Abstract
There exists a firm relation between the rotational updraught (and/or downdraught) of a convective storm and the storm-relative environmental helicity. The already-known formula, which was derived with linear approximation and on a form-preserving assumption, describes the relation qualitatively well. In this note, a similar formula is derived without the linear approximation, although the formpreserving assumption is retained. The nonlinear terms generate the acceleration of propagation and/or the temporal variation of growth rate of the storm, which are absent from the linear formula. These nonlinear effects enter the derived formula.
