alt_to_press.c

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00001 /* ***************************************************** */
00002 /* Compute standard atmosphere pressure given altitude.  */
00003 /* alt_to_press.c                                        */
00004 /* ***************************************************** */
00005 /* Author: Christian Page, CERFACS, Toulouse, France.    */
00006 /* ***************************************************** */
00011 /* LICENSE BEGIN
00012 
00013 Copyright Cerfacs (Christian Page) (2015)
00014 
00015 christian.page@cerfacs.fr
00016 
00017 This software is a computer program whose purpose is to downscale climate
00018 scenarios using a statistical methodology based on weather regimes.
00019 
00020 This software is governed by the CeCILL license under French law and
00021 abiding by the rules of distribution of free software. You can use, 
00022 modify and/ or redistribute the software under the terms of the CeCILL
00023 license as circulated by CEA, CNRS and INRIA at the following URL
00024 "http://www.cecill.info". 
00025 
00026 As a counterpart to the access to the source code and rights to copy,
00027 modify and redistribute granted by the license, users are provided only
00028 with a limited warranty and the software's author, the holder of the
00029 economic rights, and the successive licensors have only limited
00030 liability. 
00031 
00032 In this respect, the user's attention is drawn to the risks associated
00033 with loading, using, modifying and/or developing or reproducing the
00034 software by the user in light of its specific status of free software,
00035 that may mean that it is complicated to manipulate, and that also
00036 therefore means that it is reserved for developers and experienced
00037 professionals having in-depth computer knowledge. Users are therefore
00038 encouraged to load and test the software's suitability as regards their
00039 requirements in conditions enabling the security of their systems and/or 
00040 data to be ensured and, more generally, to use and operate it in the 
00041 same conditions as regards security. 
00042 
00043 The fact that you are presently reading this means that you have had
00044 knowledge of the CeCILL license and that you accept its terms.
00045 
00046 LICENSE END */
00047 
00048 
00049 
00050 
00051 
00052 
00053 
00054 #include <utils.h>
00055 
00057 void
00058 alt_to_press(double *pres, double *alt, int ni, int nj) {
00059 
00067   /*
00068     ;   Convert an array of (pressure-) altitudes (m) into an array of 
00069     ;       pressures (hPa) using the ICAO standard atmosphere definition
00070     ;
00071     ;       See <A HREF="http://www.pdas.com/coesa.htm">exact definition
00072     ;       here<\A>
00073     ;       
00074     ;       The 7 layers of the US standard atmosphere are:
00075     ;
00076     ;        h1   h2     dT/dh    h1,h2 geopotential alt in km
00077     ;         0   11     -6.5     dT/dh in K/km
00078     ;        11   20      0.0
00079     ;        20   32      1.0
00080     ;        32   47      2.8
00081     ;        47   51      0.0
00082     ;        51   71     -2.8   
00083     ;        71   84.852 -2.0
00084   */
00085 
00086 #define NLAYERS 7 
00088   int i; /* Loop counter */
00089   int layr; /* Loop counter */
00090 
00091   double limits[NLAYERS+1];
00092   double lapse_rate[NLAYERS];
00093   int isoth[NLAYERS];
00094   double presb[NLAYERS];
00095   double tb[NLAYERS];
00096 
00097   int layer = 0;
00098 
00099   /* Layer boundaries in m */
00100   limits[0] = 0.0;
00101   limits[1] = 11.0 * 1000.0;
00102   limits[2] = 20.0 * 1000.0;
00103   limits[3] = 32.0 * 1000.0;
00104   limits[4] = 47.0 * 1000.0;
00105   limits[5] = 51.0 * 1000.0;
00106   limits[6] = 71.0 * 1000.0;
00107   limits[7] = 84.852 * 1000.0;
00108 
00109   /* Lapse rates in each layer (9 means 0) */
00110   lapse_rate[0] = -6.5 / 1000.0;
00111   lapse_rate[1] = 9.0;
00112   lapse_rate[2] = 1.0 / 1000.0;
00113   lapse_rate[3] = 2.8 / 1000.0;
00114   lapse_rate[4] = 9.0;
00115   lapse_rate[5] = -2.8 / 1000.0;
00116   lapse_rate[6] = -2.0 / 1000.0;
00117   
00118   /* Flag for isothermal layers */
00119   isoth[0] = 0;
00120   isoth[1] = 1;
00121   isoth[2] = 0;
00122   isoth[3] = 0;
00123   isoth[4] = 1;
00124   isoth[5] = 0;
00125   isoth[6] = 0;
00126 
00127   presb[0] = 1013.25;
00128   tb[0] = 288.15;
00129 
00130   /* Loop over layers and get pressures and temperatures at level tops */
00131   for (i=0; i<(NLAYERS-1); i++) {
00132     tb[i+1] = tb[i] + (1-isoth[i]) * lapse_rate[i] * (limits[i+1] - limits[i]);
00133     presb[i+1] = (1-isoth[i]) * presb[i] * exp(log(tb[i]/tb[i+1]) * K_GMR / lapse_rate[i]) +
00134       isoth[i] * presb[i] * exp(-K_GMR * (limits[i+1]-limits[i]) / tb[i]);
00135   }
00136 
00137   for (i=0; i<(ni*nj); i++) {
00138     /* Now calculate which layer each value belongs to */
00139     for (layr=0; layr<NLAYERS; layr++) {
00140       if ( (limits[layr] - alt[i]) > 0.0 ) {
00141         layer = layr - 1;
00142         layr = NLAYERS + 1;
00143       }
00144     }
00145     if (layer < 0) layer = 0;
00146     if (layer > (NLAYERS-1)) layer = NLAYERS - 1;
00147     
00148     /* Corresponding pressure */
00149     pres[i] = isoth[layer] * presb[layer] * exp(-K_GMR * (alt[i] - limits[layer]) / tb[layer]) +
00150       (1-isoth[layer]) * presb[layer] * pow( ( tb[layer] / ( tb[layer] + lapse_rate[layer] *
00151                                                              (alt[i] - limits[layer]) ) ), (K_GMR/lapse_rate[layer]) );
00152   }
00153   
00154 }

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