Towards a Better Understanding of the Steering Flows Responsible for the Unusual Looping Track of Hurricane Joaquin (2015)
Abstract
Hurricane Joaquin (2015) was an unusual case because it originated as a mid-to-upper level low in the subtropical Atlantic and underwent tropical transition as it tracked southwestward into the Bahamas. This motion was climatologically rare and poorly predicted by many operational forecast models, except for the ECMWF. For example, the normally reliable GFS and UKMET models held steady with forecasting the storm to curve westward, then northwestward, and strike the mid-Atlantic coast through the 1800 UTC 30 Sep forecast cycle, when the storm was within 4 days of a potential U.S. landfall. They failed to capture the persistence of Joaquin's southwestward motion for a longer period and its eventual recurvature to the northeast away from the U.S. coast. Here, we present the results of our 1-km Weather Research and Forecasting (WRF) model prediction, which successfully simulates the full 60-h rapid intensification period and looping southwestward track. Initial conditions are generated using cycling data assimilation in which conventional, radiance, and aircraft reconnaissance mission observations are assimilated into a 3-km background forecast using the WRF Data Assimilation system (WRFDA) in hybrid 3DVAR mode. Ensemble error statistics are derived from an 80-member 9-km WRF ensemble updated using a similar set of observations and the Data Assimilation Research Testbed (DART) Ensemble Kalman Filter (EnKF). Using our model prediction, observations, and gridded global reanalyses, we examine the complex synoptic-scale steering environment surrounding Joaquin. We then identify the large-scale features that were resolved differently by the GFS model. Using a steering flow error budget equation, we find that early-cycle operational GFS model track errors resulted from 1) subtle misrepresentations in the amplitude of the complex ridge-trough pattern surrounding Joaquin and 2) an inadequately deep vortex that did not interact strongly with upper-level winds. We then confirm the sensitivity of Joaquin's track to both error sources by running WRF simulations from perturbed analyses.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.A43Q3425M
- Keywords:
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- 3315 Data assimilation;
- ATMOSPHERIC PROCESSESDE: 3360 Remote sensing;
- ATMOSPHERIC PROCESSESDE: 3372 Tropical cyclones;
- ATMOSPHERIC PROCESSESDE: 4313 Extreme events;
- NATURAL HAZARDS