BY CHER M. PAGE, NEVILLE NICHOLLS, NEIL PLUMMER, BLAIR TREWIN, MIKE MANTON, LISA ALEXANDER, LYNDA E. CHAMBERS, YOUNGEUN CHOI, DEAN A. COLLINS, ASHMITA GOSAI, PAUL DELLA-MARTA, MALCOLM R. HAYLOCK, KASIS INAPE, VICTOIRE LAURENT, LUC MAITREPIERRE, ERWIN E.P. MAKMUR, HIROSHI NAKAMIGAWA, NONGNAT OUPRASITWONG, SIMON MCGREE, JANITA PAHALAD, M.J. SALINGER, LOURDES TIBIG, TRONG D. TRAN, KALIAPAN VEDIAPAN, AND PANMAO ZHAI

CSIRO, Aspendale, AustraliaBureau of Meteorology Research Centre, Melbourne, AustraliaNational Climate Centre, Bureau of Meteorology, Melbourne, AustraliaHadley Centre, Exeter, United KingdomKorean Meteorological Administration, Seoul, Republic of KoreaNational Institute of Water and Atmospheric Research, Auckland, New ZealandClimatic Research Unit, University of East Anglia, Norwich, United KingdomNWS, Konedobu, Papua New GuineaMétéo-France, Tahiti, French PolynesiaMeteo-France, Noumea, New CaledoniaMeteorological and Geophysical Agency, Jakarta, IndonesiaFiji Meteorological Service, Nadi, FijiJapan Meteorological Agency, Climate Prediction Division, Tokyo, JapanMeteorological Department, Bangkok, ThailandPhilippine Atmospheric, Geophysical, and Astronomical Services Administration, Quezon City, PhilippinesClimate Research Center, Hanoi, VietnamMalaysian Meteorological Service, Petaling Jaya, MalaysiaNational Climate Centre, Beijing, ChinaCORRESPONDING AUTHOR: Cher Page, CSIRO Atmospheric Research, Private Bag I, Aspendale, Victoria, Australia, 3195, E-mail: Cher.page@csiro.au

Analyses taken over all observed weather conditions of daily 0600 EST climate data from a network of monitoring stations in and around the large city of Melbourne, Australia, revealed a 20-yr mean urban heat island (UHI) value of 1.13 degreesC. The UHI varied seasonally between summer (1.29 degreesC), spring (1.25 degreesC), autumn (1.02 degreesC), and winter (0.98 degreesC). Investigations undertaken with daily wind speed and cloud amount data enabled a detailed investigation of the relative importance of factors such as the turbulent and radiative exchanges on Melbourne's UHI. Analysis of variance and regression techniques were used to explore these processes and to predict the behavior of the UHI in numerical terms for mean seasonal and annual periods between 1972 and 1991. Over the 20-yr period, analyses of the association among Melbourne's UHI, wind, and cloud revealed that the UHI was inversely proportional to approximately the fourth root of both the wind speed and the cloud amount. This relationship explained more of the UHI variance during summer and the least variance during winter. Increases in the amount of cloud cover and in the frequency of wind speeds in excess of 2.0 m s(-1) resulted in a statistically significant (95% confidence level) reduction in UHI magnitude. The influence of wind in limiting Melbourne's UHI magnitude was greatest during clear to near-clear sky conditions. Similarly increases in cloud were most restrictive to UHI development during calm to low wind speeds. Unlike most previous studies, the linear regression analysis presented here revealed that cloud was more limiting than the wind speed to UHI development for all seasons except summer. Contour plots of the UHI are presented for the various associations between each category of cloud and wind. These plots enable a clear visual presentation of the most to least favorable conditions for UHI intensity and development. The analyses indicate that low wind speeds and little or no cloud were typically associated with the largest UHI development. Eight octas of cloud and wind speeds in excess of 5.0 m s(-1) were usually associated with modest (but still apparent) UHI development.

This study investigates the magnitude of the urban heat island (UHI) effect in four small towns, with populations of less than 10,000, and one large city, with a population of 3.02 million. All of the experiment sites are located in southeast Australia. Several climatic variables were measured along automobile transects from rural locations through the centres of each settlement. Some transects were repeated at different times of the day. A comparison of long-term temperature records is made from independent sites at one of the towns, and the influence of the UHI effect on the historical temperature record is discussed in a qualitative nature. The maximum UHI effect at the centre of a town over grass is related to population via a regression equation. The urban-rural temperature difference was found to increase with increasing population via the equationDeltaT(u-r(max)) =1.42 log(population)-2.09The results are discussed in the context of investigations in Europe and North America, and it is suggested that Australian towns and cities are likely to have smaller maximum UHI effects than are observed on the other two continents, for settlements with the same population. The findings of this study have implications for the compilation of historical temperature records and the maintenance of observation networks, particularly for climate change studies.

Trends in extreme daily temperature and rainfall have been analysed from 1961 to 1998 for Southeast Asia and the South Pacific. This 38‐year period was chosen to optimize data availability across the region. Using high‐quality data from 91 stations in 15 countries, significant increases were detected in the annual number of hot days and warm nights, with significant decreases in the annual number of cool days and cold nights. These trends in extreme temperatures showed considerable consistency across the region. Extreme rainfall trends were generally less spatially coherent than were those for extreme temperature. The number of rain days (with at least 2 mm of rain) has decreased significantly throughout Southeast Asia and the western and central South Pacific, but increased in the north of French Polynesia, in Fiji, and at some stations in Australia. The proportion of annual rainfall from extreme events has increased at a majority of stations. The frequency of extreme rainfall events has declined at most stations (but not significantly), although significant increases were detected in French Polynesia. Trends in the average intensity of the wettest rainfall events each year were generally weak and not significant. Copyright © 2001 Royal Meteorological Society

A number of indices have been developed to investigate recent changes in the annual frequencies of extreme temperature events in Australia. A high-quality daily temperature dataset including 88 station records is used to determine trends in these indices, generally over the period 1957 to 1996. Indices investigated include measures of the frequencies of daily maximum and minimum temperatures above and below fixed temperature thresholds, as well as frequencies above and below specified percentile levels. Trends in consecutive days and nights of extreme temperature are also considered. These trends indicate that occurrences of warm temperature extreme events have generally increased over the investigation period, whilst numbers of extremely cool temperature events have decreased. The trends are particularly strong for indices based on minimum temperature, with many statistically significant at the 95 per cent confidence level. Some of the trends display considerable regional variation. Examples of this are a downward trend in the frequency of warm extremes in parts of the far southeast, counter to the national trend, and an especially strong decrease in the frequency of relatively cool days along the east coast. A number of measures of daily temperature variability are also examined with many records showing significant declines for these indices. These trends may provide the first evidence of decreases in intraseasonal temperature variability consistent with those observed over large parts of the northern hemisphere landmass.

A semi-objective weighting scheme is developed to rank stations from the Bureau of Meteorology's synoptic network according to different climate network needs. In particular, Tasmanian stations are ranked according to how well they meet the selection criteria for a Reference Climate Station. The selected stations are then compared with the official Tasmanian Reference Climate Stations in order to determine the usefulness of the scheme in selecting stations for such a network. Various combinations of weights are used in an attempt to assess which attributes are most important and which, if any, can be left out of consideration. An optimum combination for Reference Climate Station selection is determined and may be used in future to assess the relative importance of a Reference Climate Station threatened with closure. The semi-objective selection scheme developed was found to be a useful tool in the selection of stations for climate change purposes. This study also highlights the broader observational requirements for climate monitoring.

Long-term in situ observations are widely used in a variety of climate analyses. Unfortunately, most decade- to century-scale time series of atmospheric data have been adversely impacted by inhomogeneities caused by, for example, changes in instrumentation, station moves, changes in the local environment such as urbanization, or the introduction of different observing practices like a new formula for calculating mean daily temperature or different observation times. If these inhomogeneities are not accounted for properly, the results of climate analyses using these data on be erroneous. Over the last decade, many climatologists have put a great deal of effort into developing techniques to identify inhomogeneities and adjust climatic time series to compensate for the biases produced by the inhomogeneities. It is important for users of homogeneity-adjusted data to understand how the data were adjusted and what impacts these adjustments are likely to make on their analyses. And it is important for developers of homogeneity-adjusted data sets to compare readily the different techniques most commonly used today. Therefore, this paper reviews the methods and techniques developed for homogeneity adjustments and describes many different approaches and philosophies involved in adjusting in situ climate data. (C) 1998 Royal Meteorological Society.

Analysis of the global mean surface air temperature has shown that its increase is due, at least in part, to differential changes in daily maximum and minimum temperatures, resulting in a narrowing of the diurnal temperature range (DTR). The analysis, using station metadata and improved areal coverage for much of the Southern Hemisphere landmass, indicates that the DTR is continuing to decrease in most parts of the world, that urban effects on globally and hemispherically averaged time series are negligible, and that circulation variations in parts of the Northern Hemisphere appear to be related to the DTR. Atmospheric aerosol loading in the Southern Hemisphere is much less than that in the Northern Hemisphere, suggesting that there are likely a number of factors, such as increases in cloudiness, contributing to the decreases in DTR.

In this study, observed trends and variability in climate and sea level for Australia, New Zealand and the south Pacific (Oceania) are presented. The results are derived from high quality, long-term climate data. It is found that annual surface air temperatures have increased between 0.4 and 0.8 degrees C throughout most of the region in she period 1952-1993. The fewer longer temperature series from paras of the region show an increase of 0.7 +/- 0.2 degrees C of land air temperatures and of the surrounding sea surface temperature from the beginning of the century, consistent with that observed for the Southern Hemisphere. Concurrent with observed warming over the last four decades, a significant decrease in the diurnal temperature range (DTR) has occurred over significant parts of Australia, and in the central south-west Pacific, particularly in areas where cloud cover has increased. In other areas, there is little change or small increases in DTR with associated decreases in cloud cover.Summer precipitation increases occurred over eastern Australia. Increases occurred in the south Pacific to the north-east of the South Pacific Convergence Zone (SPCZ), whilst decreases occurred to the south-west of the SPCZ. Interannual variability in temperature and precipitation is very much driven by the Southern Oscillation throughout Oceania. Since the mid-1970s, more frequent El Nino episodes have influenced longer term trends in precipitation.However the longer term warming trend observed in the sea and land temperature reflect hemispheric-wide climate warming and circulation change. Warming is also observed in surface and main thermocline waters in the oceans, with a freshening of Antarctic Intermediate water The observed sea level rises of almost 2 mm y(-1) (from tide gauges) are consistent with the observed ocean warming. The observed records of change in Oceania show some definite trends. This area provides an excellent regional monitoring platform of trends and variability in climate and sea-level in the Southern Hemisphere.

Most climate change studies to date have generally focussed on changes in climatic means rather than changes in climatic extremes and variability and yet, from an impacts perspective, changes in the latter are likely to be at least as important. Changes in the extremes and variability of high-quality surface air temperature data have been analysed over Australia for the period 1961 to 1993. The mean, maximum, and minimum temperatures and the diurnal temperature range were examined.Regional trends in intraseasonal and interannual temperature variability were mixed and generally not statistically significant, although some seasonal changes (e.g. interannual decreases in spring) were. While intraseasonal trends were generally small overall there was a tendency for winter increases and autumn decreases, and a weaker spring-increase, summer-decrease pattern of change. There has been a bias towards increases in temperature variability for the daytime and a tendency towards decreases in variability of overnight temperatures. Further, there was an association between warmer days (nights) and increased (decreased) temperature variability, particularly in the south. Through analysis of changes in the 95th and 5th daily percentile temperatures, the cooler 'relative extremes' were found to increase at a similar rate to the median, but a little larger than rises in the warmer 'relative extremes'. However, changes in the differences between these extremes (analogous to low frequency intraseasonal variability) were not significant.Perhaps surprisingly, the most significant seasonal increase in intraseasonal temperature variability has occurred in the temperate west region in winter during a period of decreased baroclinic activity and declining rainfall totals. This study indicates that the direction of change in regional temperature variability, unlike those for actual temperature itself, may be difficult to predict even if changes in broadscale atmospheric circulation are evident.

HIGH-FREQUENCY climate variability is a fundamental aspect of climate. Understanding climate change demands attention to changes in climate variability and extremes1, but knowledge of the recent behaviour of these variables has been limited by the unavailability of long-term high-resolution data. Climate simulations incorporating increased greenhouse-gas concentrations2-9 indicate that a warmer climate could result in a decrease in high-frequency temperature variability (analogous to the decrease in variability observed from the poles to the tropics, and from winter to summer10) and an increase in the proportion of precipitation occurring in extreme events. Here we analyse high-frequency temperature and precipitation data from hundreds of sites spread over Australia, China, the former Soviet Union and the United States over the past 30 to 80 years. Day-to-day temperature variability is seen to have decreased in the Northern Hemisphere, and-at least within the United States-the proportion of total precipitation contributed by extreme, one-day events has increased significantly. We find that although the notion of a recent increase in interannual temperature variability is supported by data from the past few decades11, the longer data records indicate that this trend is an aberration.

Trends in maximum and minimum temperatures over Australia from 1951 to 1992 have been examined using data adjusted for inhomogeneities. The results, showing a decrease in the diurnal temperature range (DTR) over large areas of the Australian continent, confirm earlier findings and are consistent with trends over much of the global landmass. The trends are largely a consequence of the minimum temperatures increasing more than the maximum temperatures. Decreases in the DTR were strongest over the northeastern interior of the continent and over a small area of the southwest.

Monthly mean maximum and minimum temperatures for over 50% (10%) of the Northern (Southern) Hemisphere landmass, accounting for 37% of the global landmass, indicate that the rise of the minimum temperature has occurred at a rate three times that of the maximum temperature during the period 1951-90 (0.84-degrees-C versus 0.28-degrees-C). The decrease of the diurnal temperature range is approximately equal to the increase of mean temperature. The asymmetry is detectable in all seasons and in most of the regions studied.The decrease in the daily temperature range is partially related to increases in cloud cover. Furthermore, a large number of atmospheric and surface boundary conditions are shown to differentially affect the maximum and minimum temperature. Linkages of the observed changes in the diurnal temperature range to large-scale climate forcings, such as anthropogenic increases in sulfate aerosols, greenhouse gases, or biomass burning (smoke), remain tentative. Nonetheless, the observed decrease of the diurnal temperature range is clearly important, both scientifically and practically.

RECORDS of hemispheric average temperatures from land regions for the past 100 years provide crucial input to the debate over global warming1–4. Despite careful use of the basic station data in some of these compilations of hemispheric temperature1,2,4–6, there have been suggestions7,8 that a proportion of the 0.5 °C warming seen on a century timescale may be related to urbanization influences—local warming caused by the effects of urban development. We examine here an extensive set of rural-station temperature data for three regions of the world: European parts of the Soviet Union, eastern Australia and eastern China. When combined with similar analyses for the contiguous United States9,10, the results are representative of 20% of the land area of the Northern Hemisphere and 10% of the Southern Hemisphere. The results show that the urbanization influence in two of the most widely used hemispheric data sets1,2,4 is, at most, an order of magnitude less than the warming seen on a century timescale.