Following the instructions about how to calculate CARRA precipitation rate found on
https://confluence.ecmwf.int/pages/viewpage.action?pageId=338485814
I have downloaded accumulated TP from CARRA at timesteps 6,9,12,15 and 18.
I then calculate the 3 hourly precipitation by differencing TP(@9) - TP(@6) etc.
I have noticed that occasionally this gives me small negative values.
An example is shown below. It is based on Carra West TP data from April 15th 1992 at 00:00. For the purposes of demonstration I asked for all steps (see request file below). Looking at the data for one point ([i,j]=[217,183]) in the downloaded file
( lon=307.44E, lat=52.55 N ) shows the following TP per hour from the start of the run to step=30 hours
Step(h) TP (kg/m2)
1, 8.296966552734375e-05
2, 0.0033626556396484375
3, 0.0039501190185546875
4, 0.0037994384765625
5, 0.003765106201171875
6, 0.00408172607421875
9, 0.0034637451171875
12, 0.00341796875
15, 0.0032806396484375
18, 0.0033721923828125
21, 0.0035552978515625
24, 0.003570556640625
27, 0.00335693359375
30, 0.00341796875
There are many instances where the TP actually drops from one step to the next. Neglecting transients during the first 6 hours, but after that the value at 6h is bigger than the value at 9 hours, which in turn is bigger than the value at 12h, which again is bigger than the value at 15h.
The three hourly differences are small (O(8) kg/m2s) but I am puzzled that they show up at all. How can accumulated precip decrease from one step to the next?
Sincerely
Halldór
cds request file
import cdsapi
c = cdsapi.Client()
c.retrieve(
‘reanalysis-carra-single-levels’,
{
‘domain’: ‘west_domain’,
‘level_type’: ‘surface_or_atmosphere’,
‘variable’: ‘total_precipitation’,
‘product_type’: ‘forecast’,
‘time’: ‘00:00’,
‘leadtime_hour’: [
‘1’, ‘2’, ‘3’,
‘4’, ‘5’, ‘6’,
‘9’, ‘12’, ‘15’,
‘18’, ‘21’, ‘24’,
‘27’, ‘30’,
],
‘year’: ‘1992’,
‘month’: ‘04’,
‘day’: ‘15’,
‘format’: ‘grib’,
},
‘download.grib’)