The sensitivity of TC intensity and structure to grid spacing in the Advanced Hurricane WRF

Forecasting tropical cyclone (TC) intensity continues to be one of the foremost challenges in the field of tropical meteorology. Rapid intensification is especially difficult to predict. One current hypothesis is that rapid intensity changes are a result of relatively poorly understood and simulated small-scale processes that occur in the storm's core. If such processes can be adequately represented, then one can expect improved TC intensity forecasts.

Most operational numerical models currently are run at grid spacings much greater than the scale of a TC's core. Therefore, they do not sufficiently resolve the complex processes in and near the eyewall. The Weather Research and Forecasting (WRF) model can be run at the high resolutions that are needed to better resolve the core. Several recent studies using WRF have employed an innermost grid spacing of 4 km, with a few experiments using an additional nest at either 1.33 km or 1 km grid. However, there are conflicting results as to whether the additional nest with a grid spacing of ~1 km improves intensity forecasts compared to forecasts using a 3 or 4 km innermost nest.

This paper will simulate multiple storms using the Advanced Hurricane WRF (AHW) that was developed at the National Center for Atmospheric Research (NCAR). The paper will describe the differences between simulations run using 36, 12, and 4 km nested grids and simulations made with 36, 12, 4, and 1.33 km nested grids. Differences in intensity, storm track, as well as storm structure will be described. Of specific interest is whether eyewall replacement cycles, which have a large impact on short-term TC intensity fluctuations, can be resolved with the additional nest. Previous case studies indicate that there may indeed be an improvement in simulated storm structure when a 1 km nest is included. By analyzing multiple storms, this research seeks to confirm the previous results.