GEE – How to Extract Daily Values for 3 ERA5 Climate Parameters in GEE

Question

I'm new to GEE, and I've been trying to extract daily values from 3 climate parameters (minimum_2m_air_temperature, maximum_2m_air_temperature, and total_precipitation) from the ERA5 Daily Aggregates dataset. I tried using the code provided in this question with the minimum temperature, but the temperature value was null. Is there any way to fix this?

Please see the code I used:

Map.centerObject(ILPLS_shape);

var min_temp = ee.ImageCollection('ECMWF/ERA5/DAILY')
                   .select('minimum_2m_air_temperature')
                   .filter(ee.Filter.date('2016-12-01', '2018-03-02'));

var min_temp_list = min_temp.toList(min_temp.size());

print(min_temp_list);

var study_area = min_temp_list.map(function(img){
  
  var date = ee.Date(ee.Image(img).get('system:time_start')).format().slice(0, 10);
  img = ee.Image(img).subtract(273.15);
  
  // Getting the image value
  var value_temp = ee.Image(img)
    .get('minimum_2m_air_temperature');
  
  return [date, value_temp];
  
});

var vis2mt = {
  min: -20,
  max: -10,
  palette: [
    '#000080', '#0000D9', '#4000FF', '#8000FF', '#0080FF', '#00FFFF', '#00FF80',
    '#80FF00', '#DAFF00', '#FFFF00', '#FFF500', '#FFDA00', '#FFB000', '#FFA400',
    '#FF4F00', '#FF2500', '#FF0A00', '#FF00FF'
  ]
};

var image = ee.Image(min_temp_list.get(0))
  .subtract(273.15)
  .clip(ILPLS_shape);
print(image);

Map.addLayer(image, vis2mt);

print(study_area);

var myFeatures = ee.FeatureCollection(study_area.map(function(el){
  el = ee.List(el); // cast every element of the list

  return ee.Feature(null, {
    'date': el.get(0),
    'value_temp': el.get(1),
  });
}));

print(myFeatures);

Answer 1

You don't provide ILPLS_shape so, I arbitrarily created one. The issue is produced because you forgot to write 'reduceRegion' method in your code; as it can be observed as follows:

  // Getting the image value
  var value_temp = ee.Image(img)
    .get('minimum_2m_air_temperature');

It is mandatory to have:

  var value_temp = ee.Image(img)
    .reduceRegion(ee.Reducer.mean(), ILPLS_shape)
    .get('minimum_2m_air_temperature');

Fixing this, complete code is:

Map.centerObject(ILPLS_shape);

var min_temp = ee.ImageCollection('ECMWF/ERA5/DAILY')
                   .select('minimum_2m_air_temperature')
                   .filter(ee.Filter.date('2016-12-01', '2018-03-02'));

var min_temp_list = min_temp.toList(min_temp.size());

print(min_temp_list);

var study_area = min_temp_list.map(function(img){
  
  var date = ee.Date(ee.Image(img).get('system:time_start')).format().slice(0, 10);
  img = ee.Image(img).subtract(273.15);
  
  var value_temp = ee.Image(img)
    .reduceRegion(ee.Reducer.mean(), ILPLS_shape)
    .get('minimum_2m_air_temperature');
  
  return [date, value_temp];
  
});

var vis2mt = {
  min: -20,
  max: -10,
  palette: [
    '#000080', '#0000D9', '#4000FF', '#8000FF', '#0080FF', '#00FFFF', '#00FF80',
    '#80FF00', '#DAFF00', '#FFFF00', '#FFF500', '#FFDA00', '#FFB000', '#FFA400',
    '#FF4F00', '#FF2500', '#FF0A00', '#FF00FF'
  ]
};

var image = ee.Image(min_temp_list.get(0))
  .subtract(273.15)
  .clip(ILPLS_shape);
print(image);

Map.addLayer(image, vis2mt);

print(study_area);

var myFeatures = ee.FeatureCollection(study_area.map(function(el){
  el = ee.List(el); // cast every element of the list

  return ee.Feature(null, {
    'date': el.get(0),
    'value_temp': el.get(1),
  });
}));

print(myFeatures);

and after run it, you can observe at following picture that features have the second property named 'value_temp' without any null value.

Editing Note:

You're right. Your area only covers a pixel. So, the geometry needs to take its centroid. Following code, for your area in "Cagayan de Oro" (circled in blue in below picture), fix it.

https://code.earthengine.google.com/92ed2c63228deeb2b026c748ec53f59d

After running it at GEE code editor, at the following picture it can be observed the corresponding temperature values.