带起点的流图的Cartopy platecarree和正交投影问题

Question

我想用matplotlib函数plt.streamplot和Cartopy package绘制一个速度场（u，v），作为（经度，纬度）的函数，作为流图，在板块卡里（lon-lat）和正交投影中。我希望固定流图的起点，因为我希望最终创建一些特征在球体上旋转的动画，其中流线“锚定”在某处。然而，我在这些预测中遇到了一些问题，下面将进一步详述

我使用以下包版本：Python3.7.6、matplotlib3.2.2、cartopy0.18.0

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创建包含或不包含上述预测起点的流线图的“最小”示例为：

import numpy as np
import matplotlib.pyplot as plt
import cartopy.crs as ccrs

# define longitude and latitude grids
nlon, nlat = 48, 24
dlon, dlat = 7.5, 7.5
lons = np.linspace(-180+dlon,180,nlon)
lats = np.linspace(-90,90-dlat,nlat)

# define random velocity fields
np.random.seed(999)
u = np.random.rand(nlat, nlon)
v = np.random.rand(nlat, nlon)

# get indices of latitudes north of the equator and longitudes within +/- 90 deg
ilat_north = np.where(lats >= 0)[0]
ilon = np.where(np.abs(lons) <= 90)[0]

# create a list of 5 starting points at various latitudes aligned at a longitude of 0 deg
start_points = list(zip(list(np.linspace(0,0,5)),list(np.linspace(0,80,5))))

# output plot directory and whether plots should be saved
plot_dir = '/home/proxauf/test_streamplot_cartopy'
save_plots = True

def plot_streamplot(mode, start_points=None, figtitle=None, outname=None, save_plot=False):

    fig = plt.figure()
    if mode == 'mpl-platecarree':
        # a Plate-Carree projection without Cartopy (directly using matplotlib)
        ax = plt.gca()
        ax.streamplot(lons, lats[ilat_north], u[ilat_north,:], v[ilat_north,:], start_points=start_points)
    elif 'cartopy' in mode:
        if mode == 'cartopy-platecarree':
            # a Plate-Carree projection with Cartopy
            ax = plt.subplot(projection=ccrs.PlateCarree())
            ax.streamplot(lons, lats[ilat_north], u[ilat_north,:], v[ilat_north,:], start_points=start_points, transform=ccrs.PlateCarree())
        elif mode == 'cartopy-orthographic0':
            # an orthographic projection with a latitudinal inclination of 0 deg with Cartopy
            # restrict longitudes to +/- 90 deg, otherwise an error occurs (outside of projection area)
            ax = plt.subplot(projection=ccrs.Orthographic(central_longitude=0,central_latitude=0))
            ax.streamplot(lons[ilon], lats[ilat_north], u[ilat_north[:,None],ilon[None,:]], v[ilat_north[:,None], ilon[None,:]], start_points=start_points, transform=ccrs.PlateCarree())
        if mode == 'cartopy-orthographic90':
            # an orthographic projection with a latitudinal inclination of 90 deg with Cartopy
            ax = plt.subplot(projection=ccrs.Orthographic(central_longitude=0,central_latitude=90))
            ax.streamplot(lons, lats[ilat_north], u[ilat_north,:], v[ilat_north,:], start_points=start_points, transform=ccrs.PlateCarree())
    if 'platecarree' in mode:
        ax.set_xlim(-180,180)
        ax.set_ylim(-90,90)
    ax.set_aspect('equal')
    if 'cartopy' in mode:
        # draw gridlines with label for visual orientation
        ax.gridlines(draw_labels=True)
    if start_points is not None:
        # plot starting points for the streamplot
        for i in start_points:
            if 'cartopy' in mode:
                ax.plot(i[0],i[1],marker='o',transform=ccrs.PlateCarree())
            else:
                ax.plot(i[0],i[1],marker='o')
    fig.suptitle(figtitle)
    if save_plot:
        # save the plot and close it
        plt.savefig('%s/%s' % (plot_dir, outname))
        plt.close()

# create and save streamplots for different projections, with and without starting points
plot_streamplot(mode='mpl-platecarree', start_points=None, figtitle='mpl-platecarree, without start points', outname='test_streamplot_mpl_pc_sp_no.png', save_plot=save_plots)
plot_streamplot(mode='cartopy-platecarree', start_points=None, figtitle='cartopy-platecarree, without start points', outname='test_streamplot_cartopy_pc_sp_no.png', save_plot=save_plots)
plot_streamplot(mode='cartopy-orthographic0', start_points=None, figtitle='cartopy-orthographic0, without start points', outname='test_streamplot_cartopy_ortho0_sp_no.png', save_plot=save_plots)
plot_streamplot(mode='cartopy-orthographic90', start_points=None, figtitle='cartopy-orthographic90, without start points', outname='test_streamplot_cartopy_ortho90_sp_no.png', save_plot=save_plots)
plot_streamplot(mode='mpl-platecarree', start_points=start_points, figtitle='mpl-platecarree, with start points', outname='test_streamplot_mpl_pc_sp_yes.png', save_plot=save_plots)
plot_streamplot(mode='cartopy-platecarree', start_points=start_points, figtitle='cartopy-platecarree, with start points', outname='test_streamplot_cartopy_pc_sp_yes.png', save_plot=save_plots)
plot_streamplot(mode='cartopy-orthographic0', start_points=start_points, figtitle='cartopy-orthographic0, with start points', outname='test_streamplot_cartopy_ortho0_sp_yes.png', save_plot=save_plots)
plot_streamplot(mode='cartopy-orthographic90', start_points=start_points, figtitle='cartopy-orthographic90, with start points', outname='test_streamplot_cartopy_ortho90_sp_yes.png', save_plot=save_plots)

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下面是这个最小示例创建的输出

案例1：无起点的Mpl板材搬运：工作正常：

案例2：无起点的Cartopy板卡利：工作正常：

案例3:Cartopy-Orthographic-0无起点：工作正常（？），但高纬度被切断：

案例4:Cartopy-Orthographic-90无起点：工作正常：

案例5：Mpl板材搬运，起点：工作良好：

案例6：带起点的Cartopy板卡雷：未显示两个起点的两条流线：

案例7：带起点的Cartopy-Orthographic-0：未显示任何数据（无流线、起点等）：

案例8:Cartopy-Orthographic-90，起点：仅显示北半球的一部分，流线似乎不正确：

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总结：案例6、7和8（Cartopy Plate Carree/Orthographic-0deg-倾角/Orthographic-90deg-倾角投影）给出了意外的、可能不正确的输出。有人能给我解释一下为什么情节是这样的吗？我是否误解了我应该给出的输入？

小问题：如果我想为正交投影设置轴限制（例如，0度倾角的纬度限制为[-90，+90]度，90度倾角的纬度限制为[0，+90]度，还有经度限制），例如，假设我只打印30度以上的数据，这是如何工作的ax.set_xlim或ax.set_ylim不起作用，我假设（因为他们不知道投影）。如果这需要通过ax.set_extent完成，我需要在那里设置哪些经度和纬度限制