122
The main issues concerning this approach reside in the choice of the parameters characteristic of the PDF, and in the complexity of some of the formulations typical of ESEMs,
which does not allow the analytical calculation of all terms. Therefore, unless complex
terms are calculated numerically, this approach would provide only a partial, first order
representation of intra-patch heterogeneity, while retaining however the full complexity
of the ESEM structure.
4.3. Indirect effects of surface heterogeneities
In section 4.1 we have defined the indirect effects of surface heterogeneities as those
associated with organized mesoscale circulations (OM C) induced by strong horizontal
gradients in surface forcings, e.g. latent and sensible heat flux. A typical example of
such circulations is the sea-breeze. The physical mechanism which generates the seabreeze is associated with the horizontal gradient in sensible heat flux induced by daytimc
solar heating over the cool water surface (low flux) and the warm land surface (high flux).
During the day, greater turbulent vertical mixing produces a lifting of the pressure surfaces
over land compared to ocean. This generates a positive lalld-to-ocean horizontal pressure
gradient at some level which initiates an off shore circulation with compensating on-shore
flow near the surface. This circulation tends to remove the horizontal pressure gradient
in the late afternoon until it dissipates in the early evening. At night, radiational cooling
over land produces a sinking of the pressure surfaces which, by a similar mechanism,
initiates a reverse off-shore circulation.
Although the sea-breeze has been known and studied for many decades, it was only
relatively recently that Yan and Anthes (1984) suggested that a similar OMC might
be triggered by gradients in surface vegetation, e.g. grass vs forest or irrigated vs nonirrigated surfaces. Anthes (1984) suggested that such circulations might significantly affect
local climate, especially in semi-arid regions, and that in fact using specified geometries
for vegetation cover management could be an effective way to modify local climate (Fig.
10). This hypothesis was confirmed by early modeling work (Yan and Anthes 1988), and
since then a multitude of studies have appeared in the literature to analyze the effect of
OMCs induced by surface heterogeneities. Because of the basic two-dimensional nature
of the sea-breeze process the early studies mostly employed two-dimensional mesoscale
models, along with idealized background conditions (Yan and Anthes 1988, Giorgi 1989,
Pielke and Avissar 1990, Pielke et al. 1991). Only recently, full three-dimensional models
have been used to analyze these circulations under more realistic atmospheric forcings
(Lynn 1994, Seth and Giorgi 1995). In particular, the sensitivity of OMCs to different
surface vegetation geometries (individual strips, alternating bands, or checkerboard of wet
and dry areas) has been extensively studied.
The main issues concerning this approach reside in the choice of the parameters characteristic of the PDF, and in the complexity of some of the formulations typical of ESEMs,
which does not allow the analytical calculation of all terms. Therefore, unless complex
terms are calculated numerically, this approach would provide only a partial, first order
representation of intra-patch heterogeneity, while retaining however the full complexity
of the ESEM structure.
4.3. Indirect effects of surface heterogeneities
In section 4.1 we have defined the indirect effects of surface heterogeneities as those
associated with organized mesoscale circulations (OM C) induced by strong horizontal
gradients in surface forcings, e.g. latent and sensible heat flux. A typical example of
such circulations is the sea-breeze. The physical mechanism which generates the seabreeze is associated with the horizontal gradient in sensible heat flux induced by daytimc
solar heating over the cool water surface (low flux) and the warm land surface (high flux).
During the day, greater turbulent vertical mixing produces a lifting of the pressure surfaces
over land compared to ocean. This generates a positive lalld-to-ocean horizontal pressure
gradient at some level which initiates an off shore circulation with compensating on-shore
flow near the surface. This circulation tends to remove the horizontal pressure gradient
in the late afternoon until it dissipates in the early evening. At night, radiational cooling
over land produces a sinking of the pressure surfaces which, by a similar mechanism,
initiates a reverse off-shore circulation.
Although the sea-breeze has been known and studied for many decades, it was only
relatively recently that Yan and Anthes (1984) suggested that a similar OMC might
be triggered by gradients in surface vegetation, e.g. grass vs forest or irrigated vs nonirrigated surfaces. Anthes (1984) suggested that such circulations might significantly affect
local climate, especially in semi-arid regions, and that in fact using specified geometries
for vegetation cover management could be an effective way to modify local climate (Fig.
10). This hypothesis was confirmed by early modeling work (Yan and Anthes 1988), and
since then a multitude of studies have appeared in the literature to analyze the effect of
OMCs induced by surface heterogeneities. Because of the basic two-dimensional nature
of the sea-breeze process the early studies mostly employed two-dimensional mesoscale
models, along with idealized background conditions (Yan and Anthes 1988, Giorgi 1989,
Pielke and Avissar 1990, Pielke et al. 1991). Only recently, full three-dimensional models
have been used to analyze these circulations under more realistic atmospheric forcings
(Lynn 1994, Seth and Giorgi 1995). In particular, the sensitivity of OMCs to different
surface vegetation geometries (individual strips, alternating bands, or checkerboard of wet
and dry areas) has been extensively studied.