A Blast to Tropical Cyclone Intensity Forecasts Past (Photo Credit: NOAA Hurricane Research Division)

Discussion:  An important technique to estimate the strength of tropical cyclones is known as the Dvorak technique.  The Dvorak technique, developed by Vernon Dvorak is a satellite-derived system to estimate storm strength solely based on visible and infrared satellite imagery.   The system uses cloud patterns visualized through satellite imagery and can then be used to indirectly measure wind speed and central pressure.  Today the Dvorak method is used mainly as verification to measure wind and pressure.  This important technique is even used by the National Hurricane Center.  What makes this system useful and accurate is because storms with approximately similar intensities were found to contain similar cloud pattern identified on satellite imagery.

The Dvorak technique uses patterns such as a curved band, shear pattern, central dense overcast pattern, central cold cover pattern, and eye pattern.  Once the current satellite image of a storm is compared to the list of possible patterns, an assigned T-number is given.  Smaller T-numbers are associated with weaker storms such as tropical depressions (< T-2.5), while larger numbers are associated with tropical storms (T>2.5).  Using the central dense overcast pattern, a tropical storm is identified by a T-number of 2.5 with an estimated 35 knot wind.  The starting point for a hurricane is T-4.0 with an estimated wind speed of 65 knots.  If a storm contains a visible eye, then the difference between the eye of the storm and the cloud tops around the eye is used to indicate current intensity (CI).  A larger temperature difference is often associated with a stronger storm because colder cloud tops are associated with stronger convection in the eye wall of a hurricane.

The Dvorak technique is useful when routine aircraft surveillance is not an option such as with storms in the eastern Atlantic, and also provides a more complete intensity history.  Another Dvorak technique known as the Objective Dvorak Technique (ODT) was created and uses a computer to determine the T and CI numbers, rather than a human derived method.  This technique is one of many that meteorologists use to track tropical cyclones to keep the public safe and informed. 

To learn more about other neat observational stories both directly and indirectly related to atmospheric science, be sure to click here!

©2018 Meteorologist Allan Diegan