GHCNM Spatial Sampling

SCOPE

This post is about the current spatial sampling of the stations with monthly average rainfall data in GHCNM version 2, and temperature data in GHCNM versions 3 and 4, the current source data for many “official” reconstructions of the global land rainfall and surface air temperature history.

Preliminary results are included for GHCNM version 4, which has not yet been released officially.

A figure of around 8000 stations is often quoted for GHCNM version 3, which might appear to be an adequate spatial sampling. However, most of those 8000 stations are no longer providing updates, and there are questions about the adequacy of the current spatial coverage in two distinct areas:

  • Spatial sampling of the varying climate around the globe
  • Detection and correction of inhomogeneities in the currently reporting stations

These two questions will be discussed on a per-country basis, starting with Australia.

AUSTRALIA

The Australian Bureau of Meteorology has 112 stations in ACORN-SAT(2012), intended to describe the varying temperature histories around the country:

ACORN-SAT-network-map

The currently reporting stations in GHCNMv3 (unadjusted), for monthly TAVG (average temperature) data is shown in the following list for Australia, generated by recording only those stations with data in 2018:

GHCNM_TAVG_JAN2018_AUS

These 62 stations in Australia that are currently reporting monthly TAVG to GHCNMv3 are possibly adequate to represent the varying temperatures around the country, but only if those stations remain unchanged in equipment, environment and procedures, and are free of errors.

ACORN-SAT was constructed by homogenisation involving many hundreds of other stations. This is no longer possible in GHCNMv3, which only has current data for 62 stations in Australia.

To confirm that all of the many other Australian stations in GHCNMv3 are currently non-reporting, here is a table of all Australian TAVG data (qcu: “unadjusted”) for 2017:

GHCNM_TAVG_2017_AUS

The 4-digit numbers in the table above, with separate columns for Jan to Dec, are the average temperature in hundredths of a degree C, -9999 means missing data.

GHCNM version 4 (preliminary) results are as follows. A total of 101 Australian stations contributed monthly average TAVG (unadjusted) data for January 2018, shown below in two parts:

GHCNMv4_TAVG_JAN2018_AUS_part1

GHCNMv4_TAVG_JAN2018_AUS_part2

101 stations will be a significant improvement on 62, but falls short of the 300 that is my guesstimate for the minimum number required to have a good chance of detecting and correcting inhomogeneities, and infilling missing data.

GHCNM version 2 (now containing only monthly rainfall totals)

Australian stations with rainfall data in 2018 are shown in the following list:

Australia_2018_GHCNMv2_PRCP

The above list includes the rainfall totals for January 2018, in tenths of a mm.

The 29 stations in the list are insufficient to allow analysis of contemporary rainfall, and detection/correction/infilling of anomalous/missing data.

The entirety of the 2017 rainfall data for Australia is given in the following table, with missing data (-9999) shown in red:

Australia_2017_GHCNMv2_PRCP

GHCNMv2 is no longer fit-for-purpose, and the Australian BoM has questions to answer about why so much data is missing from these stations, many/most of which are at Meteorological Offices.

More to follow later, about other countries …

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