Geoscience Reference
In-Depth Information
(wrongly) assigned to the boreal forest in Canada during the snow-covered winter
months in the ECMWF model it calculated values of net radiation that were
substantially less than observed values. Figure 25.6c shows that the snow cover
over a broad region of the western United States in March, April, and May appears
to be anti-correlated with area-average monsoon rainfall measured in the
subsequent July and August in New Mexico, a feature which has been ascribed
to the effect of snow melt on summer soil moisture.
Because the influences of transient changes in frozen precipitation cover are
physically plausible and consistent with observations, this influential mechanism
is assessed as being 'extremely likely' in Table 25.1. Over the last decade there has
been significant effort deployed toward improving the representation of frozen
precipitation in SVATS (e.g., Bowling
et al.
, 2003; Luo
et al.
, 2003; Nijssen
et al.
,
2003; Etchevers
et al.
, 2004; Niu and Yang, 2004; 2006; 2007; Fassnacht
et al.
,
2006). Because progress has been reasonably good, the classification of the
quantification and modeling of this mechanism is currently assessed as being of
'medium' quality in Table 25.1. Adequate representation of area-average behavior
of
heterogeneous
frozen precipitation cover and its influence on meteorological
feedbacks and hydrological flows remains elusive and merits further research.
4.
Combined effect of transient changes
For simplicity and clarity the mechanisms through which land surfaces can
influence climate and weather associated with soil moisture, vegetation cover and
frozen precipitation have been considered separately above. But it is important to
recognize that in practice they are intimately interrelated. This is true in the real
world and should also be true in any well-conceived model of surface processes.
The atmosphere's ability to access soil moisture is related to vegetation cover
because plants' roots, stems and leaves serve as an important conduit between
soil and the overlying air. Plants can also intercept precipitation and return it to
the atmosphere before it enters the soil, thus changing the amount of moisture
present in the soil that is available to the atmosphere. Coverings of snow and ice
and associated frozen soil influence the effect of vegetation cover by altering
surface albedo and by inhibiting transpiration from plants when soil is frozen.
Water temporarily stored in frozen form on the surface of soil and vegetation
during winter usually melts several months later and supplements the soil mois-
ture available to transpiring vegetation in the subsequent summer. The evolution
of the snow cover with time depends on what vegetation is present because tall
forests may shade snow, while snow on short grassland remains exposed to the
sun. Similarly, the seasonal cycle in vegetation growth and the extent to which
plants' controls act to moderate transpiration rate are related to the amount of
moisture in the soil.
Because of the complexity of the regional scale interaction between the
three influential mechanisms associated with soil moisture, vegetation cover
and frozen precipitation, it is perhaps not surprising that all are assessed as
'extremely likely' in Table 25.1, and that the present status of understanding and
