One of the utilities installed on the satellite image processing equipment at the Joint Typhoon Waring Center (JTWC), Guam, is an automated routine for computing Dvorak "T" numbers for TCs that possess eyes. The routine adapts the rules of the Dvorak technique as subjectively applied to enhanced infrared imagery in order to arrive at an objective T number, or "Digital Dvorak" T number (referred to as DD numbers). In the western North Pacific, infrared imagery is available hourly from the GMS satellite, and hourly DD numbers were calculated for all of the typhoons of 1996 and for some of the more intense typhoons of 1995 and 1997.
The evolution of the DD numbers for several of the more intense typhoons which occurred during 1995, 1996 and 1997, has revealed some characteristic changes that occur during the lifetimes of many of these TCs. In many cases, large fluctuations in the time series of the DD numbers can be linked to the changes in the structure of the eye -- the formation of concentric wall clouds being the most common. When compared with the best-track data, some common features of the DD numbers emerge: during the intensifying phase of the TC, the DD numbers tend to rise more rapidly and peak earlier than the operationally determined T numbers from conventional subjective Dvorak analysis.
The characteristics of the time series of the DD numbers implies short-term fluctuations in the convective behavior of TCs. These fluctuations suggest that there may be corresponding rapid and large fluctuations in the TC's intensity. If real, this behavior has major ramifications for operational warning quality and for intensity research (which depends largely on best-track data for its intensity input and for its source of validation data). This also questions our knowledge of the rates of TC spin-up and spin-down in relation to the convective fluctuations. An exploration of the behavior of the DD numbers may lay the groundwork for future modifications to current methods of estimating TC intensity from satellite imagery.