Geology Reference
In-Depth Information
Mean Annual Rainfall (1998-2007) (m/yr)
3 - 4
> 4
A
<0.5
1 - 2
0.5 - 1
2 - 3
Plat
range front
Himalayan foreland
B
<0.75
1.5 - 3
4 - 5
> 5
.75 -1.5
3 - 4.5
Pla
range front
R
Himalayan Rainfall
and Topography
Himalayan foreland
Fig. 11.22
Orographic precipitation and topography in the Himalaya.
A. Decadal record of remotely sensed rainfall from the TRMM satellite with
∼
5-km spatial resolution. Note nearly
continuous band of high rainfall along the range front (white dashed line) and second, discontinuous band lying on
the southern flank of the High Himalaya. B. Topography of the northern foreland, the Himalaya, and the southern
Tibetan Plateau. See swaths in Fig. 10.32 for relationship of topography to orographic rainfall. Modified after
Bookhagen and Burbank (2010). [A color version of this appears as Plate 10.]
Models of orographic precipitation have been
around for many decades. Ron Smith (1979)
nicely summarized the state of knowledge in the
late 1970s and has continued to participate in
advancing our knowledge of these important
interactions. In the early 2000s, Roe developed
simpler versions of these models of atmosphere-
topographic interactions that were designed to
be coupled to landscape evolution models. In
Roe's models (Fig. 11.23), one specifies the wind
direction and speed, the water content of the
atmosphere, and its temperature. The air mass
then encounters a landmass and is made to rise
at a rate dictated by the air speed and the slope
of the landscape. Here, the relevant slope is some
envelope of the topography, although later mod-
els demonstrate the role of smaller-scale steering
of the air masses by local valleys (Anders
et al
.,
2008). The water in the air mass condenses to
form “hydrometeors,” and falls at a rate governed
by the settling speed of either the raindrops or
the snowflakes. Were it not for the finite travel
distances of these hydrometeors from their site of
formation due to wind velocity, the precipitation
pattern in these models would simply reflect the
slope of the landmass. Roe and Baker (2006)
detail the pattern of precipitation that takes into
account the trajectories of the hydrometeors. The
effect is to shift the pattern of precipitation down-
wind of that associated with the slope of the
topography. This shift allows some snow or rain
to waft over the crest of a mountain range to fall
on the dry side, where the slopes alone would
suggest no precipitation should fall (Fig. 11.23).
Roe
et al
. (2002) applied this simple model to
the evolution of a river profile in an uplifting rock
mass. Using a simple stream-power rule for river
incision, in which incision rate is a function of the
product of water discharge and slope of the river,
requires that one calculate the spatial pattern of
water discharge. Although many early models of
river incision simply took the water discharge to
be a product of drainage area and some given
precipitation (in fact, most models use drainage
area as a proxy for discharge), a more proper
formulation for the discharge at a point on a river
is to integrate the product of the precipitation
with the area at all elevations from the crest of
