Thermal Convection in a Tilted Rectangular Cell with Aspect Ratio 0.5

Thermal convection in a three-dimensional tilted rectangular cell with aspect ratio 0.5 is studied using direct nu- merical simulations within both Oberbeck-Boussinesq （OB） approximation and strong non-Oberbeck-Boussinesq （NOB） effects. The considered Rayleigh numbers Ra range from 105 to 107, the working fluid is air at 30OK, and the corresponding Prandtl number Pr is 0.71. Within the OB approximation, it is found that there exist multiple states for Ra = 105 and hysteresis for Ra = 106. For a relatively small tilt angle/3, the large-scale circulation can either orient along one of the the vertical diagonal planes （denoted by Ma mode） or orient parallel to the front wall （denoted by Mp mode）. Which of the two modes transports heat more efficiently is not definitive, and it depends on the Rayleigh number Ra. For/Ta = 107 and β = 0°, the time-averaged flow field contains four rolls in the upper half and lower half of the cell, respectively, Md and Mp modes only developing in tilted cells. By investigating NOB effects in tilted convection for fixed/Ta = 106, it is found that the NOB effects on the Nusselt number Nu, the Reynolds number Re and the central temperature Tc for different β ranges are different. NOB effects can either increase or decrease Nu, Re and Tc when β is varied.

Analysis of a Large-scale Hail Process in Ulanqab City

Based on the weather monitoring data,NCEP/NCAR reanalysis data and Doppler weather radar data,the circulation background,atmospheric stability and changing characteristics of radar echoes of a large-scale hail weather process in Ulanqab City on July 5,2021 were analyzed.The results show that this hail weather process occurred in the summer afternoon of the hail-prone period in Ulanqab City,and was formed under the influence of upper trough and the circulation background of"two troughs and one ridge",which was conducive to the occurrence of strong convection weather.The low-level shear line provided the dynamic and water vapor conditions for the occurrence of the hail.In strong convection weather,water vapor was transported mainly from the Bay of Bengal,India and Nepal over the Himalayas.Before the occurrence of strong convection weather,water vapor transport increased significantly,and the low-level water vapor concentrated below 400 hPa,with obvious convergence and vertical transport.The characteristic parameters of radar echoes,such as combined reflectivity,vertically integrated liquid,and echo top height,increased significantly before and during the occurrence of hail,which had good indicative significance for the prediction and early warning of hail.

STRUCTURE FEATURES AND COMPOSITE ANALYSIS OF CONVECTIVE CELLS IN A WARM SECTOR HEAVY RAINFALL EVENT OVER SOUTHERN CHINA

This paper uses the ARW-WRF model to carry out a numerical simulation of a warm-sector heavy rainfall event over southern China on the 22-23 May, 2014. A composite analysis method was used to analyze the evolution process and structural features of the convective cells on a convection line during this rainfall event. This analysis identified three stages:(1) Stage of activation: the equivalent potential temperature surfaces as lower layers start to bulge and form warm cells and weak vertical convective cloud towers which are subject to the impact of low-level warm moist updrafts in the rainfall sector;(2) Stage of development: the warm cells continue to bulge and form warm air columns and the convective cloud towers develop upwards becoming stronger as they rise;(3) Stage of maturity: the warm air columns start to connect with the stable layer in the upper air; the convective cloud tower will bend and tilt westward with each increasing in height, and the convection cell is characterized by a "crescent-shaped echo" above the 700 h Pa plane. During this stage the internal temperature of the cell is higher than the ambient temperature and the dynamic structural field is manifested as intensive vertical upward movement. The large-value centers of the northerly and westerly winds in the middle layer correspond to the warm moist center in the cells and the relatively cold center south of the warm air column. Further analysis shows that the formation of the "crescent-shaped" convective cell is associated with horizontal vorticity. Horizontal vorticity in the center and west of the warm cell experiences stronger cyclonic and anticyclonic shear transformation over time; this not only causes the original suborbicular cell echo shape to develop into a crescent-like shape, but also makes a convection line consisting of cells that develop to the northwest.

Quantitative Analysis of Meso-β-scale Convective Cells and Anvil Clouds over North China

This paper proposes several quantitative characteristics to study convective systems using observations from Doppler weather radars and geostationary satellites. Specifically, in order to measure the convective intensity of each system, a new index, named the ＂Convective Intensity Ratio＂ （CIR）, is defined as the ratio between the area of strong radar echoes at the upper level and the size of the convective cell itself. Based on these quantitative characteristics, the evolution of convective cells, surface rainfall intensity, rainfall area and convectively generated anvil clouds can be studied, and the relationships between them can also be analyzed. After testing nine meso-β-scale convective systems over North China during 2006–2007, the results were as follows： （1） the CIR was highly correlated with surface rainfall intensity, and the correlation reached a maximum when the CIR led rainfall intensity by 6–30 mins. The maximum CIR could be at most ～30 mins before the maximum rainfall intensity. （2） Convective systems with larger maximum CIRs usually had colder cloud-tops. （3） The maximum area of anvil cloud appeared 0.5–1.5 h after rainfall intensity began to weaken. The maximum area of anvil cloud and the time lag between maximum rainfall intensity and the maximum area of anvil cloud both increased with the CIR.

Structure Analysis and Classification of Precipitation Cells by Fractal Geometry

This paper analyzes the possibility to discriminate between convective precipitation and stratiform precipitation. This study aims to improve the measurement of rainfall from teledetection data obtained both on the ground and in space. For this, two parameters, fractal dimension and fractal lacunarity, are considered. To calculate the fractal dimension, we use the approach of box-counting and show that the fractal dimension differs between convectives cells and stratiforms ones. And then the fractal lacunarity parameter is calculated by using the sliding boxes algorithm. The study for all the regions shows that precipitation cells can be described by different lacunarities whatever the scale of analysis. We deduce that the two parameters, fractal dimension and fractal lacunarity, can be used to classify precipitations in convective regime and stratiform regime.

A microphysical investigation of different convective cells during the precipitation event with sustained high-resolution observations

The growth and breakup processes of raindrops within a cloud influence the rain intensity and the sizes of raindrops on the surface.The Doppler velocity spectrum acquired by a vertically pointing radar(VPR)contains information on atmospheric turbulence and the size classification of falling hydrometeors.In this study,the four types of Convective Cells(CC)during precipitation events with more than 700 mm of precipitation in southern China are described.The characteristics of four types of CCs correspond to the isolated convection,the early stage,the mature stage,and the decline stage of organizational convection,in that order.Microphysical analysis using retrieval of vertical air motion(Vair)and raindrop evolution in clouds from Doppler velocity spectra collected by C-band VPR revealed the growth and breakup of falling raindrops with dynamic impact.Larger raindrops appear in the early stages and are accompanied by ice particles,which are impacted by the falling path᾽s downdraft.Raindrop aggregation,which is primarily related to the alternation of updraft and downdraft,accounts for the mature stage᾽s high efficiency of surface rainfall.The CCs in the decline stage originate from the shallow uplift in the weak and broad downdraft under conditions of enough water vapor.The updraft dominates the stage of isolated convection.Observations of convective cells could be more accurately represented in model evaluations.

Hydrothermally induced diagenesis: Evidence from shallow marinedeltaic sediments, WilhelmØya, Svalbard

Sedimentary basins containing igneous intrusions within sedimentary reservoir units represent an important risk in petroleum exploration. The Upper Triassic to Lower Jurassic sediments at Wilhelm?ya(Svalbard) contains reservoir heterogeneity as a result of sill emplacement and represent a unique case study to better understand the effect of magmatic intrusions on the general burial diagenesis of siliciclastic sediments. Sills develop contact metamorphic aureoles by conduction as presented in many earlier studies. However, there is significant impact of localized hydrothermal circulation systems affecting reservoir sediments at considerable distance from the sill intrusions. Dolerite sill intrusions in the studied area are of limited vertical extent(～12 m thick), but created localized hydrothermal convection cells affecting sediments at considerable distance(more than five times the thickness of the sill)from the intrusions. We present evidence that the sedimentary sequence can be divided into two units:(1) the bulk poorly lithified sediment with a maximum burial temperature much lower than 60-70 ℃,and(2) thinner intervals outside the contact zone that have experienced hydrothermal temperatures(around 140 ℃). The main diagenetic alteration associated with normal burial diagenesis is minor mechanical plastic deformation of ductile grains such as mica. Mineral grain contacts show no evidence of pressure dissolution and the vitrinite reflectance suggests a maximum temperature of ～40 ℃. Contrary to this, part of the sediment, preferentially along calcite cemented flow baffles, show evidence of hydrothermal alteration. These hydrothermally altered sediment sections are characterized by recrystallized carbonate cemented intervals. Further, the hydrothermal solutions have resulted in localized sericitization(illitization) of feldspars, albitization of both K-feldspar and plagioclase and the formation of fibrous illite nucleated on kaolinite. These observations suggest hydrothermal alteration at T> 120-140 ℃ at distances considerably further away than expected from sill heat dissipation by conduction only, which commonly affect sediments about twice the thickness of the sill intrusion. We propose that carbonate-cemented sections acted as flow baffles already during the hydrothermal fluid mobility and controlled the migration pathways of the buoyant hot fluids. Significant hydrothermally induced diagenetic alterations affecting the porosity and hence reservoir quality was not noted in the noncarbonate-cemented reservoir intervals.

Analysis of a Heavy Rainstorm in Jincheng in August of 2010

[Objective] A heavy rainstorm in Jincheng in August in 2010 was expounded. [Method] By dint of the conventional meteorological data, and automatic weather station data, and Doppler radar data, one severe torrential rainstorm was analyzed from the aspect of circulation background, physical quantity field, satellite cloud image, and radar echo, etc. [Result] The rainstorm was in the circulation field of low-west-east-obstruction, and was formed under the middle and low layer shear line and low air torrent situation. The low layer shear and convergence of wind favorable to the lift of unsteady air around Jincheng. The intrusion of cold air in the low layer of convective layer and above the ground trigered such convective weather. The torrent of the low air in the southwest sent abundant water vapor to the rainstorm area. The high temperature and the moisture accumulated much unsteady energy for the generation of rainstorm. The main precipitation system of this process was the singular of convective echo which was stimulated by the ground mesoscale shear line. Under the guidance of southwest airstream of the low and middle air, the convective echo singular formed train effect by moving towards Jincheng and formed large rainstorm. Doppler radar data suggested that the characteristics of the generation, development and movement of this mesoscale rainstorm system. The strong precipitation center was in the large value area of the gradient in the back of the TBB center. [Conclusion] The study provided references for the forecast and pre-warning of temporary rainstorm of such kind.

A New Mechanism of Convective Cell Regeneration and Development Within a Two-Dimensional Multicell Storm

In this study, based on simulations of a two-dimensional multicell storm under a ground-layer upshear （Uz〈0） by a mesoscale numerical model, a new mechanism of cell regeneration and development within the multicell storm at the ＂less than optimal shear＂ state.is proposed. In the presence of a ground-layer upshear, the circulation associated with the surface cold pool is not counteracted by that associated with the ambient wind sl^ear, and the density current extends out faster, making the multicell storm stay at the ＂less than optimal shear＂ state. As a result, a new cell is triggered by the strong vertical perturbation ahead of the mature convection, rather than by the split-up from the updraft at the leading edge of the surface cold pool as well as the gust front. The latter is the mechanism at the ＂optimal＂ state proposed by Lin et al. in 1998. In the new mechanism, the regenerated cell grows fast with the incident warm moist air from the upstream of the multicell storm, and tends to cut off the moist airflow into the mature convection at its western sector. Consequently, the mature convection would weaken, be replaced, and eventually decay. Actually, these two different mechanisms come into play in a way depending on the relationship between the circulation of the low-level shear and that of the cold pool. When the circulation of the cold pool is stronger than that of the wind shear, the multicell storm is at the ＂less than optimal shear＂ state, and the new convective cell is produced by the disturbance ahead of the mature cell. When the circulation of the cold pool is weaker, the cell regeneration is dominated by the mechanism at the ＂optimal＂ state, and the new cell is split from the gust front updraft. Therefore, these two mechanisms are not contradictive. With a moderate ground-layer upshear, they can alternately operate within a multicell storm.

VORTEX MOTIONS IN THE ATMOSPHERIC CONVECTIVE BOUNDARY LAYER

Large vortices with scales ranging from hundreds meters to tens of kilometers are generally found in the atmospheric convective boundary layer(CBL). These vortices play important roles in the vertical transport of momentum,heat,water vapor and other tracers in the boundary layer.On the basis of the view of interaction between the convection in CBL and the gravity waves in the upper stable layer the authors developed a convection-wave theory on the formation of large vortices.According to the theory the wavenumber spectrum of the large vortices mainly depends on the atmospheric conditions in both of the upper and lower layers,such as wind speed,wind direction shear,stratification as well as temperature jump. In the present paper satellite image and weather data in a case of cold air outbreak over warm ocean are analyzed to study every stage of the convective processes,such as cloud street, convective cell as well as their transformation.According to the theory the wavenumber compositions for cloud street and convective cell are calculated,respectively,on the basis of the atmospheric conditions at every stage.The distributions of vertical motions,convergent band and disturbed interface are obtained and compared with the cloud patterns in the convective processes. Thus the study seems to offer a likely explanation for the origin of large vortices in CBL.