Representation of Tropical Cyclones in the Model for Prediction Across Scales (MPAS)

Tropical cyclones (TCs) impact society in many direct ways, and play a crucial role in Earth’s energy and water cycles. Recurving TCs, and those undergoing extratropical transition (ET), are especially important in the North Atlantic and western North Pacific basins as these systems can impact major population centers outside the tropics or sub-tropics, as Irene (2011) and Sandy (2012) have recently demonstrated in the US. Further, outflow from recurving or transitioning TCs can lead to the formation or amplification of midlatitude Rossby wave trains, which can be associated with impactful weather, such as cold-air outbreaks or heavy precipitation far removed from the location of the TC itself.  Extensive previous work has investigated TC changes with climate warming; however, fewer studies have examined how climate change would affect the process, frequency, and intensity of TC extratropical transition.

Before examining how the intensity and frequency of recurving and extratropically transitioning TCs will be affected by climate change, the feasibility of representing these phenomena with numerical models, both individually and on the climatological scale, must be explored. Here, multi-seasonal simulations (April through December) are conducted using the Model for Prediction Across Scales (MPAS) with 60-km horizontal grid spacing. While it is understood that this resolution is too coarse to represent TC intensity, it allows a verification study of some aspects of climatological TC behavior, including regions of genesis, frequency, and ET locations. These simulations span 5 years representing a variety of environments (e.g., a strong El Niño, a strong La Niña, an anomalously active year, an anomalously inactive year, etc.). A tropical cyclone detection algorithm is then applied to the model output to gather information on model simulated TC density, genesis density, and ET density. These results are verified against observations (e.g., the IBTrACS dataset) and model analyses (e.g., 0.5º GFS-ANL) to determine if MPAS provides an accurate representation of climatological TC activity.

Additionally, a case study of Typhoon Nuri is conducted to investigate how MPAS represents the track, intensity, and extratropical transition of a particular tropical cyclone. Typhoon Nuri began as a tropical depression on 30 October 2014 off the coast of the Philippines, reached a peak intensity of 910 hPa on 2 November, and dissipated on 7 November after transitioning to an extratropical cyclone. Simulations of Typhoon Nuri are conducted with MPAS using a global horizontal grid with uniform 60-km grid spacing as well as a grid varying from 15-km grid spacing over the western North Pacific Ocean to 60-km over the remainder of the globe. Time and resources permitting, a simulation will be conducted using a grid with 3-km grid spacing over the western North Pacific expanding out to 15-km elsewhere. Results from these simulations are compared to a simulation of Typhoon Nuri using Global Weather Research and Forecasting Model (GWRF) and reanalyses for verification.