Satellite-Monitored Snow Cover in the Climate System - Satellite-based Applications on Climate Change - page 182

12.5 Snow-Atmosphere Coupling Experiment
Xu ( 2011 ) designed a series numerical experiment to investigate the role of snow
cover as a source of predictability at seasonal time scales over the Northern Hemi-
sphere. A global climate model is used, consisting of the fully coupled land and
atmosphere components of the Community Climate System Model. Ensembles of
boreal spring-summer climate simulations are made with specified climatological sea
surface temperatures. Following the methodology of the Global Land-Atmosphere
Coupling Experiment (GLACE), a control ensemble is created with perturbed initial
atmospheric states and realistic land surface initialization. In the test cases, snow-
cover fraction and snow water equivalent are specified in all ensemble members
based on model-simulated snow information or realistic snow data from remote
sensing and an operational land surface analysis. The snow-atmosphere coupling
strength is quantified as in GLACE as the degree to which identically constrained
snow boundary conditions reduce the ensemble spread of key meteorological
variables like precipitation and near-surface air temperature. The snow albedo effect,
snow hydrological effect, or mixed effects are estimated by different experiments and
snow stages. Metrics of potential predictability and feedback are also investigated.
From spring to early summer, the snow-covered regions demonstrate significant
coupling to the atmosphere over large portions of the Northern Hemisphere
(Fig. 12.2 ). The local coupling between snow state and atmosphere is found to
have three distinct stages: the stable-snow period before snowmelt when interactions
are through radiative processes controlled by albedo, the period after snowmelt when
interactions are through the delayed hydrologic effect of soil moisture anomalies
resulting from snow anomalies, and the intervening period during snowmelt when
both radiative and hydrologic effects are important. The coupling strength is strongest
during the snowmelt period along the transient zone between snow-covered and
snow-free areas and migrates northward with the retreating snow line. The coupling
strength due to the hydrological effect (soil moisture impact) after snowmelt is
generally stronger than the coupling strength due to the albedo effect (radiative
impact) before snowmelt. The Tibetan Plateau is a special snow-atmosphere cou-
pling region due to its high incident solar radiation caused by its high altitude and
relatively low latitude.
Figure 12.3 shows the zonal mean over land of coupling strengths shown in
Fig. 12.2 . At all latitudes, the coupling strength is strongest during snowmelt and
weakest before the snowmelt. The coupling strength generally increases with latitude
to a peak at roughly 50-65 but then decrease sharply north of 70 .Thereisalocal
maximum of coupling strength at roughly 34 caused by the Tibetan Plateau. Before
snowmelt, the coupling strength is mainly due to the snow albedo effect as shortwave
radiation increases during spring. During the snowmelt period, the SWE and SCF
have peak variability. Both albedo and hydrological effects contribute to the coupling
strength during snowmelt. The hydrological effect after snowmelt has a stronger than
the snow albedo effect before melting. The potential predictably from accurate