Three-Dimensional Tropical Cyclone Energy Exchanges: Land, Ocean, Storm-Storm, Environment

We have defined novel methods for partitioning energy within tropical cyclones (TCs) based on hurricane forecast models: volume-integrated kinetic energy (VIKE) and internal energy (VIIE). By balancing time series of these partitions with net energy inflows (including air-sea fluxes) and outflows, we find they provide unique insights on TC intensification, surface wind field expansion, and other metrics related to potential TC hazards. TC forecasts cluster based on differences in these energy balances that are related to interactions with: other TCs, land, ocean shelves, and baroclinic environmental features. We achieve these results by constructing a partial moist static energy (MSE) budget for TC forecasts from 3-hourly outputs of the Hurricane Analysis and Forecast System. This configuration allows TCs and other regions of interest to be modeled at high resolution, while interacting with one another and the environment via the fixed outer domain. Individual terms of the MSE budget for each nest are summed over grid cells with >17 m/s mean wind speed, and over vertical layers of the model from the surface to the highest level of TC vertical alignment. We analyzed real-time and retrospective forecasts for HAFS over multiple hurricane seasons, confined to cycles and forecast hours that verified reasonably well with track and intensity from National Hurricane Center (NHC) Best Track: for these cases, we find that VIKE is a skillful predictor of potential for intensification, while VIIE provides guidance on environmental interaction and transition to sub- or extra-tropical features in the model. Unlike previously published metrics such as surface integrated kinetic energy (IKE), VIKE and VIIE allow an evaluation of the volumetric evolution of a TC. This evaluation permits us to quantify the energy available for potential hazards, improving our understanding of how quickly a TC may recover from detrimental factors such as vertical wind shear, dry air intrusion, a cooling ocean surface, or interaction with land, synoptic features, or other TCs. We further partition VIKE into vertical levels (low-, mid-, and upper-troposphere), VIIE into internal energy and geopotential, air-sea enthalpy fluxes into shelf- and open-ocean partitions and within and outside radius of maximum wind, analyzing their correlations with one another. These innovations offer us a deeper understanding of processes which may further the goal of improving forecasts of TCs and their potential hazards.

Plain-language Summary: Where does the energy come from to allow hurricanes to produce broad fields of high surface winds? We know this energy is derived largely from the input of heat and moisture at the air-sea interface. What additional role is played by interaction with synoptic weather features such as baroclinic lows and continental air masses, and with other hurricanes and tropical weather systems? We approach these questions using a numerical weather model (HAFS-M), correlating changes of air-sea heat and moisture flux, kinetic energy and internal energy (heating, moistening, and lifting of air) throughout the three-dimensional hurricane; we then associate these changes with changes in surface winds (surface kinetic energy). The goal of this research is to improve the predictability of potential hazards associated with hurricanes, particularly hurricanes that make landfall one or more times.