GEE: mapping over a wind data (ERA5) image collection

Question

I want to extract average daily wind speeds over the Netherlands (NL below) at 1pm from the ERA 5 dataset on GEE.

I am trying to map over an image collection, and for some reason it is telling me that "u.pow is not a function". The formula is right (I tested it on a single image).

It's the calcwind variable that doesn't run as a result. If I understand correctly, this code should add a windspeed attribute to the dataset which I will then use to identify days with low wind speed.

I will compare this wind data to S5P NO2 data, which should contain u and v wind vectors (see Panoply screenshot below), but unfortunately this seems to be excluded in the GEE L3 NO2 dataset...

//Setup

var countries = ee.FeatureCollection("FAO/GAUL/2015/level0");
var NL = countries.filterMetadata('ADM0_NAME', 'equals', 'Netherlands');

var ERA5 = ee.ImageCollection("ECMWF/ERA5_LAND/HOURLY")
     .filter(ee.Filter.date('2020-07-01', '2020-07-02'))
     .filterMetadata('hour', 'equals', 13);

var ERA5NL=ERA5.map(function(im){ 
   return im.clip(NL)
})


// Wind calculation
var u = ERA5NL.select('u_component_of_wind_10m');
var v = ERA5NL.select('v_component_of_wind_10m');

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

var ERA5x = ERA5NL.map(calcwind);


//Mapping
var vis = {
  min: 0,
  max: 5,
  palette: [
    '#FFFFFF', '#FFFF71', '#DEFF00', '#9EFF00', '#77B038', '#007E55', '#005F51',
    '#004B51', '#013A7B', '#023AAD'
  ]
};

Map.setCenter(5.5, 52, 7.5);
Map.addLayer(ERA5x.select('windspeed'), vis, "Wind speed (m/s) 10m above surface");

Answer 1

You can map it by using an image.expression. Following code works as expected.

var countries = ee.FeatureCollection("FAO/GAUL/2015/level0");
var NL = countries.filterMetadata('ADM0_NAME', 'equals', 'Netherlands');

var ERA5 = ee.ImageCollection("ECMWF/ERA5_LAND/HOURLY")
     .filter(ee.Filter.date('2020-07-01', '2020-07-02'))
     .filterMetadata('hour', 'equals', 13);

var ERA5NL = ERA5.map(function(im){ 
   return im.clip(NL)
})

var ERA5x = ERA5NL.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(ERA5x);

var vis = {
  min: 0,
  max: 5,
  palette: [
    '#FFFFFF', '#FFFF71', '#DEFF00', '#9EFF00', '#77B038', '#007E55', '#005F51',
    '#004B51', '#013A7B', '#023AAD'
  ]
};

Map.setCenter(5.5, 52, 7.5);
Map.addLayer(ERA5x.select('windspeed'), vis, "Wind speed (m/s) 10m above surface");

After running it in GEE code editor, I got result of following image without any error.: