Action minimization algorithm for TC rapid intensification in the WRF model

Direct computer simulation of intense tropical cyclones (TCs) in climate models is limited by computational expense. Intense TCs have small-scale structures and are relatively rare, making it difficult to produce large ensembles of storms at sufficiently high resolution. Further, models often fail to capture the process of rapid intensification, which is a distinguishing feature of the most intense TCs. The problem of rapid intensification is especially important in the context of global warming, which is often postulated to in crease the frequency of intense TCs. To better leverage computational resources for the study of rapid intensification, we present an action minimization code applied to the WRF and WRFPLUS models. The algorithm adds a series of perturbations to a model trajectory over time, biasing the model toward states with some characteristic of interest (in this case, an intense TC). Each perturbation is indistinguishable from noise and consists of an adjustment to each value in several two- or three-dimensional physical fields: zonal and meridional wind, temperature, surface pressure, and geopotential.