GEE 的Sentinel CloudProbability 的去云算法 代码

以下是基于Google Earth Engine平台的Sentinel-2云去除代码，使用了Sentinel-2的B2、B3、B4、B8波段和cloud_mask，其中cloud_mask是通过Sentinel-2的QA60数据中的cloud_probability和cloud_shadow_probability计算得到的：

// Load Sentinel-2 TOA reflectance data.
var sentinel = ee.ImageCollection('COPERNICUS/S2')
                  .filterDate('2019-01-01', '2019-12-31')
                  .filterBounds(geometry)
                  .map(function(image) {
                    var cloud_mask = ee.Image(0).where(
                        image.select('QA60').bitwiseAnd(1<<10), 1).rename('cloud_mask');
                    var cloud_probability = image.select('QA60').bitwiseAnd(1024).rightShift(10).rename('cloud_probability');
                    var cloud_shadow_probability = image.select('QA60').bitwiseAnd(2048).rightShift(11).rename('cloud_shadow_probability');
                    var cloud_mask_combined = cloud_mask.or(cloud_probability.gt(20)).or(cloud_shadow_probability.gt(20));
                    return image.addBands(cloud_mask_combined);
                  })
                  .map(function(image) {
                    return image.clip(geometry);
                  });

// Function to mask clouds using the Sentinel-2 cloud mask.
var maskClouds = function(image) {
  var cloudMask = image.select('cloud_mask').not();
  return image.updateMask(cloudMask);
};

// Function to calculate the NDVI.
var calculateNDVI = function(image) {
  var ndvi = image.normalizedDifference(['B8', 'B4']).rename('ndvi');
  return image.addBands(ndvi);
};

// Function to calculate the EVI.
var calculateEVI = function(image) {
  var evi = image.expression(
      '2.5 * (nir - red) / (nir + 6 * red - 7.5 * blue + 1)',
      {
        'nir': image.select('B8'),
        'red': image.select('B4'),
        'blue': image.select('B2')
      }).rename('evi');
  return image.addBands(evi);
};

// Apply the cloud mask, calculate the NDVI and EVI, and combine the bands.
var sentinel_ndvi_evi = sentinel
                          .map(maskClouds)
                          .map(calculateNDVI)
                          .map(calculateEVI)
                          .select(['B2', 'B3', 'B4', 'B8', 'ndvi', 'evi']);

// Function to filter images based on the quality of the NDVI and EVI.
var filterQuality = function(image) {
  var ndvi_quality = image.select('ndvi').qualityMosaic('ndvi').gte(0.6);
  var evi_quality = image.select('evi').qualityMosaic('evi').gte(0.6);
  return image.updateMask(ndvi_quality.and(evi_quality));
};

// Filter the images based on the quality of the NDVI and EVI.
var sentinel_filtered = sentinel_ndvi_evi.filter(filterQuality);

// Create a median composite of the filtered images and display it.
var sentinel_median = sentinel_filtered.median();
Map.addLayer(sentinel_median, {bands: ['B4', 'B3', 'B2'], min: 0, max: 0.3}, 'Sentinel-2 Median Composite');

其中，cloud_mask_combined是通过cloud_mask、cloud_probability和cloud_shadow_probability三个变量进行计算得到的云掩膜，用来遮盖掉云和阴影。```calculateNDVI```和```calculateEVI```分别用来计算NDVI和EVI。```filterQuality```用来过滤掉质量较差的NDVI和EVI。最后，使用```sentinel_filtered.median()```来得到过滤后的影像的中值合成图。