Bulk Cloud Model Parameters

Overview

This page is part of the Bulk Cloud Model (BCM) documentation.
Since revision 3274 the bulk cloud model (before named with microphysics)
is modularized and has a own parameter namelist.
It contains a listing of all PALM input parameters used to steer the BCM.
For an overview of all BCM-related pages, see the Bulk Cloud Model main page.

Parameter list

NAMELIST group name: bulk_cloud_parameters

Parameter Name	FORTRAN Type	Default Value	Explanation
bulk_cloud_model	L	.F.	Parameter to switch on the bulk cloud model. For bulk_cloud_model = .T., equations for the total water mixing ratio and the liquid water potential temperature are solved instead of those for water vapor mixing ratio and potential temperature. The parameterization of cloud and precipitation physics can be steered with cloud_scheme. Also, cloud-top cooling by longwave radiation can be utilized (see cloud_top_radiation). bulk_cloud_model = .T. requires humidity = .T.. This bulk cloud model can not be applied if cloud droplets are simulated explicitly (see cloud_droplets).
cloud_scheme	C*20	'saturation_adjust'	Parameter to choose microphysics for bulk cloud physics (which requires cloud_physics = .TRUE.). The following values are allowed: 'saturation_adjust' Simple saturation adjustment scheme (also known as 0%-or100% scheme), in which a grid volume is either saturated or subsaturated. Detailed information about the condensation scheme is given in the description of the cloud physics. Supersaturations are instantaneously condensed to liquid water. No precipitation is produced. If precipitation is important, use 'kessler, 'seifert_beheng' or 'morrison'. 'kessler' One-moment cloud microphysics according to Kessler (1969). It is also based on the saturation adjustment scheme to diagnose cloud water. However, it allows precipitation if the liquid cloud water exceeds a threshold value. This water is instantaneously removed from the model domain. Additionally, liquid cloud water is allowed to sediment if cloud_water_sedimentation is set to true. 'seifert_beheng' Two-moment cloud microphysics according to Seifert and Beheng (2006). It is also based on the saturation adjustment scheme to diagnose cloud water. The cloud drop number concentration is set via nc_const. Rain water and hence precipitation is treated with two additional prognostic equations for rain water mixing ratio and rain drop concentration, including autoconversion, accretion, selfcollection, breakup, evaporation, and sedimentation. Ventilation effect on evaporation is steered by ventilation_effect and set to true per default. Sedimentation can be controlled via c_sedimentation, limiter_sedimentation. Turbulence effects on accretion and autoconversion are steered via collision_turbulence. Additionally, liquid cloud water is allowed to sediment too if cloud_water_sedimentation is set to true. 'morrison' Two-moment cloud microphysics according to Seifert and Beheng (2006), Khairoutdinov and Kogan (2000), Khvorostyanov and Curry (2006) and Morrison and Grabowski (2007). The 'morrison'-scheme can be understood as an extension of the implemented 'seifert_beheng'-scheme. In comparison to the 'seifert_beheng'-scheme there are three main differences. First, instead of saturation adjustment the diffusional growth is parametrized while calculating condensation/evaporation rates explicitly. However, for a appropriate usage of this scheme the time-step must smaller than the diffusional growth relaxation time. Usually, this is in the order of 1-2 seconds. Second, the activation is considered with a simple Twomey activation-scheme. For this also Koehler-theory can take into account with the parameter curvature_solution_effects_bulk = T. The background aerosol concentration, which also determines the maximum number of activated cloud droplets, can be prescribed with na_init. Thirdly, the number concentration of cloud droplets (nc) and the cloud water mixing ratio (qc) are prognostic quantities. This allows a change of the cloud droplet number which is also considered for all microphysical processes.
aerosol_bulk	C*20	'nacl'	Parameter to choose the used aerosol type. Currently three approximations are available: 'nacl' It is assumed that the aerosol is nacl. 'c3h4o4' It is assumed that the aerosol is malonic acid. 'nh4no3' It is assumed that the aerosol is ammonium sulfate. The molecular weight, denisty and the solubility (vant Hoff factor) of this specific type is considered.
call_microphysics_at_all_substeps	L	.F.	Parameter to control how often 2-moment cloud microphysics (cloud_scheme = 'seifert_beheng') are computed during a model time step. Using the default, cloud microphysics are computed once before the time step. Using call_microphysics_at_all_substeps = .T., cloud microphysics are computed before every substep of the applied time step scheme, which is, however, not necessary to gain acceptable results. Note that advection and diffusion of rainwater mixing ratio (qr) and rain drop concentration (nr) are not affected by this parameter (these processes are computed as any other scalar).
c_sedimentation	R	2.0	Courant number for sedimentation process. A Courant number that is too big inhibits microphysical interactions of the sedimented quantity. There is no need to use the limiter (limiter_sedimentation) if c_sedimentation <= 1.0. This parameter only comes into effect if the microphysical cloud scheme according to Seifert and Beheng (2006) is used (cloud_scheme = 'seifert_beheng').
curvature_solution_effects_bulk	L	.F.	Parameter to switch on an activation scheme which considers curvature and solution effects of cloud droplet activation. Therefore a parameterization of Khvorostyanov and Curry (2006) is used. The physio-chemical aerosol properties can be prescribed with aerosol_bulk, dry_aerosol_radius and sigma_bulk.
cloud_water_sedimentation	L	.F.	Parameter to consider sedimentation of cloud water according to Ackermann et al. (2009, MWR). This parameter only comes into effect if the microphysical cloud scheme according to Seifert and Beheng (2006) (cloud_scheme = 'seifert_beheng') or by Kessler (1969) (cloud_scheme = 'kessler') is used.
dry_aerosol_radius	R	0.05E-6	The mean geometric radius of the dry aerosol spectrum.
limiter_sedimentation	L	.T.	Slope limiter in sedimentation process according to Stevens and Seifert (2008). This parameter only comes into effect if the microphysical cloud scheme according to Seifert and Beheng (2006) is used (cloud_scheme = 'seifert_beheng'). If c_sedimentation <= 1.0 there is no need to use the limiter.
na_init	R	100.0E6	Background dry aerosol concentration. If cloud_scheme = 'morrison' is used this parameter replaces nc_const. Activation is parameterized assuming that the number of activated CCN cannot be larger than na_init. This parameter only comes into effect if the microphysical cloud scheme according to Morrison and Grabowski (2007) is used (cloud_scheme = 'morrison').
nc_const	R	70.0E6	Fixed cloud droplet number density (in 1/m3). The default value is applicable for marine conditions. This parameter only comes into effect if the microphysical cloud scheme according to Seifert and Beheng (2006) is used (cloud_scheme = 'seifert_beheng').
collision_turbulence	L	.F.	Turbulence effects on the collision process, namely the autoconversion and accretion according to Seifert, Nuijens and Stevens (2010). This parameter only comes into effect if the microphysical cloud scheme according to Seifert and Beheng (2006) is used (cloud_scheme = 'seifert_beheng').
sigma_bulk	R	2.0	The dispersion of the dry aerosol spectrum.
ventilation_effect	L	.T.	Parameter to consider the ventilation effect on evaporation of raindrops according to Seifert (2008). This parameter only comes into effect if the microphysical cloud scheme according to Seifert and Beheng (2006) is used (cloud_scheme = 'seifert_beheng').

References