1.
Meteorology
The model has an off-line meteorology: the meteorological fields are input every 3-hour. The fields are provided by ECMWF and FUB (see annex for abbreviations). There is a choice to select one of the two data sets. At the moment, ECMWF data sets available to the model cover the meteorological years 1990 till 2004. For the FUB data set, the period 1995-2004 is covered, and in the near future the extension to 1990-1994 will be made.
1.1 FUB data
Meteorological data are obtained from the Free University of Berlin (FUB).
The meteorological data are produced at the FUB employing a diagnostic meteorological analysis system based on an optimum interpolation procedure on isentropic surfaces. The system utilizes all available synoptic surface and upper air data (Reimer and Scherer, 1992; Kerschbaumer and Reimer, 2003).
The output on the horizontal domain of LOTOS-EUROS of this system is available at TNO. The actual vertical interpolation is performed using a preprocessor at TNO, which enables to specify the vertical resolution, e.g. the vertical extent and the number of layers within and above the mixing layer.
The available meteorological input parameters are listed in Table 7.1. Most of the parameters are used in the model. However, the height of the cloud top and base and the stability parameters are not incorporated. Cloud base and top height are excluded because the quality of the data is not good enough. The stability parameters are calculated inside the model for consistency reasons.
Table 7.1. The meteorological parameters available in the FUB data.
|
Parameter |
|
|
U-wind component |
[m/s]
|
|
V-wind component |
[m/s]
|
|
Temperature |
[K]
|
|
Water vapour |
[ppm]
|
|
Density |
[Kg/m3] |
|
Obukov-Monin length* |
[m]
|
|
Ustar* |
[m/s]
|
|
Precipitation |
[mm/3h]
|
|
10m wind speed |
[m/s]
|
|
2m temperature |
[K]
|
|
Cloud cover |
[]
|
|
Mixing layer height |
[m]
|
|
Surface temperature |
[K]
|
|
Surface humidity* |
[%] |
|
Cloud top* |
[m]
|
|
Cloud base* |
[m]
|
|
Solar radiation |
[W/m2] |
|
Snow fall |
[mm/3h]
|
|
Layer heights |
[m]
|
A few meteorological parameters are calculated or adjusted inside the model. The relative humidity is calculated from the water vapour concentration using the Claussius-Clapeyron relation. In addition, we neglect rain when the 3-hour accumulated amount of rain is less than 0.3 mm. A limit value was necessary as the rain amounts are very often negligibly small but non zero, which results in a wetted surface. A wet surface has a large impact on the dry deposition speeds for some components, e.g. ozone. Consequently, without the limit value these very small rain amounts would affect the dry deposition fluxes significantly. Finally, stability parameters are calculated online, see below.
1.2
ECMWF data
LOTOS-EUROS can also
read GRIB-files that are retrieved from ECMWF (ERA40 data) directly onto the
LOTOS-EUROS grid. After reading the meteo-parameters from specified pressure
levels, data is interpolated in the vertical to get values for the middle of
the LOTOS-EUROS vertical layers.
The following ECMWF single
layer (ground level) meteo fields are read:
§
t
§
cloud
cover
§
boundary
layer height
§
relative
humidity at 2 m
§
wind
velocity at 10 m
§
precipitation.
The following ECMWF
multi-layer (pressure levels) meteo fields are read:
§
geopotential
§
t
§
x-component
of wind velocity
§
y-component
of wind velocity
§
relative
humidity.
Most ECMWF meteorological fields are available for each 3 hours; if there
are only data available each 6 hours, an extra t
Wind components in
LOTOS-EUROS are “terrain following”. Terrain following means practically
that the ground level wind patterns follow the orography of
A few meteorological parameters are
calculated or adjusted inside the model. After the fields are read, the model
calculates the corresponding vertical velocity fields (w) according to the mass conservation law of incompressible fluids.
Further, the water vapour concentration is calculated using the
Claussius-Clapeyron relation. In addition, we neglect rain when the 3-hour
accumulated amount of rain is less then 0.3 mm. A limit value was necessary as
the rain amounts are very often negligibly small but non zero, which results in
a wetted surface. A wet surface has a large impact on the dry deposition speeds
for some components, e.g. ozone. Consequently, without the limit value the very
small rain amounts would affect the dry deposition fluxes significantly. Finally, stability parameters are calculated
online, see below.
Linear interpolation is used to derive the
meteorological fields at the interval times between the update times (0h, 3h,
etc).
1.3
Stability
and vertical diffusion coefficient
The vertical diffusion coefficient Kv is determined by:
where
κ = von Kar
UV = friction velocity
z = height
L = Monin-Obukov
length
Φ= function
proposed by Businger et al. (1971).
The Monin-Obukov
length L is determined as follows:
with a1 and
a2 being constants (0.004349 and 0.003724 respectively), z0
the surface roughness length and S and SE given by:
with b1, b2
and b3 being constants (-0.5034, 0.2310 and –0.0325 resp.). Us
is the wind speed near the surface (given as input into the model) and CE is an
exposure factor depending on cloud cover and solar zenith angle.
For a stable
atmosphere (L>0) the expression of the
For an unstable
atmosphere (L<0) the expression is:
For a neutral
atmosphere the function is equal to unity.
The friction velocity
follows from:
with
The function f in a
stable atmosphere is given by:
In an unstable
atmosphere the function f is:
with the
Aerodynamic resistance
From the stability
parameters presented above one can easily calculate the aerodynamic resistance:
It follows that:
with
fh
analogous to function f but instead of reference height the integral is taken
to the height to which the aerodynamic resistance is required.