Outer Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 254

6.3 Atmosphere-land interface

As with the ocean surface, the interaction between the
ABL and the land surface involves the transfer of momen-
tum, mass, and heat. Mass transfer of water vapor and
photosynthetic gases arises as the ABL interacts with sur-
face water in lakes, soil substrates, and vegetation canopies.
Heat transfer from land surfaces to the ABL occurs via
reradiated long-wave thermal energy and may be viewed
as a type of forced convection. In the summary in
Sections 6.3.1 and 6.3.2, we focus on sediment mass trans-
port and associated exchanges of momentum.

erosional lineations ( yardangs ) enable sand-flow distributions
to be mapped out regionally (Fig. 6.19). Ideally, a sand-flow
map should show resultant directions as flowlines and
resultant magnitudes as contours, analogous to a com-
bined wind direction and pressure map. Sand-flow maps
are also analogous to drainage maps in that they show
divides separating distinct “drainage” basins: peaks in fixed
high-pressure areas and saddles in between them. Unlike
water drainage, there is little direct relation between sand
flow and topography since winds and their sandy bedload
may blow uphill. For example, the sand-flow lines for
North Africa extend from erg to erg, implying very long
transport distances downwind. Evidence for this is pro-
vided by satellite images showing yardangs linking ergs
along sand-flow lines. All the sand-flow lines arise within
the desert itself, with the main clockwise circulatory cell
roughly corresponding to the subtropical high-pressure
zone.
Notice from Figs 6.19 and 6.20 that the Saharan
sediment-flow lines eventually lead to the ocean, coinci-
dent with the great plume of Saharan dust that extends
out for thousands of kilometers into the Atlantic Ocean.
A significant part reaches as far as the Lesser Antilles,
providing a steady flux of fine silt particles to the deep
ocean. Such mineral dust is a potent light-scattering
aerosol and has an important cooling role to play in climate
change as well as a source of nutrients (chiefly labile Fe 2 )
to the oceans. Current estimates of total global dust
flux to the atmosphere vary between 1 and 3

6.3.1 Peculiarities of the ABL over land

In detail, the flow behavior and characteristics of the local
or regional ABL over land is more variable than that over
oceans in particular because of:
the existence of commoner stable seasonal atmospheric
stratification or inversions, which cause cooler and denser
air masses to “buffer” the transmission of momentum
from any overlying gradient wind, leading to often tempo-
rary “calm” conditions and the undesirable affects of
urban pollution
local flow due to motions arising from differential land
surface heating and cooling as typified by land and sea
breezes, by the generation of thermals over deserts and
other hot plains, and by the outflow of cold dense
katabatic winds from ice, mountains, and plateaux downs-
lope into surrounding plains and ocean
topographic enhancement of local or synoptic wind speed
causing acceleration or decelerations as land surfaces inter-
sect higher up or lower down the gradient wind column
formation of lee-side effects like the well-known undu-
lating waves of cloud formed downwind from linear
mountain ranges and the tip jets characteristic of moun-
tainous capes bordering oceans, for example, Cape
Farewell, S. Greenland etc.
downslope flow of dry air driven by a pressure reduction
on the lee side of mountains, the flow accompanied by
adiabatic warming at the dry air lapse rate - the foehn effect

10 9 ton
yr 1 . It has been estimated that the total flux of Saharan
mineral dust is some 2.6 10 8 ton yr 1 , the magnitude

having varied throughout the Quaternary in response to
cycles of climate change. The evidence for this comes
from deep-sea cores, which enable dust deposition rates
to be calculated. Present-day fluxes are low compared to
those at the last glacial maximum. A worrying recent
trend has seen marked increases in levels of wind-blown
dust in the troposphere worldwide, particularly in winds
sourced from the expanding deserts and dried-up lake
basins of northern China and Mongolia. It is not known
exactly whether it is recent climate change to aridification
or the poor maintenance of land use that is the cause of
this trend.
In mountainous terrains and over the high latitude ice
sheets, storm-force winds play a major role in the transport
of particulate ice and snow, driving it into major accumu-
lations. As with the transport of sand grains in deserts, the
transporting particles exert an important feedback as flow
and solid momentum is exchanged.

6.3.2 Synoptic aeolian sand and dust transport in
the atmospheric boundary layer

In the great trade wind sand seas ( ergs ) of the Sahara and
central Australia, there is a close correspondence between
dominant wind flow and sand transport. Meteorological
observations, bedform orientations, and the trend of