The equation used to calculate the eastward and northward water-vapour flux (kg m^{-1} s^{-1}) respectively in the ERA-Interim (The ERA-Interim Archive document version 2.0):

where:

η : the model level coordinate, eta,

∂p : partial delta pressure,

u: eastward wind speed

v: northward wind speed

q: specific humidity

g: acceleration due to gravity

∂η : partial delta eta,

dη : Integration is performed with respect to eta.

The top model level is at 0.1hPa, which is in the middle of the top model layer.