A few more GHCN problems

GHCN monthly data (v2 or v3) may not always be quite what it seems. Consider Nitchequon (Canada: 40371826000), looking here at the five individual time series in GHCN v2:

Some missing years – but where’s the problem?

Palma: ties in STEP1 comb_records.py

This post is published now for “historic interest” as the issue is no longer relevant with the GISTEMP change to use GHCN v3 adjusted data as input. Some familiarity with GISTEMP STEP1 processing and log files is virtually essential to follow the detail below, although some idea of the difference in combined station time series which may arise as a result of different tie-breaking strategies may be gained by examining the two alternative series added after the e-mail to GISS included below. For this station, Palma, using an alternative tie-breaking strategy reduces the temperature in the early years of the combined record by 0.8 degrees when compared to the GISS GISTEMP combined record. As we approach the present both combined records will show the same values. In the case of Palma the alternative tie-breaking strategy reduced the temperature at the start of the combined record when compared to the standard GISS GISTEMP, but of course for other stations the alternative tie-breaking strategy increased as well as reduced early temperatures. Palma was selected for this e-mail (and post) as the station showing the greatest change from the standard GISS GISTEMP when using this particular alternative tie-breaking strategy. It is of course quite possible that a different alternative tie-breaking strategy might produce an even greater difference for this or some other station.

GISTEMP change to use GHCN v3

The initial GISTEMP steps are described in gistemp.txt in the new GISTEMP (v3) source:

GHCN v3 station location metadata – a “report card”

Just over two years ago, when notifying NASA Goddard (GISTEMP) of errors in station coordinates in their v2.inv file, derived from the corresponding GHCN file, I also contacted Russell Vose at NOAA directly, rather than assuming he would be contacted by Goddard. He replied promptly, appreciating feedback, indicated that he was no longer working on the GHCN temperature data, that he would pass on the information to those working on a new version of the temperature dataset, and that “Hopefully some of these can be fixed quickly, but others may take a little longer”.

So, two years later, is the location metadata for GHCN v3 improved when compared to that for GHCN v2? The answer unfortunately, at least in so far as non-USA data is concerned, must be “not really”. For this “report card” I will consider the changes made for non-US station locations, and in particular the response to the errors I reported.

GISTEMP and GHCN v3 – Dublin Airport downloads in March 2012

My previous post noted a substantial change in the adjusted GISTEMP output temperature series for Dublin Airport between data downloaded from GISS on March 12th when compared to data downloaded from GISS on March 6th, while the input data for this station (GHCN v2 adjusted temperature series) showed only much smaller changes.

This may simply reflect the fact that the GHCN v3 data for September to November 2011 was missing in early March download from GISS. If this is the case, it seems undesireable that missing data for a single quarter should have such a pronounced effect on the adjusted output. It could also be the case that this change arises as a result of changes to the GHCN v3 adjusted data for rural stations which GISTEMP uses to adjust Dublin Airport. This is a possibility which will take more time to check, and will be considered in a later extension of this post. For now however I will just present a comparison of the data downloaded on these two dates.

GISTEMP and GHCN v3 – two stations illustrated

I’ve not yet had time to delve into the GISTEMP code changes to use GHCN v3 in place of GHCN v2. This post simply takes a look at the treatment of two stations, Dublin Airport and Reykjavik, which have been mentioned on other blogs, and is intended in part for use to provide graphs to illustrate comments I may make or questions I may ask on those other blogs. I have some further comments to add, but I will keep these for further posts, and for now just put these images and brief comments in this post.

Are you acquainted with any of these GHCN stations?

The quality of GHCN metadata, in particular the latitude and longitude coordinates of stations, remains unsatisfactory. In fact the transition from v2 to v3 might suggest that renaming might be in order – judging by the nature of the metadata changes USAAHCN (US And Afterthoughts, rather than Global)  might seem a more apt description.

In this post I combine a rant, as you, the reader, may have already guessed, with a request for assistance from any reader who has knowledge of the actual location of certain stations (as distinct from the locations of those stations obtained from the GHCN metadata or from my own efforts to correct these locations).

Afterthought: information would also be welcome on the correct location of any GHCN station which is not listed below (such as, for example, most of the non-airport non-WMO stations) but which you know to have a location error in the GHCN inventory. It would also be helpful if any corrections also include the source of the information used to make the correction – unfortunately some of the “corrections” which I have seen posted on blogs are themselves incorrect. (The source of the information may of course simply be along the lines that “I live beside xxx airport and can see the station” rather than some published source)

The context of this request for assistance is preparation of a journal submission. I submitted a comment on Hansen, J., R. Ruedy, M. Sato, and K. Lo (2010) to Reviews of Geophysics, regarding the consequences for the Gistemp analysis of using a station inventory in which an unknown, but substantial, number of station locations are in error. The response was that “Reviews of Geophysics does not accept Comments/Replies since they are not a review”. The response however continued by suggesting that

“the topic of your Comment is potentially of great interest to the Climate Community and therefore I suggest that you submit it as a stand-alone paper to GRL or JGR. A paper that can quantify the size of the impact of the corrections would be an important contribution to the field”

Finding corrected locations for the complete GHCN v2 or v3 station inventory is an herculean task, one which of course should be undertaken by NOAA rather than by unpaid volunteers. It may be suggested that coordinates of the accuracy required by Gistemp and by my comment are not required by NOAA for their own purposes in respect of GHCN adjustments – I am not sufficiently acquainted with GHCN procedures to comment on this. If this is the case however the station inventory should have been published with corresponding precision, and certainly not with the misleading precision suggested by coordinates published with apparent precision of 0.01° in v2, now increased to 0.0001º in v3.

However, corrected coordinates for a much smaller subset of stations seem to be sufficient to demonstrate a substantial effect on the Gistemp analysis. Before submission I would like to verify and if possible refine my corrected coordinates for as many stations as possible in this subset.