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Heterogeneous surfaces
affect the PBL in several ways. Local surface properties
(water vs. land, field vs. forest) lead to differences in
surface fluxes of momentum, heat
and moisture. The resulting uneven surface fluxes combine with terrain
irregularities to generate both standing and transient
eddies, which can modify the local turbulent fluxes.
FIFE was designed in part to learn more about how to characterize
fluxes over a heterogeneous
surface. The challenge involved blending measurements from
several platforms ( Kelly et al. 1992),
as well as finding ways to relate surface
flux to flux measured from the aircraft ( Schuepp et al. 1992).
Difficulties in estimating the budgets of heat and moisture pointed to the
potential importance of mesoscale circulations and large-scale advection (e.g.,
Grossman 1992).
Mahrt et al. [1994] analyze the low-level
fluxes over a surface with well-defined land-use variations.
They found that turbulent flux is modulated by surface processes
and mesoscale (> 5km) motions. The modulation increases
the effective exchange coefficient for heat flux,
and decreases that for momentum flux. While the first result is
probably robust, the second is case-dependent.
For example, Mahrt and Ek [1993] found an increase
in effective drag coefficient over the HAPEX (Hydrological
and Atmospheric Pilot Experiment) area, which they
attributed to the added effect of form drag.
The effect of a sea-surface temperature discontinuity was reported by
Friehe et al. [l991] on the changes in boundary-layer depth
and turbulence statistics from warm-to-cold and cold-to-warm across
an ocean front.
U.S. National Report to IUGG, 1991-1994
Rev. Geophys. Vol. 33
Suppl., © 1995 American Geophysical Union