Comparative Investigation of Ensemble Forecast of Explosive Cyclone Entering Southern Yellow Sea in Spring 2018

Observations and European Centre for Medium-Range Forecast (ECMWF) reanalysis data were used to compare the ability of ensemble forecast members with a horizontal resolution of 0.5º(latitude)×0.5º(longitude) to provide dynamic and thermodynamic factors relating to a cyclone that exploded over the southern Yellow Sea in February 2018. According to validation results for the track and strength of the cyclone and surface wind, two sets of ensemble forecast members were selected as good and bad forecast members, respectively, and the main conclusions were subsequently obtained from a comparative investigation between them as follows: 1) When the cyclone had explosively developed, the trough and vortex at 500 hPa and 850 hPa, respectively, strengthened rapidly, and the southwest jet streams at lower and upper levels simultaneously increased rapidly in association with the rapid strengthening of synoptic systems; this provided a favorable condition for explosive cyclone development. 2) With the rapid strengthening of ascending motion after the cyclone had entered the Yellow Sea, convergence and divergence at lower and upper levels were respectively intensified; this promoted a pressure reduction and the cyclone then explosively developed. Following convergence of water vapor at a mid-low level, latent heat was released under the ascending motion, which promoted divergence and convergence at upper and lower levels, respectively, and then enhanced ascending motion. Therefore, the rapid strengthening of water vapor flux convergence caused the cyclone to explosively develop. The downward transmission of the high potential vorticity (PV) and baroclinicity at a lower level were strengthened, the degree of stability weakened, and cyclonic vorticity was enhanced. These factors were beneficial for explosive cyclone development, and it finally increased to a medium-strength explosive cyclone. 3) Although the forecast fields from the two ensemble sets were both weaker than those of analysis data, the good ensemble members caught the rapid strengthening of the synoptic system at a mid-upper level over the cyclone and the rapid developing processes of these key factors and physical quantities (such as vertical motion, PV, temperature advection, and water vapor). Therefore, the ability of the good members to track the strengthening cyclone was found to be superior to that of bad members.