Figure 8 . Global 2-m temperature (deg K) bias (top) and RMSE
(bottom) for 13-km SHiELD (orange) compared to contemporary GFS (blue),
both validated against ERA5 Reanalysis (Hersbach et al. 2020).
3.2 T-SHiELD North Atlantic Nest for Tropical Cyclone Prediction
T-SHiELD uses the variable-resolution capabilities of FV3 to replicate
the tropical cyclone track skill of global models and the intensity
skill of convective-scale regional hurricane models. This configuration
uses the 13-km SHiELD grid and then places a large factor-of-four
two-way nest over the tropical North Atlantic (Figure 1). The resulting
nested domain has grid cells of about 3-km width and interacts with its
parent global domain. Earlier experiments and a comprehensive evaluation
of T-SHiELD 2017 were described in Hazelton et al. (2018a, 2018b).
T-SHiELD has been used as the initial prototype for the Hurricane
Analysis and Forecast System (HAFS; Hazelton et al., 2020). Here we will
describe further evolution of T-SHiELD, including progress towards
rectifying two forecast issues in T-SHiELD 2017: an
under-intensification bias for rapidly intensifying storms, and storms
with a radius of maximum winds (RMW) that is too large. Note that there
is no 2019 version of T-SHiELD.
Hazelton et al. (2018b) found that the RMW in T-SHiELD 2017 was often
larger than observed and in particular larger than that in HWRF
simulations from the same set of cases. Zhang et al. (2015) found that
reducing the parameterized mixing in the PBL scheme reduced the size of
the RMW in HWRF. While reducing the parameterized mixing in the hybrid
EDMF scheme gave modest improvement to hurricane structure in T-SHiELD,
there was no appreciable reduction in the size of the eyewall. A
dramatic and immediate impact was instead found by using the
positive-definite (PD) advection scheme for water vapor and
microphysical tracers. Results from T-SHiELD 2018 simulations of Major
Hurricane Irma, initialized prior to its rapid intensification, show
that a simulation using the older monotonic advection scheme (Figure 9)
produces a gradually expanding vortex that does not intensify.
Meanwhile, the simulation with the new PD scheme and no other
changes to the physics or dynamics, including advection of dynamical
quantities, produces an intensifying storm with a contracting eyewall.
Notably, the vertical velocity within the eyewall is much more coherent
with the PD scheme and is continually displaced within the eyewall,
which we suspect may be driving both the intensification of Irma and a
continued contraction of the eye, as well as contributing to enhanced
precipitation within the eyewall. For this reason, the positive-definite
advection scheme was selected for T-SHiELD 2018.