Impact of Surface Potential Vorticity Density Forcing over the Tibetan Plateau on the South China Extreme Precipitation in January 2008. Part I: Data Analysis

Fig.  1.   Time–longitude cross-sections of (a) potential vorticity density (PVD) (shaded; 10^–5 s^–1) and (b) zonal PVD advection (shaded; 10^–10 s^–2) on 305-K isentropic surface averaged over 28°–34°N from 1 January to 5 February 2008. The black lines in (b) indicate 6-h accumulative precipitation (mm). The white area indicates the area with surface potential temperature greater than 305 K, i.e., the underworld.

Fig.  2.   Vertical cross-sections of PVD (shading; 10^–7 K m s kg^–1), potential temperature (contour; K), and wind (vector; m s^–1; the vertical component has been multiplied by –50) averaged over (a, c, e) 30°–36°N and (b, d, f) 110°–120°E for (a, b) January–February climate mean, (c, d) the storm period, and (e, f) the difference of PVD and wind between these two periods (c minus a, d minus b). The dotted region in (e, f) represents the PVD difference exceeding three standard deviations.

Fig.  3.   Vertical cross-sections of (a, c, e) PVD advection term (shading; 10^–13 K m kg^–1) and PVD (contour; 10^–7 K m s kg^–1); and (b, d, f) diabatic heating term (shading; 10^–13 K m kg^–1) and diabatic heating rate (contour; 10^–5 K s^–1) over 110°–120°E for (a, b) January–February climate mean, (c, d) the storm period, and (e, f) the difference between these two periods.

Fig.  4.   Vertical cross-sections of (left columns) zonal PVD advection (shading; 10^–13 K m kg^–1) and wind (vector; m s^–1), (middle columns) meridional PVD advection (shading; 10^–13 K m kg^–1) and wind (vector; m s^–1), and (right columns) horizontal PVD advection (shading; 10^–13 K m kg^–1) and PVD (10^–7 K m s kg^–1) over 110°–120°E for (a–c) January–February climate mean, (d–f) the storm period, and (g–i) the difference between these two periods.

Fig.  5.   Vertical cross-sections of vertical PVD advection (shading; 10^–13 K m kg^–1), PVD (contour; 10^–7 K m s kg^–1), and wind (vector; m s^–1) averaged over 110°–120°E for (a) January–February climate mean, (b) the storm period, and (c) the differences of PVD (shading; 10^–7 K m s kg^–1) and wind (vector; m s^–1) between these two periods. Contour in (c) indicates PVD in the storm period

Fig.  6.   Distributions of surface PVD tendency for (a) the climate mean, (b) the storm period, and (c) their difference (10^–7 K s^–2). The black line indicates the elevation of 3000 m.

Fig.  7.   Distributions of (a) the total surface PVD tendency, and the surface PVD tendency resulting from (b) PVD flux divergence, (c) diabatic heating, and (d) friction from 18 January to 2 February. The unit of PVD tendency is 10^–7 K s^–2

Fig.  8.   Distributions of (a) surface PVD flux divergence term ($- \nabla \cdot \left({W{V}} \right)$; 10^-7 K s^–2), (b) divergence term ($- W\nabla \cdot {V}$; 10^–7 K s^–2), (c) PVD advection term ($- {V} \cdot \nabla W$; 10^–7 K s^–2), and (d) air divergence ($\nabla \cdot {V}$; 10^–6 s^–1) from 18 January to 2 February.

Fig.  9.   Vertical cross-sections of (a) absolute vorticity advection (shading; 10^–10 s^–2) and its vertical gradient (contour; 10^–12 s^–2 hPa^–1), (b) relative vorticity (shading; 10^–5 s^–1) and temperature (contour; K) over 110°–120°E from 18 January to 2 February 2008. The red and blue curves in (b) indicate the zero isotherm in the storm period and that of January–February climate mean from 1980 to 2017