ERA5: large 10m winds

Last modified on May 05, 2023 10:11

A few times per year, the analysed low level winds, eg the 10m winds, become unrealistically large in a particular location, which varies amongst a few apparently preferred locations. The largest values seen so far are about 300 ms^-1. This problem occurs towards the end of the data assimilation windows (9-21 UTC and 21-9 UTC) because of an instability in the analysis method.

From 19 February 2020 onwards, the ERA5 system has examined the 10m wind components and if the magnitude of either component exceeds 50 ms^-1, then the analysed parameters are replaced with the "4v" parameters. These latter parameters are similar to the analysed ones, but don't usually suffer from such large values. For more information on 4v and data assimilation, see "Part II Data Assimilation" of the IFS documentation (CY41R2 for ERA5).

The choice of a 50 ms^-1 cut off is an arbitrary one, which could miss spurious values that are less than 50 ms^-1. This may well occur in the one or two hours just before the critical value of 50 ms^-1 is reached. It is also possible that this cut off point could penalise realistic winds, such as in hurricanes. However, given the spatial resolution of ERA5 data (with a grid spacing of 31km), it is thought that in general, 10m wind components higher than 50 ms^-1 are not realistic in ERA5, even if the observed magnitude is higher. This is illustrated in Fig. 1 for Hurricane Ivan in the Caribbean on 12 September 2004 at 20 UTC, where the analysed winds are compared with the 4v winds. The minimum value of the analysed 10 metre V wind components is -53 ms^-1 (see Event 57, Table 2), while the minimum value of the 4v 10 metre V wind components is -29 ms^-1. Even though the analysed wind speeds are not as large as the observed ones (the estimated peak wind speeds for Ivan were of order 70 ms^-1), the 4v winds are thought to give a more realistic representation of the winds, for this resolution.

Fig. 1a	Fig. 1b

Fig. 1a. The analysed 10m winds for 12 September 2004, 20 UTC, featuring Hurricane Ivan. The regional plot domain shows the Caribbean and Gulf of Mexico.	Fig. 1b. As Fig 1a, but for the 4v 10m winds.

For dates before this system check was introduced into ERA5 (i.e. for the period January 1979 to 18 February 2020), the two tables below give details when the analysed 10m wind components (the 10 metre U wind component and 10 metre V wind component) exceed 50 ms^-1. The CDS API script below can be used to retrieve the "4v" 10m wind components, which don't usually suffer from this large wind problem.

#!/usr/bin/env python
import cdsapi
 
c = cdsapi.Client()
c.retrieve('reanalysis-era5-complete', {     # do not change this!
    'stream'  : 'oper',
    'type'    : '4v',
    'levtype' : 'sfc',
    'param'   : '165.128/166.128',
    'date'    : '1986-07-23',
    'time'    : '21',
    'step'    : '12',
    'grid'    : '0.25/0.25', # Latitude/longitude grid: east-west (longitude) and north-south resolution (latitude). Default: reduced Gaussian grid
    'format'  : 'grib', # Default: grib, supported netcdf.
}, 'outfile.grib')

This CDS API script can be used to retrieve the "4v" 10m wind components from the ECMWF data archive, MARS. For more information on how to use CDS API scripts, see How to download ERA5. For the temporal specification, the CDS uses the validity date and time, whereas MARS uses date, time and step (there is further discussion of step in the section "Date and time specification" of the ERA5 data documentation). The 4v model trajectories begin from 9 and 21 UTC and you should use the hourly steps from 1 to 12 hours. In this example in the script above, the validity date/time would be given by adding the step, 12 hours, to the time, 21 UTC, thereby yielding a validity date/time of 1986-07-24 at 9 UTC, which is the 12th event in Table 1 below, for the 10m U.

Event	Date	Time	Latitude	Longitude	10m U
1	19790417	900	-74.051	164.160	7.2323150635e+01
2	19800330	700	-1.827	280.406	-5.9812698364e+01
3	19800330	800	-1.827	280.406	-9.8439865112e+01
4	19800330	900	-1.827	280.406	-7.1381820679e+01
5	19820908	700	-74.051	164.160	-6.9596405029e+01
6	19820908	800	-74.051	164.160	-5.2189117432e+01
7	19821024	2100	-74.051	164.160	-1.5377770996e+02
8	19841004	700	-74.051	164.160	-5.5174591064e+01
9	19841004	800	-74.051	164.160	-6.8660507202e+01
10	19841004	900	-74.051	165.120	-7.1647964478e+01
11	19860724	800	-74.051	164.160	-8.8280364990e+01
12	19860724	900	-74.051	164.160	-2.9203100586e+02
13	19871016	2000	-74.051	164.160	-9.4200180054e+01
14	19871016	2100	-74.051	164.160	-1.5219485474e+02
15	19890711	2100	38.080	78.333	7.0804840088e+01
16	19890912	800	-74.051	164.160	-8.3815658569e+01
17	19890912	900	-74.051	164.160	-1.6730293274e+02
18	19900402	700	-74.051	164.160	5.8469223022e+01
19	19930624	600	-74.051	164.160	6.0172592163e+01
20	19930624	700	-74.051	164.160	1.0357415771e+02
21	19930624	800	-74.051	164.160	7.8659744263e+01
22	19930624	900	-74.051	164.160	5.5145812988e+01
23	19930705	2000	38.080	78.333	8.0320663452e+01
24	19930705	2100	38.361	78.398	5.9963912964e+01
25	19940914	2100	-74.051	164.160	-7.9331817627e+01
26	19970607	700	-74.051	164.160	5.8002792358e+01
27	19970607	800	-74.051	164.160	1.0099096680e+02
28	19970923	1900	-74.051	164.160	1.3710269165e+02
29	20010423	2100	-74.051	164.160	-8.0685379028e+01
30	20040327	800	-74.051	164.160	-6.4567825317e+01
31	20050314	800	-1.827	280.406	-6.1867416382e+01
32	20050314	900	-1.827	280.406	-9.2169509888e+01
33	20060304	900	-1.827	280.125	-6.3722732544e+01
34	20060328	800	-1.827	280.406	-5.7734420776e+01
35	20060328	900	-1.827	280.406	-1.1018218994e+02
36	20070331	2100	-74.051	164.160	8.7419113159e+01
37	20090303	700	-1.827	280.406	-8.9056137085e+01
38	20090303	800	-1.827	280.406	-1.3375457764e+02
39	20090303	900	-1.827	280.406	-2.0406346130e+02
40	20091218	2100	43.138	75.000	-5.9425277710e+01
41	20100325	900	-1.827	280.688	-5.4004959106e+01
42	20110127	900	-1.827	280.406	-5.3115798950e+01
43	20111224	2100	43.138	75.000	-6.8307769775e+01
44	20130719	2100	-74.051	164.160	-6.1746520996e+01
45	20140426	700	-1.265	280.688	5.7151275635e+01
46	20140426	700	-1.546	280.406	-8.5247161865e+01
47	20140426	800	-1.265	280.688	7.2114089966e+01
48	20140426	800	-1.546	280.406	-1.4157341003e+02
49	20140426	900	-1.546	280.406	-2.0927426147e+02
50	20160522	2000	-74.051	164.160	-6.6684082031e+01
51	20160522	2100	-74.051	164.160	-5.9349365234e+01
52	20160929	2100	-74.051	164.160	6.5295028687e+01
53	20161128	2100	42.857	75.375	-5.2441085815e+01

Table 1: Occurrences when the 10 metre U wind component exceeds 50 ms^-1.

Event	Date	Time	Latitude	Longitude	10m V
1	19790401	2100	-5.480	137.812	-9.6993362427e+01
2	19800330	900	-1.827	280.406	-5.0487777710e+01
3	19800831	1900	-5.199	137.531	-6.5852249146e+01
4	19800831	2000	-5.199	137.531	-5.0360015869e+01
5	19821024	2100	-74.051	164.160	5.4236557007e+01
6	19830930	2000	42.576	84.815	5.0510147095e+01
7	19830930	2100	42.576	84.815	6.1684814453e+01
8	19840811	1800	-5.480	137.812	1.3961134338e+02
9	19840811	1900	-5.199	137.531	1.2407348633e+02
10	19840811	2000	-5.199	137.531	1.3110269165e+02
11	19840811	2100	-5.199	137.531	1.2531871033e+02
12	19850812	1900	-5.199	137.531	6.5993698120e+01
13	19850812	2000	-5.480	137.812	8.7721923828e+01
14	19850812	2100	-5.480	137.812	1.1631915283e+02
15	19851116	2100	-5.480	137.812	-8.0120529175e+01
16	19860724	900	-74.051	164.160	1.3616104126e+02
17	19871016	2100	-74.051	164.160	6.7936035156e+01
18	19890420	2000	-5.480	137.812	-5.5810119629e+01
19	19890420	2100	-5.480	137.812	-1.2454008484e+02
20	19890711	2100	38.361	78.398	1.4824528503e+02
21	19890912	900	-74.051	164.160	6.3201354980e+01
22	19901117	600	41.171	39.960	-7.1991546631e+01
23	19901117	700	41.171	39.600	-7.2909973145e+01
24	19910620	2000	-5.480	137.812	9.5385955811e+01
25	19910620	2100	-5.761	138.094	1.6087461853e+02
26	19920415	800	20.094	84.375	5.1177642822e+01
27	19921108	2100	-5.199	137.531	-5.9594894409e+01
28	19921113	1800	-5.480	137.812	-7.4539321899e+01
29	19921113	1900	-5.761	137.812	-1.4339295959e+02
30	19921113	2000	-5.480	137.531	-1.5561082458e+02
31	19921113	2100	-5.480	137.531	-1.7285772705e+02
32	19921123	2100	43.138	74.625	-7.4817504883e+01
33	19930705	2000	38.361	78.398	1.7013420105e+02
34	19930705	2100	38.361	78.398	6.0669006348e+01
35	19930815	2000	31.335	72.000	-1.2063990784e+02
36	19950127	2100	40.890	39.600	-9.3620895386e+01
37	19960710	2000	-5.480	137.812	7.3640014648e+01
38	19960710	2100	-5.480	137.812	1.1385275269e+02
39	19970422	2100	40.890	39.240	-1.3644331360e+02
40	19970617	1700	39.766	89.297	-1.0041018677e+02
41	19970626	1900	-5.480	137.812	5.7410095215e+01
42	19970626	2000	-5.480	137.812	1.2612721252e+02
43	19970626	2100	-5.480	137.812	8.5113784790e+01
44	19970817	2000	-5.480	137.812	5.6425781250e+01
45	19970817	2100	-5.480	137.812	1.4044223022e+02
46	19970923	1900	-74.051	164.160	-5.1563308716e+01
47	20010127	600	41.171	39.960	1.2166479492e+02
48	20010127	700	41.171	39.960	8.3281478882e+01
49	20020602	2100	38.923	83.320	-6.7981185913e+01
50	20030516	2100	-5.480	137.812	6.4198028564e+01
51	20030704	2000	40.328	84.240	5.8809799194e+01
52	20040711	1800	40.328	87.480	1.6328262329e+02
53	20040711	1900	40.328	87.480	7.7149871826e+01
54	20040803	2100	-5.199	137.531	-5.1051910400e+01
55	20040830	2000	-5.199	137.531	-7.2028549194e+01
56	20040830	2100	-5.199	137.250	-1.5628892517e+02
57	20040912	2000	19.532	277.031	-5.3294830322e+01
58	20050805	2000	-5.480	137.812	-9.3390945435e+01
59	20050805	2100	-5.480	137.812	-9.8295639038e+01
60	20051208	800	40.890	39.960	6.6637542725e+01
61	20060304	900	-1.546	280.406	-7.0116149902e+01
62	20060612	1900	-5.480	137.812	6.8560852051e+01
63	20060612	2000	-5.480	137.812	1.0062788391e+02
64	20060612	2100	-5.480	137.812	1.1736833191e+02
65	20061210	700	41.171	39.600	5.8087677002e+01
66	20061210	800	41.171	39.600	5.1876373291e+01
67	20061219	700	41.171	39.960	6.5212203979e+01
68	20061219	800	41.171	39.600	5.0869308472e+01
69	20071227	600	41.171	39.600	7.9303802490e+01
70	20071227	700	41.171	39.960	1.0291072083e+02
71	20071227	800	41.171	39.960	8.6426681519e+01
72	20090303	900	-1.827	280.406	-6.8449584961e+01
73	20091218	2000	43.138	74.625	-6.3902404785e+01
74	20091218	2100	43.138	74.625	-1.5077589417e+02
75	20100110	2100	40.890	39.960	-5.4371688843e+01
76	20111207	2100	41.171	39.960	-5.2954406738e+01
77	20111224	2000	43.138	74.625	-8.1969665527e+01
78	20111224	2100	43.138	74.625	-1.0727139282e+02
79	20120629	2000	-5.480	137.812	6.8156173706e+01
80	20120629	2100	-5.480	137.812	5.3109390259e+01
81	20130727	1900	38.080	78.333	6.0674041748e+01
82	20131224	600	41.171	39.960	6.8190124512e+01
83	20131224	700	41.171	39.600	6.7012496948e+01
84	20131224	800	41.171	39.600	5.1414718628e+01
85	20140426	700	-1.265	280.406	5.4881195068e+01
86	20140426	700	-2.108	280.688	-5.8466461182e+01
87	20140426	800	-1.546	280.688	-8.5272201538e+01
88	20140426	900	-1.546	280.688	-1.0531086731e+02
89	20150912	1900	-5.199	137.531	6.4281372070e+01
90	20160516	1900	-5.480	137.812	1.1651092529e+02
91	20160516	2000	-5.480	137.531	1.5173913574e+02
92	20160516	2100	-5.480	137.812	7.3943984985e+01
93	20160531	2000	-5.480	137.812	8.6714050293e+01
94	20160531	2100	-5.480	137.812	8.5311660767e+01
95	20160825	1900	-5.480	137.812	8.8643951416e+01
96	20160825	2000	-5.199	137.531	6.8540237427e+01
97	20161128	2000	43.138	74.625	-8.1585525513e+01
98	20161128	2100	43.138	74.625	-1.6854412842e+02
99	20171011	2100	-5.480	137.812	1.1839462280e+02
100	20180914	2100	-5.480	137.812	5.5520050049e+01
101	20191116	1800	-5.480	137.812	7.6125183105e+01
102	20191116	1900	-5.199	137.531	1.4759844971e+02
103	20191116	2000	-5.199	137.531	7.8542388916e+01

Table 2: Occurrences when the 10 metre V wind component exceeds 50 ms^-1.

_{This document has been produced in the context of the Copernicus Climate Change Service (C3S).}

_{The activities leading to these results have been contracted by the European Centre for Medium-Range Weather Forecasts, operator of C3S on behalf of the European Union (Delegation Agreement signed on 11/11/2014 and Contribution Agreement signed on 22/07/2021). All information in this document is provided "as is" and no guarantee or warranty is given that the information is fit for any particular purpose.}

_{The users thereof use the information at their sole risk and liability. For the avoidance of all doubt , the European Commission and the European Centre for Medium - Range Weather Forecasts have no liability in respect of this document, which is merely representing the author's view.}

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