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. 1aFig. 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.


EventDateTimeLatitudeLongitude10m 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.

EventDateTimeLatitudeLongitude10m 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

101201911161800-5.480137.8127.6125183105e+01
102201911161900-5.199137.5311.4759844971e+02
103201911162000-5.199137.5317.8542388916e+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). 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.