Geoscience Reference
In-Depth Information
clouds. The liquid and solid flux is generally significant only in localized regions
and for short time periods, such as over warm ocean currents and in cumulus clouds
in the tropics.
The convenient aspect of Equation
6.3
is that the terms on the right can be cal-
culated from vertical profiles of specific humidity and winds based on atmospheric
reanalyses. This circumvents some of the problems with the direct observations of
P and ET as well as biases in these surface variables typically present in reanalysis
outputs (see
Chapter 9
for more information). Having gridded fields of humidity
and winds means that one can obtain gridded fields of P-ET (e.g., Cullather et al.
2000
; Rogers et al.,
2001
). Before the advent of atmospheric reanalyses, most stud-
ies of P-ET using the aerological approach focused on large areal averages (e.g.,
the region north of 70°N), based on interpolating rawinsonde data to the domain
boundaries and averaging precipitable water over the domain (e.g., Walsh et al.,
1994
; Serreze et al.,
1995a
).
The atmospheric and surface branches of the hydrologic cycle can be linked by
developing a similar expression for the surface:
P - ET = ∇•
F
+ ∂S/∂t
(6.4)
where S is the water content of the column underlying the atmosphere and
F
rep-
resents lateral transports of water. The latter can be a complicated term because
it includes both surface runoff and subsurface flows in terrestrial regions and the
advection of ice and water in the oceans. Strictly speaking, P-ET is the same in
Equations
6.3
and
6.4
only if one assumes that the surface is an atmospheric col-
umn's only “source” of ET and its only “sink” of P (Walsh et al.,
1994
).
While the aerological approach is attractive, it has its drawbacks. Humidity fields
from reanalyses may contain errors related to shortcomings in data assimilation
methods (both for rawinsondes and satellite retrievals). For example, it appears that
all moderns reanalysis systems - including MERRA, the NCEP Climate Forecast
System Reanalysis (CFSR), and ECMWF ERA-Interim - have a positive cold-sea-
son humidity and temperature biases below the 850 hPa level (Serreze et al.,
2012
).
The vapor flux convergence is also sensitive to errors in the wind fields, which
reflect the available density of assimilation data and temporal changes in the report-
ing network, and quality of the atmospheric model. In general, P-ET computed from
the aerological method and from the reanalysis forecasts of P and ET do not balance
(i.e., water is not conserved in reanalyses).
Despite such shortcomings, the aerological approach applied to atmospheric
reanalysis seems to work fairly well. This is supported from the study of Cullather
et al. (
2000
), who compared the annual meridional flux of moisture across 70°N
from the NCEP/NCAR and ERA-15 reanalyses against values computed via inter-
polation of radiosonde data. Annual-average fluxes from both models are signifi-
cantly higher than the radiosonde estimates during summer, with better correspon-
dence during winter. Closer examination points to the radiosonde network being
insufficiently dense to capture the major moisture transport “pathways” into the
Arctic (Zhu and Newell,
1998
). Put differently, the reanalyses seems to provide
more realistic depictions of the flux. Aerological P-ET averaged for the region north
