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The Dvorak technique estimates of tropical cyclone intensity from satellite images. Vern Dvorak developed the scheme using a pattern recognition decision tree (Dvorak 1975, 1984). Utilizing the current satellite image of a tropical cyclone, one matches the image versus a number of possible pattern types: Curved band Pattern, Shear Pattern, Eye Pattern, Central Dense Overcast (CDO) Pattern, Embedded Center Pattern or Central Cold Cover Pattern. If infrared satellite imagery is available for Eye Patterns (generally the pattern seen for hurricanes, severe tropical cyclones and typhoons), then the scheme utilizes the difference between the temperature of the warm eye and the surrounding cold cloud tops. The larger the difference, the more intense the tropical cyclone is estimated to be. From this one gets a "T-number" and a "Current Intensity (CI) Number". CI numbers have been calibrated against aircraft measurements of tropical cyclones in the Northwest Pacific and Atlantic basins. On average, the CI numbers correspond to the following intensities
Remember, the winds are really determined by the pressure gradient not the central pressure. Small tropical cyclones (like Andrew in 1992) can have stronger winds for a given central pressure than a larger stormse with the same central pressure. The lower pressuress of the Northwest Pacific tropical cyclones results from the difference in the background climatology. Some valuable links: http://www.tpub.com/weather2/10-15.htm http://www.ssd.noaa.gov/PS/TROP/dvorak.html http://www.tropicalupdate.com/atlantic_tropic_watch_guide_to_d.htm References:NOAA web page: http://www.aoml.noaa.gov/hrd/tcfaq/H1.html Dvorak, V.F., 1975: "Tropical cyclone intensity analysis and forecasting from satellite imagery" Mon. Wea. Rev., 103, pp.420-430 Dvorak, V.F., 1984: "Tropical cyclone intensity analysis using satellite data" NOAA Tech. Rep. NESDIS 11, 47pp Return to Lesson 12 |