Wet deposition parameterization#
Overview#
Wet deposition in PALM is modelled using the parameterization of Berge and Jakobsen (1998). Scavenging rates for specific soluable gas and particulate species are calculated at the in- and below-cloud levels based on the field precipitation rate, through which concentrations of said species can be updated along each vertical column in the computational domain.
Theoretical foundations#
The convention employed in Simpson et al (2012) is used here to describe the parametrization of scavenging rates, expressed in
- In-cloud scavenging rate (
), - Under-cloud scavenging rate for gas phase species (
), and - Under-cloud scavenging rate for particulate species (
).
The in-cloud scavenging rate applies to all relevant gas and particulate species, and is calculated as follows:
where
Similarly, the below-cloud scavenging rate for gas-phase species can be modelled using the relation below:
where
On the other hand, the scavenging of particle species through entrainment of rain droplets is determined using the following equation:
where
Typically, the cloud layer is identified by using the mass fraction of liquid water, when it crosses a threshold of 10
Species | |||
---|---|---|---|
0.3 | 0.15 | N/A | |
1.4 | 0.5 | N/A | |
1.4 | 0.5 | N/A | |
1.4 | 0.5 | N/A | |
1.4 | 0.5 | N/A | |
0.1 | 0.03 | N/A | |
0.05 | 0.015 | N/A | |
1.0 | N/A | 0.02 | |
1.0 | N/A | 0.4 |
Namelist (_p3d
) Options#
The PALM wet deposition model can be activated from the namelist (_p3d
) file by setting the option chem_wet_deposition
to .TRUE.
. The model automatically matches the available gas phase and aerosol species listed in the table above with the species defined in the active chemical mechanism in the file chem_gasphase_mod.f90
by name. The user can further specify the intervals in which the scavenging rates are updated via the namelist option chem_wet_deposition_update_interval
, which is defaulted to 300 seconds.
Normally, the wet deposition model extracts information cloud layer and precipitation from the build cloud model at each update interval (defined by the chem_wet_deposition_update_interval
option) to re-calculate the scavenging rates. This dependence can be disabled by setting the
namelist option chem_wet_deposition_override
to .TRUE.
. In this way, the vertical cloud layer and precipitation rate can be supplied directly by the user. The location of the cloud layer is specified through the options chem_wet_deposition_cloud_level_lower
and chem_wet_deposition_cloud_level_uppers
, and they are defaulted to 10 and 15 respectively, denoting the vertical grid level for the lower and upper bounds of the cloud layer. Meanwhile, the precipitation rate can be specified by the option chem_wet_deposition_rain_rate
, which carries a default of 1.0
A summary of the namelist options are listed below:
_p3d_ Option |
Default | Function |
---|---|---|
chem_wet_deposition |
.FALSE. |
.TRUE. activates the wet deposition model |
chem_wet_deposition_update_interval |
300 | Seconds between scavenging rate updates |
chem_wet_deposition_model_override |
.FALSE. |
.TRUE. to use user-supplied cloud and precipitation information |
chem_wet_deposition_cloud_level_lower |
10 | User-supplied lower bound for the domain cloud layer |
chem_wet_deposition_cloud_level_upper |
15 | User-supplied Upper bound for the domain cloud layer |
chem_wet_deposition_rain_rate |
1.0 | User-supplied rate of rain precipitation for the whole domain |
References#
-
Berge, E., and Jakobsen, H.A.: A regional scale multi-layer model for the calculation of long-term transport and deposition of pollution in Europe, Tellus B, 50, 205-224, 1998.
-
Laakso, L. et al: Ultrafine particle scavenging coefficients calculated from 6 years field measurements, Atmos. Environ., 37, 3605–3613, 2003.
-
Simpson, D. et al: The EMEP MSC-W chemical transport model – technical description, Atmospheric Chemistry and Physics, 12, 7825–7865, 2012.
-
van Zanten, M.C. et al: Controls on precipitation and cloudiness in simulations of trade-wind cumulus as observed during RICO, Journal of Advances in Modeling Earth System 3(2), 2011.