Observations of Gulf of Tehuantepec gap wind events from QuikSCAT: An updated event climatology and operational model evaluation

Gap wind events in the Gulf of Tehnuantepec are the only storm-force (48 kt or greater) wind events outside of tropical cyclones in the eastern North Pacific area of responsibility of NHC's Tropical Analysis and Forecast Branch (TAFB). The availability of ocean surface vector wind (OSVW) data from the NASA QuikSCAT satellite has allowed consistent and unprecedented documentation of these events, including their frequency, duration and intensity (gale vs. storm force). A multiyear event climatology from 1999–2000 (the first cold season in which near-real time [NRT] 25-km QuikSCAT retrievals were available) through 2005–2006 shows that on average 13.4 gale-force events occur each cold season. While the 25-km NRT retrievals can detect some storm-force events, the advent of 12.5-km NRT QuikSCAT wind retrievals in 2003 resulted in an increase in the number of storm-force events detected; on average, six storm-force events per cold season. Prior to the availability of NRT QuikSCAT OSVW data, the intensities of Tehuantepec events were known only from occasional ship observations in the region. For example, from October 1999 through January 2007, QuikSCAT indicated 42 storm-force Tehuantepec events, even using only 25-km retrievals prior to 2003. Only 11 of these storm-force events (27%) would have been identified from six-hourly synoptic ship reports.

During the 2006–2007 cold season (through 20 February 2007), probability of detection (POD) of a gale-force or a storm-force event (as determined by QuikSCAT) by 10-m wind forecasts from both the NCEP Global Forecast System (GFS) and the North American Mesoscale (NAM) models was computed at 12, 24, 36 and 48 h lead times. The POD for the lowest sigma level (0.9950) winds from the GFS model was also computed, since these winds are often used by TAFB forecasters in Tehuantepec event forecasting. Results indicate that 10-m wind forecasts from the GFS and NAM are unable to predict storm-force conditions in Tehuantepec events, with average POD scores for all lead times of 0.00 and 0.05, respectively, while the lowest sigma level winds from the GFS had a slightly higher POD of 0.15. Both models show considerably more skill in detecting gale-force events, with POD values of ~0.85 for the NAM 10-m winds and ~0.50 (~0.75) for the GFS 10-m (lowest sigma level) winds. These results demonstrate the lack of reliable NWP low-level wind speed guidance available to TAFB forecasters for storm-force events. This forces TAFB forecasters to utilize pattern recognition and to interrogate model wind forecasts at levels above the boundary layer (and assume that vertical mixing will transport these winds down to the surface) to accurately forecast and warn for storm-force events.

Numerical model sensitivity tests were performed to quantify the impact of horizontal resolution and planetary boundary layer (PBL) parameterization in WRF model wind forecasts in a Tehuantepec event during the period of 16–20 January 2007. Model horizontal resolution was varied between an approximation of the grid spacing of the GFS (40 km) and that of the NAM (12 km) models, while both the Yonsei University (YSU) and Mellor-Yamada-Janjić (MYJ) PBL schemes were used. Preliminary results indicate greater sensitivity to the choice of PBL scheme than resolution in the evolution of the 10-m wind field during the event simulated. Work is underway to investigate the cause of the differing results using the two PBL schemes, including the effect of surface fluxes and sea-surface temperature on the boundary layer wind field and stability.

Finally, a preliminary evaluation of the performance of OSVW retrievals in Tehuantepec events from the passive WindSat radiometer will be presented, along with an overview of other potential sources of OSVW data for the detection of these events in the post-QuikSCAT era.