Effects of Vertical Wind Shear

Abstract

The effects of vertical wind shear on convective development have been examined for decades (e.g., Pastushkov 1975; Corbosiero and Molinari 2002; Lang et al. 2007; Ueno 2007). The delayed initiation of convection is associated with a strong vertical wind shear (e.g., Xu et al. 1992). The vertical wind shear affects convective development through the change in vertical transport of horizontal momentum (e.g., Wu and Yanai 1994). The convection becomes increasingly organized in lines because of a strong lower-tropospheric vertical wind shear (e.g., Robe and Emanuel 2001). Vertical wind shears affect convective development in two ways. First, vertical wind shear change intensity of convective systems through the change in dynamic instability. The degree of dynamic instability can be described by a secondary circulation, which is quantified by the magnitude of perturbation kinetic energy. Vertical structures of large-scale horizontal winds may alter perturbation kinetic energy through the change in energy exchange between perturbation kinetic energy and large-scale kinetic energy, which is determined by covariance between perturbation horizontal wind and vertical velocity in the presence of vertical shear of large-scale horizontal winds (e.g., Peixoto and Oort 1992; Li et al. 2002). Second, vertical structures of large-scale horizontal winds may have impacts on horizontal structures of convective systems. Unicell and multicell convections are associated with the unidirectional and reverse wind shears, respectively (e.g., Tao et al. 2003). In this chapter, sensitivity experiments with vertical wind shear excluded (BILISNWVS, PSRNVWS) are compared with the control experiments with vertical wind shear included (BILIS, PSR) to discuss effects of vertical wind shear on rainfall processes during the landfall of sever tropical storm Bilis (2006) and pre-summer heavy rainfall event over southern China in June 2008, based on Wang et al. (2009) and Shen et al. (2011). These sensitivity experiments are identical to the control experiments except in sensitivity experiments vertically varying large-scale zonal winds are replaced with mass-weighted mean large-scale zonal winds from the control experiments.