Calm winds in ISD, HadISD and GSOD

This post summarises an issue found earlier this year in the representation of calm periods (0 m/s) in the wind speed fields of ISD.  For full details, see our recently published paper on this at Environmental Research Communications.

We noted that in plots of the regional (and global) average wind speed, used in the BAMS State of the Climate report, that there was a significant inhomogeneity especially in Asian regions, see Figure 1, and wanted to understand the cause.

Fig 1: Time series of global and regional annual average wind speeds taken from stations in the HadISD.  For more details see Dunn et al, (2022) and the Surface Winds section in the BAMS State of the Climate.

The inhomgeneity is more prominent when looking at the calm fraction (percent of non-missing observations which measure 0m/s), see Figure 2. The drastic reduction for the two Asian regions, and also over Europe in 2013 is unlikely to be a natural feature of the climate at that date.

Fig 2: Time series of global and regional annual calm fraction taken from stations in the HadISD.

In looking more deeply at example stations, we noted that after 1 May 2013, there were no periods of 0m/s wind speed in the station time series in HadISD (Figure 3).  And this is a wide spread issue for stations across the globe (Figure 4).

Fig 3: Sub-daily wind measurements for the HadISD station 226760-99999 over its complete record.  [Sura, Russia, 63.58 N, 45.63E, 62.0m a.s.l.]

Fig 4a: Calm fraction for 2012

Fig 4b: Calm fraction for 2014

By tracing this back, we found this was also the case in the ISD, suggesting it has not been noticed, and affects all downstream products of the ISD (including HadISD and GSOD, the Global Summary of the Day). Investigations by our colleagues at NOAA/NCEI and their contacts in the USAF Weather Squadron found the issue as being an error in how calm winds were encoded in their outputs from the GTS.  This started on 1st May 2013, and has been corrected from 15th March 2022 going forwards.  Work is being done to correct the intevening years, and release that data into the databases, but this is still being done.

In HadISD, we can use the measurement flag which is in the ISD data files to recover calm periods assigned as missing.  This could also recover true missing data where the measurement flag has been erroneously given the value of calm, but we beleive this to be a small fraction of the observations.

By applying this simple correction to HadISD, we recover calm periods in stations between 2013 and 2022.  For those who use surface winds in their analyses, the addition of these calm periods (which used to be represented by missing data) will reduce the average wind speeds over this time range.  We show in Figure 5 how this impacts the global and regional time series, when compared to Figure 1.  There is a reduction in the magnitude of the reversal in global average surface wind speeds, which has been observed since around 2010.

 

Fig 5: Time series of global and regional annual average wind speeds taken from stations in the corrected HadISD.

There are other studies using independent data sources which reproduce both the long term decline of wind speeds since the beginning of the bulk of the HadISD records (1973) until around 2010, and also the slight reversal in global wind speeds since that date.  However, by including these previously missing calm periods means the magnitude of this reversal is reduced by around 30%.

For more details, please read the paper linked below (Open Access) or get in touch.

Reduction in reversal of global stilling arising from correction to encoding of calm periods, Dunn, Azorin-Molina, Menne, Zeng, Casey & Shen, ERC, 2021, https://iopscience.iop.org/article/10.1088/2515-7620/ac770a