Page 135 - 《高原气象》2025年第3期

3 期 盛春岩等：台风摩羯（2018）影响山东的外围螺旋雨带成因研究 693
based on the Advanced Research WRF （Weather Research and Forecasting） model and its Hybrid-3DVAR
（three-dimensional variational） data assimilation system. The model adopts 12 km and 4 km one-way nested
grids， with 44 vertical layers. The initial ensemble perturbation fields are generated by using a stochastic pertur‐
bation method， and the Ensemble Transform Kalman Filter （ETKF） method is used for the bias correction of en‐
semble forecast， providing flow dependent background errors for the Hybrid-3DVAR assimilation module. Com‐
parative experiments with and without the Aircraft Meteorological Data Relay （AMDAR） data assimilation are
conducted by adopting 100% flow-dependent error covariance and by using a 45-minute assimilation time win‐
dow. The results indicate that the outer spiral rainbands are formed by the merging and development of several
linear mesoscale convective systems （MCSs）. The outer spiral rainbands exhibit distinct characteristics of the lin‐
ear MCSs with leading stratiform precipitation， i. e. ， the linear MCSs consist of several convective cells with
back-building convection. There are several stronger linear MCSs merging laterally into other linear MCSs.
Broad stratiform echoes appear in the front （eastern part） of the linear MCS in its maturity stage， and the convec‐
tion develops up to 10 km or more. There is a weak-echo transition zone between the strong convective line and
the sub-strong stratiform echo region. Short-term heavy rainfall occurs along the linear MCS at the maturity
stage. The water vapor of heavy rainfall mainly comes from the near-surface layer （below 850 hPa） around the
typhoon， and the water vapor flux convergence is mainly concentrated near the wind field convergence line. Be‐
fore convection initiation， the middle and lower levels over Shandong are thermally unstable with high tempera‐
ture and high humidity， and the wind rotates clockwise with height， which favor the development of convective
systems. As the typhoon slowly moves northward， downward intrusion of cold air appears at 500 hPa. Below 900
hPa， on the southeast of the typhoon over central Shandong there are local convergence between southwesterly
wind and southerly wind， and between southerly wind and southeasterly wind. The convergence-induced dynam‐
ic uplift triggers the release of unstable energy， stimulating several local linear MCSs. The MCSs develop north‐
ward along the steering flow. The linear MCSs merge and strengthen for several times， and finally the elongated
spiral rainbands occur. During the convection lifetime， the updrafts are noticeably stronger than the downdrafts.
At the mature stage of the convective systems， dry and cold downdrafts appear in the lower levels in the front of
the MCS. Convective systems at the heights above 600 hPa move rapidly eastward with the upper-air steering
flow， leading to the gradual weakening and dissipation of the linear MCS. Assimilation of AMDAR can improve
the typhoon track and wind field forecasts of the WRF model， as well as the dynamical triggering mechanism of
convective systems. Thus， the occurrence of spiral rainbands in the typhoon periphery could be accurately fore‐
casted. Furthermore， central Shandong is a mountainous region， so how does the topography influence the trig‐
gering and developing of convective systems？ What are the differences between typhoon outer spiral rainbands
and the main body spiral rainbands？ What are the differences between outer spiral rainbands？ These issues de‐
serve further studies.
Key words： Typhoon Yagi； outer spiral rainbands； short-term heavy rainfall； WRF numerical experiments； lin‐
ear mesoscale convective system