NOTE: This update supersedes the previous 1.4.0.2022f version which suffered from a significant loss of observations from 2015 onwards compared to 1.3.0.2021f - see v1.4.0.2022f update blog. This was found to be caused by the frequent presence of Attachment 8 information in the new ICOADS.3.0.2 compared to ICOADS.3.0.1 which the HadISDH processing code wasn't able to deal with. The HadISDH code has now been fixed to be able to read in observations that have an Attachment 8 present. This has resulted in an overall increase in observations throughout the record compared to v1.3.0.2021f, especially from 2020 onwards. This blog compares v1.4.1.2022f with last year's v1.3.0.2021f.
1) How does the new version compare?
v1.4.1.2022f is virtually identical to v1.3.0.2021f over the 1973-2014 period for specific humidity (q) and relative humidity (RH). However, as can be seen clearly in Figs. 1 and 2 panels f to i the versions differ increasingly from 2015 onwards, especially from 2020. This divergence in the last few years likely explains the tiny consistent offset across the 1973-2014 because the years 2015 to 2020 will influence the 30 year (1991-2020) climatology period over which the anomalies are referenced.
A newer version of ICOADS has been used to provide data from 2015 onwards which includes 20% more observations overall. This has resulted in a small increase in the spatial coverage of HadISDH.marine.v1.4.0.2022f over much of the record (Figs. 1e and 2e) and a large increase from 2020 onwards. HadISDH.marine is based only on ship observations of dewpoint temperature and temperature and so does not benefit from any increase in the number of buoy observations in ICOADS.3.0.2. In terms of gridboxes with at least 70% temporal completeness, the new version v1.4.1.2022f only results in 2 fewer gridboxes and 3 additional gridbox (Figs. 1a and 2a). Figs 1e and 2e show the northern mid-latitudes and southern tropics as regions with the greatest gain in coverage.
ICOADS3.0.1 is no longer updated and suffered from a drop in drifting buoy observations as they transitioned over to being reported in BUFR format rather than TAC (Liu et al., 2022). ICOADS.3.0.1 did not use the BUFR reports. ICOADS.3.0.2 was designed to use both TAC and BUFR, generally prioritising BUFR reports when merging the many duplicate observations. ICOADS.3.0.2 brings a considerable increase in the number of surface drifting buoys and ship observations to a lesser extent.
The overall story of increasing specific humidity is concurrent in both versions but the long-term trends in v1.4.1.2022f that have an extra year of data and slight change in coverage are identical for the Northern Hemisphere but 0.01 g kg -1 decade -1 smaller for the global, tropics and Southern Hemisphere regional means (Figs. 1f to i). Across the globe (Figure 1a) the vast majority of gridboxes show trends in the same direction (98% of gridboxes - Figure 1b), with 92.4% of these both showing increasing q (Figure 1b).
The overall story of decreasing relative humidity (except over the tropics) is also still valid in both versions, with long-term negative RH trends in v1.4.1.2022f being -0.03 to -0.04 %rh decade -1 more negative than in v1.3.0.2021f (Figs 2f to i). Over the tropics, trends in RH have changed from being significantly positive, yet small, to even smaller and not significant. Across the globe (Figure 2a) there are various regions where trends have more than 1.5 times as strong (negative or positive) in v1.4.1.2022f compared to v1.3.0.2021f, especially over the tropical Atlantic. However, overall, 93% of gridboxes (Figure 2b) have trends in the same direction in both versions, with 58% of gridboxes agreeing on negative trends and 35% agreeing on positive RH trends.
Note that the decrease in RH over oceans remains an uncertain conclusion as it does not reconcile with models or theory. Conceivably, there may be regions where relatively warmer and dry air may be advected from the land which could locally lower RH, or regions where increased wind speed increases mixing which might have the effect of lowering RH. Ultimately, spatial coverage of HadISDH.marine remains limited, with very little representation over the Southern Hemisphere and so the trends in q and RH may not be truly representative of the global trend. However, the increasing specific humidity is consistent with both climate models and theoretical expectation.
2) What's New?
We use ICOADS3.0.0 (1973 to 2014) and ICOADS3.0.2 (2015 onwards) as the basis for HadISDH.marine.v1.4.1.2022f. In previous years we have used ICOADS3.0.1 (2015 onwards) but this is now superseded by ICOADS3.0.2.
This update presents a ~20% increase in coverage based on 2018. The increase is smaller for 2015 to 2017 and larger for the following years, especially 2020 onwards.
This means that HadISDH.marine.v1.4.1.2022f deviates from HadISDH.marine.v1.3.0.2021f from 2015 onwards by a very small amount and this is particularly noticeable from 2020 onwards. This increase in coverage is very welcome given the critically low levels of coverage particularly between 2014 and 2020.
A second change is the formula for calculating wet bulb temperature from dewpoint temperature and marine air temperature. Errors were found when the air temperature was very high but dewpoint temperature very low, resulting in spuriously high wet bulb temperatures. The Stull (2011) formula is now used as this has a far wider range of applicability. See the blogpost for details.
This mostly affects the wet bulb temperature fields although differences are very small, and far less than 1 degree for the most part. Differences are larger over warm, dry air conditions, which are less of a problem over ocean. The calculated wet bulb temperature is used to decide whether to calculate vapour pressure with respect to ice or water and so this new formula can lead to very small changes in vapour pressure and variables that use vapour pressure in their calculation (specific humidity and relative humidity). Stull (2011) tends to give higher wet bulb temperatures overall which will result in fewer uses of the calculations with respect to ice and therefore fractionally higher vapour pressures and related values. As it is consistently used across the time period it should not impact long-term trends in anomalies.
3) Summary of changes by level of technicality.
MAJOR CHANGES (X):
- none
MINOR CHANGES (Y):
- Change of source dataset from 2015 onwards from ICOADS3.0.1 to ICOADS3.0.2 which has increased the number of observations by ~20%.
- Change of wet bulb temperature formula to Stull (2011) - see blogpost.
BUG FIXES AND HISTORICAL DATA UPDATES (Z):
- Change in spatial coverage and regional average time series from 2015 onwards caused by new data source ICOADS3.0.2.
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