An Evaluation of Rossby Wave Breaking Predictability using Numerical Weather Prediction
Open Access
- Author:
- Long, Evan
- Area of Honors:
- Meteorology
- Degree:
- Bachelor of Science
- Document Type:
- Thesis
- Thesis Supervisors:
- Kevin Andrew Bowley, Thesis Supervisor
William Henry Brune, Thesis Honors Advisor - Keywords:
- Rossby Wave Breaking
GFS
Forecasting
Forecast Model
NWP
Atmospheric River
Precipitation
Rain
Synoptic
Meteorology
Weather - Abstract:
- Rossby wave breaking (RWB) is an atmospheric phenomenon that can be identified by the overturning of potential temperature (θ) contours on the dynamic tropopause (DT), which is the 2 potential vorticity unit (PVU; 1 PVU = 10−6 K kg−1 m2 s−1) surface. RWB events can be categorized as anticyclonic (AWB) or cyclonic (CWB) depending on the direction of overturning and can result in extreme precipitation events. In this study, the forecasting accuracy of AWB and CWB from November 2022 to March 2023 are examined with a specific focus on a three-week period of near-constant atmospheric river (AR) detection in California. RWB events are identified on the DT using the Global Forecast System (GFS) model for forecast hours 0 through 384 for each of the four model initializations per calendar day. The winter 2022-2023 season was found to be anomalously active for RWB detection when compared to existing climatologies, while the AR period was found to have anomalously high CWB activity in the Pacific Northwest region that likely contributed to the enhanced AR activity. Threat scores and size biases were also calculated for each forecasted RWB event associated with a verified event. It was observed that the GFS does not consistently detect RWB events at long lead times, but threat scores and size biases for those events that were detected were generally accurate. A case study from the AR period was also performed for a representative CWB event. The CWB event was found to be well-forecasted within 5 days of lead time but was poorly represented at longer lead times. It was hypothesized that the GFS overpredicted diabatic heating to the west of the CWB event region which acted to separate and isolate a region of low θ air from the polar air mass. Findings from this study suggest that RWB in winter 2022-2023 likely had a notable influence on the anomalous AR activity and the associated sensible weather extremes.