Figure 10. Verification of T-SHiELD 2017 and 2018 during the
2017 Atlantic hurricane season against the Best Track Dataset: intensity
(a) error and (b) bias; (c) track error; (d) 64-kt (33 m
s-1, hurricane-force) radii. Units shown (kt, nautical
miles) are standard for US operational prediction. In a–c the number of
cases (individual storms) available at each lead time is shown in
parentheses; in (d) the number in parentheses is the number of storm
quadrants available for validation.
The multiple changes in the 2018 version of T-SHiELD combined to create
tropical cyclones which are stronger overall (Figure 10a,b), with little
to no bias towards more intense storms at all lead times. There is a
minor degradation in track error in the 2018 version at longer lead
times (Figure 10c). The adoption of the PD scheme and YSU PBL scheme
likely created forecasts of more intense storms mitigated by the
introduction of the interactive mixed-layer ocean. While the weak bias
of the 2017 version was alleviated, intensity predictions were not
appreciably improved except at 120-h lead time, and in fact were
degraded between 36 and 72 hours after initialization. These results
show once again the great challenge of improving intensity prediction.
The reduction in RMWs in simulations using the PD scheme will be
discussed in more detail in a forthcoming manuscript.
3.3 C-SHiELD Nest for Continental US Convection
C-SHiELD was designed to efficiently reach convective-scale resolutions
in a global domain, in this case to replicate the capability of regional
convective-scale models for continental convection such as the 3-km NAM
Nest and the members of the
High-Resolution
Ensemble Forecast (HREF). C-SHiELD also is designed to extend
convective-scale forecasts beyond the 18-to 60-hour ranges of existing
US operational CONUS models into the medium-range timescales and beyond.
The nested domain of C-SHiELD serves as a prototype for the Regional
Forecast System (RFS; Carley et al. 2020) and the Rapid-Refresh Forecast
System (RRFS; Alexander et al. 2020), both using the regional domain
capability being developed within FV3.
The 2017 version of C-SHiELD is described in Harris et al. (2019).
Modified versions of C-SHiELD with different microphysics and PBL
schemes are described in Zhang et al. (2018) and Snook et al. (2019).
C-SHiELD 2018 saw considerable updates as shown in Table 2; C-SHiELD
2019 added incremental updates, including re-configuration of the
numerical diffusion and GFDL microphysics. We will limit our discussion
to the evolution of broad forecast characteristics, but we will perform
year-round validation instead of restricting the analysis to a single
season. The time periods evaluated are given in Table 1. The exception
is for the Surrogate Severe verification below, which is only verified
for peak severe weather season of April to August of each year.