The Dvorak technique

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.

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).

Cloud patterns

Vernon Dvorak recognized that certain cloud patterns corresponded to a certain stage of development and intensity (and thus mean maximum wind speed).

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".

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.

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.

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!