title: 多要素场消息

weight: 4

GRIB 2 的消息中第2段到第7段都可以重复，见下图。

译者注：NWPC 的 GRIB 2 数据尚未使用该功能，下面示例只是教程中的文字，尚未实际验证。

多要素场消息 - 图1

一个示例文件如下图所示：

多要素场消息 - 图2

示例

考虑 数值预报模式预报的 12 小时和 24小时的 500 hPa 高度场。

  1. Section 0: Indicator Section
  2. Section 1: Identification Section 
  3. Section 2: Local Use Section (optional) 
  4. Section 3: Grid Definition Section
  6. Section 4: Product Definition Section       (hour = 12) | repetition 1
  7. Section 5: Data Representation Section                  |
  8. Section 6: Bit-Map Section                              |
  9. Section 7: Data Section                                 |
  11. Section 4: Product Definition Section       (hour = 24) | repetition 2
  12. Section 5: Data Representation Section                  |
  13. Section 6: Bit-Map Section                              |
  14. Section 7: Data Section                                 |
  16. Section 8: End Section

注意：网格定义段（Grid Definition Section）未重复，对所有的预报时效都有效。

练习

创建一个多要素场的 GRIB 2 文件。

  1. #include <iostream>
  2. #include <eccodes.h>
  4. using namespace std;
  6. int main(int argc, char** argv)
  7. {
  8.     if(argc < 2)
  9.     {
  10.         cout<<"Usage: "<<argv[0]<<" grib_file_path";
  11.         return 1;
  12.     }
  14.     const char* file_path = argv[1];
  16.     const char* sample_filename = "regular_ll_pl_grib2";
  17.     int err = 0;
  18.     codes_handle *h = codes_grib_handle_new_from_samples(nullptr,  sample_filename);
  19.     if(h == nullptr)
  20.     {
  21.         cout<<"ERROR: unable to create handle from file "<<file_path<<endl;
  22.         return 1;
  23.     }
  25.     size_t value_size;
  26.     codes_get_size(h, "values", &value_size);
  28.     auto values = new double[value_size];
  29.     double v = 0;
  30.     for(auto i=0; i<value_size; i++)
  31.     {
  32.         values[i] = v;
  33.         v++;
  34.     }
  35.     // codes_set_long(h, "bitsPerValue", 16);
  36.     codes_set_double_array(h, "values", values, value_size);
  38.     codes_multi_handle* multi_handle = codes_grib_multi_handle_new(nullptr);
  40.     const int start_section = 4;
  42.     for (int step = 12; step <= 120; step += 12)
  43.     {
  44.         codes_set_long(h, "step", step);
  46.         codes_grib_multi_handle_append(h, start_section, multi_handle);
  47.     }
  50.     FILE* output_file=fopen(file_path, "wb");
  51.     if(!output_file)
  52.     {
  53.         cerr<<"ERROR: unable to open output file " << file_path <<endl;
  54.         exit(1);
  55.     }
  57.     codes_grib_multi_handle_write(multi_handle, output_file);
  58.     fclose(output_file);
  60.     delete [] values;
  61.     codes_handle_delete(h);
  62.     codes_grib_multi_handle_delete(multi_handle);
  64.     return 0;
  65. }

使用 grib_ls 查看生成的文件，ecCodes 将多变量消息处理成多个单变量消息。

  1. $ grib_ls multi.grib2 
  2. multi.grib2
  3. edition      centre       date         dataType     gridType     stepRange    typeOfLevel  level        shortName    packingType  
  4. 2            ecmf         20070323     af           regular_ll   12           isobaricInhPa  850          t            grid_simple 
  5. 2            ecmf         20070323     af           regular_ll   24           isobaricInhPa  850          t            grid_simple 
  6. 2            ecmf         20070323     af           regular_ll   36           isobaricInhPa  850          t            grid_simple 
  7. 2            ecmf         20070323     af           regular_ll   48           isobaricInhPa  850          t            grid_simple 
  8. 2            ecmf         20070323     af           regular_ll   60           isobaricInhPa  850          t            grid_simple 
  9. 2            ecmf         20070323     af           regular_ll   72           isobaricInhPa  850          t            grid_simple 
  10. 2            ecmf         20070323     af           regular_ll   84           isobaricInhPa  850          t            grid_simple 
  11. 2            ecmf         20070323     af           regular_ll   96           isobaricInhPa  850          t            grid_simple 
  12. 2            ecmf         20070323     af           regular_ll   108          isobaricInhPa  850          t            grid_simple 
  13. 2            ecmf         20070323     af           regular_ll   120          isobaricInhPa  850          t            grid_simple 
  14. 10 of 10 messages in multi.grib2
  16. 10 of 10 total messages in 1 files