98
Lower boundary conditions are the fluxes of water and energy at the bottom of the
ESEM soil region (e.g. provided by an hydrology model). Upper boundary conditions
are the climatic forcings provided by an AM, which usually consist of solar and infrared
incident radiation, precipitation, near surface air temperature, water vapor, air density
and wind speed. Lateral boundary conditions are soil and vegetation characteristics such
as soil texture and reflectivity (color), roughness, albedo, fractional vegetation cover, measures of the biomass (e.g. leaf area index, LA!), measures of the f3 evapotranspiration
parameter (which, as will be discussed in section 3 is related to soil and vegetation characteristics, such as the vegetation root and stomatal resistances) and runoff efficiencies.
These "lateral boundary conditions" can be either provided by coupled biosphere and
hydrology models or can be specified as input parameters.
In the next section, a review is given of available ESEMs, from the highly simplified
schemes used in early climate models to state-of-the-art, biophysically-based models.
3. Approaches to surface process modeling
In section 2 we have seen that, within the context of climate system modeling, the
role of ESEMs is to provide an interface between the different components of the climate
system. In practice, this translates into the following functions: 1) Provide fluxes at the
surface-atmosphere interface of net radiation, momentum, sensible heat and water vapor;
2) Describe the energy and water budget of a near surface soil region where biospheric
processes are important; 3) Describe the energy and water budget of a vegetative canopy;
4) Describe the cycle of snow formation and melting.
Since the development of the first general circulation models, surface schemes of increasing physical complexity have been developed to carry out these tasks, and presently,
over 30 surface process models are available in the literature. The purpose of this section
is to provide a description of the basic mathematical and physical frameworks of different
modeling approaches, from early simplified schemes to present state-of-the-art models.
3.1. Early simplified ESEMs
In the early stages of climate model development, the surface of the Earth was treated
very simply. Basically, the need was simply to provide acceptable values of surface fluxes as
lower boundary condition for AMs (see section 2). Early three-dimensional climate models
did not include the diurnal cycle and distinguished only three types of surfaces: land,
ocean and ice/snow. These types where generally specified with constant values of albedo,
emissivity, drag coefficient and surface wetness factor. The surface skin temperature over
land areas was calculated from an instantaneous energy balance equation
(1 - alSo + El Rv - wBT: - SH - LH = 0
(22)
Lower boundary conditions are the fluxes of water and energy at the bottom of the
ESEM soil region (e.g. provided by an hydrology model). Upper boundary conditions
are the climatic forcings provided by an AM, which usually consist of solar and infrared
incident radiation, precipitation, near surface air temperature, water vapor, air density
and wind speed. Lateral boundary conditions are soil and vegetation characteristics such
as soil texture and reflectivity (color), roughness, albedo, fractional vegetation cover, measures of the biomass (e.g. leaf area index, LA!), measures of the f3 evapotranspiration
parameter (which, as will be discussed in section 3 is related to soil and vegetation characteristics, such as the vegetation root and stomatal resistances) and runoff efficiencies.
These "lateral boundary conditions" can be either provided by coupled biosphere and
hydrology models or can be specified as input parameters.
In the next section, a review is given of available ESEMs, from the highly simplified
schemes used in early climate models to state-of-the-art, biophysically-based models.
3. Approaches to surface process modeling
In section 2 we have seen that, within the context of climate system modeling, the
role of ESEMs is to provide an interface between the different components of the climate
system. In practice, this translates into the following functions: 1) Provide fluxes at the
surface-atmosphere interface of net radiation, momentum, sensible heat and water vapor;
2) Describe the energy and water budget of a near surface soil region where biospheric
processes are important; 3) Describe the energy and water budget of a vegetative canopy;
4) Describe the cycle of snow formation and melting.
Since the development of the first general circulation models, surface schemes of increasing physical complexity have been developed to carry out these tasks, and presently,
over 30 surface process models are available in the literature. The purpose of this section
is to provide a description of the basic mathematical and physical frameworks of different
modeling approaches, from early simplified schemes to present state-of-the-art models.
3.1. Early simplified ESEMs
In the early stages of climate model development, the surface of the Earth was treated
very simply. Basically, the need was simply to provide acceptable values of surface fluxes as
lower boundary condition for AMs (see section 2). Early three-dimensional climate models
did not include the diurnal cycle and distinguished only three types of surfaces: land,
ocean and ice/snow. These types where generally specified with constant values of albedo,
emissivity, drag coefficient and surface wetness factor. The surface skin temperature over
land areas was calculated from an instantaneous energy balance equation
(1 - alSo + El Rv - wBT: - SH - LH = 0
(22)