Analysis of a Large-Scale Strong Convective Weather under a Weak Water Vapor Condition in Shanxi, China in Spring

This article uses NCEP 1° × 1° grid point reanalysis data, conventional meteorological observation data, FY2G satellite TBB data, radar combined reflectivity data, ground-encrypted automatic station observation data, etc., through the synoptic diagnostic analysis method for a comprehensive analysis of a large-scale underreporting of a strong convective weather process under weak water vapor conditions on the 13th April 2017. The results show that the severe convective weather process is affected by the short-wave disturbance in the northwesterly airflow, triggered by the uplift of the westerly trough, the mid-low shear line and the mesoscale front of the boundary layer in the dry northwest. The jet stream is also an important system for the development of this strong convective weather. In the case of weak water vapor and energy conditions, if there is strong dynamic uplift, vertical wind shear and large temperature differences, strong convection can still occur;the convection occurrence area corresponds to the high potential vorticity abnormal area. The movement speed and direction of the cloud cluster are also consistent with the movement of the high potential vorticity anomaly area;the potential vorticity anomaly will cause the cyclonic circulation to increase, and the upward movement will also increase, which is conducive to the development of strong convective weather. According to the position of the dew point front in the β mesoscale, the ground cold pool corresponds to the small value area of the convective cloud cluster TBB. The front of the cold pool is accompanied by a mesoscale ground convergence line, and the uplift is strengthened, which is conducive to the development and forward movement of thunderstorms;the outflow of the cold pool is guided by 700 hPa. When the wind direction is the same, the movement speed will increase, and the stronger the outflow, the faster the movement speed.

Spatial-Temporal Variations of Dominant Drought/Flood Modes and the Associated Atmospheric Circulation and Ocean Events in Rainy Season over the East of China

By using Season-reliant Empirical Orthogonal Function (S-EOF) analysis, three dominant modes of the spatial-temporal evolution of the drought/flood patterns in the rainy season over the east of China are revealed for the period of 1960-2004. The first two leading modes occur during the turnabout phase of El Nino-Southern Oscillation (ENSO) decaying year, but the drought/flood patterns in the rainy season over the east of China are different due to the role of the Indian Ocean (IO). The first leading mode appears closely correlated with the ENSO events. In the decaying year of El Nino, the associated western North Pacific (WNP) anticyclone located over the Philippine Sea persists from the previous winter to the next early summer, transports warm and moist air toward the southern Yangtze River in China, and leads to wet conditions over this entire region. Therefore, the precipitation anomaly in summer exhibits a 'Southern Flood and Northern Drought' pattern over East China. On the other hand, the basin-wide Indian Ocean sea surface temperature anomaly (SSTA) plays a crucial role in prolonging the impact of ENSO on the second mode during the ENSO decaying summer. The Indian Ocean basin mode (IOBM) warming persists through summer and unleashes its influence, which forces a Matsuno-Gill pattern in the upper troposphere. Over the subtropical western North Pacific, an anomalous anticyclone forms in the lower troposphere. The southerlies on the northwest flank of this anticyclone increase the moisture transport onto central China, leading to abundant rainfall over the middle and lower reaches of the Yangtze River and Huaihe River valleys. The anomalous anticyclone causes dry conditions over South China and the South China Sea (SCS). The precipitation anomaly in summer exhibits a 'Northern Flood and Southern Drought' pattern over East China. Therefore, besides the ENSO event the IOBM is an important factor to influence the drought/flood patterns in the rainy season over the east of China. The third mode is positively correlated with the tropical SSTA in the Indian Ocean from the spring of preceding year(-1) to the winter of following year(+1), but not related to the ENSO events. The positive SSTA in the South China Sea and the Philippine Sea persists from spring to autumn, leading to weak north-south and land-sea thermal contrasts, which may weaken the intensity of the East Asia summer monsoon. The weakened rainfall over the northern Indian monsoon region may link to the third spatial mode through the 'Silk Road' teleconnection or a part of circumglobal teleconnection (CGT). The physical mechanisms that reveal these linkages remain elusive and invite further investigation.

SIMULATION OF BOUNDARY LAYER EFFECTS ON A HEAVY RAINFALL EVENT CAUSED BY A MESOSCALE CONVECTIVE SYSTEM OVER THE YELLOW RIVER MIDSTREAM AREA

A heavy rainfall event caused by a mesoscale convective system(MCS),which occurred over the Yellow River midstream area during 7-9 July 2016,was analyzed using observational,high-resolution satellite,NCEP/NCAR reanalysis,and numerical simulation data.This heavy rainfall event was caused by one mesoscale convective complex(MCC)and five MCSs successively.The MCC rainstorm occurred when southwesterly winds strengthened into a jet.The MCS rainstorms occurred when low-level wind fields weakened,but their easterly components in the lower and boundary layers increased continuously.Numerical analysis revealed that there were obvious differences between the MCC and MCS rainstorms,including their three-dimensional airflow structure,disturbances in wind fields and vapor distributions,and characteristics of energy conversion and propagation.Formation of the MCC was related to southerly conveyed water vapor and energy to the north,with obvious water vapor exchange between the free atmosphere and the boundary layer.Continuous regeneration and development of the MCSs mainly relied on maintenance of an upward extension of a positive water vapor disturbance.The MCC rainstorm was triggered by large range of convergent ascending motion caused by a southerly jet,and easterly disturbance within the boundary layer.While a southerly fluctuation and easterly disturbance in the boundary layer were important triggers of the MCS rainstorms.Maintenance and development of the MCC and MCSs were linked to secondary circulation,resulting from convergence of Ekman non-equilibrium flow in the boundary layer.Both intensity and motion of the convergence centers in MCC and MCS cases were different.Clearly,sub-synoptic scale systems in the middle troposphere played a leading role in determining precipitation distribution during this event.Although mesoscale systems triggered by the sub-synoptic scale system induced the heavy rainfall,small-scale disturbances within the boundary layer determined its intensity and location.

Characteristics of Radar Echo Parameters and Microphysical Structure Simulation of a Short-Time Heavy Precipitation Supercell

By using the conventional observations, radar data, NCEP/NCAR FNL 1°×1° reanalysis data and numerical simulation data and with the construction and calculation of radar echo parameters, this paper presents the structural characteristics and physical processes of a short-time heavy precipitation supercell that occurred in the squall line process in Shanxi Province on 24 June 2020. The results show that this squall line event occurred in front of a surface cold front,combined with infiltration of low-level cold air and continuous increase of near-surface humidity in the afternoon. The surface mesoscale convergence line and mesoscale dew point front contributed to the development and systemization of the squall line by a large degree. The short-time extremely heavy precipitation in Pingshun County was caused by the development of a supercell from thunderstorm cells on the front side of the squall line. The characteristics of sharp increase in vertical integral liquid water content, persistent increase in reflectivity factor and continuous rise in the echo top height appeared about 23 min earlier than the severe precipitation, which has qualitative indicating significance for the nowcasting of short-time heavy precipitation. A quantitative analysis of the radar echo parameters suggests that the“sudden drop”of FV40was a precursor signal of cells’ coalescence and rapid development to the mature stage. The areal change of the echo core at the 6 km height was highly subject to the merging and developing of cells, the rapid change of hydrometeor particles in clouds and the precipitation intensity. Changes in the cross-sectional area of convective cells at different heights can indirectly reflect the changes of liquid particles and ice particles in clouds, which is indicatively meaningful for predicting the coalescing and developing-to-maturing of cells and heavy precipitation 30-45 min earlier.A comprehensive echo parameter prediction model constructed by the random forest principle can predict the magnitude of short-time heavy precipitation 40-50 min in advance. Numerical simulation reveals that large amounts of water vapor existed in the near-surface atmosphere, and that the cells rapidly obtained moisture from the ambient atmosphere and developed rapidly through maternal feeding. The cold cloud zone was narrow, upright and had a high stretch height. The upward motion in clouds was strong and deep, and very rich in liquid water content. The graupel particles had a large vertical distribution range, the coexistence area of graupel and snow was large, the height of raindrops was close to the surface with a wide horizontal scale, and the precipitation efficiency was high. These may be the important elements responsible for the occurrence of the short-time heavy precipitation that exceeded historical extreme values. On the basis of the above analyses, a comprehensive parameter(CP) prediction model is worked out, which can estimate the developing trend of supercells and the intensity of short-time heavy precipitation about 1 h in advance.

CHARACTERISTICS OF GRAVITY WAVE PROPAGATION AND ENERGY CONVERSION IN A SUDDEN HEAVY RAINFALL EVENT

In this paper, a sudden heavy rainfall event is analyzed, which occurred over the Yellow River midstream during 5-6 August 2014. We used observational, NCEP/NCAR reanalysis, high-resolution satellite, and numerical simulation data. The main results are as follows. Under an unfavorable environmental circulation, inadequate water vapor and unfavorable dynamic conditions but sufficient energy, a local sudden heavy rainfall was caused by the release of strong unstable energy that was triggered by cold air transport into middle and lower layers and the propagation of gravity waves. The distributions of rain area, rain clusters, and 10-minute rainfall showed typical mesoscale and microscale fluctuation characteristics. In the mesoscale rain area or upstream, there was a quasi-stationary wave of mesoscale gravity waves with their propagation downstream. In the course of propagation from southwest to northeast,the wavelength became longer and the amplitude attenuated. In the various phases of gravity wave development, there were evident differences in the direction of the wave front. Wave energy was mainly in the lower layers. Unstable vertical wind shear at heights of 1-6 km provided fluctuation energy for the gravity waves. The mechanisms of heavy rainfall formation were different at Linyou and Hancheng stations. Diabatic heating was the main source of disturbed effective potential energy at Linyou. The explosive short-period strong precipitation was caused by the release of strong effective potential energy triggered by the gravity waves, and its development and propagation after that energy maximized. In contrast, the latent heat release of upstream precipitation was the main source of disturbed effective potential energy at Hancheng. This formed a positive feedback mechanism that produced continuous precipitation. In the studied event, the development of westerly belt systems had disturbed the wind field. The contribution of kinetic energy generated by this disturbance could not be ignored. The Froude number, mountain shape parameter, and ratio between mountain height and temperature inversion layer thickness had various effects of atmosphere and terrain on mesoscale and microscale mountain waves. In upper and lower layers, there were five airflows that were strengthened by the terrain. All these had important influences on local heavy rainfall at Linyou and Hancheng stations.

Assessment of Soil Erosion Susceptibility Using Empirical Modeling

Soil erosion is one of the most serious land degradation problems all over the world, causing irreversible land quality reduction. In this paper, we modify the Revised Universal Soil Loss Equation （RUSLE） model by replacing the factors of slope length and gradient with Sediment Transport Index （STI）. The Digital Elevation Model, terrain parameters, Normalized Difference Vegetation Index （NDVI）, and rainfall data are used as inputs to the model. Along with the application of remote sensing techniques and ground survey measurements, erosion susceptibility maps are produced. The revised models are then used to obtain the optimal estimate of soil erosion susceptibility at Alianello of southern Italy, which is prone to soil erosion. The soil loss estimated from the modified RUSLE model shows a large spatial variance, ranging from 10 to as much as 7000 ton ha^-1 yr^-1. The high erosion susceptible area constitutes about 46.8% of the total erosion area, and when classified by land cover type, 33% is ＂mixed bare with shrubs and grass＂, followed by 5.29% of ＂mixture of shrubs and trees＂, with ＂shrubs＂ having the lowest percentage of 0.06%. In terms of slope types, very steep slope accounts for a total of 40.90% and belongs to high susceptibility, whereas flat slope accounts for only 0.12%, indicating that flat topography has little effect on the erosion hazard. As far as the geomorphologic types are concerned, the type of ＂moderate steep-steep slopes with moderate to severe erosion＂ is most favorable to high soil erosion, which comprises about 9.34%. Finally, we validate the soil erosion map from the adapted RUSLE model against the visual interpretation map, and find a similarity degree of 71.9%, reflecting the efficiency of the adapted RUSLE model in mapping the soil erosion in this study area.