HTML
-
Version 3.6 of WRF and its adjoint model are used, with the initial fields and boundary conditions derived from ERA-Interim data at a resolution of 0.25° × 0.25°. Experiments are set up in one single domain of 29 × 29 grid points, with grid spacing of 90 km. In addition, 31 eta levels are adopted in the vertical direction, and the parameterization schemes are microphysical (lscondscheme), planetary boundary layer (surfdragscheme), and cumulus convective (ducuscheme). These schemes are utilized because their respective adjoint schemes are available for calculating the NFSV-type tendency perturbation. We first compare the TC simulations under different horizontal resolutions; the results are illustrated in Fig. 1. It is shown that the simulated minimum SLPs under different resolutions are almost the same in experiencing a rapid drop within the first several hours and then gradually increasing, but all of them depart from the best track of the China Meteorological Administration (CMA). For different resolutions, the simulated minimum SLPs drop by approximately 20 hPa at the 24-h lead time, with the resolutions decreasing from 90 km to 30 km, whereas trivial differences occur in the minimum SLPs of TCs when the resolutions are further decreased from 30 km to 10 km. Obviously, the simulated minimum SLPs are sensitive to resolution. In the present study, the optimization algorithm used to calculate NFSV-tendency perturbation requires the sensitivity of model output to tendency perturbations, which is provided by the adjoint model, and the adjoint model is coded strictly according to the tangent linear model. However, the validity of the tangent linear model is verified to be much more acceptable when a 90-km horizontal resolution is used. Therefore, we have to use the WRF model with a 90-km horizontal resolution. Although the 90-km resolution is coarse and induces additional model errors with respect to the TC intensity forecast, it provides an opportunity for the sensitivities of NFSVs to demonstrate their applicability in reducing model errors. That is, it is investigated in the present study whether the ability to simulate TC intensity can be greatly improved using the sensitivity of the NFSVs even though the horizontal resolution of WRF is relatively coarse.
Figure 1. Simulated minimum SLP at horizontal resolutions of 90 km (black), 30 km (blue), and 10 km (green) for the TC case Dujuan. The simulation is generated by the WRF model and the best-track data (red) are from the CMA.
There are nine TC cases for investigation, all of which originated over the western North Pacific. Their basic information is detailed in Table 1. Among these TCs, three cases [i.e., Dujuan (2015), Parma (2009), and Meranti (2016)] experienced rapid intensification within 24 h, i.e., their near-surface maximum wind speed (MWS) increases by more than 15 m s−1 during this period. Another three cases [i.e., Fungwong (2014), Megi (2010), and Tembin (2012)] underwent obvious weakening during the 24 h, and the remaining three cases [i.e., Neoguri (2014), Nanmadol (2011), and Jangmi (2008)] maintained their intensity and had no obvious variation during this period. For all these nine cases, the model simulates much weaker storms than in reality, with a higher average minimum SLP of 82.5 hPa at a lead time of 24 h.
Name Start time (0 h; UTC) End time (24 h; UTC) Intensity at start time Intensity atend time Dujuan (201521) 0000 26 Sep 0000 27 Sep 965 hPa/38 m s−1 930 hPa/55 m s−1 Parma (200917) 1200 29 Sep 1200 30 Sep 994 hPa/20 m s−1 970 hPa/35 m s−1 Meranti (201614) 0000 12 Sep 0000 13 Sep 955 hPa/42 m s−1 910 hPa/65 m s−1 Neoguril (201408) 0000 8 Jul 0000 9 Jul 935 hPa/52 m s−1 966 hPa/38 m s−1 Nanmadol (201111) 1200 26 Aug 1200 27 Aug 920 hPa/60 m s−1 945 hPa/42 m s−1 Jangmi (200815) 0000 29 Sep 0000 30 Sep 970 hPa/35 m s−1 990 hPa/23 m s−1 Fungwong (201416) 0000 21 Sep 0000 22 Sep 982 hPa/28 m s−1 985 hPa/25 m s−1 Megi (201013) 0000 20 Oct 0000 21 Oct 940 hPa/52 m s−1 940 hPa/52 m s−1 Tembin (201214) 0000 26 Aug 0000 27 Aug 965 hPa/38 m s−1 975 hPa/33 m s−1 Note: The numbers (No.) and intensities are from the best-track data of the CMA, the latter of which include the minimum SLP and maximum surface wind speed at the corresponding time. Table 1. Nine TC cases used in this study.