Changes between Version 63 and Version 64 of doc/tec/bc
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- Jan 14, 2019 1:23:01 PM (6 years ago)
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doc/tec/bc
v63 v64 116 116 }}} 117 117 118 Currently, there are three different options to calculate the Obukhov length and the surface fluxes which are steered via the NAMELIST parameter [wiki:doc/app/inipar#most_method most_method]. 119 120 121 === Method 1: circular === 122 123 The traditional implementation in PALM ({{{most_method = 'circular'}}}) requires the use of data from the previous time step. The following steps are thus carried out in sequential order. First of all, ''θ'',,*,, and ''q'',,*,, are calculated by integration of the corresponding vertical derivation functions mentioned above using the value of ''z'',,MO,,/L from the previous time step. Second, the new value of ''z'',,MO,,/L is derived from the equation for ''L'' using the new values of ''θ'',,*,, and ''q'',,*,, but using ''u'',,*,, from the previous time step. Then, the new values of ''u'',,*,,, and subsequently the momentum fluxes are calculated by integration, respectively. At last, the above equations for the scaling parameters are employed to calculate the new surface fluxes by using ''θ'',,*,, and ''q'',,*,,, and ''u'',,*,,. In the special case, when surface fluxes are prescribed instead of surface temperature and humidity, the first and last steps are omitted and ''θ'',,*,, and ''q'',,*,, are directly calculated from ''u'',,*,, and the surface fluxes. 124 125 In summary, the following actions are performed in sequential order: 126 1. calculate ''θ'',,*,, and ''q'',,*,, 127 2. calculate ''u'',,h,, 128 3. determine Obukhov length 129 4. calculate ''u'',,*,, 130 5. derive surface fluxes 131 132 133 === Method 2: Newton iteration / lookup table === 134 135 Alternatively, the Obukhov length can be calculated by solving an implicit equation relating the ''L'' to the bulk Richardson number. This can be achieved either by a Newton iteration algorithm ({{{most_method = 'newton'}}}) or by using a lookup table ({{{most_method = 'lookup'}}}). Note that the latter is faster than the Newton iteration method and the results of both methods are more precise compared to the circular method. However, it can only be used when the roughness lengths are homogeneously set on each processor. 118 The Obukhov length can be calculated by solving an implicit equation relating the ''L'' to the bulk Richardson number. This is achieved using a Newton iteration method. The surface fluxes are calculated based on the following sequence of actions: 136 119 137 120 Both methods require a different sequential order to derive the surface fluxes: 138 121 1. calculate ''u'',,h,, 139 2. determine Obukhov length (Newton iteration or lookup table)122 2. determine Obukhov length 140 123 3. calculate ''u'',,*,, 141 124 4. calculate ''θ'',,*,, and ''q'',,*,, … … 172 155 \end{equation*} 173 156 }}} 174 In case of {{{most_method = 'newton'}}}, the above equations are solved for ''L'' by Newton iteration, i.e. finding the root of the equation157 The above equations are solved for ''L'' by Newton iteration, i.e. finding the root of the equation 175 158 {{{ 176 159 #!Latex … … 195 178 until ''L'' meets a convergence criterion. 196 179 197 If {{{most_method = 'lookup'}}} is used, a table of ''Ri'',,b,, against ''z'',,MO,,/''L'' is created at model start, based on the prescribed values of the roughness lengths. During the model run, ''Ri,,b,,'' is calculated and the respectively value of ''z'',,MO,,/''L'' is retrieved from the lookup table and using linear interpolation between the discrete values in the table. In order to speed up this method, the value of ''Ri,,b,,'' from the previous time step is used as initial value. Due to the fact that the lookup table is created at model start, it is essential that the roughness lengths are 1) homogeneous on each processor (limiting this method to homogeneous surface configurations) and should not vary during the simulation (should thus not be used when using dynamic roughness length over water surfaces). For more details, see [source:palm/trunk/SOURCE/surface_layer_fluxes.f90 surface_layer_fluxes.f90]. 198 199 Furthermore, the flat bottom of the model can be replaced by a Cartesian topography (see Sect. [wiki:doc/tec/bc#Topography Topography]). 180 The flat bottom of the model can be replaced by a Cartesian topography (see Sect. [wiki:doc/tec/bc#Topography Topography]). 200 181 201 182 By default, lateral boundary conditions are set to be cyclic in both directions. Alternatively, it is possible to opt for non-cyclic conditions in one direction, i.e., a laminar or turbulent inflow boundary (see Sect. [wiki:doc/tec/bc#Laminarandturbulentinflowboundaryconditions Laminar and turbulent inflow boundary conditions]) and an open outflow boundary on the opposite site (see Sect. [wiki:doc/tec/bc#Openoutflowboundaryconditions Open outflow boundary conditions]). The boundary conditions for the other direction have to remain cyclic. A complete overview about the non-cyclic lateral boundary conditions is given in Sect. [wiki:doc/tec/bc#Non-cycliclateralboundaryconditions non-cyclic lateral boundary conditions]
