Mapping issue with pcolor()

When I use pcolor() to show the raster data, there seems something wrong with the data, as shown in the figure below. But using pcolorm(), the data can be correctly mapped. Does anyone know why? The following code is provided by Austin M. Weberhttps://www.mathworks.com/matlabcentral/answers/2092826-changing-the-longitude-of-netcdf-from-0-360-to-180-180?s_tid=mlc_ans_email_view#comment_3101196.

% Repeating the first part
load cpc.mat
lat = double(lat);
lon = double(lon);
mask = lon > 180;
lon(mask) = lon(mask) - 360;
lat = flip(lat);
tmax = flipud(tmax');

% Repeating making a new netcdf file
file_name = 'new.nc';
ncid = netcdf.create(file_name, 'NETCDF4');
lat_len = length(lat);
lon_len = length(lon);
lat_dim = netcdf.defDim(ncid, 'lat', lat_len);
lon_dim = netcdf.defDim(ncid, 'lon', lon_len);
lat_var = netcdf.defVar(ncid, 'lat', 'double', lat_dim);
lon_var = netcdf.defVar(ncid, 'lon', 'double', lon_dim);
tmax_var = netcdf.defVar(ncid, 'tmax', 'double', [lat_dim, lon_dim]);
netcdf.endDef(ncid);
netcdf.putVar(ncid, lat_var, lat);
netcdf.putVar(ncid, lon_var, lon);
netcdf.putVar(ncid, tmax_var, tmax);
netcdf.close(ncid);

% Visualize
mync = ncread('new.nc','tmax');
lat = ncread('new.nc','lat');
lon = ncread('new.nc','lon');

figure(1)
subplot(2,1,1)
    pcolor(lon, lat, mync)
    title('Cartesian Axes with pcolor()')
    colorbar
    shading interp
    axis equal tight
    grid on
subplot(2,1,2)
    axesm('eqdcylin')
    pcolorm(lat, lon, mync)
    title('Map Axes with pcolorm()')
    colorbar
    shading interp
    tightmap
    gridm
    mlabel('south') % Longitude tick labels
    plabel('on') % Latitude tick labels
    

Answer 1

[Voss]

The discontinuity in lon where it changes from +180 to -180 causes the problem with pcolor, which you see as the blue streak in the upper part of the plot. The pcolor documentation says that, in order "to create a rectangular grid of vertices", if input X is a vector it must be "A vector containing values that are increasing or decreasing". With that discontinuity, the values of lon are not all-increasing or all-decreasing.

A simple way fix the pcolor plot is to plot based on a sorted copy lon and a copy of mync where its columns are rearranged in the same order. See lon_plot and mync_plot below.

% Repeating the first part
load cpc.mat
lat = double(lat);
lon = double(lon);
mask = lon > 180;
lon(mask) = lon(mask) - 360;
lat = flip(lat);
tmax = flipud(tmax');

% Repeating making a new netcdf file
file_name = 'new.nc';
ncid = netcdf.create(file_name, 'NETCDF4');
lat_len = length(lat);
lon_len = length(lon);
lat_dim = netcdf.defDim(ncid, 'lat', lat_len);
lon_dim = netcdf.defDim(ncid, 'lon', lon_len);
lat_var = netcdf.defVar(ncid, 'lat', 'double', lat_dim);
lon_var = netcdf.defVar(ncid, 'lon', 'double', lon_dim);
tmax_var = netcdf.defVar(ncid, 'tmax', 'double', [lat_dim, lon_dim]);
netcdf.endDef(ncid);
netcdf.putVar(ncid, lat_var, lat);
netcdf.putVar(ncid, lon_var, lon);
netcdf.putVar(ncid, tmax_var, tmax);
netcdf.close(ncid);

% Visualize
mync = ncread('new.nc','tmax');
lat = ncread('new.nc','lat');
lon = ncread('new.nc','lon');

figure(1)
subplot(2,1,1)
    [lon_plot,idx] = sort(lon);
    mync_plot = mync(:,idx);
    pcolor(lon_plot, lat, mync_plot)
    title('Cartesian Axes with pcolor()')
    colorbar
    shading interp
    axis equal tight
    grid on
subplot(2,1,2)
    axesm('eqdcylin')
    pcolorm(lat, lon, mync)
    title('Map Axes with pcolorm()')
    colorbar
    shading interp
    tightmap
    gridm
    mlabel('south') % Longitude tick labels
    plabel('on') % Latitude tick labels    

[Qian]

It works great! Thank you so much.

[Voss]

You're welcome!