Geoscience Reference
In-Depth Information
The discussion that follows draws strongly on the study of Serreze et al.
( 2007 ), who examined the energy budgets for polar cap and Arctic Ocean domains
( Figure 3.6 ) based on data from the ERA-40 and NCEP/NCAR reanalyses (see
Chapter 9 ) for the 1979-2001 period, emphasizing the former product, along with
satellite and oceanographic data compiled from a variety of sources. The reanalyses
provide for gridded fields of atmospheric budget terms (data for each grid node rep-
resenting a column); the atmospheric terms for each domain are based on averaging
data over the nodes contained in each domain.
3.3
Energy Budget of the Polar Cap Domain
3.3.1
Understanding the Seasonal Cycle
Monthly atmospheric energy budget terms for the polar cap from ERA-40 are pro-
vided Table 3.1 . We also make use of CERES satellite data for R top and AVHRR
data for planetary albedo during months with a significant solar flux. CERES can be
viewed as a follow-on to the earlier ERBE (Wielicki et al., 1996 ). It must be stressed
at this point that, although atmospheric reanalyses like ERA-40 provide for useful
evaluations of the energy budget, even today, there is considerable uncertainty in
some of the terms. For example, from Table 3.1 , the monthly energy budget residual
from ERA-40 (the sum of all of the budget terms; if the budget is closed, the sum is
zero) can be as large as 24 W m −2 . The story is the largely the same for other atmo-
sphere reanalysis. Mass correction errors applied to the atmosphere transports and
errors in the surface fluxes needed to get F sfc are known to be particularly problem-
atic. Although, as seen from Table 3.1 , there can be large differences in R top between
ERA-40 and the CERES satellite observations (although the latter are only for a
short period, 2000-2005), satellite observations themselves have uncertainties.
With these caveats in mind, consider first the annual cycle in the rate of atmo-
spheric energy storage of energy storage ∂A E /∂t , starting with August, which repre-
sents late summer. According to ERA-40, there is a loss of energy from the atmo-
spheric column in this month of −17 Wm −2 . The loss is at its maximum in September
(−27 W m −2 ), is roughly maintained in October (−22 W m −2 ), then declines through
the winter. It turns positive in February. The atmospheric energy gain increases dur-
ing spring to a maximum of +25 Wm −2 in April, and is slightly positive in July. The
annual cycle in the atmospheric energy content can be readily seen in atmospheric
variables averaged for the region north of 70°N, such as the mean height of the 300
hPa pressure surface ( Figure 3.7 ) and the mean precipitable water ( Figure 3.8 ) as
evaluated from NCEP/NCAR data.
The annual cycle in the energy storage of the atmosphere is determined by the
interactions of the three terms on the right of Equation 3.2 . A useful starting point
is to consider an idealized (but obviously unrealistic) polar cap for which there
is no horizontal atmospheric heat flux convergence and no vertical heat transfers
between the atmosphere and the underlying column ( F A and F sfc equal zero).
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