether the hurricane climate is changing is complicated by two
main factors, and recent observational and modeling studies have sometimes pro-
duced conflicting results (Knutson et al. 2010 ). First, tropical cyclone (TC) activity in
the Atlantic (and other regions as well) undergoes significant natural variability on
annual and multi-decadal time scales (Kossin et al. 2010 ). Second, the observing
systems for measuring TC activity have varied extensively in the last century
(Landsea et al. 2010 ). Before the mid-1940s, the primary source for TC information
was ship reports. Aircraft reconnaissance became available in the mid-1940s, low-
earth orbiting (LEO) satellites in the mid 1960s, and operational geostationary
satellites in the 1970s. In addition, the instrumentation from aircraft reconnaissance
has varied considerably, and this data was routinely available only for the Atlantic
and western North Pacific through 1987. After 1987, west Pacific TC reconnaissance
was discontinued. Thus, it is not always obvious whether long-term TC increases are
physical or are due to improvements in the ability to monitor them.
Because TCs spend most of their lifetime over the tropical and subtropical oceans
and the limited availability of in situ and aircraft observations, satellite data is
fundamentally important for the analysis and forecasting of TCs. In fact, just a few
years after the launch of the first meteorological satellite (TIROS-1) in 1960, methods
began to be developed to estimate TC intensity from satellite imagery (Hubert and
Timchalk 1969 ). These early attempts were not completely satisfactory, but about a
decade later the very successful Dvorak method was developed (Dvorak 1975 ). In this
chapter, the use of satellite data for estimating TC intensity is described. Satellite data
also have many other TC applications, including position and structure analysis and
atmosphere and ocean numerical forecast model initialization, but these topics are
beyond the scope of this chapter. Accurate TC intensity estimation is important for
both short-term forecasting and for monitoring changes in global TC activity.
This chapter begins with a description of the Dvorak intensity estimation
technique, which is still a cornerstone of operational TC analysis around the
globe. The intensity of a TC is quantified as the maximum sustained surface wind
speed associated with the storm. Another indicator of TC intensity is the minimum
sea-level pressure near its center. Newer methods that make use of passive micro-
wave sensors are also presented. General methods for satellite wind estimation and
their application to TCs are also briefly described. This chapter concludes with a
summary of how forecasters combine TC information from many sources and a
look toward future satellite capabilities.
10.2 The Dvorak Tropical Cyclone Intensity Estimation
Method
The Dvorak technique estimates tropical cyclone intensity using satellite imagery.
It was one of the first innovative applications of meteorological satellite imagery,
and it is still widely used today at tropical cyclone forecast centers throughout the
world (Velden et al. 2006 ). The Dvorak technique was developed in the early 1970s
by Vernon Dvorak and his colleagues at NOAA NESDIS.