Abstract:As the latest generation of geostationary meteorological satellites in our country, a significant development has been made for Fengyun-4A (FY-4A). Compared with the previous generation (Fengyun-2), FY-4A has better observation accuracy and a shorter scanning time. Taking full advantage of the advanced geosynchronous radiation imager (AGRI) data, the level of weather and meteorological disasters forecasting in countries along the “The Belt and Road Initiatives” will be effectively improved. The interface for the FY-4A AGRI data assimilation is complemented in Weather Research and Forecasting Data Assimilation (WRFDA) v3.9.1 model before investigating the bias characteristics based on RTTOV v11.3 model and GFS analysis. Bias-correction experiments of FY-4A AGRI data in infrared Channels 8–14 were further conducted. The results show that: (1) Channels 8–10 and 14 have warm biases. There are cold biases in Channels 11–13. The biases and standard deviation of the water vapor Channels 9 and 10 are small. The characteristics of the biases show obvious differences between land and ocean in Channels 11–14. Land’s biases are more complex than the ocean’s. For these channels, observations on land can be eliminated in quality control. (2) The slope of the linear regression equation between bias and satellite zenith angle is less than 0.035. There is no obvious dependence of biases on the satellite zenith angle. (3) The bias in Channels 8 and 11–14 show more obvious dependence on the scene temperature than those in Channels 9 and 10. (4) The variational bias correction tested during 1800 UTC on May 13–15, 2018 shows that the systematic bias was effectively corrected.

Abstract:An extreme rainfall event [maximum of 1056.7 mm (24 h)^-1] induced by an ultra-long-duration, linearly-shaped mesoscale convective system (β-MCS) occurred over Gaotan town (GT) of the Guangdong Province on 30–31 August 2018. This event broke the highest record for the Guangdong Province, caused a severe flash flood, and created social concern. Using multiple observations and NCEP/NCER_FNL data, we performed an evaluation of the precipitation, convection, and environmental conditions as well as initiation and maintenance for the β-MCS. The data showed that the tropical cloud clusters moved northward and induced large-scale heavy rainfall, with a background of a monsoon depression in a mesoscale favorable environment. A linearly-shaped β-MCS, characterized with a back-building, ultra-long-duration, quasi-stationary, low echo-top-height, and low echo-convective-centroid was responsible for the record-breaking rainfall over GT. Analysis using a rotation rate equation of sea and land breezes indicated the convection initiation and organization are closely related to the near-surface flow affected by multiscale systems. Southerly flow was sustained for a long period that was determined by reverse forces between the monsoon depression, local terrain, and barometric gradient, while all three came to a balance. Strengthening of the southerly flow on the HJ river valley side over the terrain slope helped warm-ridge development of the temperature field, blocking cold pooling. The outflow boundary moved southeastward on the mountain side over the terrain slope, leading to a sharp temperature gradient in that region. Quantitative diagnosis using a mesoscale atmospheric dynamics equation demonstrated the dynamic mechanism sustaining the convection maintenance and β-MCS organization to local vertical wind shear, causing a sharp temperature gradient.

Abstract:The characteristics of the meridional movement of western Pacific subtropical high (WPSH) ridge lines at different pressure levels under the influence of a single tropical cyclone (TC), i.e., Megi, and its possible mechanisms are analyzed by conducting sensitivity experiments with WRF. The results show that the WPSH ridge lines shift southward under the influence of TC Megi. Moreover, the higher the ridge lines are, the more southward they move. The possible mechanisms are that the shift of WPSH ridge lines is directly affected by the zonal wind anomalies in its vicinity, and the zonal wind and temperature gradient anomalies near the ridge lines caused by TC Megi generally satisfy the thermal wind relationship. Therefore, under the influence of the temperature gradient anomalies near the ridge line caused by TC Megi, the zonal wind anomalies will change with altitude, which will affect the vertical distribution of WPSH ridge lines. In addition, the diagnostic analysis results of the temperature tendency equation show that the different physical processes near the ridge, which are stimulated by TC activities, exhibit quite different temporal and spatial distributions. Moreover, the horizontal advection and nonadiabatic heating anomalies caused by TC Megi mainly lead to the abnormal increase of atmospheric temperature. Meanwhile, the vertical transport anomalies mainly lead to the abnormal decrease of atmospheric temperature. Therefore, the thermal effect of TC activities plays an important role in the process of changing the vertical distribution of WPSH ridge lines.

Abstract:Based on the NCEP/NCAR daily reanalysis data for the period of 1958–2017, this study comparatively analyzes the stratospheric and tropospheric evolutions during the lifecycle of both strong and weak stratosphere polar vortex events (SPV and WPV events, respectively). Moreover, the atmospheric circulation and dynamical characteristics of two types of WPV events, namely events with and without stratospheric sudden warming (SSW), were also analyzed. The results show that the formation of SPV events follow a slow development and then a rapid intensification stage, while the WPV events are established dramatically. Compared with the SPV events, the WPV events are stronger and have a higher anomaly center when they reach a peak. Moreover, the occurrence of SPV and WPV events is closely related to the positive feedback of wave–mean flow interaction. For the SPV events, a Pacific–North American teleconnection-like pattern weakens the wave-1 of planetary waves during the growth stage. When the stratospheric westerly winds are strengthened to a certain extent, upward propagating planetary waves are greatly suppressed; thus, the polar vortex is intensified rapidly and reaches the peak stage. For the WPV events, a wave-1 pattern enhances the upward propagating planetary waves in the growth stage, which soon leads to weak westerly winds in the stratosphere by exerting a drag on the zonal flow. More planetary waves then propagate into the stratosphere, and thus, the polar vortex is dramatically weakened and even broken down. In addition, for the WPV events with SSW, enhanced upward wave-1 Eliassen-Palm (EP) flux in the stratosphere occurs in the growth stage. Through the positive feedback of wave–mean flow interaction, both the upward propagating wave-1 and wave-2 EP fluxes are increased, which leads to the breakdown of the polar vortex. For the WPV events without SSW, the upward propagating wave-1 EP flux is weak in the growth stage, while the wave-2 flux plays an important role. Hence, the total upward propagating planetary waves are much smaller than the WPV events with SSW. For the WPV events without SSW, a Eurasian (EU) teleconnection-like pattern in the height field appears in the upper troposphere during the growth and peak stages, accompanied by strong anomalous poleward EP flux, which leads to extreme negative Arctic oscillation (AO) in the troposphere. For the WPV events with SSW, a wave train from the lower latitude over the North Pacific in the height field appears in the upper troposphere mainly during the growth stage. In the later stages, the tropospheric influence of the WPV events with SSW is relatively delayed and not robust, and the magnitude of AO index is much smaller than that for the WPV events without SSW.

Abstract:Using rocket sounding data, a new generation of weather radar data and meteorological data is utilized to comprehensively analyze the hail cloud in Baota District, Yan'an County, on July 17, 2015. The results show the following aspects. (1) At 0800 BT (Beijing time), low vortex in the Hetao region split eastward, with a strong cold advection; the fast movement and a rising of surface temperature at 1400 BT caused this hail. (2) The hailstorm conditions inside the backward position, such as temperature and humidity, convection index (Tg), whole layer special humidity integral index (IQ), and total totals index (TT), are lower than those in the external natural atmosphere. The stratification stability index (K), lifted index (LI), and Showalter index (SI) show that hail clouds inside are smaller than those outside. The storm severity index (SSI) of the hail cloud, a thermal parameter, is lower than that in the natural atmosphere. The internal energy parameter, i.e., convective available potential energy (CAPE) is considerably lower than those in the natural atmosphere. The height of the 0℃ layer inside the hail cloud is lower than the natural atmosphere outside the hail cloud. (3) Location of the rocket detection is opposite to the back of the hail cloud. The wind direction changes counterclockwise from downward to upward in the hail cloud.At the -20℃ high-temperature layer, air flow is higher and stronger, and the whole layer has partial sinking airflow. (4) The temperature range that is near the 0℃ layer in hail cloud is -1.8℃–5.0℃. The maximum humidity area is within the depth of 1.0 km, where humidity is over 80% and the maximum humidity is 87.1%. These provide water vapor conditions for hail formation. (5) There is a maximum horizontal wind speed of 19 m s^-1 rapid flow and a thickness of 0.022 km close to the lower layer of 0℃. The layer also maintains a horizontal wind speed of 13 m s^-1 or above in the temperature range of -4.8℃–5.0℃ and a thickness within 1.6 km. These provide a dynamic field condition for hail formation. (6) There is a weak wind zone of ≤ 2 m s^-1 in areas where the temperature range is -8.7℃–9.2℃ and the thickness is within 0.2 km. Below the weak wind zone, where the temperature range is -4.6℃–8.8℃ and the thickness is within 0.889 km, it is the updraft. Here, the average rising speed is 1.79 m s^-1, and the maximum rising speed is 4 m s^-1. This configuration provides an environmental field for the growth of hail.

Abstract:Using daily station data and Japanese 55-year Reanalysis Project (JRA-55) data from 1959 to 2013, a three-dimensional background circulation structure of summer heavy rainfall in the middle–lower reaches of the Yangtze River (MLYR) was analyzed. Composite analyses of circulation in advance of 373 heavy rainfall days revealed that a prominent warm anomaly with a center at 300 hPa emerged in the upper troposphere over the MLYR. Because of hydrostatic and quasi-geostrophic equilibriums, an anticyclonic (cyclonic) anomaly formed above (below) the warm center. On the one hand, the warm anomaly strengthened the westerly winds to the north of the warm center by a high-level anticyclonic circulation anomaly, which resulted in the jet stream in the upper-level over East Asia shifting southward and eastward to the north side of the MLYR. This enhanced the upper-level divergent anomaly field over the MLYR. On the other hand, the cyclonic anomaly below the warm anomaly reinforced the low-level southwesterly winds to the MLYR, which transported more water vapor to it and strengthened convergence. The favorable configuration of high and low altitude circulation anomalies caused by warm anomalies played an important role in the formation of strong precipitation in the MLYR. The 300 hPa warm anomaly existed at 400–300 hPa in the eastern part of the Qinghai-Tibet Plateau 48 hours before the precipitation in the MLYR. The 700 hPa cyclonic circulation appeared in the middle and lower layers over the Sichuan Basin 24 hours ahead of schedule. The high and low circulation elements cooperated with each other and moved eastward with time. The warm anomaly first reached the MLYR, and cooperated with the low-level cyclonic circulation and water vapor convergence area, resulting in strong precipitation over the MLYR.

Abstract:The authors researched the interdecadal trend shifts of latent heat flux (LHF) over the Kuroshio Extension (KE) and Gulf Stream (GS) regions during the warming and warming hiatus periods using the LHF data and relevant variables obtained from the Objectively Analyzed Air–Sea Fluxes Project of the Woods Hole Oceanographic Institution and the Ishii subsurface temperature and salinity data obtained from the Japan Agency for Marine–Earth Science and Technology. The small perturbation method, empirical orthogonal function analysis, and International Thermodynamic Equation of Seawater—2010 are applied in this research. Contrasting interdecadal trend shifts of LHF exist in the KE and GS regions. The interdecadal LHF trend of the KE region shifts from positive to negative around 2001, whereas that of the GS region shifts from negative to positive around 1993. The variation of the KE region primarily resulted from sea surface temperature change (ocean-induced), whereas that of the GS region resulted from wind speed (1979–1992; atmosphere-induced) and sea surface temperature (1993–2013). The interdecadal variations of ocean heat content (OHC) in the KE and GS regions are also different: The interdecadal variation of surface heat content in the KE region is consistent with the mixed layer, whereas that in the GS region is different from that in the deep layer. Meanwhile, the changes below the surface layer are more consistent. The internal heat content changes in both regions reflect the warming hiatus phenomenon. The internal heat content in the KE region influences first the lower layer and then the upper layer, whereas that in the GS region influences first the upper layer and then the lower layer. The difference between the surfaces of the KE and GS regions can be attributed to the difference between ocean and atmospheric factors. Moreover, the vertical difference of the interdecadal variation of internal heat content can be attributed to the structural difference between the two regions. All of these variations are associated with the warming hiatus and may affect the warming hiatus conversely.

Abstract:On the basis of the daily snow depth (SD) data provided by the Japanese 55-year Reanalysis project, the global reanalysis data from the European Centre for Medium-Range Weather Forecasts and sea temperature data from Hadley Center (Hadley), this study analyzes the asymmetric modes of the spring SD anomaly in Eurasia and its teleconnection with the North Atlantic sea surface temperature (SST). Results are verified by numerical simulation analysis. Significant differences are found between the first two spring SD modes over the Eurasian continent that are represented as two asymmetric forms in this study: the first mode features a zonal uniform distribution, and the second displays an obvious west–east contrast distribution. North Atlantic “tri-polar” and “saddle” SST modes have a significant correlation with the first and second SD modes, respectively. Corresponding to the two SST modes, the wave activity fluxes in the mid-high latitudes over the northern hemisphere are characterized by two kinds of propagation characteristics: Silk Road pattern (SRP) and Eurasian teleconnection pattern (EU), both of which have different effects on the position and intensity of the westerly air flow in middle–high latitudes, and thus exert different remote influences on the SD distribution in Eurasia. The localized multiscale energy and vorticity analysis shows that the kinetic energy (KE) of source region in the North Atlantic has a transform process from bottom to top. In addition, the average KE conversions enhance over the exit region of the westerly jet, benefiting high-level KE accumulation and outward divergence and creating remote effects on downstream areas. The CAM5.1 model simulation is used to study the effects of the two SST modes on the propagation characteristics of wave activity fluxes. Simulation results verify the observation results well. SST modes may be responsible for the SRP and EU propagation characteristics of wave activity fluxes. Meanwhile, changes in the climatic field elements of the two SST modes are consistent with the distribution characteristics of the corresponding SD modes.

Abstract:To obtain accurate winter precipitation data, this study focuses on the correction and error calculation of the influence of near-surface horizontal wind during snowfall measurement using particle size and velocity (PARSIVEL²). Revised results show that under certain wind speeds, ignoring the influence of wind can cause the significant underestimation of large particles’ diameter. By contrast, large wind speeds indicate that the underestimation of same-sized particles’ diameter during calculation is evident. When the wind speed does not exceed 2 m s^-1, the calculation error of the falling speed of snowfall particles is approximately 3%, and the calculation error of diameters is within 7%. In the analysis of the real snowflake spectrum obtained during a snowfall in Nanjing on 4 January, 2018, ignoring the influence of wind shifts the peak of the snowflake spectrum and narrows the spectrum, resulting in the overestimated concentration of small particles and underestimated concentration of large particles, which in turn affect the calculation of microphysical quantities. Specifically, radar reflectivity factor Z and snowfall intensity I are underestimated, and the actual value of the Z–I relationship fitting coefficient a is greater than the calculated value, whereas b is small. However, when the wind speed is large, the flow near the ground is complicated, and the vertical turbulent motion cannot be ignored. This correction method may no longer be applicable. Adding windbreakers in future observations or making corrections in subsequent data processing is recommended to eliminate the impact of wind on snowfall measurements.

Abstract:The spatial and temporal distribution characteristics of different types of thermodynamic variables and potential vorticity during precipitation were compared and analyzed in this study. On the basis of the heavy rainfall events in Jilin Province on 13–14 July 2017, the following five types of potential temperature were calculated with model outputs: conventional potential temperature (θ), equivalent potential temperature (θ_e), generalized potential temperature containing a condensation probability function (θ_Gao), generalized potential temperature containing a freezing probability function (θ_Wang), and potential temperature covering condensation and freezing (θ_Gu). The relationships between five associated types of potential vorticity [PV(θ), PV(θ_e), PV(θ_Gao), PV(θ_Wang), and PV(θ_Gu)]and precipitation were also analyzed. Results showed that the generalized potential temperature introducing a freezing probability function (θ_Wang) and its potential vorticity [PV(θ_Wang)] corresponded well with heavy rainfall. The differences between θ_Wang and θ_Gao were observed at 5–11 km in the mid-upper troposphere over the rainfall region. θ_Wang was always greater than θ_Gao, with the maximum difference reaching 2.5 K. Hence, the introduction of the freezing probability function extends the application scope of the generalized potential temperature and offers a reliable depiction of the thermodynamic state of nonuniform saturated moist air over rainfall regions. The differences among the five types of potential vorticity were mainly observed under 12 km over the rainfall region. The positive and negative anomaly centers for potential vorticity PV(θ_Gao) and PV(θ_Wang) respectively defined by θ_Gao and θ_Wang were increasingly visible. The anomaly value of PV(θ_Wang) was greater than that of PV(θ_Gao), and the differences could reach ±0.2 PVU. Such difference was due to the enhancement of the generalized potential temperature over the rainfall region resulting from the introduction of the freezing probability function. This condition led to the abnormal enhancement of the moist potential vorticity in the freezing region.

Abstract:The spatial–temporal variation characteristics of the daytime cloud fraction and cloud optical thickness of various cloud types over central eastern China in summer are explored using ERA5 reanalysis data and CERES satellite data during the period of 2001–2017. The effects of various cloud types on the near-surface air temperature are quantitatively analyzed using a radiative–convective model. The observations show that the annual mean daytime total cloud fraction and its optical thickness decrease gradually from south to north, while the upper-middle cloud fraction dominates the total cloud fraction. For annual mean changing rates, the total cloud fraction shows a significant decrease of 0.3% a^-1 with the largest contribution from low clouds (-0.27% a^-1). The increasing trend of total cloud optical thickness ranges from 0 to 0.1 a^-1, where low and lower-middle cloud optical thickness show an increase of 0.06 a^-1 and 0.03 a^-1, respectively, while the upper-middle and high cloud optical thickness show a decreasing trend of 0.08 a^-1 and 0.03 a^-1, respectively. The model results show that the annual mean CET (Cloud Effect Temperature) of the four different cloud types are negative, with values of 2.9℃, 2.7℃, 2.2℃, and 1.7℃ for low, lower-middle, upper-middle, and high clouds, respectively, indicating the cooling effects of various cloud types. The low cloud CET in the North China Plain is up to -5℃, while the lower-middle and upper-middle clouds are up to -7.8℃ in the Sichuan Basin and Yunnan–Guizhou Plateau. The interannual variations of CETof different cloud types and near-surface air temperature have good consistency. The near-surface air temperature decreases (increases) before (after) 2004, while the CET of different cloud types decrease (increase) during this period, which indicates good correspondence between the strengthening (weakening) of the cloud cooling effect and the decrease (increase) of the near-surface temperature. Specifically, a positive correlation of the four cloud types and near-surface air temperature over central eastern China occurs during the daytime in the summer. The annual mean daytime upper-middle cloud fraction plays an important role in all types of clouds over central eastern China in the summer, and the correlation coefficient between the CET and near-surface air temperature is as high as 0.63. In summary, the effects of different cloud types on the near-surface air temperature are different, but all show positive correlations. The quantitative analysis of the influence of different cloud types on the near-surface air temperature can provide a scientific reference for the accurate measurement of global warming, the role of cloud feedback in regional warming, and accurate prediction of regional warming scenarios.

Abstract:The Beijing Broadband Lightning NETwork (BLNET) is a regional total flashes 3D (three-dimensional) location network that combines research and business. In 2015, the BLNET hardware, station network layout, and location algorithm were updated and upgraded to improve the sensitivity of the sensor and improve computational efficiency and detection performance. BLNET features the functions of IC (Intra-Cloud) flashes, CG (Cloud-to-Ground) flash-pulse-type identification, and current peak estimation, as well as 3D real-time location of lightning radiation pulses and the fine location of the channel-resolvable lightning discharge process. The analysis of the real-time 3D location results for the lightning radiation source pulse during the thunderstorm that occurred on 7 July 2017 shows a total of 11,902 lightning flashes during the thunderstorm process. Most of these flashes were dominated by IC flashes, with CG flashes accounting for just 28% of the total. PCG (Positive Cloud-to-Ground) flashes account for only 5% of the total number of CG flashes. During the mature period of the thunderstorm, the maximum lightning frequency was 927 flashes (6 min)^-1. By comparing and analyzing the location of the lightning radiation source and the radar echo at the corresponding time, the radiation source was found to be basically concentrated in the strong echo range. The fine location results of the PCG flashes indicate that the initial stage involved a clear pre-breakdown process. The origin of the lightning radiation source was about 5.4 km above sea level, and then the channel developed upward. At about 10 km, the channel began to exhibit a horizontal development. The fine location results of the NCG (Negative Cloud-to-Ground) flashes indicate that the discharge first originated from a height of about 7.1 km, then the channel developed to the south, and some negative pilot branches developed downward. After about 38 ms, the channel stopped developing for a short time. After 17 ms, the channel development began again and the air was re-energized. The above results show that BLNET can locate and monitor 3D real-time lightning activity of the whole thunderstorm life history, as well as obtain the fine location of the lightning 3D discharge channel.

Abstract:Aiming at promoting the application of new types of sounding data in NWP (numerical weather prediction) models, this paper presents a basic research work of return radiosonde data. Based on archived return radiosonde observation datasets in China, a quality control scheme for future operational implementation purposes is established. By comparing and analyzing the statistical characteristics of observation samples before and after the quality control, the rationality of the quality control method is demonstrated. After the quality control procedure, the sampling distribution of the detection variables is more reasonable, and the inner-consistency of variables is also improved. An uncertainty analysis of return radiosonde data is then carried out by referring to the high-resolution NWP model forecast field and the conventional sounding observation data of the same site. The results show that the precision of return radiosonde reaches the breakthrough target defined by the WMO (world meteorological organization). Some detection variables even achieve the ideal target. Finally, the assimilability of the return radiosonde data is discussed based on the background field of the NWP model. The results show that wind field observations at all times and night temperature observations satisfy the Gaussian and unbiased assumptions of the variational assimilation system and can be assimilated directly. To play a more effective role in the data assimilation system, air pressure, humidity observations, and daily temperature need to be corrected before data assimilation. This work lays a foundation for the future assimilation application of return radiosonde.

Abstract:Based on the Himawari-8 satellite TBB (Black Body Temperature) data from the Japan Meteorological Agency, the ERA5 (the fifth generation of European Centre for Medium-Range Weather Forecasts Reanalysis) reanalysis data from the European Centre for Medium-Range Weather Forecasts, and a new energy diagnostic method that uses a temporal scale for scale separation, the evolutionary characteristics of a convective cloud cluster from 0000 UTC 5 June to 1500 UTC 6 June 2016 (lasted 40 hours) that formed over the Tibetan Plateau (TP), moved eastward, and induced heavy precipitation in downstream regions were investigated in this study. The main findings are as follows. The main influencing systems for the eastward-moving convective cloud cluster at different stages of its lifespan were different. Before moving out of the plateau, the cloud cluster was mainly affected by a plateau vortex and a short-wave trough. As the cloud cluster vacated the plateau, the plateau vortex dissipated, whereas the short-wave trough intensified with time, and finally, the short-wave trough became the main influencing system of the cloud. The deep convection features of the eastward-moving cloud cluster were significant. The eastward-moving cloud cluster induced a series of precipitations from west to east, with the strongest precipitation occurring after the cluster had moved out of the plateau and its convective had lowered in height. The energetics characteristics of the eastward-moving convective cloud cluster experienced notable changes during the cluster lifespan, and the associated precipitation characteristics were also significantly different. When the cloud cluster was over the TP (the first stage), the contribution from the background field was a dominant factor. The background field provided energy for the evolution of eddy flow (by downscale kinetic energy cascade), which directly induced heavy precipitation. During the second stage, the precipitation-related latent heating was greatly enhanced, which significantly increased the APE (available potential energy) of eddy flow. Under the influences of vertical motion, the APE of eddy flow was released and converted to the KE (kinetic energy) of eddy flow. This acted as a dominant factor for the sustainment of the precipitation-related eddy flow. When the cloud cluster moved out of the TP, the influence of the background field on eddy flow was enhanced again; however, the influence was not direct as in the first stage. In this stage, the influence from the background environment favored the persistence of precipitation-related eddy flow indirectly. First, the APE of the background field was transferred to the APE of eddy flow through a downscale energy cascade. Then, a baroclinic energy conversion from the APE of the eddy flow to its KE occurred. This acted as a dominant energy source for the KE of eddy flow. Furthermore, in the third stage, a remarkable upscale energy cascade of KE was found, which reflected the feedback of eddy flow on its background field. However, the feedback intensity was not enough to significantly affect the evolution of the background field.