python - 近侧透视投影缺失数据轮廓图

Question

def plotMap():
        
    proj=ccrs.NearsidePerspective(central_longitude=270, central_latitude=80)
    fig, ax = plt.subplots(subplot_kw=dict(projection=proj), figsize=(12,10))
    #ax.set_extent([-179, 179, 20, 90], crs=ccrs.PlateCarree())
    
    ax.add_feature(cfeature.OCEAN.with_scale('10m'), facecolor='black',  zorder=12)
    ax.add_feature(cfeature.LAKES.with_scale('50m'), facecolor='black',  zorder=12)  
    ax.add_feature(cfeature.BORDERS.with_scale('10m'), zorder=10)
    ax.add_feature(cfeature.COASTLINE.with_scale('10m'), zorder=13)
  

    states_provinces = cfeature.NaturalEarthFeature(
            category='cultural',  name='admin_1_states_provinces_lines',
            scale='50m', facecolor='none', linewidth=1.2)
    ax.add_feature(states_provinces, edgecolor='gray', zorder=15)
    
    return fig, ax 

fig, ax = plotMap()
cmap=my_cmap1
levels= [-48,-36,-24,-18,-12,-9,-6,-4,-2,2,4,6,9,12,18,24,36,48]
tick_labels = ['-4ft \n -1220mm','-3ft \n -905mm','-2ft \n -610mm','-1.5ft \n -457mm','-1ft \n -305mm','-9in \n -229mm','-6in \n -152mm','-4in \n -102mm','-2in \n -51mm',
                   '+2in \n +51mm','4in \n 102mm','+6in \n +152mm','+9in \n +229mm','+1ft \n +305mm','+1.5ft \n +457mm','+2ft \n +610mm','+3ft \n +905mm','+4ft \n +1220mm']
ticks=levels
contourf = ax.contourf(wrap_lons,lat, lanina_snow_anom_final, levels=levels, cmap=cmap, extend='both', zorder=1.1, transform = ccrs.PlateCarree())
cb = plt.colorbar(contourf, pad=0.01, orientation='horizontal', aspect=30, ticks=ticks)
cb.ax.tick_params(labelsize=13)
cb.ax.set_xlabel('Snowfall Anom', weight='bold')
cb.ax.set_xticklabels(tick_labels, fontsize=8)
cb.outline.set_edgecolor('black')

plt.title('Composite Snowfall Anom DJF 2005-06, 2007-08, 2010-11, 2016-17', fontsize=16, weight='bold')
plt.savefig('/root/research_imgs/winter_2020/laninas_snow_anom_NH', dpi=150, bbox_inches="tight")
plt.close()

此图中的结果：

为什么只绘制正值？我不明白这里发生了什么。如果我将中心经度指定为 -90，也会发生同样的事情。

标签： pythonmatplotlibcartopy