Zhongwei Yan

Trends of Extreme Temperatures in Europe and China Based on Daily Observations

Ten of the longest daily temperature series presently available in Europe and China are analysed, focusing on changes in extremes since pre-industrial times. We consider extremes in both a relative (with respect to the time of year) and an absolute sense. To distinguish changes in extremes from changes affecting the main part of the temperature distribution, a percentile smaller than 10 (and/or larger than 90) is recommended for defining an extreme. Three periods of changes in temperature extremes are identified: decreasing warm extremes before the late 19th century; decreasing cold extremes since then and increasing warm extremes since the 1960s. The early decreases and recent increases of warm extremes dominate in summer, while the decrease of cold extremes for winter persists throughout the whole period. There were more frequent combined (warm plus cold) extremes during the 18th century and the recent warming period since 1961 at most of the ten stations, especially for summer. Since 1961, the annual frequency of cold extremes has decreased by about 7% per century with warm extremes increasing by more than 10% per century but with large spatial variability. Compared with recent annual mean warming of about 2–3 ° C/century, the coldest winter temperatures have increased atthree times this rate, causing a reduced within-season range and therefore less variable winters. Changes in the warmest summer temperatures since 1961 exhibit large spatial variability, with rates of change ranging from slightly negative to 6 ° C/century. More extensive station observations since 1961 indicate that the single site results are representative of larger regions, implying also that the extremes studied are the result of large-scale changes. Recent circulation changes in daily gridded pressure data, used as an indicator of wind speed changes, support the results by explaining some of the trends.

A new estimate of the China temperature anomaly series and uncertainty assessment in 1900–2006

While global warming during the last century has been well recognized, the magnitude of the climate warming in regions such as China over the past 100years still has some uncertainty due to limited observations during the early years. Several series of temperature anomalies for the 20(th) century in China have been independently developed by different groups. The uncertainty arises mainly from the sparse observations before 1950, where statistics are sensitive to the small and potentially biased sample. In this study, BSHADE-MSN (Biased Sentinel Hospitals Areal Disease Estimation and Means of Stratified Nonhomogeneous Surface), a combination of two novel distinct statistical methods that are applicable with different sample situations to a spatial heterogeneous surface, is applied to estimate annual mean temperature anomalies for China. This method takes into account prior knowledge of geographical spatial autocorrelation and nonhomogeneity of target domains, remedies the biased sample, and maximizes an objective function for the best linear unbiased estimation (BLUE) of the regional mean quantity. For the period 1900-1999, the overall trend estimated by BSHADE-MSN is 0.80 degrees C with a 95% confidential interval between 0.41 degrees C and 1.18 degrees C. This is significantly lower than that calculated by Climate Anomaly Method (CAM) and Block Kriging. The new temperature anomaly series for China exhibits slightly warmer conditions for the period before 1950 than existing studies. All the methods applied so far agree well with each other for the period after 1950, when there are sufficient stations across the country for the estimation of temperature anomaly series. Cross validation shows that the new regional mean temperature anomaly series has smaller estimation error variance and higher accuracy than those based on the other methods assessed in this study.

The last 140 ka in the Afro-Asian arid/semi-arid transitional zone

Abstract During the last ten years, a great number of geological or other proxy records provided radiocarbon, U/Th or TL dated information on the past climatic oscillations in the arid/semi-arid zones extending between the Atlantic and the Pacific. Comparisons and a synthetic view of these results can now be attempted for the last 140 ka, and compared with global changes, as registered in oceanic or ice cores, and with palaeomonsson models: for this purpose, wide spatial and time scales have to be used. On the whole, arid/humid alternations roughly fit global changes, cold phases corresponding to an extension of the arid areas to the South, warm phases corresponding to the shrinking of the same to the North. The last interglacial is associated with an increase of precipitation throughout the area considered. Isotopic stage 4 brings no evidence whatsoever of humid conditions. Two wetter episodes are registrated during stage 3. A major rainfall decrease is everywhere associated with the Last Glacial Maximum (21-15 ka in most regions), the arid or semi-arid zones extending several hundred kilometers southwards, relative to the present-day pattern. The two abrupt deglaciation steps and the Younger Dryas are recorded in all of the most sensitive regions, at the margins of the present-day monsoonal range. During the Holocene, the precipitation increases everywhere (by 100–400 mn, relative to the present-day values), the optimum being at 8.5-6.5 ka. A climatic deterioration follows with an irregular pattern of dry/wet episodes, according to the different geographic conditions. The humid phase terminates at 3.5-3 ka in the whole transitional zone. The major southward shift of the monsoonal precipitation range since its optimum, some 8000 years ago, fits the astronomical neoglacial trend and may possibly be correlated with past stage 5d, although its rapidity and spatial importance may just reflect one of the sharp successive cold/warm variations registered by GRIP during the whole stage 5.

Nested high-resolution modeling of the impact of urbanization on regional climate in three vast urban agglomerations in China

[1] In this paper, the Weather Research and Forecasting Model, coupled to the Urban Canopy Model, is employed to simulate the impact of urbanization on the regional climate over three vast city agglomerations in China. Based on high-resolution land use and land cover data, two scenarios are designed to represent the nonurban and current urban land use distributions. By comparing the results of two nested, high-resolution numerical experiments, the spatial and temporal changes on surface air temperature, heat stress index, surface energy budget, and precipitation due to urbanization are analyzed and quantified. Urban expansion increases the surface air temperature in urban areas by about 1C, and this climatic forcing of urbanization on temperature is more pronounced in summer and nighttime than other seasons and daytime. The heat stress intensity, which reflects the combined effects of temperature and humidity, is enhanced by about 0.5 units in urban areas. The regional incoming solar radiation increases after urban expansion, which may be caused by the reduction of cloud fraction. The increased temperature and roughness of the urban surface lead to enhanced convergence. Meanwhile, the planetary boundary layer is deepened, and water vapor is mixed more evenly in the lower atmosphere. The deficit of water vapor leads to less convective available potential energy and more convective inhibition energy. Finally, these combined effects may reduce the rainfall amount over urban areas, mainly in summer, and change the regional precipitation pattern to a certain extent.

An Analysis of Daily Maximum Wind Speed in Northwestern Europe Using Generalized Linear Models

Abstract The basic climatological pattern and recent trends in daily maximum wind speed (DMWS) for the region 47.5°–65°N and 12.5°W–22.5°E are studied using gamma distributions within a generalized linear model. Between 1958 and 1998, DMWS has increased over the ocean in winter but weakened over continental Europe in summer. Large-scale circulation changes such as those of the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) account for the strengthening wind over the ocean. Global warming may have impacted the regional wind climate. In particular, Southern Hemisphere temperature exhibits a significant effect on the distinct oceanic and continental trends in DMWS. It is suggested that the steady warming of the Southern Hemisphere during the last few decades may have forced the North Atlantic storm track to shift in such a way that storms are enhanced toward the northwestern oceanic area, but weakened throughout most of the European continent.

The East Asian summer monsoon circulation anomaly index and its interannual variations

Based on the concept of East Asia-Pacific (EAP) teleconnection which influences East Asian summer monsoon, an index for East Asian summer monsoon circulation anomaly was defined and it was pointed out that this index can describle the interannual variation character of summer climate in East Asia, especially in the Yangtze River and Huaihe River Valley.

Recent trends in weather and seasonal cycles: An analysis of daily data from Europe and China

A wavelet analysis is conducted to investigate daily variability (DV, timescales of less than 3 days), weekly (weather) variability (WV, timescales of 5 days up to 2 months), and seasonal variability (SV, timescales of 8 to 17 months) in five temperature series from Europe and China and two westerly indices for the European/North Atlantic sector back to the 18th century. DV exhibits local features so that it is sensitive to any inhomogeneity in each series. Analysis of DV shows the potential for further homogenization of the data and suggests that for the present study, daily series are only truly homogeneous back to the 19th century. WV is responsible for extremes of large-scale cold/warm variations in the daily series and explains about 80% of the total variance. WV is found to be significantly weaker in northern Europe by 7-10% during warming periods, especially for winter and autumn, but summer temperature correlates positively with WV, with a maximum coefficient of 0.52 for central England. This indicates that for warming periods, WV is reduced in the cold season, but is potentially increased in the warm season. The principal timescale of weather, about 16 days in Europe and 11 days in China, does not exhibit significant trends. Changes in SV from cold to warm periods often result in weaker seasonal cycles, with an unprecedented reduction of up to 3°C at St. Petersburg during the warm period since 1988. The analysis of the westerly indices supports the recent unusually anomalous seasonal cycles, with stronger winter westerlies over the northern Atlantic and Europe. The trends in WV of the westerly indices coincide with the temperature data, implying they are responsible for the large-scale changes over northern Europe.

Trend of Surface Air Temperature in Eastern China and Associated Large-Scale Climate Variability over the Last 100 Years

AbstractUsing the reconstructed continuous and homogenized surface air temperature (SAT) series for 16 cities across eastern China (where the greatest industrial developments in China have taken place) back to the nineteenth century, the authors examine linear trends of SAT. The regional-mean SAT over eastern China shows a warming trend of 1.52°C (100 yr)−1 during 1909–2010. It mainly occurred in the past 4 decades and this agrees well with the variability in another SAT series developed from a much denser station network (over 400 sites) across this part of China since 1951. This study collects population data for 245 sites (from these 400+ locations) and split these into five equally sized groups based on population size. Comparison of these five groups across different durations from 30 to 60 yr in length indicates that differences in population only account for between 9% and 24% of the warming since 1951. To show that a larger urbanization impact is very unlikely, the study additionally determines ho...

Impact of Anthropogenic Heat Release on Regional Climate in Three Vast Urban Agglomerations in China

We simulated the impact of anthropogenic heat release (AHR) on the regional climate in three vast city agglomerations in China using the Weather Research and Forecasting model with nested high-resolution modeling. Based on energy consumption and high-quality land use data, we designed two scenarios to represent no-AHR and current-AHR conditions. By comparing the results of the two numerical experiments, changes of surface air temperature and precipitation due to AHR were quantified and analyzed. We concluded that AHR increases the temperature in these urbanized areas by about 0.5°C—1°C, and this increase is more pronounced in winter than in other seasons. The inclusion of AHR enhances the convergence of water vapor over urbanized areas. Together with the warming of the lower troposphere and the enhancement of ascending motions caused by AHR, the average convective available potential energy in urbanized areas is increased. Rainfall amounts in summer over urbanized areas are likely to increase and regional precipitation patterns to be altered to some extent.

Statistical Downscaling of Summer Temperature Extremes in Northern China

Two approaches of statistical downscaling were applied to indices of temperature extremes based on percentiles of daily maximum and minimum temperature observations at Beijing station in summer during 1960–2008. One was to downscale daily maximum and minimum temperatures by using EOF analysis and stepwise linear regression at first, then to calculate the indices of extremes; the other was to directly downscale the percentile-based indices by using seasonal large-scale temperature and geo-potential height records. The cross-validation results showed that the latter approach has a better performance than the former. Then, the latter approach was applied to 48 meteorological stations in northern China. The cross-validation results for all 48 stations showed close correlation between the percentile-based indices and the seasonal large-scale variables. Finally, future scenarios of indices of temperature extremes in northern China were projected by applying the statistical downscaling to Hadley Centre Coupled Model Version 3 (HadCM3) simulations under the Representative Concentration Pathways 4.5 (RCP 4.5) scenario of the Fifth Coupled Model Inter-comparison Project (CMIP5). The results showed that the 90th percentile of daily maximum temperatures will increase by about 1.5°C, and the 10th of daily minimum temperatures will increase by about 2°C during the period 2011–35 relative to 1980–99.