Regional And Mesoscale Meteorology Team
Daily Satellite Discussion
Thursday August 7, 1997
The Digital IR Technique extracts two temperatures from the IR image. The Eye Temperature is the warmest pixel near the center (15.8C on this image). Warmers eyes indicate greater intensity. The Surrounding Temperaure is the warmest pixel on a circle, which measures the cold ring asssociated with the eyewall. The colder the Surrounding Temperature the greater the intensity. This measurement is done for different size circles in the size range shown by the two yellow circles on the above image. The coldest Surrounding Temperature is then used along with the Eye Temperature and a lookup table to assign an intensity. (With this image the Surrounding Temperature is -76.2C at radius of 60 km). The final answer is expressed in terms of a T-no. (7.6 for this image) which are standard units from a qualitative operational intensity assignment method often referred to as the Dvorak Technique (after Vern Dvorak who worked on its development at NESDIS in the 70s and 80s). The operational Dvorak technique typically gives intensity estimates at 6 hr intervals to the nearest 1/2 T-No. for which standard conversions to Maximum Surface Wind Speed and Minimum Sea-Level Pressure can be used. For example, a T-No. of 2.5 is a 35 kt minimal tropical storm, a T-No. of 4.0 is a minimal hurricane, and T-6.0 is a 115 kt hurricane. That conversion for this image gives 158 kt maximum surface wind speed. However experience with the digital IR results indicate that a 6-12 hr running mean best represents current intensity, which was around 7.3-7.4 for Guillermo. The operational Dvorak Technique and the 6-hr NHC advisories for Hurricane Guillermo's maximum intensity gave a T-No. of 7.0 corresponding to 140 kt (76 m/s) surface wind speed.