Surface Rainfall Processes during the Genesis Period of Tropical Cyclone Durian (2001)

The rainfall processes during the formation of tropical cyclone (TC) Durian (2001) were investigated quantitatively using the three-dimensional (3D) WRF-based precipitation equation. The rain rate ( P S ) decreased slightly as the TC approached to formation, and then increased as Durian began to intensify. The rate of moisture-related processes ( Q WV ) in the equation contributed around 80% to P S before TC genesis, and made more contribution during and after TC genesis. The rate of hydrometeor-related processes ( Q CM ) contributed about 20% before TC formation, followed by less contribution during and after TC formation. Q WV were dominated by the 3D moisture flux advection rate ( Q WVA ), while the surface evaporation rate ( Q WVE ) also played an important role. Just before TC genesis, moisture from Q WVA and Q WVE helped the local atmosphere moisten (negative Q WVL ). Q CM were determined by the 3D hydrometeor advection rates ( Q CLA and Q CIA ) and the local change rates of hydrometeors ( Q CLL and Q CIL ). During TC formation, Q CM largely decreased and then reactivated as Durian began to intensify, accompanied by the development of TC cloud. Both the height and the strength of the net latent heating center associated with microphysical processes generally lowered before and during TC genesis, resulting mainly from lessening deposition and condensation. The downward shift of the net latent heating center induced a more bottom-heavy upward mass flux profile, suggesting to promote lower-tropospheric convergence in a shallower layer, vorticity amplification and TC spin-up.