Updating Australia's High-Quality Annual Temperature Dataset
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Aust.Met. Mag. 53 (2004)15-93 Updating Australia's high-quality annual temperature dataset Paul Della-Marta and Dean Collins National Climate Centre,Bureau of Meteorology,Australia and Karl Braganza CSIRO AtmosphericRese arch, Aspendale, Australia (Manuscriptreceived June 2003;revised September 2003) An updated and improved version of the Australian high-qual- ity annual mean temperature dataset of Torok and Nicholls (1996)has been produced. This was achievedby undertaking a thorough post-1993homogeneity assessment using a number of objective and semi-objectivetechniques, by matching closed records onto continuing records, and by adding some shorter duration records in data-sparseregions. Each record has been re-assessedfor quality on the basis of recent metadata, result- ing in many records being rejected from the dataset. In addi- tion, records have been re-examinedfor possibleurban conta- mination using some new approaches.This update has high- lighted the need for accurate and completestation metadata. It has also demonstrated the value of at least two years of over- lapping observations for major site changes to ensure the homogeneityof the climate record. A total of 133 good-quality, homogenisedrecords have been produced. A non-urban subset of 99 stationsprovides reliable calculations of Australia'sannu- al mean temperature anomalies with observation error vari- ancesbetween 15 and 25 per cent of the total variance and decorrelation length scalesgreater than the ayerageinter-sta- tion separation. trntroduction Torok and Nicholls (1996) produced a homogenised primary purposeof this datasetwas to enablethe 'high-quality' or datasetof annual mean maximum reliablemonitoring of climatetrends and variability and minimum temperatureseries for Australia. The at annual and decadaltime-scales. Consequently, eachstation record was correctedfor discontinuities Correspondingauthor address:D. Collins, National Climate Centre. causedby changesin sitelocation and exposure, and Bureau of Meteorology,GPO Box 1289K, Melbourne,Vic. 3001, other known data problems(Peterson Australia. et al. 1998). Email: [email protected] Suchdiscontinuities can be as large,or largerthan, 16 Australian Meteorological Magazine 53:2 June 2004 real temperaturechanges (e.g. the approximate 1"C Assessmentof station quality drop in maximum temperaturesassociated with the switch to Stevensonscreen exposure (Nicholls et al. Torok and Nicholls (1996) subjectively ranked the 1996)) and consequently confound the true long- quality of records on a scale of 1 to 6, with rank 1 term trend. Temperaturerecords for 224 observation being very good, 5 being very poor and 6 being urban. locations were homogenised, 170 of which were This update re-assessedeach station, mainly on the classified as non-urbani.e. free from the influence of basis of post-1993metadata, but also using metadata urban warming. Generally the high-quality records compiled by Torok (1996). All stationswere ranked were homogenisedfrom 1910,by which time most on the original 1 to 5 scale,but insteadof a rank 6, all stations are believedto have been equippedwith the stations were noted as either urban or non-urban, current standard instrument shelter. The dataset enabling urban stationsto also be given a quality rank. endedin 1993. Rankswere subjectivelyassigned, taking into account At the end of each year Australia's annual mean the current and historical standardof site exposure, temperature anomaly is calculated using all avail- number of site moves, number of instrument changes, able high-quality, non-urban temperature records. recent standardof observationsand amount of miss- This is routinely provided to the public via an annu- ing data. al climate summary, a media statement and the Since the development of the initial high-quality Bureau of Meteorology's internet site (e.g. datasetthere have been changesin the way station http ://www.bom. gov. aulannouncements/media- history information is recorded by the Australian releases/climate/change/20030106.shtml). The Bursau of Meteorology. Traditionally station metada- datasethas also beenused extensively in the calcu- ta have beenrecorded on paper history files. With the lation of atmospheric indicators for Australia's developmentof SitesDB (Lane 1995), an electronic 20Ot State of the Environmenl report (Collins and metadata database, the process of assessing station Della-Marta 2002). In order for such analyses to quality hasbeen streamlined.However, at present,the remain credible. the climate records used need to be information in the databaseis only reliable since the routinely checkedfor homogeneity. mid to late 1990s. Also, there is currently great varia- Since 1993, the high-quality dataset has been tion in the quality of information held in SitesDB, updatedusing data that have undergonebasic quality reflecting different levels of use and appreciation of assurance checks but no checks for homogeneity. needs throughout the Bureau. Many stationsare likely to have undergonechanges The introduction of SitesDB has resultedin paper requiring data adjustmentin this time. Also, the sta- history files generally not being used routinely. If tion exposure and quality of observations may have SitesDB is not supported adequately this will result changed since the initial assessmentof Torok and (and possibly already has resulted) in the loss of Nicholls (1996). Until this update, the number of important information. Also, most communication high-quality stations still operating has gradually about observation stations is now via email. rather declined due to site moves and closures,resulting in than formal inter-office memoranda.Such communi- gaps in the network. For the 2002 anntal summary cations often contain useful information for future only 138 stations had a full year of data available; 99 assessmentsof the climate record, but are generally of theseclassified as non-urban.The intention of this not placed on file. A concerted effort is currently update was to assessthe quality and homogeneityof underwaywithin the Bureau of Meteorologyto popu- open stations,to re-open'closed' recordsusing near- late SitesDB with historical metadatafrom paper his- by open stationsand to fill datavoids if possible. tory files and to archive station-specificemail com- Most of the homogeneitytechniques employed by munication. Torok and Nicholls (1996) have been replicated. After this latest assessment,sixteen of the origi- However, a number of new methodologies have also nal non-urban stations were consideredto have bet- been used to assesswhether a record is likely to have ter quality than the assessmentof Torok and been contaminated by urban warming, and to utilise Nicholls (1996).This was usually due to the station availablecomparison observation data in caseswhere having moved to a location with better exposure. a station has moved. The developmentof web-based However, 99 stations were ranked with lower quali- technology and greater digitisation of metadata since ty. Predominantly this has occurred at manual sta- the production of the original datasethave allowed tions, possibly due to diminishing resourcesbeing summary information, relevantanalyses and metada- used to maintain the cooperative network as ta to be conveniently combined into a webpage for reliance on the automatic weather station network each individual station.This has enabledthe effrcient increases.This has resulted in documented. but assessmentof eachrecord. uncorrected, exposure problems, less interest from Della-Martaet al.: UpdatingAustralia's annual temperature dataset 17 cooperativeobservers and the downgrading of man- ' Assessmentof recent homogeneity ual observationprograms, such as the cessationof weekendobservations. Torok (1996) provides a brief metadatahistory up to Some of the apparentdecline in quality is simply 1993 for each high-quality station. These histories due to more stringent assessmentcriteria during the were updatedfor this assessmentusing the metadata latestassessment. Some records contain many years contained in SitesDB and hardcopy station files to in which annual mean values were estimatedby identify potential datesof new discontinuities.Torok Torok and Nicholls (1996) using a referenceseries and Nicholls (1996) and Torok (1996) also used a basedon highly correlatedneighbour series. Often numberof objectivestatistical tests to help determine this amountedto more than twenty contiguousyears datesof discontinuity.Using the metadatawhere pos- of estimated data. The amount of in-filling appears sible,decisions were madeas to which discontinuities to have had no bearing on the original station rank. would be correctedto homogenisea record. Torok However, for this assessmentit was decided that (1996) providesa record of the datesat which homo- substantialperiods of estimateddata could be mis- geneity adjustmentswere made, the magnitude and leading to potential users. Generally, any records sign of adjustments,the reasonsfor adjustmentsand with around twenty or more years of estimated which testswere usedto identify them. annual mean values were ranked as poor or very In an attempt to validate previous adjustmentsit poor in the updated dataset(see Table 1). was decidedto reconstructthe unhomogenisedcom- Stations assessedas poor or very poor were not positecandidate series using raw stationdata for each removed from the original datasetin order to max- high-quality station.The raw datasetwas createdby imise the number of long-term records available merging data from the Australian