def plot(self, ax):
# Get global u/v wind data
ug, vg = self.chart.get_data(self.gfs_vars, apply_domain=False)
# Get level coordinates for U/V wind components
ug_lv_coord = gfs_utils.get_level_coord(ug, self.level_units)
vg_lv_coord = gfs_utils.get_level_coord(vg, self.level_units)
# Constrain to specified level(s)
ugc = gfs_utils.get_coord_constraint(ug_lv_coord.name(), self.level)
ug = ug.extract(ugc)
vgc = gfs_utils.get_coord_constraint(vg_lv_coord.name(), self.level)
vg = vg.extract(vgc)
# Apply smoothing
ug_data = wrf_smooth2d(ug.data, 6)
ug.data = ug_data.data
vg_data = wrf_smooth2d(vg.data, 6)
vg.data = vg_data.data
# Set up windspharm wind vector object
w = VectorWind(ug, vg)
xi = w.vorticity()
div = w.divergence()
# Constrain to specified domain
domain_constraint = gfs_utils.get_domain_constraint(self.chart.domain)
xi = xi.extract(domain_constraint)
div = div.extract(domain_constraint)
# Calculate strain S following Schielicke et al 2016
u_x, u_y = w.gradient(ug)
v_x, v_y = w.gradient(vg)
D_h = u_x + v_y # horizontal divergence
Def = u_x - v_y # stretching deformation
Def_s = u_y + v_x # shearing deformation
ss = D_h**2 + Def**2 + Def_s**2
ss = ss.extract(domain_constraint)
S = np.sqrt(ss.data) / math.sqrt(2)
# Okubo-Weiss parameter
# vorticity - (div + strain)
okw = (div + S) - xi
# Set mask to inspect O-W parameter in relevant region i.e. < 20N
self.set_coord_mask(lat_max=20)
# Define mask for O-W parameter
mask = self.coord_mask | (okw.data < self.thres)
# Apply mask
okw_masked = iris.util.mask_cube(okw, mask)
ax.contourf(self.lon, self.lat, okw_masked.data, **self.options)