Gravity Changes over Russian River Basins from GRACE - Planetary Exploration and Science: Recent Results and Advances - page 56

the scope of this study. Mass increase has been observed in some Siberian regions,
such as sources of Lena and Irtysh, which may be related to the degradation of
permafrost (Frappart et al. 2010 ; Landerer et al. 2010 ). Potentially as a result of
global warming, melted ground ice is replaced by water, which increases the density,
mass, and, consequently, gravity field. Negative anomaly over the Caspian Sea can
be related to its level decrease, as reported in Zonn et al. ( 2010 ).

3.4 Discussion

For large territories like Russia, satellite gravity field data represent an important
source of hydrologic information. In this study monthly gravity field solutions from
GRACE in terms of mass (EWH) changes were processed by MSSA and averaged
over the 15 largest Russian river basins. Annual component (PC 1) shows amplitude
increase since 2009 (Figs. 3.4 and 3.6 ). Unprecedented maximum in spring 2013
is caused by the huge snow accumulation over the territory of Russia (Figs. 3.5
and 3.6 ). Trend component shows increase since 2003, maximum in 2009, followed
by the decrease. This behavior is mostly defined by Siberian river basins. Map for
the trend (Fig. 3.7 ) shows mass anomalies increase in Siberia and decrease over the
Caspian Sea.
We cannot answer the question why precipitation increased in winter-spring
2013 in Russia, causing unprecedented snow accumulation. It could be related to
the anomalies of atmospheric mass transfer from Atlantic, Gulf Stream circulation,
El Niño/La Niña, or conditions in the Arctic. There are some evidences in favor
of global warming pause (hiatus). Such questions could be answered only after
extended interdisciplinary research, involving climatology, meteorology, and other
sciences. Some issues and projections for precipitation and anomalous weather
conditions into the future could be found in IPCC Fifth Assessment Report ( 2013 )
or the report of Hydrometeorological Center of Russia (Report on Climate 2013 ).
In this work we did not plan to find explanation of the causes of meteorological
and hydrological changes over Russia. Our goal was to present observational data,
its processing technique, and show its usefulness for hydrological and climatological
studies, exploration of our planet.
We used MSSA to filter data and separate meaningful components, founding
it a promising method for GRACE data processing. MSSA method has greater
flexibility than simple EOF and could be useful in the analysis of other satellite
observations, such as altimetry, water vapor, and precipitation data (Zotov 2012 ).
Exact physical interpretation of the obtained signals requires comparison to
hydrological models (GLDAS, WGHM) and ground-based observations. The
remaining questions are (i) what is the useful part of the signal in PC 1 and
PC 2, (ii) how to reduce boundary effects for the first and the last points of PCs, and
(iii) how to better separate secular change from annual and, probably, other periodic
signals? As for the last question, fortunately, we already have 11 years of GRACE
data, and quite a good separation can be achieved by the appropriate choice of L.