Atmospheric boundary layer characteristics during the BOBMEX-Pilot experiment

The atmospheric boundary layer characteristics observed during the BOBMEX-Pilot experiment are reported. Surface meteorological data were acquired continuously through an automatic weather monitoring system and manually every three hours. High resolution radiosondes were launched to obtain the vertical thermal structure of the atmosphere. The study area was convectively active, the SSTs were high, surface air was warm and moist, and the surface air moist static energy was among the highest observed over the tropical oceans. The mean sea air temperature difference was about 1.25‡C and the sea skin temperature was cooler than bucket SST by 0.5‡C. The atmospheric mixed layer was shallow, fluctuated in response to synoptic conditions from 100 m to 900 m with a mean around 500 m.     

Numerical simulation of the marine boundary layer characteristics over the Bay of Bengal as revealed by BOBMEX-98 Pilot experiment

An attempt has been made to study the marine boundary layer characteristics over Bay of Bengal using BOBMEX (Bay of Bengal and Monsoon Experiment) pilot experiment data sets, which was conducted between 23rd October and 12th November 1998 on board ORV Sagar Kanya. A one-dimensional multilevel atmospheric boundary layer with TKE-ε closure scheme is employed to study the marine boundary layer characteristics. In this study two synoptic situations are chosen: one represents an active convection case and the other a suppressed convection. In the present article the marine boundary layer characteristics such as temporal evolution of turbulent kinetic energy, height of the boundary layer and the airsea exchange processes such as sensible and latent heat fluxes, drag coefficient for momentum are simulated during both active and suppressed convection. Marine boundary layer height is estimated from the vertical profiles of potential temperature using the stability criterion. The model simulations are compared with the available observations.     

The mean and turbulence structure simulation of the monsoon trough boundary layer using a one-dimensional model withe-l ande-ε closures

An attempt has been made here to study the sensitivity of the mean and the turbulence structure of the monsoon trough boundary layer to the choice of the constants in the dissipation equation for two stations Delhi and Calcutta, using one-dimensional atmospheric boundary layer model withe-ε turbulence closure. An analytical discussion of the problems associated with the constants of the dissipation equation is presented. It is shown here that the choice of the constants in the dissipation equation is quite crucial and the turbulence structure is very sensitive to these constants. The modification of the dissipation equation adopted by earlier studies, that is, approximating the Tke generation (due to shear and buoyancy production) in theε-equation by max (shear production, shear + buoyancy production), can be avoided by a suitable choice of the constants suggested here. The observed turbulence structure is better simulated with these constants. The turbulence structure simulation with the constants recommended by Aupoixet al (1989) (which are interactive in time) for the monsoon region is shown to be qualitatively similar to the simulation obtained with the constants suggested here, thus implying that no universal constants exist to regulate dissipation rate. Simulations of the mean structure show little sensitivity to the type of the closure parameterization betweene-l ande-ε closures. However the turbulence structure simulation withe-ε. closure is far better compared to thee-l model simulations. The model simulations of temperature profiles compare quite well with the observations whenever the boundary layer is well mixed (neutral) or unstable. However the models are not able to simulate the nocturnal boundary layer (stable) temperature profiles. Moisture profiles are simulated reasonably better. With one-dimensional models, capturing observed wind variations is not up to the mark.     

Conserved variable analysis of the marine boundary layer and air-sea exchange processes using BOBMEX-Pilot data sets

The present study is based on the observed features of the MBL (Marine Boundary Layer) during the Bay of Bengal and Monsoon Experiment (BOBMEX) — Pilot phase. Conserved Variable Analysis (CVA) of the conserved variables such as potential temperature, virtual potential temperature, equivalent potential temperature, saturation equivalent potential temperature and specific humidity were carried out at every point of upper air observation obtained on board ORV Sagar Kanya. The values are estimated up to a maximum of 4 km to cover the boundary layer. The Marine Boundary Layer Height is estimated from the conserved thermodynamic profiles. During the disturbed period when the convective activity is observed, the deeper boundary layers show double mixing line structures. An attempt is also made to study the oceanic heat budget using empirical models. The estimated short-wave radiation flux compared well with the observations.     

Marine boundary layer characteristics during a cyclonic storm over the Bay of Bengal

During the period 12–16 June 1996 a tropical cyclonic storm formed over the southwest Bay of Bengal and moved in a north-northeasterly direction. The thermodynamic characteristics of this system are investigated by utilizing the surface and upper air observations collected onboardORV Sagar Kanya over the Bay of Bengal region. The response of the cyclonic storm is clearly evident from the ship observations when the ship was within the distance of 600–800 km from the cyclonic storm. This study explores why (i) the whole atmosphere from surface to 500 hPa had become warm and moist during the cyclonic storm period as compared to before and after the formation of this system and (ii) the lower layer of the atmosphere had become stable during the formative stage of the cyclonic storm.     

Atmospheric boundary layer studies at Jodhpur during MONTBLEX using sodar and tower

A monostatic sodar was set up at Jodhpur, near the western end of the monsoon trough, to investigate the atmospheric boundary layer dynamics. A 30 m instrumented tower was also located close to the sodar antenna. Data were collected from June to August during the monsoon period of 1990, as also from July 1992 to September 1993. Thermal plumes, surface-based stable layers (both flat or short spiky top and tall spiky top), elevated/multi-layers with or without undulations and dot echo structures were seen; however, erosion of the morning inversion layer in the form of a rising layer with growing thermal plumes under it was rarely seen, and that too only during the winter period. The observed structure of the stable layer with tall spikes and its depth have been found to be correlated with the intensity of the monsoon spell; the dot echoes have been found to be correlated with the approach of a monsoon depression near Jodhpur; and the elevated/multilayers have been attributed to the formation of a subsidence (shear instability).     

Simulation of marine boundary layer characteristics using a 1-D PBL model over the Bay of Bengal during BOBMEX-99

The characteristic features of the marine boundary layer (MBL) over the Bay of Bengal during the southwest monsoon and the factors influencing it are investigated. The Bay of Bengal and Monsoon Experiment (BOBMEX) carried out during July–August 1999 is the first observational experiment under the Indian Climate Research Programme (ICRP). A very high-resolution data in the vertical was obtained during this experiment, which was used to study the MBL characteristics off the east coast of India in the north and south Bay of Bengal. Spells of active and suppressed convection over the Bay were observed, of which, three representative convective episodes were considered for the study. For this purpose a one-dimensional multi-level PBL model with a TKE-ε closure scheme was used. The soundings, viz., the vertical profiles of temperature, humidity, zonal and meridional component of wind, obtained onboard ORV Sagar Kanya and from coastal stations along the east coast are used for the study. The temporal evolution of turbulent kinetic energy, marine boundary layer height (MBLH), sensible and latent heat fluxes and drag coefficient of momentum are simulated for different epochs of monsoon and monsoon depressions during BOBMEX-99.The model also generates the vertical profiles of potential temperature, specific humidity, zonal and meridional wind. These simulated values compared reasonably well with the observations available from BOBMEX.     

Study of near surface boundary layer characteristics during pre-monsoon seasons using micrometeorological tower observations

Studying the boundary layer is imperative because severe weather in this portion of the atmosphere impacts on environment and various facets of national activities and affects the socioeconomic scenario of a region. Near surface boundary layer characteristics are investigated through the vertical variation of fluxes of heat, moisture, momentum, kinetic energy and Richardson number during the pre-monsoon season (April-May) at Kharagpur (22° 30’ N, 87° 20’ E) and Ranchi (23° 32’ N, 85° 32’ E) with 50 and 32 m tower data, respectively, on thunderstorm and non-thunderstorm days. The temporal variation of fluxes within the boundary layer and the kinetic energy at different logarithmic heights are observed to vary significantly between thunderstorm and non-thunderstorm days. The heat and momentum fluxes show a maximum peak while the moisture flux shows a sudden attenuation just before the occurrence of thunderstorms. The wind field depicts to play a crucial role at the inland station Kharagpur, which is in the proximity of the Bay of Bengal, compared to the station Ranchi, situated over hilly terrain on Chotanagpur. The micrometeorological study of the boundary layer reveals a significant finding pertaining to observe the passage of thunderstorms. It is observed that the ratio of the potential temperature (θ) and equivalent potential temperature (θe) remains confined within a critical range between 0.85 and 0.90 during the passage of thunderstorms.     

波流边界层水沙运动数值模拟——I:水动力模拟

波流边界层水动力模拟对研究波流相互作用和泥沙运动具有重要的理论意义和实践价值。开发了波流边界层1DV垂向一维水动力数值模型,可用于模拟漩涡沙波床面和平底床面水动力特征。模型的构建基于边界层控制方程,平底床面采用k-ε模型,沙波床面采用双层模型,提出了漩涡层和紊动扩散层交界面紊动动能和紊动耗散率表达式。试验资料验证表明,模型较好地模拟了波浪-水流-床面共同作用下的边界层水动力特征,包括波周期内不同相位流速分布、紊动动能、剪切应力等以及波致时均流速分布和波流相互作用下的时均流速分布等。根据所建模型,讨论了不同床面和波流组合条件下的水动力特征。该模型可为研究波流边界层内水动力特征提供工具。     

Sensitivity of tropical cyclone intensification to boundary layer and convective processes

This study examines the role of the parameterization of convection, planetary boundary layer (PBL) and explicit moisture processes on tropical cyclone intensification. A high-resolution mesoscale model, National Center for Atmospheric Research (NCAR) model MM5, with two interactive nested domains at resolutions 90 km and 30 km was used to simulate the Orissa Super cyclone, the most intense Indian cyclone of the past century. The initial fields and time-varying boundary variables and sea surface temperatures were taken from the National Centers for Environmental Prediction (NCEP) (FNL) one-degree data set. Three categories of sensitivity experiments were conducted to examine the various schemes of PBL, convection and explicit moisture processes. The results show that the PBL processes play crucial roles in determining the intensity of the cyclone and that the scheme of Mellor-Yamada (MY) produces the strongest cyclone. The combination of the parameterization schemes of MY for planetary boundary layer, Kain-Fritsch2 for convection and Mixed-Phase for explicit moisture produced the best simulation in terms of intensity and track. The simulated cyclone produced a minimum sea level pressure of 930 hPa and a maximum wind of 65 m s⁻¹ as well as all of the characteristics of a mature tropical cyclone with an eye and eye-wall along with a warm core structure. The model-simulated precipitation intensity and distribution were in good agreement with the observations. The ensemble mean of all 12 experiments produced reasonable intensity and the best track.     

Thermal and wind structure of the monsoon trough boundary layer

Radiosonde data from Jodhpur, taken at 0530, 1730 and around 1100 hr IST during MONTBLEX 1990, reveal that the distribution of virtual potential temperature0 _v below about 500 hPa has a structure characterized by up to three layers each of approximately constant gradient. We are thus led to introduce a characterization of the observed thermal structure through a sequence of the symbolsN, S andU, standing respectively for neutral, stable or unstable conditions in the different layers, beginning with the one closest to the ground. It is found that, of the 29 combinations possible, only the seven classes,S, SS′, SNS′, NS, NSS′, USS′ andUNS are observed, whereS′ stands for a stable layer with a different gradient of0 _r. than in the layerS. It is also found that, in 90% of the launches at 0530 hr, 48% of the launches at 1730 hr and 69% of the launches around 1100 hr, the first radiosonde layer near the ground is stable; the classical mixed layer was found in only 11 % of the data set analysed, and, if present on other occasions, must have been less than 250 m in height, the first level at which radiosonde data are available. Supplementing the above data, sodar echograms, available during 82% of the time between June and August 1990, suggest a stable layer up to a few tens of metres 48% of the time. A comparative study of the radiosonde data at Ranchi shows that the frequent prevalence of stability near the surface at Jodhpur cannot be attributed entirely to the large scale subsidence known to be characteristic of the Rajasthan area. Further, data at Jodhpur reveal a weak low level jet at heights generally ranging from 400 to 900 m with wind speeds of 6 to 15 m/s. Based on these results, it is conjectured that the lowest layers in the atmosphere during the monsoons, especially with heavy clouding or rain, may frequently be closer to the classical nocturnal boundary layer than to the standard convective mixed layer, although often with shallow plumes that penetrate such a stable layer during daytime.     

Development and propagation of a pollution gradient in the marine boundary layer during INDOEX (1999)

The development and propagation of a pollution gradient in the marine boundary layer over the Arabian Sea during the Intensive Field Phase of the Indian Ocean Experiment (1999) is investigated. A hypothesis for the generation of the pollution gradient is presented. Infrared satellite images show the formation of the pollution gradient as the leading edge of a polluted air mass in the marine boundary layer and also its propagation over the Arabian Sea and the northern Indian Ocean. Aerosol data measured from two research vessels over the Arabian Sea show a variation in the concentrations caused by the passage of this pollution gradient. Depth of the pollution gradient was found to be about 800 m. A numerical model was used to simulate the development of this gradient and its propagation over the ocean. Results show that its formation and structure are significantly influenced by the diurnal cycle of coastal sea-land breeze circulations along India’s west coast. Transport of aerosols and gases over the Arabian Sea in the lower troposphere from land sources appears to be through this mechanism with the other being the elevated land plume.     

波流边界层水沙运动数值模拟——II.泥沙运动模拟

为解析波流边界层内泥沙运动,建立了基于水动力-泥沙-床面互馈过程的波流边界层1DV泥沙数学模型,可用于模拟不同床面形态下粉沙-沙的含沙量过程。床面形态模块提供床面形态类型和相应参数;给出了平底和沙波床面粗糙高度和泥沙扩散系数的确定方法;采用了适宜粉沙及沙的制约沉速、底部参考浓度和起动剪切应力等公式;引入含沙量层化效应和制约沉降反映水动力与泥沙之间的相互影响。水槽试验资料验证表明,建立的模型较好地模拟了不同床面不同波流组合条件下的含沙量剖面。在此基础上,讨论了不同床面含沙量剖面模拟方法的差异,指出床面形态是决定含沙量变化的重要因素之一,仅通过改变床面粗糙高度不足以反映漩涡沙波床面的含沙量剖面特征。该模型可为研究波流边界层内泥沙运动和物质输运提供工具。     

弹性波数值模拟的混合边界与频散抑制

针对刚性边界条件、衰减边界条件与透射边界条件对弹性波场的影响及计算中产生的数值频散现象,将衰减边界与透射边界双重作用于人工边界,引入了混合边界条件,并在计算中采用了微量阻尼的方法,结果表明,混合边界条件能更好地吸收人工边界的反射;数值频散的阻尼算法能大大减弱由网格剖分、计算误差造成的频散现象。该方法可有效提高波场模拟精度。     

扩散边界层对沉积物盐分释放的阻滞影响

扩散边界层是水环境沉积物与上覆水之间物质交换的必经区。通过无风和有风条件下两组沉积物盐分释放实验,研究海湾水库砂质沉积物-水界面扩散边界层的形成情况,并通过水动力扰动下穿过扩散边界层的实测盐分通量与Fick第一定律理论计算通量的比较,以及扰动与无扰动时通量的比较,探讨扩散边界层对沉积物盐分释放的影响。研究结果表明,在稳定风场条件下,沉积物-水界面处存在厚度约3.5 cm的扩散边界层,有风条件下的界面盐分交换通量略大于无风时的通量;分子扩散是边界层内盐分运移的主要机制,扩散边界层对沉积物盐分释放具有阻滞作用,并且边界层越厚,阻滞效应越显著。另外,由于扩散边界层的存在,限制了水动力对沉积物与上覆水之间物质交换的增强作用,所以海湾水库沉积物中蓄积盐分对上覆水的影响将是长期的过程。     

土层随机地震反应分析中侧向边界的影响分析

研究了随机地震激励下土层地震反应及土-结构相互作用分析中有限土域的取值范围对计算精度的影响。分别以基岩白噪声谱和杜修力修正基岩谱作为输入,进行了土层随机地震反应分析,探讨了自由边界、固定边界和黏弹性边界3种人工边界条件下土域的取值范围、土介质阻尼和土体长深比（L/H）对土层随机地震反应的影响,给出了侧向人工边界合理位置的具体建议。在此基础上,选取土体的合理边界范围,建立土-结构相互作用分析模型,分别以基岩白噪声谱、杜修力修正基岩谱以及实测地震波功率谱作为输入,进行了土-结构相互作用体系随机地震反应分析,计算结果验证了侧向人工边界取值在土-结构相互作用体系随机地震反应分析中的有效性。     

广义塑性梯度模型的变分原理和边界条件

在广义塑性力学的双屈服面模型的体积屈服面和剪切屈服面中引进应变梯度项,构造梯度依赖的双屈服面,可以建立广义塑性梯度模型的理论框架,此时的位移率 、剪切塑性乘子率 、体积塑性乘子率 成了各自独立的变量。为了建立相应的数值模型,构造了增量泛函,建立并证明了3类变量( , , )变分原理,得到应力边界条件,同时塑性乘子作为自然边界条件给出。     

Numerical simulation of desiccation cracking in a thin clay layer using 3D discrete element modeling

Desiccation cracking of clay soil is of critical importance in many applications, such as industrial waste containment, hydraulic barriers, road embankments, and agricultural operations. The factors that influence cracking are known qualitatively, but it is not clear how to predict the initiation and propagation of cracks. This study presents a discrete element approach to modeling desiccation cracking in thin clay layers, considering material property changes. First, an aggregate shrinkage model based on the aggregate structure of clay was proposed, and the drying shrinkage of clay soil was modeled by imposing drying shrinkage kinetics for each aggregate at the micro-scale. Second, the clay soil was represented by an assembly of aggregates linked by bonds, and desiccation cracking of the clay layer was modeled using a three-dimensional discrete element code (PFC3D), with the aid of the embedded programming language FISH. When the clay layer is sufficiently thin, the water content gradient along the section can be neglected; thus, the shrinkage kinetics are the same for all of the grains of clay. In the model based on the discrete element method (DEM), the bond strength and contact stiffness changed during drying. Their changes were determined by matching the simulation results with the experimental data. Third, the DEM approach was validated by reproducing experimental desiccation tests performed on a thin clay layer in a disk shape. The geometric parameters of surface cracks were quantified using image analysis techniques and were compared with experimental observations. Fourth, some factors of influence, such as the sample thickness, the properties of the soil–base interface, micro-mechanical parameters, and shrinkage parameters, were investigated using the DEM model. The results obtained from the DEM analyses were compared with the results of prior research in this field of study. The approach used in this study is very promising for simulating desiccation cracking in thin clay soil because the model captures the initiation and propagation mechanism of desiccation cracks. Although this study was carried out on surface cracking in a thin clay layer, the extension of this methodology is of potential benefit not only for predicting three-dimensional desiccation cracking in real clay liners but also for modeling cracking in other materials with properties that vary with water content or temperature, such as concrete and rock.     

A model study on the decay of volcanic aerosol layer and verification with Pinatubo and El Chichon data

The time evolution of stratospheric aerosol layer formed after a volcanic eruption is studied taking into account the aerosol microphysical processes of growth, coagulation and sedimentation. Using a simple model we could explain the observed evolution of the Pinatubo volcanic layer which decayed in about 3 years. The experimental data obtained by Nd:YAG backscatter lidar over Ahmedabad further supports this finding. The data obtained after the El Chichon volcanic eruption also showed that the El Chichon aerosol layer decayed in about 3 years time. Thus, though the amount of SO₂ injected has been higher, in the case of Pinatubo, about two to three times more than El Chichon, it has resulted in the production of larger aerosol particles due to faster growth and coagulation processes, and subsequently a faster removal rate, to give more or less a similar background aerosol amount at the stratosphere in about 3 years time.     

古尔班通古特沙漠及周边区域冬季大气边界层高度对地表积雪的响应

古尔班通古特沙漠是中国唯一冬季存在长期积雪的沙漠,在此特殊地理环境下,沙漠及周边区域冬季雪深和边界层高度的时空变化特征和相互关系尚未明确。本文利用1980—2019年SMMR（Scanning Multichannel Microwave Radiometer）、SSM/I（Special Sensor Microwave/Imager）、SSMI/S（Special Sensor Microwave Imager/Sounder）被动微波遥感雪深数据、古尔班通古特沙漠腹地雪深观测数据和ERA5再分析资料（the Fifth Generation ECMWF Reanalysis）边界层高度数据,分析了沙漠及周边区域冬季雪深和边界层高度的时空变化特征与相互关系。结果表明：古尔班通古特沙漠及周边区域冬季雪深年均值为8.45 cm,整体呈现东北部和南部积雪较深,其他区域积雪较浅并呈现出由沙漠中心区域向四周逐渐减少的特点,雪深在古尔班通古特沙漠及其东北、南边的邻近区域呈升高趋势,剩余地区呈下降趋势。古尔班通古特沙漠及周边区域冬季边界层高度年均值为105.54 m,呈现东南部和西北部高,中心沙漠区域、东北部、西南部较低的特点,边界层高度在沙漠及周边区域升高而其他区域降低。古尔班通古特沙漠的冬季雪深和大气边界层高度时空变化整体呈负相关,其中93.17%以上的沙漠区域呈负相关,平均相关系数为-0.32,最大相关系数绝对值为-0.58,空间相关系数为-0.42（P<0.05）。