Vertical helicity

The example to calcluate vertical helicity.

print('Open data files...')
f_uwnd = addfile('D:/Temp/nc/uwnd.2011.nc')
f_vwnd = addfile('D:/Temp/nc/vwnd.2011.nc')
f_omega = addfile('D:/Temp/nc/omega.2011.nc')

print('Calculate vertical helicity...')
tidx = 173    # Jun 23, 2011
t = f_uwnd.gettime(tidx)
print(t)
level = '1000:100'
lat = '15:55'
lon = '70:135'
uwnd = f_uwnd['uwnd'][tidx,level,lat,lon]
vwnd = f_vwnd['vwnd'][tidx,level,lat,lon]
omega = f_omega['omega'][tidx,level,lat,lon]
wd = meteolib.vorticity(uwnd, vwnd)
lx = -(wd*omega*10.)/12.64*1e6
lx1 = lx[:,'40',:]
lev1 = lx1.dimvalue(0)
lev2 = meteolib.pressure_to_height_std(lev1)
levels = []
for i in range(0, len(lev1)):
    levels.append('%i' % lev1[i])
lx1.setdimvalue(0, lev2)

print('Plot...')
layer = contourf(lx1, 20)
title('Vertical helicity - Lat north 40 (' + t.strftime('%Y-%m-%d') + ')')
yticks(lev2, levels)
xlabel('Longitude')
ylabel('Pressure (hPa)')
colorbar(layer)