Geoscience Reference
In-Depth Information
(Kane et al.,
1991
; Hinzman et al.,
1996
). The small negative values in MERRA
likely reflect shortcomings in MERRA's depiction of ET in very cold conditions
with a stable boundary layer. Consistent with the presence of open water underly-
ing a fairly cold and dry atmosphere, January ET over the Norwegian Sea locally
exceeds 100 mm. However, these values pale in comparison to monthly means of
more than 200 mm further south along the coast of North America where sea surface
temperatures are fairly high and there are strong winds linked to the North Atlantic
cyclone track. Similarly high January ET totals are found over the north Pacific.
Compared to winter, ET total over land areas are higher in spring, but remain very
low over the ice-covered Arctic Ocean.
ET peaks over land areas in summer. A broad band of valued from 120-160 mm
extends across over the Eurasian subarctic, with values of 60-80 mm extending up
to the Arctic coast. ET over the melting sea ice surface remains low; the skin tem-
perature cannot depart from the freezing point, and the saturation vapor pressure for
a melting (pure water) surface is only 6.1 hPa, limiting the magnitude of vertical
vapor gradients. ET over the Norwegian Sea is lower than in winter, manifesting
smaller vertical gradients in humidity and weaker winds. The field for October cap-
tures the transition back to the winter pattern.
6.2.3
Net Precipitation from the “Aerological Method”
In hydrologic analyses, net precipitation, or P-ET, is itself is a valuable term, and
can be obtained in the absence of direct surface measurements of the two variables.
Consider the moisture budget of an atmospheric column, extending from the surface
to a height above which moisture content is negligible (e.g., 300 hPa). The budget
can be expressed as
∂W/∂t = ET - P - ∇•
Q
(6.2)
where the moisture content of the atmosphere, W, is expressed as precipitable water
(the equivalent water depth of the vapor in the column). Time is given by t, ET is the
surface evapo-transpiration rate,
Q
is the vertically integrated water vapor flux (a
vector) from the surface to the top of the column and ∇•
Q is its divergence
, and P
is the precipitation rate. The equation hence summarizes the processes contributing
to a time change in the column precipitable water (∂W/∂t). If there is ET from the
surface into the column, precipitable water increases unless there is a correspond-
ing moisture outflow. If there is precipitation, precipitable water decreases. It fol-
lows that if there is a horizontal divergence, precipitable water decreases, while it
increases if there is convergence.
Rearrangement yields an expression for P-ET:
P - ET = −∇•
Q
- ∂W/∂t
(6.3)
For long-term annual means (and assuming a stationary climate), the last term of
Equation
6.3
(the time change) is zero and may hence be dropped. The aerological
approach neglects the atmospheric flux of water in the liquid and solid phases in
