Exporting ERA5 hourly data from Earth Engine not working

Question

My overall objective is to extract all areas that have a night time temperature <= 0 degrees C from the ERA5 hourly collection on Earth Engine, consideirng the time period 1981-2020 over a entire country (not points, polygons). The steps I have followed are -

1. Filter entire collection to have images only from 10pm-6am (night time defined in my project) (lines 141)
2. Convert images to degrees celcius (lines 146-149)
3. Reclassify all pixels such as pixels with night time temp <=0 is 1 and rest is 0 over the entire collection (lines 151-155)
4. Reduce the collection by sum to get the number of times a pixel has night time temp <=0 in a day (lines 159-175) 5)Again reclassify like step 3 such that if temperature dropped <=0 in a day, pixel value is 1, else 0 (lines 180-183)
5. Again reduce by sum like step 4, such that pixel value is number of days in which nigh time temp dropped <=0 in a month (lines 185-201)
6. Finally reduce by sum again, pixel value is number of days temp dropped <=0 in a year (lines 205-221)
7. Reclassify again such that all pixels with value > 15 is 1 and rest is 0. This means that all pixels that had more than 15 days of night time temp dropping to <=0 in a year is 1 and rest is 0.
8. Last reduction by sum where pixel value means number of years in which there was >15 days of nght time temp dropping <=0 10 ) Export to drive.

I have completed above for India as an example and I have initiated the script only for 2 years- Jan 1981 to dec 1982.

Everything runs. However the export stops after 2.5-3.5 hours with the error

Error: reduce.sum: Error in map(ID=0): Image.gt: If one image has no bands, the other must also have no bands. Got 0 and 1. (Error code: 3)

How do I figure out which images (hours) do not have any bands and exclude these images at any step? Alternatively how do I correct this script so it runs and exports out a binary raster, for a country, indicating areas where min 15 days of a year had night time temp dropping below freezing?

Here is the code editor script starting lines 122

Answer 1

This usually happens to me when one of my filters is not working properly. For example, an ImageCollection filter might not be returning any images.

In this case, I think the cause is that mapping a function over an ImageCollection (e.g., (image.subtract(273.15) in line 148) doesn't conserve the image properties, including the system:time_start and system:time_end properties that GEE uses to filter by date. The later operations that apply a .filterDate() filter then fail to find any images, hence the error.

The workaround I use is to manually set a new system:time_start and system:time_end property for each image when you map a function over an ImageCollection, e.g., in line 152:

var filtered_tempToDegrees = filtered_data.select('temperature_2m').map(function(image){
  return image.subtract(273.15);
});

becomes

var filtered_tempToDegrees = filtered_data.select('temperature_2m').map(function(image){
  return image.subtract(273.15)
              .set("system:time_start", image.get("system:time_start"),
                   "system:time_end", image.get("system:time_end"));  

And so on for each operation.

It is a bit cumbersome to apply this to each step, but it should preserve the date-time property and allow the filters to work. I'd love to know if there is a better way!

Answer 2

You can explicitly filter for images with certain band names:

var result = collection.filter(ee.Filter.listContains('system:band_names', 'temperature_2m'))