Decomposing Forcing Mechanisms and Impacts of Simulated Diurnal Pulses

Diurnal pulses, which can be cooling pulses (CPs) or warming pulses (WPs), depending on their infrared brightness temperature trend, are ubiquitous within tropical cyclones (TCs) worldwide. Recent research indicates that CPs are often coupled with convective rainbands in nature. Analysis of simulations of historic TCs corroborates this link between CPs and hazardous weather, while noting that CPs can also be restricted to the cirrus canopy without a connection to sensible weather at the surface. Precise generation mechanisms and impacts of CPs on TC structure and intensity, however, are less understood.

The current study will analyze global simulations of Hurricane Dorian (2019) with the Model for Prediction Across Scales (MPAS) to improve the understanding of CP generation and impact. Only CPs that occur at least five days following initialization will be considered for analysis to remove the effect of model spin-up. First, to test CP impact on TC intensity and structure, several variables will be calculated before and after CP onset (e.g., maximum 10-m wind speed, radius of maximum wind, cloud shield area). Second, since previous studies and preliminary results using the Weather Research and Forecast Model (WRF) identify pulses in simulations with no solar radiation (i.e., night only), the current study seeks to corroborate these results with MPAS night-only and day-only simulations. We hypothesize that without boundary conditions adding external forcing to WRF simulations multiple times a day, our night-only and day-only MPAS simulations will not show diurnal pulses occurring “on-the-clock.” They may, however, feature sporadic pulses “off-the-clock,” which could provide additional insight into how pulses manifest in the first place.