Finding areas with at least 7-days/year of sub-zero temps over 30 years using ERA5 in Google Earth Engine

Question

I'm looking to make a climate zone map in Google Earth Engine (GEE). One of the criteria I'm using is frost occurrence- which is defined by having at least 7 days per year with a minimum temperature below 0 degrees Celsius. I'm doing this ideally over a 30-year period as well.

I'm using ERA5's daily minimum air temperature dataset to calculate this. I have created an image collection that has grouped all images of the same year together but am stuck on the next step.

How would I go about filtering down areas that have at least 7 days of minimum temps below zero per year (and vice versa- a variable containing global areas that do not have frost conditions) into variables from this grouped image collection?

// Daily mean 2m air temperature
var era_min_t = ee.ImageCollection("ECMWF/ERA5/DAILY")
  .filterDate('1990-01-01', '2020-01-01')
  .select('minimum_2m_air_temperature');

// Reduce the collection to a set of unique year representatives
// to serve as the primary collection in a join with the complete
// collection.
var distinctYear = era_min_t.distinct('year');
print(distinctYear, "Distinct Year");
// Define a filter that identifies which images from the complete collection
// match the 'year' from the distinctYear collection.
var filter = ee.Filter.equals({leftField: 'year', rightField: 'year'});

// Define a join.
var join = ee.Join.saveAll('sameYear');

// Apply the join and convert the resulting FeatureCollection to an
// ImageCollection.
var joinCol = ee.ImageCollection(
  join.apply(distinctYear, era_min_t, filter)
);
print(joinCol);

Answer 1

I would iterate over the years, mask non-freezing pixels and count:

var collection = ee.ImageCollection("ECMWF/ERA5/DAILY")
  .select('minimum_2m_air_temperature')
  
var freezingTemperature = 273.15

var freezingYearCount = ee.ImageCollection(ee.List.sequence(1990, 2019)
  .map(function (year) {
    var startDate = ee.Date.fromYMD(ee.Number(year), 1, 1)
    return collection
      .filterDate(startDate, startDate.advance(1, 'year'))
      .map(function (dailyTemperature) {
        return dailyTemperature
          .updateMask( // Mask days that are not freezing
            dailyTemperature.lt(freezingTemperature)
          )
      })
      .reduce(ee.Reducer.count()) // Number of freezing days
      .gte(7) // At least 7 freezing days 
  })
).reduce(ee.Reducer.count()) // Number of freezing years

Map.addLayer(freezingYearCount, {min: 0, max: 30, palette: '#e8fa5b, #fbb43d, #eb7958, #b15f82, #744992, #2c3395'}, 'freezingYearCount')

https://code.earthengine.google.com/ee845e74ee16d5e9039129ced31c5b80