Abstract:Seasonal change in wind strength and direction is one of the most significant characteristics of meteorological elements in the monsoon region. Lu and Chan (1999) defined an index to characterize the monsoon intensity in the South China Sea (SCS) by using the low-level wind field. For simplicity, only the meridional component was used. The study shows that the index has a good correlation with the summer rainfall in South China (SC). To modify and verify this index, by using the monthly mean NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis data and the daily precipitation data of the national reference or basic synoptic stations in SC, the authors analyzed the wind projection in different directions, not the restricted southerly direction, so that the performance of the index can be improved. To quantify the relationship between the monthly mean wind projection component intensity of the SCS in different directions and the monthly precipitation in SC, the generalized linear models and two forms of the probability distribution hypothesis, namely, normal distribution, and gamma distribution, have been used. Results show that the wind projection component intensity of the SCS in the northwest (11.3°) direction has a better correlation with the summer precipitation in SC then original index. Moreover, a significant nonlinear relationship between precipitation in SC and the improved index has been observed. The generalized linear model with gamma distribution hypothesis for precipitation can obtain better realistic regression results than the generalized linear model with normal distribution hypothesis.

Abstract:Based on conventional observations, L-band soundings, and NCEP reanalysis data, the key mechanisms involved in initiating and enhancing the first severe regional snowstorm of the early winter of 2016 in Henan Province, China, were diagnosed. Results showed a large-scale circulation that included a full-latitude shortwave trough and a stable cold vortex to the northeast. Southerly mid- to upper-air and northerly lower-level winds played an important role in the snowstorm. When an 850-925 hPa northeasterly jet provided a “cold cushion” by forcing warm and humid air upward, it and a 500-700 hPa southwesterly jet formed a strong vertical wind shear and a deep frontogenetic zone. This was conducive to ascension and frontal secondary circulation, which considerably enhanced the snowstorm. The warm and humid southwest 700 hPa jet was a key feature of the storm, transporting beneficial water vapor and convergence with the cold air behind the northeast cold vortex in the Huang-Huai area. This caused considerable dynamic lifting. The north-south migration of the convergent shear at 700 hPa caused two snowfall centers in midwestern and southeastern Henan. The snow zone moved with the southward migration of the “cold air wedge.” The atmosphere above the snow zone showed a positive wet barotropic term and a negative wet baroclinic term with large absolute values of both during the entire process. This was conducive to the release of symmetric unstable energy and the development of the snowstorm. The essential and significant conditions of the snowstorm are the upper-air shortwave trough, the mid-air convergent shear, the lower-level “cold cushion” and a frontogenetic zone.

Abstract:In this paper, the atmospheric circulation field, vertical shear of ambient horizontal wind, and dynamical and thermal conditions within the typhoon circulations of East China-landfalling tropical cyclones (TC) for the period 2005-2014 were compared and analyzed. Using CMORPH precipitation data, the TC precipitation was classified into two categories: left side of track (L) and right side of track (R). Results showed that the 20 landfalling TCs included twelve L TCs and eight R TCs. In general, atmospheric circulation factors played a major role in the TC precipitation distributions. During landfall of the L TCs, in the upper troposphere, the South Asian high was zonally distributed and the flow was divergent on the left side of the track area, a westerly trough featured to the east mid-troposphere, and the western Pacific subtropical high was located to the south, with a southwesterly vertical speed shear of the ambient horizontal wind. In contrast, for the R TCs, the South Asian high was longitudinally oriented in the upper troposphere, then broke down. On the left side of track, wind was relatively uniform, except for a strong divergent flow on the right. The subtropical high was much further north and was blocking, and the vertical shear of the ambient horizontal wind was northeast, which was conducive to R-type precipitation. Thermal advection and water vapor convergence within the typhoon circulation corresponded well with the rainfall area. Warm advection occurred in the lower layers of the southwest quadrants of the L TCs, leading to decreasing atmospheric stability. At the same time, the water vapor convergent zone was also located in the southwest quadrant, contributing to precipitation on the left side of the TC track. For R TCs, the subtropical high was further north and transported warm, moist air on southeasterly flow to the west of the typhoon, and the northeastern quadrant always maintained warm advection, which was favorable for rainfall on the right side of the track.

Abstract:Atmospheric aerosols play a critical role in many aspects of climate, environmental, and human health problems and have attracted increasing attention in recent years. It is of great importance to retrieve aerosol optical depths (AODs) accurately. Calibration of the direct solar radiation data obtained by sun photometers is a key step in obtaining accurate AODs. Unfortunately, there are significant errors in the AODs obtained for heavily polluted regions, such as most parts of China, by the use of incorrect solar calibration constants determined by the widely used Langley method. To overcome this problem, in this paper, we propose a method for performing an objective assessment of Langley calibrations that combines the surface shortwave radiation observed by a pyrheliometer with the SBDART atmospheric radiative transfer model. We verified the feasibility of the proposed method using two and a half years of solar radiation data obtained by the multi-filter rotating shadowband radiometer and the normal incidence pyrheliometer. The results showed that the accuracy of the AOD retrievals was improved and the solar calibration constants could be constrained to a set of stable values.

Abstract:In this paper, WRF (V3.9) is used to simulate the mountain-plain breeze circulation in eastern Chengdu (Longquan mountain) during a heavy pollution event on 7 December, 2016. The structure and evolution of the mountain-plain breeze circulation are discussed. Besides, the influence of the Aerosol Optical Depth (AOD) on the mountain-plain breeze circulation is examined by conducting a sensitivity experiment in which the AOD value is reduced. The Longquan mountain is fairly long from north to south and relatively narrow from east to west with low elevation. Results show that strong ground inversion existed in the winter during the heavy pollution event. Cross-mountain downslope wind circulation occurred in the night and late morning due to the imbalance of radiative heating between western and eastern slope and the dynamic forcing of background wind. The plain breeze began after the noon and ended at around 1700 LST. The maximum horizontal stretch of the plain breeze circulation could reach 3 to 4 times of the mountain width. Cross-mountain downslope wind circulation and mountain-plain breeze circulation varied greatly in the north-south direction. The former was stronger in the southern part of the mountain whereas the latter in the central and southern part was easily covered by vigorous turbulence. In the northern part, on the contrary, there existed significant plain breeze circulation. After reducing the AOD in the sensitivity experiment, the solar radiative heating and planetary boundary layer height both increased to a certain extent. Plain breeze circulation in the central and southern part became more ambiguous due to amplified turbulences. In the northern part, plain breeze circulation enhanced and lasted longer due to the elevated planetary boundary layer and the strengthened difference in sensible heat flux between the mountain and plain areas.

Abstract:The implement of "one belt and one road" program has made the starting station of the Silk Road Xi'an become the focus of the world. The air quality in Xi'an also attracts attention from the government and the public. Taking a strong dust period in northern China in May 2017 as a case, we firstly used the aerosol and atmospheric chemistry model developed by the Institute of Atmospheric Physics (IAP-AACM) to simulate the spatial and temporal distribution of fine particulate matter (PM_2.5) in the Central Shaanxi area. Combined with hourly surface PM_2.5 observation data, we explored the relationship between dust aerosol and the PM_2.5 simulation. Results show that adding the dust component to anthropogenic PM_2.5 significantly improves simulation accuracy, through which the correlation can be elevated by 0.4-0.6, and the sudden increase of PM_2.5 during the strong dust period can be well reproduced. During strong dust and general periods, the contribution of dust aerosol to PM_2.5 ranges from 60%-80% and 10%-30%, respectively. High-resolution simulations improve the model’s ability to capture the spatiotemporal changes of the pollutants.

Abstract:In this paper, the authors examined the sprite phenomenon over the southern Tibetan Plateau (22°-30°N, 86°-98°E) by comparing the lightning detection data from the World-Wide Lightning Location Network (WWLLN) and observations from the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) instrument aboard the FORMOSAT-2 satellite during 2004-2015. It was found that most of the location accuracy of ISUAL was sound after analysis of 17 samples, and the deviations from WWLLN were ＜ 50 km. It was consistent with the results obtained in North America and its vicinity. Based on these results, the authors analyzed the characteristics of the parent lightning strokes of the sprites combined with the cloud-top brightness temperature data from the FY 2 satellite. The authors found that sprites over the southern Tibetan Plateau formed not only from mesoscale convective systems, but also from smaller-scale convective systems.

Abstract:The estimated internal variability of near-surface air temperature was compared using three widely adopted methods [pre-industrial control (piControl) simulations, polynomial fit method, and analysis of variance method , based on 37 models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) and 40 large-ensemble simulations from the Community Earth System Model (CESM). The associated influences on the time of emergence (TOE) of near-surface air temperature in future climate projections were also quantified. The results showed that for multimodels from the CMIP5, the estimated internal variability was comparable based on the piControl simulations and the polynomial fit method, while variability estimated by the analysis of variance method was exaggerated in terms of the magnitude because of its inclusion of model uncertainty. Polar amplification was evident in the spatial distribution of estimated internal variability of surface temperature, with considerably larger magnitudes in the mid- to high-latitudes than the low-latitudes. The internal variability of surface temperature did not vary significantly with time or emission scenarios, except for in the tropics, estimated by the analysis of variance method. Moreover, the estimated internal variability showed high consistency among the three methods, based on large-ensemble simulations from the CESM. The different estimated internal variabilities further affected the TOE in future climate projections, mainly in the North Atlantic Labrador Sea and the Weddell and Ross Seas in the Southern Ocean where deep overturning circulations occur. Specifically, the internal variability was estimated to be less than 15% of the forced signals over China based on all three methods in the CESM large-ensemble simulations. This result was comparable to those estimated by the piControl simulations and polynomial fit method based on the CMIP5 multimodels but tended to be overestimated by the analysis of variance method.

Abstract:Based on daily mean air temperature (T _m) and daily minimum air temperature (T _min) data from 762 Chinese stations, analysis of the probability density distribution of annual minimum air temperature in China showed a clear bimodal distribution. It was found that there are significant regional differences in the distribution of annual minimum air temperature during the East Asian winter monsoon. Inspections of the spatial distribution of the frequency of annual minimum air temperature in the two peaks revealed significant regional differences, which were roughly divided into two climatic zones by the 0℃ line of the long-term mean winter air temperature. By analyzing the date of annual minimum air temperature in the two climatic zones, the boreal winter was divided into an early winter that spans from November 16th to January 15th of the following year, and a late winter that spans from January 16th to March 15th. Following this definition, the interannual variations of early and late winter air temperatures in China were studied by season-reliant empirical orthogonal function (SEOF). Meanwhile, the associated atmospheric stationary and transient waves and their interactions were analyzed based on the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data. It revealed that the first two SEOF modes of surface air temperature from early to late winter depicted the in-phase evolution and the out-of-phase evolution. The spatial morphology of the atmospheric circulation anomaly in early and late winter in the in-phase evolution mode is relatively consistent, and intensity of the circulation anomalies strengthens gradually from early to late winter. However, the atmospheric circulation anomaly in early and late winter in the out-of-phase evolution mode is quite different, as is their spatial morphology. Dynamic and thermal forcing effects of transient waves and the dispersion of the stationary waves play an important role in the maintenance and development of stationary waves from North Atlantic to Eurasia. Waves on the North Atlantic in the in-phase evolution mode spread energy from North America to Europe, which strengthens the European high. Energy dispersions of this center are significantly enhanced in late winter, which forms the wave from Europe to the east of the Kara Sea and further to Lake Baikal. Dynamic and thermal forcing effects caused by atmospheric transient waves make a positive contribution to the maintenance and development of the atmospheric centers of action in Europe and the Baikal area. Characteristics of atmospheric stationary and transient waves and their interactions in early winter in the out-of-phase evolution mode are similar to those in late winter in the in-phase evolution mode. However, the waves in late winter spread from the center of the North Atlantic to Greenland, and further eastward through the Ural region to the northern part of the Tibetan Plateau. Movement of the storm track can lead to abnormal activities of the transient waves, making a positive contribution to the maintenance of the atmospheric centers of action in the southern North Atlantic.

Abstract:Herein, we compare the primary simulation differences when considering the electric field effect (EFE) in the 3D dynamic-electric coupling numerical cloud model, wherein EFE was introduced into the thermodynamic equations and falling velocities of different hydrometeors. In the EFE calculations, owing to weak initial electrical activity, the dynamical field and precipitation intensity changed only slightly. With enhanced electrical activity, the updraft and downdraft speeds increased and the precipitation intensity exhibited both increase and decrease periods, whereas the total precipitation increased only slightly. Additionally, the amount of lightning nearly doubled, and it was generated earlier and lasted longer; hence, its feedback effect cannot be ignored. The results show that the peaks of the hail and graupel particles’ grid-scale mass-weighted instantaneous falling velocities were 10 m s^-1 and 7 m s^-1, respectively. Because of the narrow range of the strong electrical field and the direct transient effect on particles of the electric field force, the maximum falling velocities of hydrometeors only fluctuate slightly, but differences in the falling velocities are more obvious among small-size particles. By controlling the hydrometeor falling velocities, the electric field changes the particles’ primary production, increases the production rates of raindrops and ice crystals, decreases the production rates of graupel and hail, adjusts the spatiotemporal distributions of the vapor liquid and solid state hydrometeors, increases water vapor by 9% and latent heating by 7%, and provides internal energy for the further development of thunder clouds.

Abstract:Variations in Asian upper tropospheric temperature during summer is closely related to, and may, indeed, serve as a useful predictor of East Asian precipitation. The predictability of these interannual variations in summer UTT (upper tropospheric temperature, represented by 500-200 hPa thickness) for the period 1960-2005 in the ENSEMBLES multi-model seasonal forecast, initiated 1 May every year, was examined in this study. Results showed that the interannual variability of Asian UTT in summer was skillfully predicted by ENSEMBLES, as measured by the good prediction of its standard deviation centers in the mid-latitude and high correlation coefficient of its first two leading interannual variability modes compared with observations. The main deficiency of the multimodel ensemble mean (MME) was that the temperature at high-latitudes could not be captured. The correlation coefficients of the first (PC1) and second (PC2) principle components in the MME with those from NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis were 0.63 and 0.77, respectively. The first two leading models of Asian UTT in summer in observation were dominated by (1) the silk-road teleconnection at the upper troposphere forced by Indian monsoon precipitation anomalies in an ENSO-developing summer, and (2) the Pacific-Japan teleconnection forced by northwestern Pacific Ocean rainfall anomalies in an ENSO-decaying summer, respectively. These processes were well-predicted by ENSEMBLES; thus, a high prediction skill for Asian summer UTT was shown in ENSEMBLES. The first two leading modes of Asian summer UTT well-represented the zonal and meridional thermal contract variation. A comparison with two previous widely-used East Asian Summer Monsoon (EASM) indices was performed; results showed a much better predictive outcome for PC1 than for the traditional zonal thermal contrast. Using PC1 of Asian summer UTT and the traditional meridional EASM index as two predictors of summer precipitation over eastern China, a dynamical-statistical forecast model was established. The cross-validation results showed that the new forecast model significantly improved the predictive skill of summer precipitation over Northeast China and the upper stream of the Yangtze River. The first leading mode of Asian summer UTT and corresponding PC could well represent the zonal thermal contrast, which has a correlative relationship with summer precipitation over eastern China, and can be well-predicted by climate models. It can effectively serve as one predictor of summer precipitation over mid-latitude China, particularly northeastern China.

Abstract:Atmospheric energetics is an important part of atmospheric science. Understanding the spatiotemporal characteristics of atmospheric energy can provide new ideas and methods for atmospheric research, especially research on climate change. This work explains the comprehensive features of global atmospheric energy changes on the basis of the distribution, trends and dominant mode changes shown by total energy, internal energy, potential energy, latent heat energy, and kinetic energy as inferred from NCEP monthly reanalysis data for 1948 to 2016. The main conclusions are as follows: (1) The total energy decreases from the equator to the poles and from high-altitude areas, and energy in most parts of the world increases. The distribution and variation of internal energy and potential energy are closely related to the total energy. The maximum area and significant change zones of latent heat energy are located in the equator and low latitudes. The maximum area of kinetic energy is located in the long-wave trough of the middle latitudes and the outlet zone of westerly jets. In addition, kinetic energy located in double westerly jets in the southern hemisphere presents the most pronounced variations. (2) The total energy shows discontinuous periodic leap growth. The total energy of the Northern Hemisphere is more than that of the Southern Hemisphere. The speed-up of the Northern Hemisphere, however, is slower than that of the Southern Hemisphere. That is, the energy between the Northern Hemisphere and Southern Hemisphere tends to be homoplastic. The total energy above the ocean is more than that above land, and the gap between the total energy above the ocean and that above land has widened. Volcanic eruptions may have an important effect on the interannual reduction in atmospheric energy. (3) The spatial characteristics and distribution trends of the first leading mode of each component of atmospheric energy coincide, and they underwent a decadal catastrophe approximately in 1975. As a whole, the second leading modes of the total energy, internal energy, and potential energy of the atmosphere reflect that the changes in the north and south poles oppose those in other regions. The trend exhibited by the change in latent heat energy in some lower-latitude areas contradicts that exhibited by the change in the rest of the world. Kinetic energy mainly shows a meridional wave train distribution from the tropical Pacific to the north and south poles. The time series of the second leading mode possesses the characteristics of multidecadal variations that may be related to the internal variability of the climate system.

Abstract:In this study, we temporally and spatially obtain the cloud types from an active sensor product (2B-CLDCLASS-LIDAR) and the cloud properties and the associated radiation fluxes at the top of the atmosphere from a passive sensor product (CERES-SSF) (01/2007-12/2010). Further, we statistically and globally analyze the properties and instantaneous radiative forcings of various types of single- and two-layer clouds overlapping with high clouds located at the top of atmosphere. Although the aforementioned two products are independent, the passive sensor results denote reasonable contrasts between various types of single-layer clouds in terms of their optical depth, phases, and other parameters. The differences between the cloud properties of various types of single-layer clouds can significantly affect the radiation budget at the top of the atmosphere. By comparing the distributions of the CERES sample numbers with the short- and longwave radiative forcings of clouds, we denote that their high-value regions differ in terms of the shape, location, and area; further, we indicate the particular radiative characteristics of different single-layer cloud types. The dense area of the CERES-footprint sample number in case of the stratocumulus type cloud is shaped somewhat similar to an ellipsoid, whereas those of the remaining single-layer cloud types tend to resemble an exponential curve. Further consideration of the impacts of the overlapping high clouds reveals that that the overlying high clouds have more opaque and thicker cloud bodies when compared with those contained in single-layer high clouds, that the underlying cloud types tend to be more transparent and thinner when compared with their corresponding single-layer cloud types, and that the two-layer cloud types overlying high clouds, excluding high clouds that overlap cumulus, exhibit weaker cooling impacts on the Earth's atmosphere when compared with those exhibited by their underlying single-layer cloud types. The study results provide a detailed understanding of the cloud-radiation feedback and an observational basis for improving the cloud parameterization schemes in models.

Abstract:At present, ETKF (ensemble transform Kalman filter) method used in the operational GRAPES (Global/Regional Assimilation Prediction Enhanced System) regional ensemble prediction system of National Meteorological Center uses pseudo observation information, and the number and distribution of observations are fixed. To improve the ETKF method, real observation data are introduced into the ETKF process. The real observational radio-sounding data enable the perturbation field to represent uncertainty information about the observational state. Considering that the number and distribution of real observational data change daily, this can cause instability in the perturbation amplitude for the ETKF calculation. Therefore, the authors introduce a new self-adjustment amplitude rescaling factor. In this study, the authors analyzed ETKF schemes based on pseudo observations, real observations, and real observation plus the new rescaling factor and compared them in terms of their perturbation characteristics, ensemble verifications, and precipitation forecast skills. The results show that real sounding data can be effectively applied to the GRAPES regional ensemble forecasting system. Compared with pseudo observation data, real observation data is sparse, so large initial perturbation amplitudes can be obtained. The use of real observation data can help to improve the spread of the regional ensemble, but the improvements in the ensemble prediction accuracy and probability forecast skill are limited, as is the improvement in the precipitation prediction. The authors designed a new perturbation-amplitude rescaling factor to adaptively adjust the initial perturbation amplitude based on the spread and root-mean-square-error relationship in the grid space. Our investigation of the adaptive rescaling factor for adjusting the perturbation amplitude shows that this new rescaling factor can effectively obtain a stable initial perturbation amplitude and maintain the ETKF-generated perturbation structure. Since the real-observation-based ETKF scheme exhibits over-spread characteristics and limited improvement with respect to the pseudo-observation-based ETKF, the use of real observation data combined with the adaptive rescaling factor can effectively improve the skill of the regional ensemble in terms of the probabilistic forecast results while also effectively improving the precipitation forecasting skill.

Abstract:The root-water-uptake process plays an important role in maintaining the surface energy balance and water cycle. Currently, the influence of different root-water-uptake parameterization schemes on the simulation of the land surface processes in the Qinghai-Tibet Plateau is unclear. This study intends to explore the influence of these parameterization schemes and provide a reference for establishing root parameterization schemes for the future development of a land-surface-process model. Using the Beijing Climate Center Land Model (BCC_AVIM), we applied different root-water-uptake parameterization schemes and used the meteorological data observed at the Maqu station in the Qinghai-Tibet Plateau from June 1, 2010, to September 30, 2010, as the forcing data to simulate sensible heat flux, latent heat flux, soil temperature, and soil water content at the Maqu station. We subsequently compared the simulation results obtained using different parameterization schemes in case of the Qinghai-Tibet Plateau. We divided the root-water-uptake parameterization scheme into a root distribution model and a soil-water-availability function for roots. Further, we based our root distribution model on the Jackson and Schenk schemes and the soil-water-availability function for roots on the Li, LSM1.0 (Land Surface Model 1.0), and CLM4.5 schemes. A comparison of the results denotes that different parameterization schemes have little impact on the soil temperature and the soil water content but a considerable impact on the sensible and latent heat fluxes, especially with respect to canopy transpiration. We observed that the differences between the simulation results were related to precipitation. During the rainy period, the simulated root distribution model is considerably sensitive, with a large difference being observed between the sensible and latent heat fluxes simulated by the original model. During the less rainy period, the simulated soil-water-availability function for roots is more sensitive, with a large difference being observed between the sensible and latent heat fluxes simulated by the original model.