Abstract:Accurate quantification of land-atmosphere exchange is crucial for the simulation of regional scale carbon flux and CO₂ concentrations. In this study, a coupling model, WRF-GHG (Weather Research and Forecasting Model with Greenhouse Gases Module) is employed to simulate regional net ecosystem exchange (NEE) and atmospheric CO₂ concentrations over the Yangtze River Delta (YRD) from July 28 to August 2, 2010. In the modeling system, several "tagged" species are defined to trace the contribution of various sources to atmospheric CO₂ concentrations. The simulated NEE fluxes and CO₂ concentrations are evaluated against in-situ measurements. The results show that VPRM can reproduce spatial-temporal variation patterns of NEE over different types of vegetation. Overall, the diurnal patterns in CO₂ concentration compared quite well with field measurements, indicating that the model captures the major features in the diurnal variations of CO₂. However, the model underestimates CO₂ concentrations by 5~15 ppm (10^-6). This is most likely due to an underestimation of anthropogenic emissions, the uncertainties of parameters defined in VPRM, and meteorological inputs. Local meteorological conditions, such as land-lake breeze, exert an important impact on CO₂ concentrations. While lake-atmospheric interactions over Lake Taihu and vegetation-atmospheric interactions over mountain areas in northern Zhejiang province play a dominate role in carbon sink, urban-related anthropogenic emissions act as a major source of carbon. The WRF-GHG modeling system demonstrated its capability of simulating local and regional variations in CO₂ fluxes and concentrations.

Abstract:Using 18 years of Band-pass filtered sea surface height (SSH) data from satellite altimeters, the relationship between the mesoscale eddy kinetic energy (EKE) in the Kuroshio Extension (KE) region and North Pacific atmospheric storm track is investigated by introducing the EKE area index and using regression analysis. A stronger North Pacific storm track was found to be associated with intensified Kuroshio Extension mesoscale eddy activity. The North Pacific storm track shifts northward (southward) as the maximum EKE core in the KE region shifted to the north (south). However, the variation between the EKE and North Pacific storm track is reversed in the zonal position. Moreover, a lead-lag correlation between the EKE and North Pacific storm track variations exists. The strength of the EKE exhibits a 3-4 year lag positive (negative) correlation with the EOF1 and EOF3 (EOF2) of the North Pacific storm track, while the opposite is true for the position of the EKE. This can arise from the westward propagation of sea surface height anomalies, driven by Atmospheric Teleconnection Patterns associated with the North Pacific storm track.

Abstract:To study the effect of the troposphere-to-stratosphere transport (TST) caused by severe convection and the associated background synoptic system, two severe convection events, one that occurred at low latitude and one at midlatitude, were simulated. In the low-latitude case, in Guangxi Province, the proportion of TST caused directly by severe convection was about 18%, while the TST caused by the synoptic background system of the severe convection was about 82%. In the midlatitude case, in Hebei Province, the proportion of TST caused directly by severe convection was about 0.17%, while the TST caused by the synoptic background system of the severe convection was about 99.83%. In terms of the motion of particles transported from the troposphere, the particles transported directly by severe convection moved towards the southwest in both events, while the particles transported by the synoptic background systems moved towards the east. In general, the amount of TST caused by the synoptic background system of the severe convection was much larger than that caused directly by the severe convection. The TST caused by the synoptic background systems occurred two days after the severe convection initiation and reached a maximum six or seven days later.

Abstract:Monthly precipitation records at 353 stations in the middle and lower reaches of the Yangtze River during 1961-2012, provided by the National Climate Center of the China Meteorological Administration, were used to calculate the Standardized Precipitation Index in summer. The Mann-Kendall (MK) statistical test was applied to detect the trend in the number of stations at which summer SPI reached a value of -1.0 or less. According to the MK test results, the whole time series could be divided into three stages, separated by abrupt change points. Furthermore, based on National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data and National Oceanic and Atmospheric Administration extended reconstructed sea surface temperature (SST) data, the evolution of the large-scale atmospheric circulation was analyzed from pre-winter to summer, and a conceptual model established for each stage. The results showed that: (1) the first stage (1961-1973) was in an obvious state of drought, the second stage (1974-1986) was a dry-to-wet transition phase, and the third stage (1987-2012) was basically wet; (2) the atmospheric circulation changed notably in the second stage, leading to an opposite phase of the circulation anomaly between the first and third stages; and (3) global SST remained abnormally cold from pre-winter to summer, and Indian Ocean SST was especially colder in summer, which led to a dramatically weaker South Asian high and western Pacific subtropical high. The high pressure ridge over the northern Tibetan Plateau was weaker, resulting in weaker high pressure over Mongolia in pre-winter. Low pressure over India was stronger, the southern branch of the trough deepened, and southerly moisture transported by the summer monsoon prevailed over eastern China. Meanwhile, there was straight westerly wind over the mid-high latitudes of Asia, which was unfavorable for the cold air to extend into southern China. Under the above evolution and configuration of the atmospheric circulation, the middle and lower reaches of the Yangtze River were prone to widespread drought in summer during the first stage. However, the circulation from pre-winter to summer during the third stage was opposite to that during the first stage. Global SST was warmer from pre-winter to summer, especially in the Indian Ocean, resulting in a stronger western Pacific subtropical high. The stronger high pressure ridge over the northern Tibetan Plateau reinforced the high pressure in pre-winter over Mongolia. Moreover, the low pressure over India and the southern branch of the trough weakened, and water vapor stagnated over the Yangtze River basin because of the weaker summer monsoon; plus, the high ridge over Lake Baikal was favorable for the cold air to extend into southern China. Therefore, the circulation from pre-winter to summer during the third stage led to more precipitation in summer over the middle and lower reaches of the Yangtze River.

Abstract:The capabilities of 46 CMIP5 (the Coupled Model Intercomparison Project Phase five) models for simulating the annual, summer, and winter precipitation climatology over China are first examined using the outputs of these models from historical data for the period 1986-2004. Eighteen models are then chosen to project the changes of monsoon area, monsoon precipitation, and monsoon precipitation intensity over China under the Representative Concentration Pathways 4.5 (RCP4.5) scenario. The results show that the monsoon area, monsoon precipitation, and monsoon precipitation intensity will increase in the 21st century, particularly during 2081-2099. The increase in the monsoon precipitation is mainly derived from the increase in the monsoon area. Both thermal and dynamic conditions will be favorable for increased monsoon precipitation intensity and greater water vapor transport into eastern China, resulting in the expansion of the monsoon area in China.

Abstract:Utilizing daily precipitation data from 24 meteorological stations and results from the Weather Research and Forecasting (WRF) model/Urban Canopy Model (UCM), the impact of urban surface characteristics on summer rainfall in the Beijing-Tianjin-Hebei area was investigated. Results indicated that precipitation at most sites in this region has reduced during the last 30 years, and those sites whose precipitation has reduced the most are mainly centered in the Beijing-Tianjin-Tangshan metropolis. Urbanization is one of the possible factors affecting the precipitation in the Beijing-Tianjin-Hebei area. Comparison of the model results from the control run and sensitivity run indicated that rainfall and rainfall frequency clearly decreased in the Beijing-Tianjin-Tangshan metropolis due to the urban surface. Meanwhile, an increase in rainfall and rainfall intensity was apparent downwind of the urban agglomeration; precipitation above 50 mm changed significantly due to the urban surface, and the contribution to the total could be more than 60%. The percentage of rainfall above 50 mm declined by 6%-20% in the Beijing-Tianjin-Tangshan metropolis, while it increased by 8% downwind. The diurnal structure of rainfall changed due to urbanization: precipitation in Beijing and Tangshan mainly reduced due to urbanization, and the increase downwind occurred mainly in daytime. The findings of this study suggest that the inhibition or enhancement of deep convection, as influenced by changes in latent heat flux and convective available potential energy due to the urban surface, may explain the changes in precipitation.

Abstract:The Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO) are generally referred to as the first two EOF modes of Sea Surface Temperature (SST) in the North Pacific (20°-60°N, 120°E-120°W). In the present study, the authors compared the first two EOF modes of winter SST anomalies in North Pacific for the period before and after 1990. Based on the impacts of key regional SST changes, the North Pacific Oscillation (NPO), Central Pacific Warming (CPW), as well as Artic Oscillation (AO), the authors discuss the possible causes of the first mode of the NPGO after 1990. Our results suggest that before 1990, the first two EOF modes of SST exhibit the PDO and NPGO pattern, respectively. However, after 1990 the center of the maximal load of the first EOF tilts to the dateline, and gives rises to positive SST anomalies north of 40°N, resulting in a negative phase of NPGO; meanwhile, the previous NPGO changes from a dipole pattern to the triple mode. Analyses suggest that the negative correlation between the changes of SST in the northern (44°-49°N, 151°-177°W) and central (28°-36°N, 152°-178°W) North Pacific could have resulted in the internal shift of the dominant SST modes after 1990. Evidence shows that the NPO played an important external forcing in enhancing the NPGO after 1990. Based on wind-driven oceanic current theory, the increased amplitude of NPO in Sea Level Pressure (SLP) may intensify the polar gradient of the SLP along 45°N, enhancing the zonal sea surface winds, and causing the NPGO mode to prevail after 1990. The increase in CPW and AO may have enhanced the NPO by intensifying the southern and northern branch of the NPO in Hawaii and Alaska after 1990, but there is no evidence to show that both these factors were closely related to the NPO before 1990.

Abstract:A two-dimensional cumulus model coupled with an aerosol module is used to simulate a case of tropical convection in Nanjing. Numerical simulations at a resolution of 250 m are performed to investigate the effect of aerosol concentration on the electrification and lightning flash rate in the thunderstorm clouds. In this aerosol module, the distribution of aerosol particles is fitted by superimposing three log-normal distribution functions, and the activation of aerosol particles to form cloud droplets is considered. The results show that: (1) The charge structure in the thundercloud remains a triple charge structure as the aerosol concentration increases. (2) When the aerosol concentration is changed from 50 to 1000 cm^-3, a stronger formation of cloud droplets, graupel and ice crystals results in an increasing charge separation and lightning flash rate. (3) In the range of 1000-3000 cm^-3, the decrease in ice crystals caused by vapor competition leads to a reduction in upper positive charge, while the enhancement of graupel and cloud droplets results in the contribution of inductive charge to the middle negative charge region and lower positive charge region increasing with greater aerosol concentration. The flash rate shows a slight change. (4) At very high aerosol concentrations (above 3000 cm^-3), the magnitude of the charge and lightning flash rate, which remains steady in the thundercloud, is insensitive to aerosol concentration.

Abstract:Using equatorial wave modes and short-range forecast error samples of the global GRAPES (global/regional assimilation and prediction system) model, this study investigated the balance characteristics of mass and wind fields in the tropics and analyzed the problem of imposition of the Linear Balance Equation (LBE) in the tropics based on such characteristics. The results show the following: (1) Equatorial wave modes can explain a substantial fraction of the tropical forecast error variance, which ranges from 60%-80% in the mid-troposphere to more than 80% in the tropopause and lower stratosphere. (2) Only 30%-55% of the explained variance can be represented by the Equatorial Rossby wave (ER) modes, suggesting that contributions of other equatorial waves should not be ignored. (3) Based on the ER modes, introducing other equatorial waves, especially inertial-gravity and Kelvin wave modes, greatly reduces the coupling between the mass and wind. In this situation, the correlation between geopotential height and wind is close to zero in the mid-troposphere, and the correlation between geopotential height and zonal wind in the lower stratosphere is dominated by Kelvin wave modes. (4) The coupling between mass and wind was overestimated when using LBE in the tropics, because LBE mainly expresses the balance characteristics of the ER modes. Therefore, when LBE is used as the balance constraint, extra steps should be taken to reduce the fallacious correlation and allow for a decoupling of mass and wind in the tropics.

Abstract:A stable negative relationship between boreal summer precipitation over northeastern China and the sea surface temperatures (SSTs) in previous seasons, especially in spring, in the subtropical southeastern Pacific (SSEP) is revealed in this paper. Scale separation results show that the SSTs and precipitation possess anti-phase variations at both the interannual (less than 7 years) and interdecadal (more than 7 years) timescales. This close relationship is mainly determined by the interdecadal components, though the interannual components play an important role. Correlation and composite analyses show that the lower SSTs in the SSEP cause an anticyclonic wind anomaly over the study region, and a cyclonic wind anomaly in its northwestern region. At the same time an anticyclonic and a cyclonic anomaly exist over the Solomon Islands and southern Philippines, respectively. Under this circulation situation, the western Pacific subtropical high (WPSH) is situated more westward and stronger compared to its climatology, and the northeastern cold vortex (NECV) is stronger than its norm too. Thus, the southerly anomaly in the west of the WPSH strengthens the warm and wet moisture transportation from the South China Sea and western Pacific. The cold air in the west of the NECV becomes more powerful. Together, these lead to stronger convergence and more precipitation over northeastern China. On the contrary, higher SSTs in the SSEP generate opposite anticyclonic and cyclonic wind anomalies over the study region. This opposite wave train leads to a weaker WPSH and NECV, weaker moisture convergence in the lower troposphere, and less rainfall in the region. The results indicate that the SSTs in the SSEP exert an alternate cyclonic-anticyclonic wave train from the Southern Hemisphere to the eastern Northern Hemisphere. This wave train can impact the general circulation in northeastern Asia and ultimately affect precipitation over northeastern China.

Abstract:On the basis of daily mean temperature data from 566 National Meteorological Information Center stations and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data during 1961-2011, an investigation was conducted into the relationship between the variation of the winter North Atlantic Storm Track (NAST) and the frequency of cold waves in China on the annual timescale. The results from singular value decomposition analysis show that, on the annual timescale, the position of the NAST is closely linked to the cold wave frequency in winter: When the NAST moves northeastward (southwestward), the cold wave frequency at most stations decreases (increases) significantly, but the correlation between the intensity of the NAST and the cold wave frequency is low. Further analysis indicates that change in the NAST position may lead to anomalous propagation of the Synoptic-Scale Transient Eddy Activity (STEA). This could not only affect the cold air activity of China directly, but, after first having an impact on the general circulation via feedback, it could also have an indirect influence. The possible mechanism of influence is as follows: As the NAST shifts more northeastward (southwestward), it is far from (close to) the Asian westerly jet, which is unfavorable (favorable) for the direct propagation of strong STEA over the North Atlantic region via this westerly waveguide all the way to China. Meanwhile, the North American westerly jet shifts more northward (southward) and the index of the North Atlantic Oscillation is in its positive (negative) phase. Via propagation, the downstream STEA could cause the divergence (convergence) of horizontal Eliassen-Palm fluxes over high latitudes and the convergence (divergence) over mid-latitudes, which in turn will cause acceleration (deceleration) of the westerly over high latitudes and deceleration (acceleration) of the westerly over mid-latitudes. As a result, the polar vortex will (will not) be confined to the polar region so that the cold air will uneasily (easily) move southward, and then the temperature of the low-level cold mass over the Siberian high will increase (decrease). All of the above anomalies occurring in the circulation and propagation of STEA may ultimately contribute to the reduction (enhancement) of the cold wave frequency at most stations in China.

Abstract:The Shuffled Complex Evolution Algorithm is used to estimate the parameters in the Noah model, and soil hydraulic parameters are also estimated by soil texture components to evaluate the sensitivity of model hydraulic parameters in simulating soil moisture and soil temperature. The results show that soil hydraulic parameters are more sensitive than parameters related to vegetation, and more sensitive to soil moisture than soil temperature. When surface soil moisture is used to calculate the objective function, the estimated hydraulic parameters can not only improve the soil moisture, but also soil temperature. However, when deep soil moisture is used to calculate the objective function, the soil temperature simulation is even worse than with default parameters. Soil texture components are also used to calculate soil hydraulic parameters; and when they are used as estimated parameters, the results can improve both soil moisture and soil temperature simulation. Parameter estimation experiments in which soil temperature is used to calculate the objective function cannot improve the simulation of soil moisture. When soil texture components are used as estimated parameters and soil moisture and soil temperature are used to calculate the objective function, the estimated hydraulic parameters can improve both soil moisture and soil temperature simulation. Decreasing the number of parameters has little impact on the results. Accordingly, when soil texture components are used as estimating parameters in this land surface model, the estimated hydraulic parameters show good consistency-more so than when estimating hydraulic parameters directly.

Abstract:Based on daily maximum and minimum temperature data from 740 homogenized surface meteorological stations, the present study investigates the regional characteristics of the temperature trend and the dependence of maximum and minimum temperature and diurnal temperature range changes on the altitude during the period 1963-2012. It is found that the magnitude of minimum temperature increase is larger than that of the maximum temperature increase. The significant warming areas are located at high altitude, all of which increase remarkably in size during the study period. The maximum and minimum temperature and diurnal temperature range trends increase with altitude, except in spring. The correlation coefficients between the maximum temperature trend and altitude are the highest. At the same altitude, the amplitudes of maximum and minimum temperature show inconsistency: They exhibit increasing trends in the 1990s, with significant change at low altitude; they change minimally in the 1980s; and at high altitudes (above 2000 m), the magnitudes of their changes are weak before the 1990s but stronger in the last 10 years of the study period. The seasonal variability of the diurnal temperature range is large over 2000 m, decreasing in summer but increasing in winter. Before the 1990s, there is no significant variation between maximum and minimum temperature and altitude. However, their trends almost all decrease and then increase with altitude in the last 20 years. Additionally, the response to climate in highland areas is more sensitive than that in lowland areas.

Abstract:The heavy rainfall event along the Meiyu front on June 24, 2009 was analyzed using data from auto weather stations, high resolution radars, and simulations. The results show that several surface disturbance vortexes formed after the weak rainfall in the southern warm section of the Meiyu front. With the stabilizing and upward development of the disturbance vortex, a short duration rainstorm accompanied by gale occurred. After the strengthening and lifting of the westerly wind, a westerly jet formed with the jet axis between the planetary boundary layer and the low troposphere. Convergence and shear occurred after the southerly flow encountered the northerly wind. The southerly flow could have turned into convection through lifting by northerly wind. The vorticity of the planetary boundary layer increased gradually followed by synchronous enhancement of frontogenesis and convergence under the influence of surface shear and convergence. Then, the disturbance vortex could form and develop in the planetary boundary layer below the troposphere. The intensification of wind speed, shear, and convergence near the surface were the dynamic factors responsible for the formation of the mesoscale disturbance vortex.

Abstract:Studying the characteristics of freezing weather in China ultimately helps people to understand freezing weather more comprehensively. It also contributes to warning and forecasting capabilities for freezing weather, and thus disaster reduction. Using data from hourly or half hourly routine and special observations from civil aviation airports in China for the period 2011-2013, the characteristics of temporal and spatial distribution, duration and weather conditions of freezing weather events were analyzed. It was found that the freezing weather events during the study period occurred in most regions of China. The high-frequency region for freezing rain or freezing drizzle events corresponded well to that of high intensity. In the low-frequency region for freezing fog, events tended to last longer. Freezing weather usually occurred in the winter season (December to February), and mostly in January. Events occurred frequently in the early morning [from 0000 to 0700 local time (LT)] and infrequently in the afternoon (from 1300 to 1400 LST). The duration of each freezing precipitation event was usually less than 2 h. Most freezing weather events occurred when temperatures were -3℃ to -1℃ and dew point was -4℃ to -1℃. The highest frequency of freezing weather events occurred when the dew point drop was 0℃ to 1℃. The dominant winds were northeasterly or northerly when freezing precipitation occurred, and the wind speed was usually 2-3.9 m s^-1. Most freezing rain and freezing drizzle was mixed with fog, whereas freezing fog was not mixed with other precipitation.

Abstract:Based on ERA-40 monthly mean reanalysis data over the period 1958-2002, this paper studies the relationship between the spring Arctic Oscillation (AO) and the summer surface air temperature (SAT) over the Yangtze River valley by adopting the year-to-year increment approach. The winter ENSO signal is filtered out in advance. Results show that the AO in May is significantly and positively correlated with the SAT over the Yangtze River valley in August. The possible physical mechanism for this relationship is investigated by means of regression analysis. The air-ocean interaction over the middle and low latitudes might play an important role. Following a stronger positive AO in May, an anomalous subsidence flow appears over the region 10°-15°N and the Equator. At the same time, Hadley circulation over the western Pacific becomes weaker, leading to an anomalous anticyclonic divergent flow in the lower troposphere. Accordingly, significant easterly anomalies appear over the equatorial western Pacific. Since the easterly anomalies are located at the local maximum center of the climatological mean sea surface temperature (SST), the warm SSTs pile up to the west via advection, inducing the above-normal SST over the equatorial western Pacific. The positive SST anomalies persist from May to August. This can trigger an anomalous cyclonic circulation to the northwest through a Gill-like atmospheric response (Gill, 1980), which is conducive to the maintenance of the western Pacific subtropical high over the Yangtze River valley, resulting in the positive SAT anomalies over the region, and vice versa.