GEE: Compute monthly sum of GPM mm/hr

Question

I need to compute the mean monthly precipitation sum between 2001 and 2021 (May to September). I know ho to compute the mean over all months of the period but I don't know how to handle the unit. My first attempt to compute the monthly sum was to multiply each value (mm/hr) with the number of hours (24*31) but there are months with only 30 days so this leads to higher values in June and September. Is there any way to solve this problem?

Before that I tried to use the half-hour GPM Dataset (which I preferred first because I need to do this with 2022 as well to compute the anomaly) to just compute the sum of all values per month but I always got the error "User memory limit exceeded" while computing so I changed my strategy.

var gpmMonthly = ee.ImageCollection("NASA/GPM_L3/IMERG_MONTHLY_V06").select('precipitation').filterDate('2001-01-01', '2021-09-01')

var gpmMonthly = gpmMonthly.map(function(image) {
  return image.multiply(31*24).copyProperties(image, ['system:time_start'])
})

var months = ee.List.sequence(1, 12);

var monthlyMean = ee.ImageCollection.fromImages(
      months.map(function (m) {
        return gpmMonthly.filter(ee.Filter.calendarRange(m, m, 'month'))
                    .select(0).mean()
                    .set('month', m);
}))

monthlyMean = monthlyMean.filter(ee.Filter.rangeContains('month', 5, 9))

Answer 1

You can convert your function:

var gpmMonthly = gpmMonthly.map(function(image) {
  return image.multiply(31*24).copyProperties(image, ['system:time_start'])
})

in the following function:

var gpmMonthlySum = gpmMonthly_list.map(function(image) {
  
  var date = ee.Date(ee.Image(image).get('system:time_start'));
  var year = date.get('year');
  var month = date.get('month');
  
  var typeYear = ee.Number(year).mod(4).eq(0).and(ee.Number(year).mod(100).neq(0)).or(ee.Number(year).mod(400).eq(0));
  
  var days = ee.Algorithms.If(month.eq(1)
                                   .or(month.eq(3))
                                   .or(month.eq(5))
                                   .or(month.eq(7))
                                   .or(month.eq(8))
                                   .or(month.eq(10))
                                   .or(month.eq(12)), 31, 
             ee.Algorithms.If(month.eq(4)
                                   .or(month.eq(6))
                                   .or(month.eq(9))
                                   .or(month.eq(11)), 30, 
            ee.Algorithms.If(month.eq(2).and(typeYear.eq(0)), 28, 29)));
  
  return ee.Image(image).multiply(ee.Number(days).multiply(24)).copyProperties(image, ['system:time_start']);

});

where days per month are assigned as follows: 31 for January, March, May, July, August, October and December (1, 3, 5, 7, 8, 10, 12); 30 for April, June, September, November (4, 6, 9, 11); 28 for February (2) on not leap year (365 days) and 29 for February (2) on leap year (366 days).

Complete code looks as follows:

    var gpmMonthly = ee.ImageCollection("NASA/GPM_L3/IMERG_MONTHLY_V06")
      .select('precipitation')
      .filterDate('2001-01-01', '2021-09-01');
    
    print(gpmMonthly);
    
    var gpmMonthly_list = gpmMonthly.toList(gpmMonthly.size());
    
    var gpmMonthlySum = gpmMonthly_list.map(function(image) {
      
      var date = ee.Date(ee.Image(image).get('system:time_start'));
      var year = date.get('year');
      var month = date.get('month');
      
      var typeYear = ee.Number(year).mod(4).eq(0).and(ee.Number(year).mod(100).neq(0)).or(ee.Number(year).mod(400).eq(0));
      
      var days = ee.Algorithms.If(month.eq(1)
                                       .or(month.eq(3))
                                       .or(month.eq(5))
                                       .or(month.eq(7))
                                       .or(month.eq(8))
                                       .or(month.eq(10))
                                       .or(month.eq(12)), 31, 
                 ee.Algorithms.If(month.eq(4)
                                       .or(month.eq(6))
                                       .or(month.eq(9))
                                       .or(month.eq(11)), 30, 
                ee.Algorithms.If(month.eq(2).and(typeYear.eq(0)), 28, 29)));
      
      return ee.Image(image).multiply(ee.Number(days).multiply(24)).copyProperties(image, ['system:time_start']);
    
    });
    
    gpmMonthlySum = ee.ImageCollection(gpmMonthlySum);
    
    var months = ee.List.sequence(1, 12);
    
    var monthlyMean = ee.ImageCollection.fromImages(
          months.map(function (m) {
            return gpmMonthlySum.filter(ee.Filter.calendarRange(m, m, 'month'))
                        .select(0).mean()
                        .set('month', m);
    }))
    
    monthlyMean = monthlyMean.filter(ee.Filter.rangeContains('month', 5, 9));
    
    print(monthlyMean);