Binhui Liu

A spatial analysis of pan evaporation trends in China, 1955-2000

[1] Pan evaporation, an indicator of potential evaporation, has decreased during the last several decades in many parts of the world. This trend is contrary to the expectation that global warming will be accompanied by an increase in terrestrial evaporation, known as the pan evaporation paradox. In this paper we present an analysis of changes in the spatial patterns of pan evaporation in China based on data from 85 weather stations from 1955 to 2000. We found that pan evaporation decreased in China from 1955 to 2000. The decrease was statistically significant in all of China’s eight climatic regions except northeast China. We also found that the decrease in solar irradiance was most likely the driving force for the reduced pan evaporation in China. However, unlike in other areas of the world, in China the decrease in solar irradiance was not always accompanied by an increase in cloud cover and precipitation. Therefore we speculate that aerosols may play a critical role in the decrease of solar irradiance in China. By subdividing China into eight climatic regions, we found that the rate of decrease in pan evaporation was highest in the northwest and lowest in the southwest. Although changes in solar irradiance are the main cause of decreasing pan evaporation, water conditions influence the sensitivity of pan evaporation to the change in solar irradiance in comparing the eight climatic regions. Thus the spatial trends of pan evaporation differ from those of solar irradiance among these regions. INDEX TERMS: 1620 Global Change: Climate dynamics (3309); 1818 Hydrology: Evapotranspiration; 1878 Hydrology: Water/energy interactions; 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); KEYWORDS: pan evaporation, trends, China

Spatiotemporal change of diurnal temperature range and its relationship with sunshine duration and precipitation in China

We examined the spatiotemporal variation in diurnal temperature range (DTR) and discussed the reasons for the changes of DTR in China based on data from 479 weather stations from 1962 to 2011. Results showed that DTR decreased rapidly (0.291°C/decade) from 1962 to 1989 due to slightly decreased Tmax and significantly increased Tmin, but the decrease in DTR has stopped since 1990 as Tmax and Tmin kept pace with each other. During 1990–2011, DTR remained trendless, with slight increase in the 1990s and slight decrease after 2000. During the whole study period from 1962 to 2011, DTR decreased at a rate of 0.157°C/decade nationally. Spatially, decreases in DTR were greatest in Northeast China and lowest in Southwest China with a transect running from northeast to southwest showing the decreasing trends change from high to low. Seasonally, DTR decreases were greatest in winter and lowest in summer, and the magnitudes of decrease reduced from the north to south of China. The changes in DTR were closely correlated with changes in sunshine duration (SD) in China except the Tibetan Plateau, suggesting that SD decrease is an important contributor to the decrease of DTR through its influence on Tmax. In addition to the contribution of SD decrease, the increasing of precipitation played an important role in DTR decrease in Northwest China, the most arid region of China. It appeared that changes of cloud cover (CC) were not the reasons for DTR changes in the past 50 years as CC has decreased during the study period.

Spatiotemporal change in China's frost days and frost‐free season, 1955–2000

[1] From 1955 to 2000, China has experienced a decrease in the number of frost days, while the length of the frost-free season between the last spring freeze and the first fall frost has increased. Three distinct regimes can be detected in the time series: up to about 1973, the annual number of frost days was about 2 d higher than the 1961-1990 average; from 1973 to 1985, the annual number of frost days held close to that average with remarkably little interannual variability; and after 1985, the annual number of frost days decreased rapidly with distinct reversal around 1992. The dates of first and last frost show two patterns: before 1980, these dates fluctuated around the 1961-1990 average, but after 1980 (and especially from 1993) the frost-free season was rapidly lengthened. The numbers of frost days are highly correlated with minimum temperature (Tmin) in north China in spring and fall; while in south China frost dates correlate with minimum temperatures in winter. Generally, the seasonal relationships between Tmin and frost days are significant in both the temporal and spatial domains when seasonal average Tmin falls within a range of +/- 10 degrees C. Analyzing annual and seasonal influences on the number of frost days, we find that water vapor plays a significant role. Regionally, the greater influence on the length of the frost-free season in south China has been the delayed onset of the autumn frost, while in north China the spring and autumn dates each have a comparable influence on the length of the frost-free season. The initial lengthening of the frost-free season lagged about 10 years behind the rapid increase in daily minimum temperatures, while the decrease in the annual number of frost days lagged by about 15 years.

Effects of land use/land cover on diurnal temperature range in the temperate grassland region of China

As a fragile ecological zone, the temperate grassland region of China has experienced dramatic land use/land cover (LULC) changes due to human disturbances. So far, the impacts of LULC change on climate especially the diurnal temperature range (DTR) in this region are still not well understood. Based on the OMR (observation minus reanalysis) method, this study investigated the effects of LULC on DTR in the temperate grassland region of China. Considering the possible uncertainty of the results due to spatial resolution of the reanalysis dataset, two reanalysis datasets with different spatial resolutions were utilized. Results showed that LULC generally contributed to the decline of DTR in the temperate grassland region of China during 1980 to 2005. Due to different warming effects on monthly maximum temperature (Tmax) and minimum temperature (Tmin), grassland and forest tend to slightly decrease monthly DTR (approximately -0.053 to -0.050°C/decade and approximately -0.059 to -0.055°C/decade, respectively), while bare land has a slightly positive effect on DTR (approximately 0.018-0.021°C/decade). By contrast, cropland and urban tend to slightly decrease Tmax, obviously increase Tmin and thus result in a rapid decline of DTR (approximately -0.556 to -0.503°C/decade and approximately -0.617 to -0.612°C/decade, respectively). In the temperate grassland region of China, grassland vegetation changes due to human disturbances can have some effects on DTR mainly by changing the Tmax. Conversion from grassland to cropland could decrease the DTR by slowing down the increase of Tmax. But the conversion from grassland to bare land, as well as the reduction of grassland vegetation cover will increase Tmax, and consequently the DTR. The results suggest that grassland degradation is likely to result in daylight warming and increased DTR in the temperate grassland region of China.

Impact of Climate Change on Temperate and Alpine Grasslands in China during 1982–2006

Based on GIMMS NDVI and climate data from 1982 to 2006, this study analyzed the impact of climate change on grassland in China. During the growing season, there were significant effects of precipitation on the growth of all the grassland types ( ), except for meadow vegetation. For the air temperatures, there existed asymmetrical effects of maximum temperature ( ) and minimum temperature ( ) on grassland vegetation, especially for the temperate grasslands and alpine steppe. The growing season NDVI correlated negatively with but positively with for temperate grasslands. Seasonally, these opposite effects were only observed in summer. For alpine steppe, the growing season NDVI correlated positively with but negatively with , and this pattern of asymmetrical responses was only obvious in spring and autumn. Under the background of global asymmetric warming, more attention should be paid to this asymmetric response of grassland vegetation to daytime and night-time warming, especially when we want to predict the productivity of China’s grasslands in the future.

Using GIMMS NDVI time series to estimate the impacts of grassland vegetation cover on surface air temperatures in the temperate grassland region of China

Abstract Exploring the impacts of vegetation cover on regional climate is very important to assess the relationship between vegetation and climate, and to predict regional climate change in vegetated areas. Using the time series of Global Inventory Modelling and Mapping Studies (GIMMS) normalized difference vegetation index (NDVI), we estimated the impacts of grassland vegetation cover on surface air temperatures based on the observation minus reanalysis (OMR) approach. The theory of the OMR approach is that reanalysis data are not sensitive to local land surface properties, thus the changes in OMR can be attributed to land effects. The method used in this letter is that rates of change in OMR are regressed (using stepwise regression) on average NDVI and the rate of NDVI change. We applied this multiple stepwise regression analysis in the temperate grassland region of China during the growing season from 1982 to 2005. To evaluate the accuracy of regress equations, we estimated the monthly OMR temperatures during 1983–2005 and then compared the monthly estimated and actual OMR temperatures. The results showed that the estimated OMR temperatures were well consistent with actual OMR temperatures in all the months, with the correlation coefficients between them ranging from 0.83 to 0.98. It indicates that combined with the OMR approach, GIMMS NDVI can be used to accurately estimate the impacts of grassland vegetation on surface air temperatures in temperate grassland areas of China.