Level 2 Contour Plot Issue Using Python

[yiboj]

There is an issue/bug with python matplotlib package with the filled contour plot as shown in Figure 1. The horizontal lines runs across the map when the longitude data values are mixed across the boundary (0 or 360 degree).
Here is the code to create Figure 1, which shows the wrong contour plot by using RSS SMAP Level 2 data on April 1, 2015.

Code: Select all

import matplotlib.pyplot as plt
from mpl_toolkits.basemap import Basemap
from netCDF4 import Dataset
from pylab import *

# Read in SMAP sea surface salinity L2 granule file
filename = 'RSS_smap_SSS_L2C_r00872_v02.0_70km.nc'
ncin = Dataset(filename, 'r')
lons = ncin.variables['cellon'][:]
lats = ncin.variables['cellat'][:]
sss = ncin.variables['sss_smap'][:]
ncin.close()

lats = lats[:,:,1]
lons = lons[:,:,1]
sss = sss[:,:,1]
lats = lats.squeeze()
lons = lons.squeeze()
sss = sss.squeeze()

# Setup plot
plt.figure(figsize=(2, 1.12), dpi=300)

# Contour levels
clevs = np.linspace(31, 39, 101)

cmap=plt.get_cmap("jet")
cmap.set_under("black")
cmap.set_over("DarkViolet")

m = Basemap(projection='cyl', llcrnrlon=0.0, llcrnrlat=-90,
        urcrnrlon=360.0, urcrnrlat=90)
m.bluemarble()

# Filled contour
cs=m.contourf(lons,lats, sss, clevs, cmap=cmap, extend='both')

# Create extended colorbar
cb = m.colorbar(cs, 'right', size='2%', pad='0.5%')
cb.ax.set_yticklabels(cb.ax.get_yticklabels(), fontsize=4)
cb.set_label('SSS (PSU)', fontsize=4,fontweight="bold")
cb.set_ticks(range(30,40,1))

plt.title("RSS_smap_SSS_L2C_r00872_v02.0_70km.nc", fontsize=4,fontweight="bold")

plt.subplots_adjust(left=0.05, right=0.94, top=0.98, bottom=0.01)

plt.show()


Figure 1, wrong contour plot

l2_bad.png (241.46 KiB) Viewed 2812 times

The easy and fast way to fix the contour plot is to separate the data before and after the boundary (360.0 degree). User may also fix the contour plot routine contourf in the matplotlib package. Here is the code to create Figure 2, which shows the correct contour plot.

Code: Select all

import matplotlib.pyplot as plt
from mpl_toolkits.basemap import Basemap
from netCDF4 import Dataset
from pylab import *

# Read in SMAP sea surface salinity L2 granule file
filename = 'RSS_smap_SSS_L2C_r00872_v02.0_70km.nc'
ncin = Dataset(filename, 'r')
lons = ncin.variables['cellon'][:]
lats = ncin.variables['cellat'][:]
sss1 = ncin.variables['sss_smap'][:]
sss2 = ncin.variables['sss_smap'][:]
ncin.close()

lats = lats[:,:,1]
lons = lons[:,:,1]
sss1 = sss1[:,:,1]
sss2 = sss2[:,:,1]
lats = lats.squeeze()
lons = lons.squeeze()
sss1 = sss1.squeeze()
sss2 = sss2.squeeze()

idx1 = np.where(lons>=180.0)
sss1[idx1] = np.nan

idx2 = np.where(lons<180.0)
sss2[idx2] = np.nan

# Setup plot
plt.figure(figsize=(2, 1.12), dpi=300)

# Contour levels
clevs = np.linspace(31, 39, 101)

cmap=plt.get_cmap("jet")
cmap.set_under("black")
cmap.set_over("DarkViolet")
m = Basemap(projection='cyl', llcrnrlon=0.0, llcrnrlat=-90,
        urcrnrlon=360.0, urcrnrlat=90)

m.bluemarble()


# Filled contour
cs=m.contourf(lons,lats, sss1, clevs, cmap=cmap, extend='both')
cs=m.contourf(lons,lats, sss2, clevs, cmap=cmap, extend='both')

# Create extended colorbar
cb = m.colorbar(cs, 'right', size='2%', pad='0.5%')
cb.ax.set_yticklabels(cb.ax.get_yticklabels(), fontsize=4)
cb.set_label('SSS (PSU)', fontsize=4,fontweight="bold")
cb.set_ticks(range(30,40,1))

plt.title("RSS_smap_SSS_L2C_r00872_v02.0_70km.nc", fontsize=4,fontweight="bold")

plt.subplots_adjust(left=0.05, right=0.94, top=0.98, bottom=0.01)


Figure 2, correct contour plot

l2_good.png (286.57 KiB) Viewed 2812 times