raerod Function

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 Calculation of the aerodynamical resistance ra from ground up to href  *
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 AUTHOR: Matthias Langer, modified by Andreas Stohl (6 August 1993)     *
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 Literature:                                                            *
 [1]  Hicks/Baldocchi/Meyers/Hosker/Matt (1987), A Preliminary          *
         Multiple Resistance Routine for Deriving Dry Deposition        *
         Velocities from Measured Quantities.                           *
         Water, Air and Soil Pollution 36 (1987), pp.311-330.           *
 [2]  Scire/Yamartino/Carmichael/Chang (1989),                          *
         CALGRID: A Mesoscale Photochemical Grid Model.                 *
         Vol II: User's Guide. (Report No.A049-1, June, 1989)           *
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 Variable list:                                                         *
 L     = Monin-Obukhov-length [m]                                       *
 ust   = friction velocity [m/sec]                                      *
 z0    = surface roughness length [m]                                   *
 href  = reference height [m], for which deposition velocity is         *
         calculated                                                     *
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 Constants:                                                             *
 karman    = von Karman-constant (~0.4)                                 *
 ramin = minimum resistence of ra (1 s/m)                               *
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 Subprograms and functions:                                             *
 function psih (z/L)                                                    *
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