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Using ERA5 data, I am trying to extract the average annual temperature for a region. I am interested in a single value for each year. This is the code I tried to use:

//Define area
var area = ee.FeatureCollection("FAO/GAUL/2015/level2")
        .filter(ee.Filter.eq('ADM0_NAME', 'Germany'))
        .filter(ee.Filter.eq('ADM1_NAME', 'Nordrhein-Westfalen'))
        .filter(ee.Filter.eq('ADM2_NAME', 'Koeln'));
        
//Add area
Map.addLayer(area, {color: 'green', width: 1}, 'area');

var startDate = ee.Date('1979-01-01');
var endDate = ee.Date('2021-12-31')

//Get ERA5 data
var era5_2mt =ee.ImageCollection("ECMWF/ERA5/MONTHLY")
                   .select('mean_2m_air_temperature')
                   .filterDate(startDate, endDate)
                   .filterBounds(area)
                   .distinct('year');
                   
print(era5_2mt);

var era_2mt_mean = era5_2mt.mean().clip(area);
print(era_2mt_mean);
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  • You had a tag for the ArcGIS Spatial Analyst but do not appear to be using it so I removed that.
    – PolyGeo
    Sep 2, 2022 at 22:23

1 Answer 1

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The trick to obtain the desired output is to map a function that gets the pixel mean value of each annual mean temperature raster for all the years of interest. So the first step is to construct a list of the years of interest and then map a function to those years.

var startYear = ee.Number.parse(startDate.format('Y'));
var endYear = ee.Number.parse(endDate.format('Y'));
var years = ee.List.sequence(startYear, endYear, 1);
print('years', years);

//Get ERA5 data
var era5_2mt = ee.FeatureCollection(years.map(function(year){
  // Create start date using year value
  var start = ee.Date.fromYMD(ee.Number(year), 1, 1);
  var resul = ee.ImageCollection("ECMWF/ERA5/MONTHLY")
                   .filterDate(start, start.advance(1,'year'))
                   .filterBounds(area.geometry())
                   .select('mean_2m_air_temperature')
                   // Calculate mean raster
                   .mean()
                   // Calculate the mean of the pixel values
                   .reduceRegion({
                     reducer: ee.Reducer.mean(),
                     geometry: area.geometry(),
                     // Resolution in m for the calculation
                     scale: 27830
                   });
  // Return as Feature without geometry but with the value of interest as a property
  return ee.Feature(null, resul);
}));

print('era5_2mt', era5_2mt);

// Export as csv to drive                   
Export.table.toDrive({
  collection: era5_2mt,
  description: 'mean_2m_air_temperature'
});

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