Performance of Basin-scale HWRF within the Marsupial Paradigm

Since 2008, the Montgomery Research Group (MRG) at the Naval Postgraduate School (NPS) has employed the overarching framework of the marsupial paradigm for tropical cyclogenesis to track a wide range of tropical disturbances, termed “wave-pouches” using a suite of operational global weather forecast models. In 2011, the Hurricane Research Division (HRD) added their experimental Basin-scale HWRF to this multi-model ensemble. In order to provide guidance to users and model developers for future model modifications, we assess the Atlantic 2011-2014 seasons using the 2013 version of Basin-scale HWRF, focusing on the model's representation of dynamical processes that lead to tropical cyclone formation while also highlighting a few recurring model error characteristics. The MRG provides dynamical model forecasts of co-moving streamlines overlaid on relative vertical vorticity, Okubo-Weiss (OW), relative humidity, and two variations of vertical shear. Basin-scale HWRF values of these five variables are averaged over a 3x3 degree box centered on the pouch at the tracked level, providing a relatively simple diagnostic tool for the likelihood of genesis and further development.

An initial assessment of 26 pouches during 30 August – 31 October 2014 with a total of 150 forecast cycles includes 11 cases for developed systems at the time of the analysis. This small subset suggests that Basin-scale HWRF depicts minimal developed systems having an OW value of 9x10-9 s-2 and relative humidity over 70%. Of the 150 total forecasts, 110 were for pouches that never developed into TDs. Using the OW threshold of 9x10-9 s-2 for tropical depression formation in Basin-scale HWRF, 27 of the 110 forecasts had high values of OW at some point (up to 120 hours), resulting in a 25% false alarm rate. Subjective assessments suggest that Basin-scale HWRF has a tendency to produce storms in the tropics that are too strong in the OW field, often quickly over-developing small vortical hot tower-like features and merging small-scale circulations, especially in the eastern Atlantic ITCZ. In addition, the model appears to underforecast the strength of systems over land as well as developed and/or recurving systems in the western Atlantic. The MRG has developed also new Lagrangian analyses, including the time integral of an eigenvalue of the velocity gradient tensor taken along particle trajectories (Lagrangian OW), and advected equivalent potential temperature. These quantities are together used to diagnose the formation and robustness of Lagrangian pouch boundaries and offer an explanation for some of the false alarms as well as an additional comparison between forecasts and verifying analyses. The genesis products may provide dynamic insight for the model development direction.