This form was first hinted when protecting Nami from Enel by eating his lightning. The second time was before the Straw Hats saved Camie and Hacchan. It was first shown during the fight with Kuma when he got angry at Kuma for attacking them. After the Straw Hat separation he can activate this form by increasing his flames to create a dirty thunderstorm. This form increases his speed and power by using the electricity within his body and also electricity outside his body to attack people with faster and stronger attacks. After the Timeskip he can use this form without having to absorb any outside source of lightning.
Karyu no Dendo Todoroki (火竜の咆哮,Karyū no Dendo Todoroki?, literally meaning Electric Roar of the Fire Dragon)}: This is a trademark technique of Riku that is used as the first move in everyone of his fights. In this move Riku releases large beam of fire that burns everything in its path. The first thing that Riku does is that he gathers a large amount of fire in his body that later releases in few seconds. After gathering the fire and lightning he then blows big beam of fire and electicity that can burn and shock everything in its path. Riku has shown true strength of this fruit that he is even capable of damaging Pacifista's metallic body. In both Viz Manga, and the FUNimation dub it's called Electric Roar of the Fire Dragon.
Karyū no Dendo Kobushi (火竜の拳,Karyū no Dendi Kobushi?, literally meaning Electric Fist of the Fire Dragon): This is another trademark technique used by Riku. He first creates fire around his fist(s) and by concentrating a little more the fire around his fist(s) becomes hotter and bigger. When Riku hits his opponent aside from burning the place where he hit, he also causes great physical pain on opponent's body. This move can be used with both fist's or only one, with two fist's Riku can deal multiple attacks and with only one fist he can cause greater damage. In both Viz Manga, and the FUNimation dub it's called Electric Fist of the Fire Dragon.
Karyū no Dendo Tatsumaki (火竜の鉤爪,Karyū no Dendo Tatsumaki?, literally meaning Fire Dragon's Electric Tornado): After having entered Dirty Thunder Mode, the user engulfs one hand with lightning and the other with the flames. Then, by swinging their arms around, the user extends such elements in length, creating a vortex of fire and lightning, which greatly damages the opponent and pushes them away from the user with great force. In both Viz Manga, and the FUNimation dub it's called Electric Vortex of the Fire Dragon