Convective boundary layer in the region of the Monsoon Trough—A case study

A case study of the convectively driven monsoon boundary layer has been carried out using the aerological ob-servations at four stations in the region of monsoon trough during Monsoon Trough Boundary Layer Experiment (MONTBLEX) 1988. The Convective Boundary Layer (CBL) in the region of monsoon trough did not show double mixing line structure. A single mixing line representing the CBL with different stabilities with respect to the convective activities was observed.     

南海季风建立前后珠江三角洲的陆气热量交换与热力边界层结构特征

根据动力与热力指标,2004和2005年南海季风建立前后可分成明显的4个阶段——季风建立前的雨期、非雨期;季风建立后的活跃期与非活跃期。对2004和2005年南海季风建立前后的广州番禺综合外场观测资料进行分析,得到了这4个阶段陆气热量交换与热力边界层的主要特征:净辐射与净短波辐射的变化趋势基本一致,净短波辐射与净长波辐射之比为3.49—4.81,净短波辐射是净辐射的主要贡献项,云量与降水是控制净短波辐射与净辐射的直接因素;季风活跃期间午间对流云系对太阳辐射衰减显著,造成了辐射各分量以及热通量的峰值区变窄,量值急剧变小;季风建立前后感热与潜热均是净辐射的主要消耗项,占净辐射的90%以上,潜热明显大于感热,2005年较2004年潜热的分配额有明显的增加,其原因可能与近地层的风速较大,总是维持向上的湿度梯度有关;季风建立前后除季风活跃期外边界层位温结构均具有明显的日变化特征,午间混合层可发展至1070m,而季风活跃期间午间混合层发展受到对流云释放潜热的抑制,导致季风活跃期混合层消失的现象,分析还发现季风建立前后各阶段夜间残余混合层均不明显。分析表明引起陆气能量过程及边界层热力结构差异的关键因素之一是云系与降水,加强边界层过程与降水宏微观过程相互作用的研究是深入认识陆气过程与边界层结构特征的关键。     

由非季风区向季风区过渡过程中大气边界层结构的变化分析

利用中国西北中部具有代表性的非季风区、夏季风影响过渡区和季风区的7个高空站的2013年夏季晴天07时、13时、19时（北京时）的大气边界层资料,通过分析大气边界层位温、比湿、风速的垂直结构,发现大气边界层结构及厚度在不同区域的分布特征:稳定边界层厚度、残余层顶高度和对流边界层厚度从非季风区、夏季风影响过渡区至季风区出现阶梯性大幅降低,从非季风区至夏季风影响过渡区,以及从夏季风影响过渡区至季风区,对流边界层厚度降幅依次为25.6%和81.8%,稳定边界层厚度降幅依次为58.3%和41.8%;在稳定边界层条件下,可观察到低空急流的存在,非季风区低空急流出现高度明显高于夏季风影响过渡区和季风区,且非季风区的低空急流风速也明显大于夏季风影响过渡区和季风区。通过分析与大气边界层发展最为密切的陆面热力因素在不同气候区的分布,净辐射值、日地-气温差最大值以及感热通量值在非季风区大于夏季风影响过渡区和季风区,从陆面热力过程为非季风区大气边界层厚度大于夏季风影响过渡区和季风区提供了理论依据。     

利用TWP-ICE试验资料对比两种边界层参数化方案

利用高分辨率WRF单气柱模式,选取了两种边界层参数化方案(YSU,MYJ),对TWP-ICE(Tropical Warm Pool International Cloud Experiment)试验期间的个例进行数值模拟,比较了两种方案对边界层结构、云和降水模拟的影响。结果表明:季风活跃期,YSU方案模拟的湍流交换系数较小,湍流混合偏弱,边界层内热通量偏小,使地表热量和水汽不易向上输送,水汽含量在近地表明显偏多,而在边界层及其以上大气层具有显著的干偏差,因此该方案模拟的云中液态水和固态水含量偏低,云量偏少,降水率偏小;MYJ方案对于季风活跃期的边界层结构具有较好的模拟能力,其模拟的云和降水更为准确。季风抑制期,MYJ方案模拟的夜间边界层结构存在较大误差,这是因为该方案模拟的夜间湍流交换系数较大,湍流混合偏强,边界层内热通量偏大,模拟的位温和水汽混合比在边界层内随高度变化较小,而观测廓线在边界层内存在较大梯度。季风抑制期两种方案模拟的云和降水均比观测值偏多,方案之间的差异较小。     

Multi-year Simulations and Experimental Seasonal Predictions for Rainy Seasons in China by Using a Nested Regional Climate Model (RegCM_NCC). Part Ⅰ: Sensitivity Study

A modified version of the NCAR/RegCM2 has been developed at the National Climate Center (NCC), China Meteorological Administration, through a series of sensitivity experiments and multi-year simulations and hindcasts, with a special emphasis on the adequate choice of physical parameterization schemes suitable for the East Asian monsoon climate. This regional climate model is nested with the NCC/IAP (Institute of Atmospheric Physics) T63 coupled GCM to make an experimental seasonal prediction for China and East Asia. The four-year (2001 to 2004) prediction results are encouraging. This paper is the first part of a two-part paper, and it mainly describes the sensitivity study of the physical process parameterization represented in the model. The systematic errors produced by the different physical parameterization schemes such as the land surface processes, convective precipitation, cloud-radiation transfer process, boundary layer process and large-scale terrain features have been identified based on multi-year and extreme flooding event simulations. A number of comparative experiments has shown that the mass flux scheme (MFS) and Betts-Miller scheme (BM) for convective precipitation, the LPMI (land surface process model I) and LPMII (land surface process model Ⅱ) for the land surface process, the CCM3 radiation transfer scheme for cloud-radiation transfer processes, the TKE (turbulent kinetic energy) scheme for the boundary layer processes and the topography treatment schemes for the Tibetan Plateau are suitable for simulations and prediction of the East Asia monsoon climate in rainy seasons. Based on the above sensitivity study, a modified version of the RegCM2 (RegCM_NCC) has been set up for climate simulations and seasonal predictions.     

Multi-year Simulations and Experimental Seasonal Predictions for Rainy Seasons in China by Using a Nested Regional Climate Model （RegCM_NCC）. Part Ⅰ： Sensitivity Study

A modified version of the NCAR/RegCM2 has been developed at the National Climate Center （NCC）, China Meteorological Administration, through a series of sensitivity experiments and multi-year simulations and hindcasts, with a special emphasis on the adequate choice of physical parameterization schemes suitable for the East Asian monsoon climate. This regional climate model is nested with the NCC/IAP （Institute of Atmospheric Physics） T63 coupled GCM to make an experimental seasonal prediction for China and East Asia. The four-year （2001 to 2004） prediction results are encouraging. This paper is the first part of a two-part paper, and it mainly describes the sensitivity study of the physical process paraxneterization represented in the model. The systematic errors produced by the different physical parameterization schemes such as the land surface processes, convective precipitation, cloud-radiation transfer process, boundary layer process and large-scale terrain features have been identified based on multi-year and extreme flooding event simulations. A number of comparative experiments has shown that the mass flux scheme （MFS） and Betts-Miller scheme （BM） for convective precipitation, the LPMI （land surface process model I） and LPMII （land surface process model Ⅱ） for the land surface process, the CCM3 radiation transfer scheme for cloud-radiation transfer processes, the TKE （turbulent kinetic energy） scheme for the boundary layer processes and the topography treatment schemes for the Tibetan Plateau are suitable for simulations and prediction of the East Asia monsoon climate in rainy seasons. Based on the above sensitivity study, a modified version of the RegCM2 （RegCM_NCC） has been set up for climate simulations and seasonal predictions.     

Model evaluation of the atmospheric boundary layer and mixed-layer evolution

In the present study, an attempt is made to assess the atmospheric boundary-layer (ABL) depth over an urban area, as derived from different ABL schemes employed by the mesoscale model MM5. Furthermore, the relationship of the mixing height, as depicted by the measurements, to the calculated ABL depth or other features of the ABL structure, is also examined. In particular, the diurnal evolution of ABL depth is examined over the greater Athens area, employing four different ABL schemes plus a modified version, whereby urban features are considered. Measurements for two selected days, when convective conditions prevailed and a strong sea-breeze cell developed, were used for comparison. It was found that the calculated eddy viscosity profile seems to better indicate the mixing height in both cases, where either a deep convective boundary layer develops, or a more confined internal boundary layer is formed. For the urban scheme, the incorporation of both anthropogenic and storage heat release provides promising results for urban applications.     

对流边界层的大涡模拟研究

本文建立了一个均匀平坦地面上对流边界层的大涡模式,模式考虑了水汽,采用了考虑浮力和固壁影响订正的一阶闭合。并用所建模式进行了由热扰动发展的对流边界层的模拟及其对地表热状况变化响应的初步探讨性模拟工作。通过模拟认为,模式较好地反映了对流边界层的主要结构。     

Evidence Of Dynamical Coupling Between The Residual Layer And The Developing Convective Boundary Layer

The diurnal cycle of the atmospheric boundary layer (ABL) hasbeen documented on 8 August 1998 in the framework of the Étude et Simulation de la QUalité de l'air en Ile-de-France (ESQUIF) experiment that took place in the Paris area. The ABL structure was documented by means of a ground-based lidar, surface meteorological stations and soundings. The interaction between the residual layer and the convective boundary layer is investigated using the collected data as well as mesoscale modelling. As opposed to the generally accepted concept, we find evidence of entrainment at the top ofthe residual layer. High temporal simulations of the 8 August 1998 casemade with the mesoscale atmospheric model Meso-NH also show evidenceof mixing at the top of the residual layer (RL). This mixing is believed to be related to the presence of convective (gravity) waves in the RL.     

Mechanisms controlling seasonal mixed layer temperature and salinity in the Southwestern Tropical Indian Ocean

We use a heat- and salt-conserving ocean state estimation product to study the seasonal cycles of the mixed layer (ML) temperature (MLT) and salinity (MLS) balances over the southwestern tropical Indian Ocean (SWTIO) thermocline ridge (STR; 50°–75°E, 12°–5°S). For seasonal MLT, surface heat flux and ocean processes are both important. They tend to re-enforce each other during peak cooling (May–June) and warming (November) periods, but not during transition periods. The dominant ocean process is wind-driven vertical mixing. It is modulated by the variable strength of the monsoon winds (which affect the vertical diffusivity), and to a lesser extent by variability of thermocline depth (which influences the vertical stratification across the ML base). For example, thermocline shoaling in April–July alters the vertical stratification near the ML base; thus, when the monsoon winds heighten (June–September) and the vertical diffusivity increases (deepening the ML base), relatively cool subsurface water is near the ML base and easily incorporated into the ML by vertical mixing. However, vertical advection as a direct response to thermocline shoaling has little affect on MLT. This explains why MLT and thermocline depth are not positively correlated here on the seasonal timescale (as they are on the interannual timescale). Meridional advection associated with Ekman transport driven by the monsoon winds plays a secondary role. Seasonal MLS, however, is dominated by meridional advection. Vertical process effects on MLS are small, due to a weak salinity gradient near the ML base throughout the year.     

Summertime Mixing Heights At Vienna, Austria, Estimated from Vertical Soundings and by a Numerical Model

Mixing heights calculated by the Danish OML meteorological pre-processor are compared to those diagnosed from radio- and tether-sonde vertical potential temperature profiles. All methods give reliable estimates of noon mixing heights deduced from radiosoundings, especially when the boundary layer is fully convective. Differences are larger during convective conditions without a well-defined capping inversion in the radiosonde potential temperature profile or when OML calculates a mechanical mixing height. The OML model is also able to calculate the daily course of the mixing height as expected. The tethersonde-derived mixing heights are especially valuable during the morning rise of the elevated inversion. Modifications to all three methods to improve mixing-height predictions are discussed.     

Mixing-height estimation in the convective boundary layer using sodar data

Sodar has been used for about 20 years to determine mixing height. However, estimation of the height of a convective boundary layer (CBL) that exceeds the sodar-probing range is still an unsolved question. As one possible way, it is suggested that one adapt a simple mixed-layer model to sodar observations during the morning growth period of the CBL, when its top can be clearly detected. Results are compared with other methods for CBL-height estimation from sodar data that have been proposed in the literature. Finally, some prognostic aspects are discussed.     

Forward-in-time and Backward-in-time Dispersion in the Convective Boundary Layer: the Concentration Footprint

The turbulence field obtained using a large-eddy simulation model is used to simulate particle dispersion in the convective boundary layer with both forward-in-time and backward-in-time modes. A Lagrangian stochastic model is used to treat subgrid-scale turbulence. Results of forward dispersion match both laboratory experiments and previous numerical studies for different release heights in the convective boundary layer. Results obtained from backward dispersion show obvious asymmetry when directly compared to results from forward dispersion. However, a direct comparison of forward and backward dispersion has no apparent physical meaning and might be misleading. Results of backward dispersion can be interpreted as three-dimensional or generalized concentration footprints, which indicate that sources in the entire boundary layer, not only sources at the surface, may influence a concentration measurement at a point. Footprints at four source heights in the convective boundary layer corresponding to four receptors are derived using forward and backward dispersion methods. The agreement among footprints derived with forward and backward methods illustrates the equivalence between both approaches. The paper shows explicitly that Lagrangian simulations can yield identical footprints using forward and backward methods in horizontally homogeneous turbulence.     

Simulation Study of Cloud Properties Affected by Heterogeneous Nucleation Using the GRAPES＿SCM during the TWP-ICE Campaign

This study used the Global/Regional Assimilation and PrEdiction System Single-Column Model(GRAPES_SCM)to simulate monsoon precipitation with deep convective cloud and associated cirrus during the Tropical Warm Pool International Cloud Experiment(TWP-ICE), especially during the active and suppressed monsoon periods. Four cases with different heterogeneous nucleation parameterizations were simulated by using the ensemble method. All simulations clearly separated the active and suppressed monsoon periods, and they reproduced the major characteristics of monsoonal cloud such as the total cloud hydrometeor mixing ratio distribution,and precipitation and radiation properties. The results showed that the number concentration production rate of different heterogeneous nucleation parameterizations varied substantially under the given temperature and water vapor mixing ratio. However, ice formation and precipitation during the monsoon period were affected only slightly by the different heterogeneous nucleation parameterizations. This study also captured clear competition between different ice formation processes.     

Nonlocal Turbulent Mixing Based on Convective Adjustment Concepts (Ntac)

A nonlocal turbulent mixing parameterization is introduced in this study and denoted by the acronym NTAC, which stands for Nonlocal parameterization of Turbulent mixing using convective Adjustment Concepts. NTAC uses the average value of quantities in the turbulent domain in much the same way that local convective adjustment schemes use the average potential temperature. Averages are determined in the region with non-convective turbulence using information from the two end layers (denoted by TLA, Two Layer Average), while all layers contribute to the average in regions with convective turbulence (denoted by CLA, Convective Layer Average). The NTAC parameterization estimates the mixing percentage and uses this percentage as a mixing coefficient. These percentages are determined from a simplified turbulent kinetic energy equation. The scheme is versatile, conservative, and when programmed efficiently the proposed parameterization is a computationally acceptable nonlocal procedure that can be used in many existing numerical weather prediction forecast models.Numerical weather forecast model simulations using the NTAC parameterization and traditional K-theory are compared against radiosonde data. The accuracy of the proposed NTAC parameterization is found to be competitive with K theory. The greatest improvement of the NTAC over K-theory occurs during the daytime and early nighttime hours when (dry) convective activity is high. Also, areal cloud coverage is increased by the NTAC parameterization. Our findings show that the greatest nonlocal vertical mixing occurs between the layer nearest the earth's surface and the remaining layers making up the planetary boundary layer.     

Downwind non-uniform mixing in shoreline fumigation processes

The assumption of vertical uniform mixing in shoreline fumigation models is tested using two types of modifications to a base statistical model that takes into account non-uniform mixing. One of the modifications involves the use of a downdraft velocity scheme developed for a convective boundary layer. The other modification is based on an empirical adjustment factor derived from water tank experiments. Results from all three models are compared with field observations. Comparisons indicate that the modifications do not improve the results of the base dispersion model.     

Thermal Structure of a Coastal–Urban Boundary Layer

We use various temperature profilers located in and around New York City to observe the structure and evolution of the thermal boundary layer. The primary focus is to highlight the spatial variability of potential-temperature profiles due to heterogeneous surface forcing in an urban environment during different flow conditions. Overall, the observations during the summer period reveal the presence of thermal internal boundary layers due to the interaction between the marine atmospheric boundary layer and the convective urban environment. The summer daytime potential-temperature profiles within the city indicate a superadiabatic layer is present near the surface beneath a mildly stable layer. Large spatial variability in the near-surface (0–300 m) potential temperature is detected, with the thermal profile in the lower atmosphere uniquely determined by the underlying surface forcing and the distance from the coast. The summer and winter average night-time potential-temperature profiles show that the atmosphere is still convective near the surface. The seasonal averages of mixing ratio show large variability in the vertical direction.     

AN EVALUATION OF SEVERAL TURBULENCE SCHEMES FOR THE PREDICTION OF MEAN AND TURBULENT FIELDS IN COMPLEX TERRAIN

A prognostic three-dimensional mesoscale model has been developed andused in one- and two-dimensional modes to evaluate ten local turbulenceclosure schemes. The schemes ranged from first-order to the two-equationprognostic schemes. Predictions by the models were compared for aone-dimensional convective boundary layer using mixed layer scaling andmeasurements to interpret the results. Two-dimensional simulations were alsoperformed for a sea-breeze flow and for flow over a hill. The results showedthat for all of the models considered, minor differences were produced in themean meteorological fields and in the vertical scalar fluxes, but majordifferences were apparent in the velocity variances and dissipation rate.Predicted tracer concentrations were very sensitive to the turbulence modelformulation for dispersion from a point source in the convective boundarylayer, particularly for the prediction of maximum concentrations. Predictedtracer concentrations from a surface volume source for the two-dimensionalsimulations were similar for all models, although the degree of mixing in themorning growth period produced some differences. Generally, good results forthe mean meteorological fields can be obtained with first-order schemes, evenif they underpredict the magnitude of turbulence in the convective boundarylayer, and reasonable tracer concentrations can also be obtained with thesemodels provided near-source effects are not important. The two-equationprognostic models performed best for the prediction of turbulence in theconvective boundary layer.     

Aircraft and ship observations of the mean structure of the marine boundary layer over the Arabian Sea during MONEX 79

Analysis of the mean wind, equivalent potential temperature and virtual potential temperature profiles observed by the National Center for Atmospheric Research (NCAR) Electra aircraft and obtained from dropwindsondes and ship-launched radiosondes were made in conjunction with synoptic observations to study the structure of the monsoon boundary layer over the Arabian Sea during MONEX 79. Comparison of mean profiles indicates the monsoon boundary layer to be much different from the trade wind boundary layer. Results confirm the existence of a boundary-layer jet known as East African or Somali Jet. Regions of multiple cloud layers at roughly the height of the capping inversion layer were associated with the jet. Regions in which a more well-mixed layer was observed showed a jet structure depressed in height. A free-jet surface-layer model appears to describe the mean wind structure of this jet observed during the present study and by others. An approximate balance of forces was found in the monsoon boundary layer between friction, advective acceleration, Coriolis and pressure gradient forces. Friction and advective acceleration terms were significant in the lower levels of the boundary layer. Forces in a typical trade wind boundary layer were found to be approximately one order of magnitude smaller than those observed in the monsoon boundary layer.     

Effects of atmospheric boundary layer mixing representations on vertical distribution of passive and reactive tracers

Summary A one-dimensional chemistry-boundary layer model was used to study the effects of differing representations of atmospheric boundary layer (ABL) processes on simulated concentrations of passive and chemically reactive tracers. Two local- and two nonlocal-closure ABL schemes were used to perform numerical simulations during convective conditions in the ABL. Observational data from the First International Statellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) were used to provide initial meteorological conditions while representative chemical concentrations and surface and elevated emission rates were used to provide initial chemical conditions and chemical sources to the one-dimensional model. Two sets of numerical simulations were performed using the four ABL schemes. The first set simulated bottom-to-top mixing characteristics, and the second set simulated top-to-bottom mixing characteristics. Model simulations were performed for 12h starting from 0700LT 11 July 1987.Our analysis indicate that the simulated concentrations of both passive and reactive chemical species were sensitive to the type of ABL scheme used to represent turbulent mixing processes. Characteristic features associated with each scheme (e.g., growth and intensity of mixing in the ABL) contributed to the differences among the simulated species concentrations. For some of the chemical species these differences were large, particularly in the surface layer and in the interfacial layers of the ABL. In turn, differences caused by the differing mixing representations resulted in different chemical production/destruction rates. As a consequence, the simulated species concentrations differed among the simulations. We also found that chemical species concentrations were more sensitive to the type of ABL scheme in the bottom-to-top mixing simulations than in the top-to-bottom simulations.With 10 Figures