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Data

The NetCDF file generated at the end of a successful cloud-slicing simulation has the following contents:

netcdf file:/path/to/file/tropomi-ut-no2-fresco-07-1x1-jja-2019-v010101.nc {
  dimensions:
    lon = 361;
    lat = 181;
  variables:
    float lon(lon=361);
      :units = "degrees_east";
      :long_name = "longiitude";

    float nobs(lon=361, lat=181);
      :units = "unitless";
      :long_name = "Number of observations in each gridsquare used to obtain cloud-sliced UT NO2 mixing ratios";

    float cld_top_p_ceil(lon=361, lat=181);
      :units = "hPa";
      :long_name = "Gridded mean ceiling of cloud top pressures used to cloud-slice TROPOMI NO2";

    float cld_top_p_range(lon=361, lat=181);
      :units = "hPa";
      :long_name = "Gridded mean range in cloud top pressures used to cloud-slice TROPOMI NO2";

    float utno2err(lon=361, lat=181);
      :units = "pptv";
      :long_name = "Standard error of the NO2 mixing ratios in the UT (180-450 hPa) obtained using cloud-slicing";

    float utno2(lon=361, lat=181);
      :units = "pptv";
      :long_name = "NO2 mixing ratios in the UT (180-450 hPa) obtained using cloud-slicing";

    float lat(lat=181);
      :units = "degrees_north";
      :long_name = "latitude";

  // global attributes:
}

Variables output in the NetCDF file include:

lon

Centre longitude values for the target grid (1 degree in the above example) [1D]

lat

Centre latitude values for the target grid (1 degree in the above example) [1D]

nobs

The number of cloud-sliced NO2 data points in each gridbox [2D]

utno2

Gridded mean cloud-sliced NO2 mixing ratios in pptv (seasonal means for a single year in the above example) [2D]

utno2err

Gridded mean standard error on the cloud-sliced NO2 mixing ratios in pptv [2D]

cld_top_p_range

Gridded mean cloud top pressure range in hPa [2D] Useful if comparing cloud-sliced NO2 to in situ or model output,as ensures data cover consistent altitude ranges.

cld_top_p_ceil

Gridded mean cloud top pressure ceiling in hPa [2D] Useful if comparing cloud-sliced NO2 to in situ or model output,as ensures data cover consistent altitude ranges.

Sample plots

Plots output with uptrop using cartopy include three panels of global maps showing cloud-sliced nitrogen dioxide (NO2) mixing ratios at the pressure range of interest (top), the estimated error on the cloud-sliced NO2 mixing ratios, and the number of cloud-sliced values. These are not publication quality plots, but are merely for sanity checking and benchmarking.

The example below is from cloud-slicing TROPOMI NO2 in June-August 2019 at 450-180 hPa (~8-12 km):

Log file

The log file tracks the progress of the code and at the end of the simulation outputs metrics. These include the maximum number of satellite pixels in the target grid, the number of satellite pixels removed in each data filtering step, the total number of successful cloud-slicing retrievals compared to the total number that could have been retrieved, and the percent of total TROPOMI pixels used for cloud slicing:

Max no. of data points in a gridsquare:  64.0
(1) Too few points:  280605
(2) Low cloud height range:  260019
(3) Low cloud height std dev:  2105
(4) Large error:  0
(5) Significantly less than zero:  15664
(6) Outlier (NO2 > 200 pptv):  0
(7) Non-uniform stratosphere:  133461
(8) Successful retrievals:  83516
(9) Total possible points:  775370
Mean % points retained:  2.141713715255334

The printout above indicates that the gid with the most cloud-sliced NO2 retrievals has 64 data points, that, of all valid clusters of satellite pixels within the pressure range of interest, 280,605 have too few coincident points, 260,019 have a cloud height range that is less than required, 2,105 have a cloud height stnadrd deviation that is less than required, 0 have too large a cloud-sliced NO2 error, 15,664 have cloud-sliced NO2 that is statistically significantly less than zero, that 0 are outliers, and that for 133,461 the overlying stratospheric NO2 is not uniform. Also included in the printout is that there were 83,516 successful retrievals out of a total of 775,370, and that of all satellite pixels 2.14% are good quality, fall within the cloud pressure range of interest over optically thick clouds.