The following response
to "Hot Air At Low Head" was received
from Neil Plummer
of the National Climate Centre in Melbourne. It is given here in full.
- John L. Daly
Subject: Re: Low Head LIghthouse
Date: Tue, 31 Dec 1996 15:16:44 +0000
From: "Neil Plummer"
To: daly@vision.net.au
CC: climate_cdqi@bom.gov.au,
nnn@bom.gov.au, Simon Torok
Dear John
Thanks for your e-mail and attached report.
As I will explain, Low Head Lighthouse provides a long record of temperature data which, after careful scrutiny and adjustment for non-climatic influences, is useful for climate variability analysis.
You wrote:
> I read your
web page re the selection criteria for climate
> reference stations, and am puzzled as to how Low Head could have
> met such criteria. It has a strong daytime warming not present at
> Launceston. From the photo of Low Head you will notice high
> bushes near the instrument. These were not at such a height years
> ago, and have effectively created a mini sun trap as they screen
> the instrument from the prevailing northwesterly wind.
Back in July 1990, when the Reference
Climate Station (RCS) network was being developed, Low Head Lighthouse
was the first choice selection for north-central Tasmania for both the
Bureau's National Climate Centre and the Tasmanian Regional Office.
It is important to note that there are no RCS's which fully satisfy the selection criteria and selections were sometimes based on a balance between the various criteria (sometimes, for example, long record versus station relocation). It was always known that data for many stations would require adjustments at some time during their history. In addition, because RCS's were to provide data for long-term climate monitoring and research well into the future, the likelihood of continued operation was also an important consideration.
I believe that the important question here is whether or not the maximum and minimum temperature data for Low Head is of use for climate change studies. Given that records extend back to the late 19th century, then it is certainly worth examining this question further. The quality of Low Head's temperature data has been carefully examined in a number of studies in recent years - all using local climatological knowledge, statistical techniques and historical station information. These methods assess the homogeneity of a data series, i.e. attempt to identify any non-climatic jumps or trends which disguise real climatic variations. These tests also provide an estimate for the adjustment required to the time series to remove the inhomogeneity.
Here are the results of the examinations on the recent Low Head record - all values represent the adjustment required to annual mean temperatures on and before the given year to make the series homogeneous.
Tasmanian Reg.Office (Unpublished) 1991------- MaxT +0.5 degC 1969
Plummer et al. 1993, 1995 ---------------------------- MaxT +0.7 degC 1968
Torok and Nicholls 1996 ------------------------------- MaxT +0.7 degC 1969
Historical station information suggests that there was a site move during the late 1960s although the exact nature of the move is not clear from the documentation. Note from your Figure 2 (in "Hot Air at Low Head") that the differences in max temperature between Launceston and Low Head become less after the late 1960s - Launceston is consistently warmer before this period but about the same after. In addition, the analysis of Torok and Nicholls (1996) also detected a +0.3 degC discontinuity at Low Head during the late 1950s - again reducing the differences in interannual variability between the two sites. The abrupt increase in maximum temperatures at Low Head in the late 1960s is more suggestive of a site change than an influence due to the growth of vegetation although the latter may still have had some effect.
Nevertheless, the results from climate studies do suggest that non-climatic influences have affected these data to the tune of about +1 degC since the late 1950s. It must be emphasised that the aforementioned studies were performed independently using different methods. Nevertheless, although the year of the inhomogeneity differs slightly, the magnitude of the artificial warming compares very well. The Australian temperature time series derived in Plummer et al. (1993, 1996) and Torok and Nicholls (1996) have been corrected for these inhomogeneities and so too have data for hundreds of other stations around Australia.
Torok and Nicholls (1996) have provided the most reliable historical temperature record to date. This paper is soon to be published and station-by-station details will be provided in Simon Torok's PhD thesis.
In your report, you query the selection of 49 mostly 'rural' stations over eastern Australia as used in Jones et al. 1990. This network was selected on the basis of stations having few missing monthly mean values over the 1930-1988 analysis period. None of the data used had been assessed for homogeneity and the paper makes no claims in this regard. It is interesting to note, however, that the high quality data set of Torok and Nicholls (1996) confirm a warming over this region.
Through close cooperation between the Observation and Climate programs, the Bureau of Meteorology is making every effort to ensure that high quality data is available for future climate monitoring. Wherever possible new sites are being established where exposure is sub-standard - options for Low Head are currently being examined. Changes in instrumentation and/or in site location are being accompanied by comparison observations collected from both the old and new sites/instruments. In the not too distant future, historical station information - including photographs and schematics - will be archived on a relational database management system. In summary, the Low Head temperature record - after suitable adjustment for non-climatic influences - is useful for climate variability analysis. The adjusted temperature records have been used in studies of climate change over Australia since 1993, following a published homogeneity analysis of Australian temperature data. A subsequent study, which now provides the most reliable Australian temperature data for climate change studies, also found the Low Head record to be suitable following adjustment.
Yours sincerely,
Neil Plummer
References -
Jones, P.D. 1990. Assessment of urbanization effects in time series of surface air temperature over land. Nature, 347, 169-172.
Plummer, N., Lin, Z. and Torok, S. 1993. Recent changes in the diurnal temperature range over Australia. In: Asymetric change of daily temperature range: Proceedings of the international MINIMAX workshop (Kukla, G., Karl, T.R. and Riches, M.R., eds), September 1993, Maryland, US, pp127-143.
Plummer, N., Lin, Z. and Torok, S. 1995. Trends in the diurnal temperature range over Australia since 1951. Atmospheric Research, 37, 79-86.
Torok, S.J. and Nicholls, N. 1996. An historical annual temperature data set for Australia. Aust.Met.Mag. (in press).
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Neil Plummer
Climate Analysis Section
Ph: +61 03 9669 4457
National Climate Centre
Fx: +61 03 9669 4678
Bureau of Meteorology
GPO Box 1289K
e-mail: n.plummer@bom.gov.au
Melbourne Victoria 3001 Australia
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