Growth and decay in surface precipitation

In Lagrangian frame of reference, accuracy of rainfall systems predicted by nowcasting algorithms can be improved through incorporating the growth and decay of the rainfall systems. The scale dependence of predictability of growth and decay of continental scale precipitating systems is studied with the help of the U.S. National radar composites. The growth and decay of precipitating systems is estimated in a time interval τ by correcting the precipitation image for advection and rotation at time t+τ with respect to precipitation image at time t and then subtracting the former from later. Rather than using an optimised advection algoriths such as the Variational Echo Traking, here system motion is defined by a solid rotation and translation i.e., the motion and rotation of the system is adjusted using a single translation vector and a single rotation angle.

Growth and decay is defined as the mismatch in precipitation patterns after taking into account its translation and rotation. Results show that the two dimensional correlation of growth and decay has elliptical structure indicating that growth and decay is non isotropic. The probability distribution function of precipitation intensities (in dBZ) of growth and decay is Gaussian. The scale dependence analysis for the continental scale systems indicates that for scales larger than 100 km, growth and decay has some predictability for a lead-time of more than 1 h.