Still clouds after cloud masking in MODIS daily reflectance 250m

Question

I try to use "QC_250m" to mask out cloud pixels from MODIS daily reflectance 250m collection, but it did not completely remove cloud pixels.

Here is the code

var terra = ee.ImageCollection("MODIS/061/MOD09GQ").filterDate("2020", "2023");
var cloud_mask = function(img){
  
  var qa = img.select("QC_250m");
  var cloudBitMask = ee.Number(2).pow(0).int()
  var mask = qa.bitwiseAnd(cloudBitMask).eq(0)
  return img.updateMask(mask)
}

var modis = terra.map(cloud_mask)

Map.addLayer(modis.first(), {}, "Cloud-free")

Here is the code link https://code.earthengine.google.com/43d8627435697b4fb11f1600e9ae650b

Answer 1

The MOD09GQ product doesn't contain a cloud indicator. The QA band only tell you if the pixel could be processed, not whether it has clouds.

Typically, people combine the MOD09GA and GQ collections, and use the state_1km band to get at a cloud mask.

var maskClouds = function(image) {
  var QA = image.select('state_1km')
  var bitMask = 1 << 10;
  return image.updateMask(QA.bitwiseAnd(bitMask).eq(0))
}

var dataset = ee.ImageCollection('MODIS/061/MOD09GQ')
  .linkCollection(ee.ImageCollection("MODIS/061/MOD09GA"), ["state_1km"])
  .filter(ee.Filter.date('2018-01-01', '2018-05-01'))
  .map(maskClouds)