ncl语言等高线图数值提取

在 NCL 中，可以使用 `cnFillOn` 和 `cnFillDraw` 函数来绘制等高线图。要提取等高线图上的数值，可以使用 `gsn_contour_labelbar` 函数来标注等高线上的数值。下面是一个示例代码：

begin
  ; 读取示例数据
  f = addfile("file.nc", "r")
  data = f->data

  ; 绘制等高线图
  res = True
  res@cnFillOn = True
  res@cnFillDraw = True
  res@cnLinesOn = False
  res@cnLevelSelectionMode = "ManualLevels"
  res@cnMinLevelValF = -2.0
  res@cnMaxLevelValF = 2.0
  res@cnLevelSpacingF = 0.4
  res@cnFillMode = "RasterFill"
  res@cnRasterSmoothingOn = False
  res@cnInfoLabelOn = False
  res@tiMainString = "Contour plot"
  plot = gsn_csm_contour_map(wks, data(0,:,:), res)

  ; 标注等高线上的数值
  res_labelbar = True
  res_labelbar@gsnDraw = False
  res_labelbar@cnLabelBarOn = False
  res_labelbar@cnLinesOn = False
  res_labelbar@cnLineLabelsOn = True
  res_labelbar@cnLineLabelFontHeightF = 0.01
  res_labelbar@cnLineLabelPlacement = "Center"
  res_labelbar@cnLineLabelDensityF = 0.5
  res_labelbar@cnLineLabelBackgroundColor = "Transparent"
  res_labelbar@cnLineLabelPerimOn = False
  res_labelbar@cnLineLabelFontColor = "black"
  res_labelbar@cnLineLabelFormat = "%.1f"
  res_labelbar@lbLabelStrings = (/"-2.0","-1.6","-1.2","-0.8","-0.4","0.0","0.4","0.8","1.2","1.6","2.0"/)
  gsn_contour_labelbar(plot, data(0,:,:), res_labelbar)

  ; 输出等高线上的数值
  labels = plot@cnLineLabels
end

这个代码会绘制出一个等高线图，并在等高线上标注数值。你可以使用 `gsn_contour_labelbar` 函数的输出参数 `labels` 来获取标注的数值。如果需要将标注的数值保存到文件或变量中，可以使用 `labels` 变量。