Vernon Francis Dvorak passed away on September 19, 2022, at the age of 93. He was an American meteorologist who developed a satellite data-based method for determining the intensity of tropical storms. It revolutionized the forecasting of their development and saved many lives over the years.
First anniversary of the death of Vernon Dvorak
On this occasion, today we want to take a look at the Dvorak technique named after him. It is based on satellite images of tropical storms in the visible and infrared range of light.
Fig. 1: Vernon Francis Dvorak (15.11.1928 bis 19.9.2022); Source: Wikipedia
In contrast to large-scale extratropical depressions , tropical cyclones are often much smaller; moreover, they form on the open ocean over warm water. Before the age of satellite observation, the predictability was limited and the warning time was short. Particular problems were caused by statements about the development of a storm – weakening or further strengthening. With the first weather satellites, forecasting of tracks began to improve, but without measurement data from buoys, ships, and aircraft, it was still difficult to predict current intensity. In the 1970s, Vernon Dvorak developed a method based on changes in the cloud structure of a hurricane that allowed statements to be made about current intensity as well as short-term development. This forecasting technique has been continuously improved and adapted for different ocean regions over the last decades. Originally, the first approaches were based on data from the Northwest Pacific(typhoons); the method was adapted accordingly for the Atlantic and East Pacific(hurricanes).
Vernon Dvorak recognized that certain cloud patterns corresponded to a certain stage of development and intensity (and thus mean maximum wind speed).
Fig. 2: Original catalog of the different cloud structures in tropical cyclones according to Dvorak.; Source: Wikipedia
The method is based on three pillars. The first is kinematics, focusing on rotation and the inclination of the axis of rotation. A symmetrically round vortex indicates a "healthy" structure, a high organization of the storm. The diameter of the rotating region gives clues to the dynamics; a small storm may intensify more rapidly than a widely extended one. And the tilt of the axis of rotation suggests vertical wind shear (increase in wind with height and/or change in direction). The ideal for a tropical storm is to have as little wind shear as possible, otherwise the development will be disrupted.
The second pillar is thermodynamic behavior, the appearance of convection at the center of the storm. Over the warm sea(at least 26 to 27 degrees), a lot of water evaporates, the moist air rises, cooling – the gaseous water vapor condenses back to liquid water. This releases a lot of energy, which is available to the storm – quasi the engine. From an initially completely disorganized thunderstorm area, a tropical depression with first rotation forms, then a tropical storm. The further stages of increase are then hurricanes, typhoons and cyclones with ever increasing rotational speed and a forming eye. The strongest convection is concentrated around the center of the storm, here the clouds are highest and at the upper edge very cold – partly top temperatures below -70 degrees are found. The symmetry and frequency of the convection is considered, whether it is pulsating or permanent "boiling up".
Fig. 3: Eye of Hurricane Isabel (2003) and the surrounding Eyewall; Source: pixabay
Fig. 4: Hurricane Dorian Eye and Eyewall (2019); Source: Wikipedia
In the eye of the cyclone, the air sinks and warms in the process (and it also becomes drier – therefore cloud-free). The temperature difference between the very cold upper edge of the eyewall and the warm eye is also a measure of the current intensity.
Finally, the third pillar is pattern recognition, where the first two aspects merge. Each pattern corresponds to a T-number, which again directly relates to the current wind speed. T8 is the highest level and is rarely reached. An example of this is Super Typhoon Haiyan from 2013.
Fig. 5: Infrared image of Haiyan in the special black and white area of the Dvorak technique. Perfect symmetry with closed and very wide convection, strong temperature contrast to the warm eye.; Source: Wikipedia
As mentioned above, the method had to be adapted and adjusted accordingly for the different ocean areas. The reason for this is that the large-scale air pressure in the Pacific Northwest is generally lower on average than in the Atlantic, for example.
Fig. 6: Dvorak T-numbers with appropriate intensity; Source: Wikipedia
In the eastern Pacific and Atlantic, Hurricane Hunter (USA) measurement flights provide real data that are enormously important for short-term forecasting and high-resolution computer models. However, these flights are costly and are not available everywhere in the world, and there is also a limit to the range of the aircraft. Thus, over the open Atlantic, Pacific or Indian Oceans, Dvorak technology provides a critical tool for assessing the current situation. Satellite data have improved massively in the last decades, and the method found by Vernon Dvorak in the 1970s has also benefited accordingly. This article is intended to pay tribute to him accordingly!
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