Geology Reference
In-Depth Information
Figure 9.16
Mean snowmelt onset averaged from passive microwave data from 1979 through 1998, using the
algorithm presented in
Drobot et al
. [2001]. The melt onset is shown in terms of the day of the year (day 190 is
9 July) according to the attached gray tone bar [
Drobot and Anderson
, 2001, Figure 5, with permission from AGU].
Julian day 110 and 130 and progresses northward.
Prior to this period snowmelt would appear along
coastlines of many seas and bays. The total melt onset
area nearly doubles from Julian day 150 through Julian
day 170. The latest snowmelt onset dates occur in the
Lincoln Sea north of Greenland where MY ice is con-
centrated. The advantage of using passive microwave is
the availability of the daily coverage of the entire Arctic
basin, which allows extensive mapping of the onset of
melt day.
Belchansky et al
. [2004] used a modified version of
the AHRA to estimate the regional and interannual
variability of the melt season in the Arctic from 1979
to 2001. They incorporated surface air temperature,
obtained from the International Arctic Buoy Program/
Polar Exchange, to circumvent two factors: the anoma-
lous estimates arising from false
T
b
and the relative
insensitivity of the AHRA to the choice of the thresh-
olds. They presented maps of the average sea ice melt
onset, melt duration, and freeze onset data for the entire
Arctic basin, averaged over two periods: 1979-1988
during a low index Arctic oscillation (AO) and 1989-
2001 during a high index AO. The authors found a cor-
relation between the ice melt duration and the seasonal
strength of the AO. As expected, melt onset occurs
early (first weeks in May) at the far east of the Arctic
region (the Kara and Barents Seas) and far west (the
Chukchi and Beaufort Seas). The melt duration in these
areas continues for 140-180 days, exceeding its duration
in the central Arctic (between 30 and 60 days) and
the peripherals of the central Arctic (between 60 and
140 days).
Markus et al
. [2009] used a different approach where
three indicators of surface melt onset, all derived from
passive microwave observations, are combined. All indi-
cators are sensitive to different features of surface melt.
The algorithm that employs this approach is called the
passive microwave algorithm (PMA). A key theme is
that emissivity increases sharply when snow becomes wet
as moisture absorbs more energy. [
Ulaby et al
., 1986]
(see also section 7.7.3.2).
Three indicators employed in the PMA are expressed
by the following equations:
T
T i Ti
bV bV
(
1
)
()
(9.17)
,
37
,
37
bV
,
37
where
T
b
,37
V
is the brightness temperature from the
37 GHz channel with vertical polarization and Δ is the
absolute difference between observations from day
i
and the following day
i
+ 1. The premise behind using
this equation is that Δ
T
b
,37
V
shows a significant increase
in temporal variability (up to 30 K) when melt begins.
The second indicator is
