Abstract:
This study investigates the role of snow cover as a source of predictability at
seasonal time scales over the Northern Hemisphere. 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
xv
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. 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 coupling region due to its high incident solar radiation
caused by its high altitude and relatively low latitude. The potential predictably
from accurate knowledge of snow distribution is highly correlated with the snowxvi
atmosphere coupling strength. Conceptual models are proposed to explain the
mechanisms behind the timing and spatial distribution of snow‐atmosphere
coupling.