Figure 4: AGRB (top) and PGRB (bottom) observed and simulated DA and
CTRL streamflows for the four analyzed WYs.
3.3. Albedo and SWE Updating with
DA
As stated in section 5.4.2, satellite albedo DA improvements were
observed in years when DA streamflow predictions were lower than CTRL
and bias was positive. The benefit of satellite DA was caused by a
decrease in modelled glacier albedo combined with an increase in
snowcover in spring and summer (Figures 5, 6, and Table 2). This
mechanism occurs because the associated melt decreases considerably
while snowcover covers glacier ice, considering the relatively high
albedo snow generates less meltwater than relatively low albedo ice for
the same shortwave insolation fluxes. The DA streamflow used for
evaluation was the mean of all 20 ensembles. This mean is lower if more
ensembles have prolonged spring and summer snowcover over ice. Since the
benefits of DA stem from a decrease in streamflow, snowcover over ice
has a larger influence on melt than the decrease in albedo introduced by
DA. In AGRB, the ice albedo decrease introduced by DA was larger than in
PGRB. At the same time, the DA snowcover over ice was smaller in AGRB
(Figures 5 and 6). The integrated response was that AGRB streamflow was
less sensitive to DA than in PGRB. These results indicate that the
benefit of satellite albedo DA stems not only from the albedo itself,
but also from associated changes in other model states and glacier
configuration.