I think that you can use geometrical method where ; Risetime: (10 − 90%) = Tr and Pulse duration: (50 − 50%) = Td.
The double exponential pulses happen to be very simple to treat. The effect of variation of pulse amplitude, rise time and pulse duration can easily be studied. The real pulses have a large variation in appearance, which makes it feasible to describe bands within which spectra can fall with various probability. The double exponential time function can be characterized as: f(t) = A ∗ (exp(−at) − exp(−bt))
For more informations see Wik's article : Manuel W Wik, Double Exponential Models for Comparison of Lightning, Nuclear and Electrostatic Discharge Spectra, Defence Materiel Administration,S-115, Sweden, 1985.