Water vapor flux divergency

The example to calcluate water vapor flux divergency.

print 'Open data files...'
f_air = addfile('D:/Temp/nc/air.2011.nc')
f_uwnd = addfile('D:/Temp/nc/uwnd.2011.nc')
f_vwnd = addfile('D:/Temp/nc/vwnd.2011.nc')
f_rhum = addfile('D:/Temp/nc/rhum.2011.nc')

print 'Read data array...'
tidx = 173    # Jun 23, 2011
t = f_air.gettime(tidx)
lidx = 3    # 700 hPa
air = f_air['air'][tidx,lidx,:,:]
uwnd = f_uwnd['uwnd'][tidx,lidx,:,:]
vwnd = f_vwnd['vwnd'][tidx,lidx,:,:]
rhum = f_rhum['rhum'][tidx,lidx,:,:]

# Calculate
print 'Calculate...'
prs = 700
g = 9.8
es = 6.112*exp(17.67*(air-273.16)/(air-29.65))
qs = 0.62197*es/(prs-0.378*es)
q = qs*rhum/100
test = cdiff(q, True)
qhdivg = hdivg(q*uwnd/g,q*vwnd/g)
qv = rhum*es/100
uv = magnitude(uwnd, vwnd)
uvq = uv*qv/(9.8*1000)

#Plot
print 'Plot...'
axesm()
geoshow('country', edgecolor='black')
#layer = contourf(qhdivg, 20)
layer = contourf(qhdivg, cmap='grads_rainbow')
title('Water Vapor Flux Divergency (' + t.strftime('%Y-%m-%d') + ')')
colorbar(layer)
xlim(0, 360)
ylim(-90, 90)