Export hourly wind speed ERA5 data

Question

I am trying to export hourly wind speed data from ERA5 hourly dataset via GEE. But when I am exporting the dataset it gives me daily average. I want a CSV file with dates, times in separate columns along with the wind speed value. I am trying to modify this code.

var ERA5 = ee.ImageCollection("ECMWF/ERA5_LAND/HOURLY")
     .filter(ee.Filter.date('2017-09-01', '2017-10-01'));
     
     
var ERA5DOM = ERA5.map(function(im){ 
   return im.clip(AOI);
});

//var u = ERA5DOM.select('u_component_of_wind_10m');
//var v = ERA5DOM.select('v_component_of_wind_10m');

//var calcwind = function(feature) {
//    return feature.set({windspeed:((u.pow(2)).add(v.pow(2)).sqrt()) })
//};

// Wind calculation

var ERA5windspeed = ERA5DOM.map(function(image){
  var wind_10m = image.expression(
    'sqrt(u**2 + v**2)', {
      'u': image.select('u_component_of_wind_10m'),
      'v': image.select('v_component_of_wind_10m')
  }).rename('windspeed');
  
  var time = image.get('system:time_start');
  return wind_10m.set('system:time_start', time) } );
  
print(ERA5windspeed);


Map.setCenter(-61.34, 15.4, 12);

var vis = {
  min: 0,
  max: 30,
  palette: [
    '#FFFFFF', '#FFFF71', '#DEFF00', '#9EFF00', '#77B038', '#007E55', '#005F51',
    '#004B51', '#013A7B', '#023AAD'
  ]
};

Map.addLayer(ERA5windspeed.select('windspeed'), vis, "Wind speed (m/s) 10m above surface");

//var Wind_chart = ui.Chart.image.series(ERA5windspeed, AOI, ee.Reducer.mean(), 1000, 'system:time_start')
//  .setOptions({
 //title: 'Wind speed Full Time Series',
 //vAxis: {title: 'm/s'},  });
//print(Wind_chart);

// Export the FeatureCollection to a csv file.
// Export.table.toDrive({
//   collection: ERA5windspeed.select('windspeed'),
//   description: 'DOM_mean_wind',
//   fileformat: 'csv',
//   selectors: ['date', 'windspeed', 'hour']
// });

                    

Answer 1

If your aim is just to get the csv you mentioned, you don't need to map ERA5DOM to an ImageCollection and to make a chart. Instead, it is adequate to get a FeatureCollection from the function as shown below.

var AOI = ee.Geometry.Polygon(
        [[[28.827, 41.076],
          [28.827, 40.891],
          [29.162, 40.891],
          [29.162, 41.076]]]);

var ERA5DOM = ERA5.map(function(im){return im.clip(AOI);});

// Wind calculation
var ERA5windspeedTable = ERA5DOM.map(function(image){
  var wind_10m = image.expression(
    'sqrt(u**2 + v**2)', {
      'u': image.select('u_component_of_wind_10m'),
      'v': image.select('v_component_of_wind_10m')
  }).rename("windspeed");
  
  // date of the image
  var date = ee.Date(image.get('system:time_start')).format('yyyy-MM-dd');
  // hour of the image
  var hour = ee.Date(image.get('system:time_start')).format('HH');
  
  // return a feature with date, hour, windspeed attributes
  return ee.Feature(null, {
    date: date,
    hour: hour,
    windspeed: wind_10m.reduceRegion(ee.Reducer.mean()).get('windspeed')
  })
});

// Check the results from Console  
print(ERA5windspeedTable);

// Export the FeatureCollection to a csv file.
Export.table.toDrive({
  collection: ERA5windspeedTable,
  description: 'DOM_mean_wind',
  fileFormat: 'csv',
  selectors: ['date', 'hour', 'windspeed']
});

Results for Istanbul:

date     hour   windspeed
2017-09-01  0   3.541915279
2017-09-01  1   3.503340651
2017-09-01  2   3.469190862
2017-09-01  3   3.365405375
........