Abstract:The effect of sea surface temperature (SST) front on the vertical structures of the marine atmospheric boundary layer (MABL) and marine stratiform clouds is not well understood especially on a daily time scale due to the lack of in-situ observations over the ocean. A research vessel, Dongfanghong 2 of Ocean University of China, captured a rapid development of stratocumulus near the Kuroshio Extension (KE) front on 12 April 2014. The cloud top and cloud base increased and the cloud coverage expanded significantly on the warm flank of the front. The present study used a suite of on board observations to analyze the mechanism for the cloud development. The results are as follows. A positive heat flux center formed on the warm flank (downwind) of the KE front due to the slow atmospheric response to the forcing of the KE front. A weak pressure trough that was associated with the heat flux center intensified, leading to vertical motions in the MABL. The turbulent mixing in the MABL was favorable for the downward transport of westerly momentum from aloft to sea surface, which in turn further intensified the surface heat flux. This positive feedback process provided favorable conditions for the increase in the thickness of the mixed layer in MABL and the height of the cloud base/top downwind the KE front. On the other hand, a warm air advection from Honshu, Japan arrived at the heat flux center. The heat flux was combined with the warm advection, producing a pronounced warm center near the surface at 0900-1200 UTC 12 April. This warm center further led to the deepening of the mixing layer, the rising of the cloud base/top and the rapid development of the clouds. This study helps to understand the mechanism of the response of the MABL and the marine stratiform clouds to SST fronts under synoptic disturbances.

Abstract:The Weather Research and Forecasting (WRF) model with the hybrid ETKF-3DVAR (ensemble transform Kalman filter-three-dimensional variational data assimilation) data assimilation system is used to investigate the impact of different ensemble generation schemes on a squall line forecast in the Huanghe-Huaihe region in the summer by assimilating the surface observations. Ensembles are created in three different ways-by using the different initial ensemble samples (RCV), by using different model physical process schemes (PPMP), and by combining the first two ensembles (BLE). Based on the increments of model states, assimilating the surface observations mainly adjusts the spatial structures of wind and water vapor mixing ratio below the level of 850 hPa; the RCV scheme mainly updates the wind distribution, the PPMP scheme updates the water vapor mixing ratio, and the BLE scheme has the characteristics of both RCV and PPMP. Assimilating surface observations can also improve 6-h precipitation forecasts, and the PPMP scheme can give a relatively better performance compared to PPMP and BLE, especially for the prediction of rainfall location and intensity. RCV and BLE schemes present a better simulation for the bow echo, and the performance with BLE is similar to that with RCV. PPMP and RCV schemes can adjust the position and intensity of the cold pool, and also influence the times of appearance and disappearance of the squall line. Generally, PPMP scheme has a greater impact on the squall line than RCV and BLE.

Abstract:The interannual variation (IAV) in aerosols mass outflow from East Asia to Pacific driven by meteorological fields in the winter-spring time and the associated atmosphere circulations are explored by the global three-dimensional Goddard Earth Observing System (GEOS) chemical transport model (GEOS-Chem) over 1986-2006. More (less) East Asian aerosols outflow to the downstream North Pacific region is concurrent with the enhanced (depressed) stationary wave center over Northeast Asia/western Pacific region (NAWP) at 500 hPa. More East Asian aerosols mass tends to be transported downstream through a path 5-10 degrees north of the climatological path in the years with negative geopotential height anomalies over NAWP. The anomalies of the geopotential height at 500 hPa over NAWP regions are accompanied by anomalies of westerly winds and baroclinicity in the lower troposphere, which induce the IAV of the flux and the path of the East Asian aerosols outflow.

Abstract:Based on the merged hourly precipitation product at 0.1° resolution in China during 2008-2014, the diurnal variation of precipitation over North China in the summer has been revealed in this study. The precipitation data is derived from hourly precipitation observed at automatic weather stations (AWS) in China and retrievals from CMORPH[CPC (Climate Prediction Center) morphing technique] satellite data. The results show that the diurnal cycles of precipitation amount and precipitation frequency over northern China in the summer have two peaks that occur in the early morning and late afternoon, respectively. Diurnal variations of precipitation over various regions with different terrains differ from each other. The diurnal cycles of precipitation amount and precipitation frequency have a single peak (two diurnal peaks) over the mountainous areas in northern Hebei and the northern part of the North China Plain (other areas). Diurnal peaks of precipitation amount and precipitation frequency both occur around 1800 BT (Beijing time) and display an obvious consistency over the areas to the west of Taihang Mountains. Over the plains and coastal areas in eastern North China, the amplitude of the morning peak is larger than that of the late afternoon peak. Analysis of the contribution of precipitation with different duration time to the total precipitation indicates that short-duration (long-duration) precipitation has a greater impact on the precipitation peak in late afternoon (morning). The precipitation with the intensity of 0.1-10 mm h^-1 makes greater contribution to the total precipitation amount compared to that with the intensity over 10 mm h^-1. The number of diurnal peaks increases with the increase in precipitation intensity.

Abstract:The MLS (Microwave Limb Sounder) satellite observations of carbon monoxide (CO) mixing ratios from 2005 to 2014 have been analyzed to study the inter-annual variation of carbon monoxide (CO) in the tropical stratosphere. The results show that the quasi-biennial oscillation (QBO) in the tropical stratospheric CO mixing ratios has a phase change at 30 hPa. The results from a chemistry-climate model show that the CO QBO is primarily resulted from combined effects of dynamical and chemical processes associated with CO, while the dynamical process is mainly associated with the transport of CO by the QBO-induced meridional circulation. The combined effects of dynamical and chemical processes lead to a reverse of the CO vertical gradient in the tropical stratosphere at 30 hPa. Consequently, the CO QBO exhibits a phase change around this level. The model results also show that the chemical processes associated with CO not only have a weakening effect on the amplitude of the CO QBO, but also lead to a QBO phase difference by 3 months between zonal wind QBO and CO QBO at 10-30 hPa.

Abstract:The characteristics of typhoon Matmo (1410) are analyzed before and after its landfall on Taiwan and Fujian based on the analysis data from NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) Global Forecast System (GFS), the reanalysis data from ECMWF ERA-Interim and rainfall observations derived from station observations and satellite retrievals of precipitation from 18 to 26 July 2014. The results indicate that the trajectory of Matmo was mostly influenced by the steering flow of subtropical high. The deep vortex structure of Matmo still maintained around the time of its landing at Fujian, cooperating well with the vertical circulation formed by the ascending motion over southeastern Fujian and the descending motion near Taiwan Strait and western Fujian. Besides, the water vapor originated from the Bay of Bengal and South China Sea were continuously transported into Matmo by the southwesterly jet (wind speed greater than 16 m s^-1 at 850 hPa), which provided the necessary energy required for maintaining the typhoon circulation around landing time. The southwesterly jet transported energy and momentum from the South China Sea to the coastal region of southeastern China. Meanwhile, sufficient water vapor transport and convergence of moisture at the low levels were conductive to the occurrence and maintenance of heavy rain caused by Matmo. The result also shows that the weak vertical wind shear, which was favorable for the maintenance of warm core structure and divergence at upper levels, was another reason for the slow weakening of Matmo after its landfall.

Abstract:Wetland is a natural complex of land and water, which has important functions in ecology, hydrology and biogeochemistry. The alpine wetland in the source region of the Yellow River (SRYR) is a key water conservation area of the Yellow River, and it is important to understand the characteristics of water and heat exchanges and their crucial influencing factors in this region. In this paper, the characteristics of water and heat exchanges are analyzed using the field observation data provided by Maduo Climate and Environment Monitoring Labotory of Nothwest Institute of Eco-Environment and Resources, CAS (Chinese Academy of Sciences) for June-August 2014 over the alpine wetland in the SRYR. The Community Land Model (CLM) is applied to simulate heat flux. A roughness length scheme, which aims at the alpine wetland, is designed based on the simulation results. The main results are as follows:(1) The upward and downward short-wave radiation and net radiation fluxes in the warm season have similar diurnal variation patterns, and the diurnal fluctuations of the upward and downward long-wave radiation fluxes are small. The increase in surface temperature lags the increases in downward shortwave radiation flux, and the latent heat flux is always positive and greater than the sensible heat flux. (2) The soil layer in which temperature has significant changes is the top shallow layer in 0-20 cm depth, where there exists an obvious diurnal cycle of temperature variation. The soil temperature starts increasing when heat is transferred down to 5 cm depth at 0900 BJT (Beijing time), and the heat reaches 10 cm depth at 1100 BJT and 20 cm depth at 1300 BJT. Soil temperature starts decreasing because of the upward transfer of heat after 1800 BJT. The soil layer below 40 cm is hardly influenced by the radiation flux, and the general characteristics of heat transfer is from the shallow top layer to deep layers. (3) The diurnal variation of soil moisture is small, and the soil at 5 cm and 10 cm depths have the minimum and maximum moisture content, respectively. (4) The roughness length for momentum (Z_0m) over the alpine wetland in Maduo during the warm season is stable, which can be set to be a constant Z_0m=0.0143 m. (5) The newly added damping kB^-1 schemes are designed for local alpine wetland, which have a better performance in simulating the surface heat flux than the original scheme of CLM. All these results have significant contributions to the research of land surface process over the wetland.

Abstract:Sever floods occurred in southern China and droughts were prevalent in northern China in the summer of 2014. Most predicted models in China missed the southern rain band in their flood season predictions conducted in March 2014, which led to relatively low prediction accuracy. Based on the higher prediction skill for summer sea level pressure of climate models and the significant relationship between the preceding winter Tibetan Plateau Snow and summer precipitation in the south, a new Hybrid Statistical Downscaling Prediction (abbreviated as HSDP) method for summer precipitation anomaly prediction in China was proposed in this paper. The method can integrate the information of the highly predictable circulation from climate models and the influential signal of Tibet Plateau Snow in the preceding winter to improve the dynamical-statistical combination prediction for summer precipitation in the south. Using this method, a statistical downscaling model was established based on the climate prediction model of National Climate Center of China. The cross validation of seasonal prediction for the summer precipitation in the south was performed and the results showed that the HSDP improved the multi-year average of anomaly correlation coefficient from -0.006 to 0.24, and it had a higher predicting skill than the original climate model in most years. Using HSDP, the precipitation prediction for the summer of 2014 could well capture the basic situations, i.e. floods in southern China and droughts in northern China, and the positive precipitation anomaly in the south. The anomaly correlation coefficient could reach 0.43. This result indicated that the HSDP has a great operational application prospect with regard to summer precipitation prediction in China.

Abstract:In this paper, the PODEn4DVar-based radar data assimilation scheme (PRAS) was improved according to the theory of NLS (Non-Linear Least Squares)-4DVar (four-dimensional variational analysis) scheme. This work aims to deal with the application problem of PRAS under highly nonlinear conditions. As a result, a new radar data assimilation scheme, i.e. NLS-4DVar-based radar data assimilation scheme (NRAS), was developed. To evaluate whether NRAS can further improve the performance compared to PRAS, the Observing System Simulation Experiments (OSSEs) and real radar data assimilation experiments for two heavy rain events (July 8, 2010, central China;March 30, 2014, southern China) were conducted in this study. The results demonstrate that, for both the OSSEs and the real radar data assimilation experiments, NRAS can further improve the assimilation result in comparison to PRAS. By increasing iteration times, NRAS can adjust the wind field and water vapor field. This leads to further improvements on the forecast of intensity and location of the rainfall. However, with increases in the iteration times, the adjustment for the initial condition by NRAS becomes smaller, which leads to a smaller improvement on the rainfall forecast. The results indicate that NRAS can effectively deal with the application of PRAS under highly non-linear condition. With fewer iteration times, NRAS can obtain approximate convergence result. NRAS is expected to better assimilate radar data in numerical weather predictions, and thus further improve the prediction of meso-micro scale weather systems.

Abstract:Using the ERA-interim reanalysis data of ECMWF as the reference, numerical errors of the GRAPES global model are evaluated first. A numerical correction of the systematic model errors is then performed based on the historical observation data by using the Anomaly Numerical-correction with Observations (ANO) method. The effect of the ANO on the global forecast is tested by several case studies, and significant improvements on the global forecasting quality are confirmed. The ANO application in numerical results from 15 to 24 July during 1984-2014 shows significant positive effects in the circulation forecasts compared to those without the ANO application. For example, the potential height and temperature forecasts have been improved in various regions. Analysis of the geopotential height forecasts at 200 hPa shows that the anomalous correlation coefficient (ACC) increases by 0.05 and the root mean square error (RMSE) decreases by 12 gpm on average for all the 31 cases. Similar results can be found at other levels. The above results verify the validity of the ANO method in the improvement of 10-day numerical weather forecasting skill of the GRAPE global model. Compared with the MOS method, the ANO method is more efficient and maneuverable for application in the operational forecast.

Abstract:This study investigates the relationship between the spring-winter snow cover anomalies over the northern and southern Tibetan Plateau (TP) and summer precipitation in East China (EC) based on numerical experiments of CESM (Community Earth System Model). The results show that snow cover anomalies over the southern and northern TP can lead to two patterns of summer precipitation in EC. Larger snow cover in the southern TP leads to excessive rainfall in the Yangtze River basin and northeastern China and less precipitation in southern China; larger snow cover in the northern TP results in enhanced rainfall in northern and northeastern China and less rainfall in the south of the lower reaches of Yangtze River. The possible mechanism of the impacts of snow cover anomalies in the northern and southern TP on summer precipitation in EC is that, changes in the diabatic heating over TP in the summer caused by snow cover anomalies results in weakened temperature gradient and westerly jet on the northern side of TP, and subsequent anomaly of westerly in the exit region. These anomalies further influence the West Pacific Subtropical High (WPSH) in the mid-troposphere and lead to anomalous anticyclonic circulation over EC. Heavier snow cover in the southern TP leads to enhanced westerly jet in the exit region, while heavier snow cover in northern TP results in weakened westerly jet. The degree of the WPSH weakening caused by larger snow cover in the northern TP is greater than that caused by the larger snow cover in southern TP. Thereby the anomalous anticyclonic circulation is located more northward. As a result, the summer rainfall belt shifts northward.

Abstract:Appropriate description of turbulence in PBL (planetary boundary layer) is essential for numerical weather prediction and simulation. Proper treatment of turbulence may have an important influence on the simulation of meteorological fields, such as temperature, moisture, wind speed and precipitation. Heavy rainfall event is frequent in the lower reaches of the Yangtze River and it is necessary to obtain a suitable PBL scheme for this region due to its dense habitability. In this study, based on the QNSE (Quasi-Normal Scale Elimination) planetary boundary layer scheme in the WRF (Weather Research and Forecasting) Model, the coefficients of the TLS (turbulent mixing length scale) in the Mellor-Yamada formulation were modified to be varying based on the application of the Prandtl formula. The MCTLS (modified coefficients of TLS) are dependent on the PBL height and surface stability, and a nonlocal term was imposed on MCTLS, thereby emphasizing the comprehensive impacts of the atmospheric dynamic and thermal structures on turbulent mixing. WRF model simulations using the original PBL scheme and the PBL scheme with the MCTLS were conducted over the lower reaches of the Yangtze River and results were compared to measurements. Improvements in the near-surface temperature, the planetary boundary layer structure and the rainfall simulations have been found. More specifically, the simulations with the MCTLS were shown to alleviate biases in the potential temperature and water vapor mixing ratio by altering turbulent mixing.

Abstract:A new assimilation module of AMSR2 (Advanced Microwave Scanning Radiometer 2) radiance data was constructed in WRFDA-3DVAR (Weather Research and Forecasting model's 3-dimensional variational data assimilation). With this module, the impact of AMSR2 assimilation on the analysis and forecast of Typhoon Son-Tinh was evaluated. AMSR2 data is able to detect Son-Tinh's cloud band pattern. Compared to the experiment without AMSR2 assimilation, AMSR2 assimilation provided a better typhoon structure, and significantly improved the central sea level pressure, maximum wind speed and typhoon track forecasts.

Abstract:The Poyang Lake basin is one of the inland areas in China that are under the most frequent influences of post-landfall tropical cyclones (TC). The interaction between TC and the underlying surface over this region is investigated using the TC best track data, surface and sounding observations and global meteorological re-analysis data. Statistical analysis of the data from 1949-2012 shows that the moving speed of TC reduced once they entered the Poyang Lake basin and their decaying rate became lower while the associated precipitation increased. Analysis of large-scale circulation indicates that TC entering Poyang Lake basin were located at the "saddle region" formed between the Pacific subtropical high and East Asian continental high. The weakened steering flow and reduced vertical shear of horizontal winds in the "saddle region" are favorable for the slowing of TC movement and decaying rate and precipitation increase. Several typical TC cases are selected to further examine the condition changes in the underlying surface during periods of TC influence. It is found that both the top layer soil temperature and lower atmosphere temperature decreased with reduced diurnal variations, while the soil water content and air humidity increased obviously due to the rainfall induced by TC. Relatively large heat storage over the Poyang Lake basin is favorable for the maintaining of TC. The convective available potential energy (CAPE) decreased significantly over this region under the impact of TC, indicating that TC triggered the release of CAPE and thus increased rainfall.

Abstract:Based on analysis of observations, we defined the starting and ending time and the total rainfall of the spring persistent rainfall in Jiangnan of China (SPRJ) from 1982 to 2014, and investigated the climatic characters of the SPRJ as well as its interannual variation. The relationship between the SST anomaly of Nino3.4 region in the preceding winter and the SPRJ and its physical mechanism were further studied. Results show that there is a significant positive correlation between the preceding winter Nino3.4 index and total SPRJ. The warm water can trigger an anomalous Walker circulation that leads to significant abnormal descending motions and corresponding low-level anticyclonic circulation near 120°E at the equator. The strengthened southwesterly winds of the anticyclone in the low level of the South China Sea facilitate more water vapor transport from the South China Sea to Jiangnan of China. On the other hand, the Indian Ocean SST anomaly associated with El Niño events can induce abnormal low level easterly winds in tropical Indian Ocean and anticyclone in the north of the Bay of Bengal, which also promotes the water vapor transportation. Meanwhile, the westerly anomalies in the upper troposphere above South Asia enhances divergence and pumping above Jiangnan of China, and thus are favorable for ascending motions and more SPRJ. In contrast, there is less SPRJ following a La Niña event. Moreover, the influence of El Niño on the SPRJ changes with its original intensity. With a strong El Niño in the preceding winter, the SSTA in the Pacific can persist to the following spring and there will be more SPRJ rainfall; with a weak El Niño in the preceding winter, however, the SSTA in the Pacific cannot persist to the following spring and the SPRJ total rainfall will decrease. Besides, when considering the combined effects of Nino3.4 index and Antarctic Oscillation Index and Indian Ocean Basin Mode of the preceding winter, the seasonal prediction is improved. Thus the multiple linear regression of the three predictors is useful for the prediction of the SPRJ.

Abstract:AERONET (Aerosol Robotic Network) level 2.0 and CM_21 data in SACOL (Semi-Arid Climate and Environment Observatory of Lanzhou University) for 2006-2012 were used to analyze optical properties and radiative effects of aerosols over Northwest China. Meanwhile, the reason for the positive TOA (Top of Atmosphere) radiative forcing was examined by using the SBDART (Santa Barbara DISORT Atmospheric Radiative Transfer). The annual averages of BOA (Bottom of Atmosphere), TOA, and atmosphere radiative forcing are -59.43 W m^-2, -17.03 W m^-2, and 42.40 W m^-2, respectively. The annual averages of AOD (aerosol optical depth, 550 nm) and α (angstrom exponent, 440-675 nm) are 0.37 and 0.91, respectively. α and AOD have opposite phases. The α value is within 0.0-0.2, which is very small, when the AOD is within 0.3-2.2. The annual averages of SSA (single scattering albedo, 675 nm), g (asymmetry factor, 675 nm) and the real part of the complex refractive index (675 nm) are 0.93, 0.68, 1.48, respectively. The annual trend of the real part is consistent with that of AOD while that of the imaginary part is opposite to that of AOD. Thereby the coarse-particle mode aerosols that have strong scattering effects dominate Northwest China. The maximum atmospheric heating rate occurs in 0-2 km and decreases with height based on the simulations with and without aerosol effects. The heating rates in the winter half year and summer half year are 2.6 K d^-1 and 0.6 K d^-1 on the surface, respectively. In the four seasons, the heating rate is the largest in the winter, followed by that in the autumn and spring, and smallest in the summer with values of 2.5 K d^-1, 1.4 K d^-1, 1.2 K d^-1, 0.2 K d^-1 on the surface in the winter, autumn, spring, and summer, respectively. The absorption by aerosols is stronger in the autumn than in the spring. Contributions of the surface albedo and SSA account for 22.5% and 77.5% of the positive radiative forcing at TOA, respectively.