Differential heating of the land and the ocean has greatly influenced the streng
th, duration, and spatial distribution of large-scale monsoons. The land surfac
e processes are crucial in modulating the monsoon processes. Several GCM studie
s, which explore potential role of land surface processes in hydrometeorological
predictability, have consistently shown that soil moisture and vegetation proce
sses have the great impact on the climate in monsoon regions. However, differen
t models exhibit very different sensitivity to the land surface forcing. Diffic
ulty and challenge in study of land surface/monsoon interactions will be discuss
ed.
Two case studies will be presented to show how vegetation process affects intras
easonal monsoon variability. We compare simulations by a NCEP GCM coupled with
two different land surface parameterizations, a two-layer soil model with no exp
licit vegetation representation, and a more comprehensive biophysical model. T
he results show that feedbacks between vegetation and the atmosphere are crucial
ly important for proper simulation of intra-seasonal monsoon evolution, includin
g its intensity, the spatial distribution of precipitation, and associated circu
lation at the continental scale.
In the East Asian study, the role of land surface processes in the abrupt monsoo
n northward jump and turning of low-level circulation has been investigated. Si
mulation results suggest that vegetation processes produce different spatial dis
tributions of the Bowen Ratio, in turn influencing the low-level temperature and
pressure gradients, wind flow (through geostrophic balance), and moisture trans
port. This process creates the great east-west thermal gradient, which may cont
ribute to the abrupt northward jump of the East Asian monsoon. It also causes t
he latitudinal heating gradient that contributes to the clockwise and counterclo
ckwise turning of the low-level wind on the East Asian and African continents du
ring the early stages of the monsoon development. In the South American case, t
he impact of land surface processes on the South American monsoon system (SAMS)
onset and evolution are investigated. In the SAMS simulation, the model without
explicit vegetation did not produce a clear southward movement of the precipita
tion in monsoon onset. Instead, it produced a very strong and early northward S
AMS movement and a very wet monsoon mature season.
However, these studies are based on a single GCM model and a specific year. Rec
ent studies with different models show that different GCM coupled with different
land models exhibit very different sensitivity to the land surface forcing. Pr
oper simulations of land/atmosphere interaction are challenge and difficulty. T
he factors and issues related to modeling land/atmosphere interactions will be d
iscussed.
Submittal Information
Name :
Date :
Yongkang Xue
30-Jul-04-03:44:58
Organization :
Theme :
University of California, Los Angeles
Theme 1
Address :
Presentation :
1255 Bunche Hall, Department of Geography, UCLA, Los Angeles, CA 90095-1524