Abstract:Lidar measurements were performed at a heavily polluted site between 5 Nov 2009 and 8 Nov 2009. Aerosol extinction coefficients, AOT (Aerosol Optical Thickness), and depolarization ratio were measured, temperature and relative humidity profiles were acquired from a microwave radiometer. Lidar is an efficient tool for detecting the ABL (Atmospheric Boundary Layer) and atmospheric aerosols. In this study, the aerosol extinction coefficient was retrieved from the measured signal of an ALS300 lidar system using the Fernald method. The ABL height was determined according to the maximum inflexion point of the inversed aerosol extinction coefficient (the height of the maximum decline rate). Atmospheric pollutant characteristics were analyzed using the depolarization ratio from the lidar. The temperature and relative humidity structure of air were obtained from microwave radiometer data. The results show that under the air pollution conditions, the atmosphere had a strong temperature inversion layer, with an inversion intensity up to 1 K (100 m)^-1, and a very low atmospheric relative humidity above 500 m; the atmosphere was highly stable. After the pollution events and before the start of snowfall, the inversion structure disappeared and the relative humidity increased significantly to reach saturation. Lidar depolarization ratio analysis indicated the pollution type to be a typical soot particulate matter pollution with regional characteristics. There was a significant linear relationship between PM2.5 and AOT, with a correlation coefficient of 0.72. Our results show that lidar systems can detect air pollution characteristics and ABL height in polluted weather before snowfall in autumn in urban Beijing.

Abstract:This study investigates the responses of the Atlantic Meridional Overturning Circulation (AMOC) simulated by the FGOALS-g2 (grid-point version 2 of Flexible Global Ocean-Atmosphere-Land System model) to three different RCPs (Representation Concentration Pathways) of CMIP5 (Coupled Model Intercomparison Project Phase 5). In this investigation, two indices are used: (1) The AMOC index, defines as the maximum value of AMOC between 15°N and 65°N, at depths greater than 500 m, and (2) AMOC extending index, which is the maximum value of AMOC between 15°N and 65°N, and between 2000-m and 2500-m depth. It is found that under RCP2.6 and RCP4.5, both the AMOC index and the AMOC extending index show decreasing trends from 2006 to 2040, following which the AMOC index presents a increasing trend and the AMOC extending index maintains its high value between 2041 and 2100. In contrast, under RCP8.5, rapid decreasing trends are been identified in the time series of both the AMOC index and the AMOC extending index. Through analysis of the mechanism of formation of the North Atlantic deep water, the main component of AMOC, it can be concluded that the long-term trends in AMOC are determined by deep water formation in the Greenland-Iceland-Norwegian (GIN) Seas, while the decadal variability of AMOC is modulated by deep water formation in the Labrador Sea. Meanwhile, it can be shown that due to the stratification stability at around 2000-m depth in the Northern Atlantic Ocean increasing by 30%-40% under RCP2.6 and RCP4.5, the increasing deep water, indicated by the recovering AMOC index, could not sink down any further, which is the reason for the non-recovery of the AMOC extending index.

Abstract:In order to research the validity of using the AMDAR (Aircraft Meteorological Data Relay) data in the GRAPES (Global Regional Assimilation and Prediction System) numerical forecast system, the bias and standard deviation of the operational AMDAR data from the National Meteorological Information Center (NMIC) were calculated, and the impact of the AMDAR data on GRAPES was analyzed. We conducted our study using data from May to June 2013 (two months), with an assimilation window of 6 h and a validity period of 7 d. The bias of the temperature was 0.2 ℃, and the standard deviation was 2 ℃. The bias of the wind was 0.2 m/s, and the standard deviation was 3 m/s. The AMDAR data error distribution is almost normal. The results show an improvement in the assimilation and forecast by GRAPES when using the AMDAR data.

Abstract:Satellite observation data are valuable for model evaluation, but to readily compare satellite-based data with model simulations, swath pixel data must first be gridded. In this study, data distortions caused by gridding (0.1°, 0.25°, 0.5°, 1.0°, and 2.5° gridding resolutions) are investigated using instantaneous and monthly pixel data from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) liquid water path (LWP) data. Results from this case study show that data gridded at grid scales of 0.1°, 0.25°, and 0.5° retain more local details in the instantaneous pixel data, while the details tend to be smoothed out at coarser grid resolutions. So data with grid scales no coarser than 0.5° are suitable for analyzing weather activities from the mesoscale to the synoptic scale. In terms of the monthly pixel LWP, data with gridding resolutions of 0.1°, 0.25°, and 0.5° also retain more detail. Although, probability density functions (PDF) of the LWP show similar patterns at all the gridding scales considered. So when analyzing monthly LWP data, the impact of gridding scales is not significant. Finally, this study compared the gridded TMI LWP data with the European Centre for Medium-range Weather Forecasts Interim reanalysis (ERA-Interim) and NCEP Climate Forecast System Reanalysis (CFSR), and found that both of these reanalyses overestimate the magnitudes of the LWP. Gridded TMI LWP data, the ERA-Interim, and the CFSR show reasonably similar regularities in the variance of the LWP.

Abstract:The heterogeneous responses of cropland Normalized Difference Vegetation Index (NDVI) to temperature and precipitation were analyzed using Advanced Very High Resolution Radiometer (AVHRR) NDVI time series and station- observed climate data from 1981 to 2008 during 12 phenophases including eight for winter wheat and four for summer maize over the North China Plain. The results showed that the lagged response of NDVI to precipitation was more noticeable than that to temperature and that NDVI was more sensitive to the precipitation (temperature) during the last and previous two phases (current and last phases). Additionally, the response varied among crop species and phenophases. The correlation coefficients between NDVI and precipitation were positive at most sites in the mid-late growing season (medium growing season) of winter wheat (summer maize). NDVI was also positively correlated to temperature at the significance level of 0.05 in the early-mid growing season of winter wheat. Furthermore, NDVI was strongly related to temperature (precipitation) during germination and turning-green (turning-green) phases. Therefore, germination and turning-green were the phases during which crop growing was largely influenced by climatic factors in the region.

Abstract:The persistent heavy rainfall events (PHREs) over southern China during 1981-2011 are classified by utilizing the daily precipitation data at 752 stations in China. PHREs in the Yangtze and Huaihe River Valley (YHRV) and PHREs in South China (SC) during 1981-2011 are selected on the basis of the following criteria: More than 10 grids [0.25°(latitude)×0.25°(longitude)] in the area with daily precipitation of more than 50 mm lasting more than five days and rainband coincidence degree greater than 20%. The statistics reveal that 31 PHREs in the YHRV were concentrated between mid-June to mid-July with an average duration of 8.29 days, whereas 34 non-typhoon PHREs in SC were concentrated in June and July with an average duration of 6.24 days. Moreover, the frequency and intensity of these two types changed in decadal growth. PHREs affected by the typhoon systems in the YHRV were concentrated between mid-July and early August. Those occurring in SC were concentrated from July to September, and their frequency and intensity have increased since 2000. Objective pattern correlation statistics are used to classify and discuss the non-typhoon PHREs in detail. PHREs in the YHRV are divided into three types: (A) main rainbelt to the south of the Yangtze River (YR), (B) main rainbelt to the north of the YR, (C) main rainbelt along the YR, and PHREs in SC are classified into two types: (E) main rain belt to the east of the Yunnan-Guizhou Plateau and (F) main rainbelt located over the Yunnan-Guizhou Plateau and Guangxi Province. On the basis of such classification, the mechanisms of PHREs will be discussed in future studies.

Abstract:Considering the anthropogenic heat (AH) flux sources from human metabolism and activity, factories, vehicular traffic, this work has estimated the diurnal and annual variations of AH flux in Guangzhou from 2000 to 2011 using 2001-2012 data obtained from the Statistics Annals of Guangzhou. The results show that the averaged AH flux of factories and vehicular traffic accounts for 55% and 36%, respectively. The AH flux value shows an increasing trend during the 12-year period from 2.7×10¹⁷ J to 4.4×10¹⁷ J, with an exception after 2006 when the value presents a slight decline. This result can be attributed to a higher energy efficiency leading to a lower emission rate of energy consumption as manufactories are the major source of AH flux. The peak values of diurnal variation is shown to occur at 1000 LST and 1400 LST. During the 12-year period, the overall diurnal variation shows a decreased trend, which can be attributed mainly to a faster pace of urbanization than the increasing rate of energy consumption, population, and vehicles. Compared with the diurnal variation parameters of AH flux in the urban module of the Weather Research and Forecasting (WRF) model, this discrepancy is closely related to discrepancies in work and life span between Guangzhou and cities in western countries.

Abstract:The Topographic Index Model (TOPMODEL) has been implemented into land surface models (LSMs) to improve modeling of hydrological process components. In recent years, great effort has been made by many researchers for adjusting some of the assumptions contained in the classic TOPMODEL, which will broaden its application scope. For example, the general power law has been used to describe the variation of saturated hydraulic conductivity with depth as an extension of the classic TOPMODEL. Known as power law TOPMODEL, its derivation is based on the assumption of spatially uniform land surface. In this paper, the power law TOPMODEL is extended to a spatially inhomogeneous land surface with spatially non-uniform saturated hydraulic conductivity at the ground surface (K₀), effective soil depth (m), and water recharge rate to the ground water (R). In addition, numerical experiments with assumed spatial variable patterns of K₀, m, and R are conducted to evaluate the hydrological effects of the spatial heterogeneity. The main conclusions are as follows: 1) The assumed spatial distributions of K₀, R, and m affect daily surface runoff, daily baseflow, and daily total runoff. Among these, the spatial distribution of m significantly increases daily surface runoff and flood discharge. 2) The assumed spatial distributions of K₀, R, and m do not significantly affect the total runoff and evaporation averaged over many years, although they change the partition of total runoff between surface runoff and baseflow. In particular, the spatial distribution of R₀ enhances averaged surface runoff of many years and significantly reduces the averaged baseflow during the same period. The other two variables change surface runoff and baseflow less than does the assumed spatial distributions of R₀.

Abstract:Using a static opaque chamber method, a field experiment was conducted in-situ in the Hulunbuir meadow grassland, Inner Mongolia, during the seeding period (2011 and 2012) and flowering period (2012) to investigate the effects of grazing and mowing on the diurnal variation in methane flux and ecosystem respiration rate. The results show that the Hulunbuir meadow grassland soils were a sink of CH₄, with the average CH₄ diurnal fluxes ranging from -23.98±6.40 to -95.96±28.57 μgCm^-2 h^-1 in the three experimental periods. The relationship between the diurnal variation in CH₄ flux and temperature is complex in the study area, and some differences were observed in the diurnal variation of soil-plant system respiratory CO₂ emission fluxes during the experimental periods. During the seeding period in 2011, the daily mean soil-plant system respiration CO₂ emission fluxes were higher than in 2012, as a result of the interaction of water and temperature. The influence of grazing on the diurnal variation of CH₄ fluxes was very limited, however, the observations suggest that grazing could have increased the daily average CH₄ flux from 12.05% to 93.35% during the seeding period in 2011 and the flowering period in 2012, but reduced it from 23.32% to 30.43% during the seeding period in 2012. Mowing could be responsible for a decrease in the average daily CH₄ flux, from 11.55% to 60.62%. Considering the cumulative daily carbon emissions, CH₄ absorption accounts for only 0.35%-2.62% of the total in the Hulunbuir meadow grasslands. Moreover, the impact of grazing and mowing on the mean cumulative daily carbon emissions differed significantly between the different phenological periods and vegetation types.

Abstract:A rainstorm process that occurred during 24-25 Jul 2011 is analyzed by numerical simulation performed with NCEP data used for the initial field. The numerical simulation accurately simulates this rainfall process and essentially reflects the rainfall distribution characteristics. The moist thermodynamic advection parameter is chosen to perform diagnostic analysis for the rainfall region. Results show that in the meridional-vertical cross section of the zonal mean of the moist thermodynamic advection parameter, the high-value areas, and the centers of the moist thermodynamic advection parameter coincide well with the heavy rainfall region and that the gradient and the upward height can qualitatively indicate the intensity. The vertical integral of the moist thermodynamic advection parameter can trace the heavy rainfall region more accurately than the light rainfall region. The centers of the moist thermodynamic advection parameter do not completely overlap the heavy rain centers; instead, the high-value areas of the gradient correspond to the centers. Moreover, the moist thermodynamic advection parameter has a spatial distribution and time evolution trend similar to those of the simulated 6 h accumulated precipitation, and its change trend can reflect the increase and decrease of precipitation.

Abstract:Observational day and night precipitation data from 89 stations in Xinjiang province during 1961-2005 were used to assess the temporal and spatial changes in daytime and nighttime precipitation processes. The goal of this investigation is to understand the change characteristics and trends of the precipitation process in both the daytime and nighttime in this area, under a background of increasing annual precipitation. The results show that the multi-year average of annual precipitation at nighttime was larger than at daytime during 1961-2005 and that precipitation increased significantly in both the day and the night. The increasing trend of precipitation is larger in the nighttime than in the daytime, with the annual increasing trend in the daytime and nighttime respectively accounting for 49% and 51% of the total increasing trend in annual precipitation. In addition, there are significant regional differences between southern and northern Xinjiang. The amounts of daytime and nighttime precipitation both show significant jumps, but at different times: In 1986 for the daytime, and in 1991 for nighttime precipitation. Further, the annual number of rainy days during the daytime is less than it is at nighttime, although they both show a clear increasing trend. This trend of increasing rainy days is greater for daytime precipitation than it is for the nighttime. Precipitation intensity in the daytime and nighttime also show a significant increase, with both precipitation intensity and the growth rate of precipitation intensity being greater at night than they are during the day. The increase in the number of rainy days is the main cause of the total increase in precipitation, while changes in precipitation intensity have little influence on the total. Meanwhile, the annual number of drizzle days show a distinct decrease in both the daytime and the nighttime. Drizzle days during the daytime are greater than at nighttime, and the decreasing rate is the same between day and night. Finally, the numbers of heavy precipitation days and their intensity both show an increasing trend. The daytime and nighttime heavy precipitation has a significant correlation with their respective total precipitation amounts, thus, heavy precipitation has a considerable influence on total precipitation. Further study is required to establish the causes of these differences in precipitation change characteristics between the daytime and nighttime.

Abstract:The categorization method was used to evaluate the performance of the Coupled Model Inter-comparison Program Phase 5 (CMIP5) climate models in simulating the decadal variation of summer monsoon rainfall in eastern China. Out of the 38 models that were examined, only six (category-1 models) captured the decadal variation of summer rainfall in eastern China in the late 1970s, i.e., increased rainfall in the Yangtze River basin and decreased rainfall in North and South China. The category-1 models simulate the decadal variation of summer rainfall in eastern China well because they are able to capture the decadal weakening of the East Asian Summer Monsoon (EASM) along with the northerly anomaly in the coast of East Asia and changes in the western Pacific subtropical high in the late 1970s. In contrast, the category-2 models, which poorly simulate the decadal variation of summer rainfall in eastern China, fail to reproduce the decadal weakening of the EASM. Further analysis indicates that while most models can reproduce the spatial distribution of the Pacific Decadal Oscillation (PDO), their accuracy at reproducing the decadal variation of the PDO varies. Category-1 models can reproduce the tropical ocean warming in the late 1970s and the relative phase transition of the PDO. Category-2 models fail to capture the phase transition of the PDO, the tropical ocean warming in the central-eastern Pacific Ocean, and the cooling trend in the Yangtze River basin, which results in low accuracy of these models for reproducing the weakened EASM and the decadal variation of summer rainfall in eastern China after the late 1970s. This indicates that the accuracy of climate models for simulating the decadal variations of sea surface temperature plays an important role in the accuracy of the models' simulation of the decadal variation of summer rainfall.