Objective Tropical Cyclone Intensity Estimation from Satellite Images ...

The Third International Workshop on Climate Informatics
National Center for Atmospheric Research, Boulder, Colorado, September 26-27, 2013
https://www2.image.ucar.edu/event/ci2013
Objective Tropical Cyclone Intensity Estimation from Satellite Images using
Data Mining Techniques
Gholamreza Fetanat and Abdollah Homaifar
Electrical and Computer Engineering Department, North Carolina A&T State University
Kenneth R. Knapp
NOAA's National Climatic Data Center
1 Introduction
Tropical cyclones (TCs) are a significant threat to life and property. Developing and improving objective
techniques to estimate a TC’s intensity remain a challenge. The Dvorak technique (DT) is the state-ofthe-art
method that has been used over three decades for estimating the intensity of TCs. Based on the
success of the DT, the objective Dvorak technique (ODT), the advanced objective Dvorak technique
(AODT) and the advanced Dvorak technique (ADT) were derived, which used computer based analysis to
provide an objective estimate of TC intensity. Unlike the ODT and AODT, whose focuses were to mimic
the subjective technique, the ADT concentrates on extending the method beyond the original application
and constraints (see [1] for a complete review).
In light of the historical use of the various Dvorak techniques, investigation of other approaches
to help increase the accuracy and precision of automated estimation of TCs’ intensities is needed. A
recently developed method, called the deviation angle variance (DAV) technique [2], uses the gradient of
the brightness temperature (BT) field to determine the level of symmetry of the TC’s cloud structure
which correlates with the intensity of the TC.
The algorithm describes herein estimates TC intensity from the intensities of TCs that are
analogous to it. Analogous storms are determined from BT profiles at the current time as well as during
recent development [3, 4]. This new technique – the Feature Analogs in Satellite Imagery (FASI)
technique – is inspired by the availability of satellite imagery for TCs from the HURSAT dataset [5]. Our
goal is to develop a new objective technique for estimating the intensity of TCs using historical satellite
imagery centered on TCs.
2 Technical Approach and Results
Data mining has attracted a great deal of attention in the information industry due to availability of large
amounts of data and the urgent need to extract useful knowledge from such data. This study applies the
techniques that are routinely used in data mining. The process for estimating intensity can be expressed
as functions in time and space:
. The intensity ( ) is estimated from spatial
analysis of satellite image g ( x,
y)
that is constrained in time ( t ) by the other function ( ). For example,
the Dvorak technique is the estimate of a final T-number based on spatial analysis (e.g., the g function),
which is then constrained in time and by other rules (e.g., the f function). This study focused solely on
the analysis of satellite imagery ( g ). Further constraints on time are left to future endeavors.
We develop a new technique for estimating intensity of TCs from feature analogs in satellite
imagery. The technique uses a TC’s center location (provided by HURSAT-B1 data [5]), and the mean
and standard deviation of satellite BTs in 14 azimuthal rings. This information from a current image as
well as those from the preceding 6, 12 and 24 hours are used as predictors to estimate TC intensity.
Instead of regression techniques, the intensities of 10 closest analogs (determined using a K-nearestneighbor
algorithm) are averaged to estimate the intensity.
The FASI technique is trained and validated using intensity data from aircraft reconnaissance in
the North Atlantic Ocean, where the data were restricted to include storms that are over water and south
INT f ( g(
x,
y),
t)
INT
f

of 45˚N. This subset comprised 2016 observations from 165 storms during 1988 – 2006. Several tests
are done to statistically justify the design of the algorithm using n-fold cross-validation. The resulting
averaged mean absolute error is 10.9 kt (50% of points are within 10 kt) or 8.4 mb (50% of points are
within 8 mb); its accuracy is on par with other objective techniques.
3 Conclusion and Future work
Further improvements of FASI are also possible. Given the global coverage of HURSAT, it is possible
that a training set could be constructed for the Western North Pacific during the period of aircraft
reconnaissance (1980-1987). Such an approach would be unique to FASI since other objective
algorithms mentioned here have been developed over the North Atlantic. This approach will also allow a
climatological application: a global estimate of TC activity based on FASI that could be validated in two
separate basins.
Temporal constraints on changes in intensity could also be applied to the algorithm in the future.
The FASI algorithm has no dependency upon time except that which is implied by the use of features
from previous satellite images (6, 12, and 24 hours prior). Other objective algorithms use temporal
constraints on intensity changes (e.g., the ADT and Subjective DT). Including similar constraints in FASI
would likely decrease some of the random error reported herein.
Another application of the FASI technique would be to include it in a satellite consensus
(SatCon) technique, which incorporates strengths and weaknesses of several objective estimates to
compute an intensity estimate that is better than the any of its individual parts [6]. The FASI technique
may well provide independent information for such an approach.
Finally, the FASI technique can be combined with an algorithm to detect the TC center of
circulation for full automation of the technique. But again, this would require some further analysis of the
center location algorithms performance using HURSAT data.
In short, the FASI technique provides a unique approach to estimate the intensity of tropical
cyclones. The error statistics of the technique are on par with other algorithms. This approach has the
potential to provide global TC intensity estimates that could augment intensity estimates made by other
objective techniques.
Acknowledgements This work was supported by the Expeditions in Computing Program of the
National Science Foundation under award CCF-1029731. Special thanks are given to Jim Kossin and
Carl Schreck who provided valuable insight during the development of this work. The opinions
expressed in this paper are those of the authors. They do not necessarily reflect the official views or
policies of NSF, NOAA, Department of Commerce, or the US Government.
References
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