Microphysical Properties of Maritime Squall Line Observed on June 2, 2008 in Taiwan

Abstract

The physical characteristics of rain are reflected by the shape of the raindrop size distribution (RDSD). Specifically, the RDSD is the result of different precipitation formation processes. We measured the RDSD at the surface in heavy rainfall during SoWMEX/TiMREX (2008) in Taiwan. The heavy rainfall was characterized by a squall line accompanied by trailing stratiform precipitation, and it was partitioned into three regions based on radar reflectivity patterns: convective line, stratiform, and reflectivity trough. The convective line was further partitioned into the convective center, leading edge, and trailing edge using a threshold rainfall intensity of 20 mm h^-1.
The leading edge, which belongs to the convective line, had upward motion from the surface and contained many small drops. The leading edge was characterized by a small median volume diameter (D₀) and a linear shape of gamma RDSD. In the convective center, a strong updraught rose to the top of the cell with time, and many large drops over 4 mm were observed. The convective center, which had large D₀ and normalized intercept parameter (N_W), was characterized by an upward convex shape of gamma RDSD. The range of raindrop diameter decreased toward the trailing edge, with no updraught or only weak updraughts at high altitudes. The trailing edge had large shape and slope parameters, along with a more upward convex shape of gamma RDSD. A bright band was observed in the stratiform region with continual downward motion from the bright band to the surface, even though the intensity became weak. In the stratiform region, D₀ and N_W were small and the gamma RDSD had an upward convex shape.
The different RDSDs in each region of a maritime squall line suggest the existence of different cloud microphysical processes described by the change of RDSD parameters: the coalescence process induces an increase of D₀ and shape parameter and a decrease of N_W, while the break-up process induces a decrease of D₀ and an increase of N_W.
In comparison with other maritime storms, the convective center has small log₁₀N_W and large D_m value. And the convective edge region is positioned between convective center and stratiform region in the D_m-log₁₀N_W scatter plot.