A numerical investigation of the JASIN atmospheric boundary layer

A numerical model of the cloudy marine boundary layer is described and used to investigate the role of entrainment instability on the developing boundary layer. In general, previous studies have been limited to boundary layers capped by convectively stable inversions or have described only cumulus fields. Here we extend a stratus-capped boundary-layer model to consider the transition to a convectively unstable cloud layer capped by an inversion across which there is a rapid decrease in wet-bulb or equivalent potential temperature. In this case, the inversion is very active and the entrainment rate is determined by the active instability at the interface, in contrast to the mean turbulent motion within the boundary layer.The model is used to interpret the observed boundary layer from the JASIN experiment. Cool, dry air is modified by prolonged passage over increasingly warmer ocean which leads to the development of a convectively unstable cloud layer.     

‘Evolution of a Storm-driven Cloudy Boundary Layer in the Arctic’

To investigate the processes of development and maintenance of low-level clouds during major synoptic events, the cloudy boundary layer under stormy conditions during the summertime Arctic has been studied using observations from the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment and large-eddy simulations (LES). On 29 July 1998, a stable Arctic cloudy boundary-layer event was observed after the passage of a synoptic low pressure system. The local dynamic and thermodynamic structure of the boundary layer was determined from aircraft measurements including the analysis of turbulence, cloud microphysics and radiative properties. After the upper cloud layer advected over the existing cloud layer, the turbulent kinetic energy (TKE) budget indicated that the cloud layer below 200 m was maintained predominantly by shear production. Observations of longwave radiation showed that cloud-top cooling at the lower cloud top has been suppressed by radiative effects of the upper cloud layer. Our LES results demonstrate the importance of the combination of shear mixing near the surface and radiative cooling at the cloud top in the storm-driven cloudy boundary layer. Once the low-level cloud reaches a certain height, depending on the amount of cloud-top cooling, the two sources of TKE production begin to separate in space under continuous stormy conditions, suggesting one possible mechanism for the cloud layering. The sensitivity tests suggest that the storm-driven cloudy boundary layer is possibly switched to the shear-driven system due to the advection of upper clouds or to the buoyantly driven system due to the lack of wind shear. A comparison is made of this storm-driven boundary layer with the buoyantly driven boundary layer previously described in the literature.     

Measurements Of Fine-Scale Structure At The Top Of Marine Stratocumulus

During the Dynamics and Chemistry of the MarineStratocumulus (DYCOMS) experiment in July–August 1985, the NCAR Electra aircraft flew a series of flight legs just at the top of the marinestratocumulus cloud decks that cap the mixed layer off the coast of southernCalifornia. Because of the corrugated structure of the cloud-top, the aircraft, which was flown at a nearly constant level and adjusted only to maintain its altitude at the average cloud-top height, was alternately within and above the clouds – roughly half the time in each domain. These legs were used to examine the structure of the cloud-top by compositing the segments on either side of the cloud/clear-air interface, which was identified by the transitions of liquid water measured by the Forward Scattering Spectrometer Probe (either increasing or decreasing) through a threshold of 0.04 × 10^-3 kg m^-3.An equivalent vertical distance (EVD) from the cloud-top was obtained from the horizontal flight legs by estimating the average slope of the cloud-top from the cloud-top radiation temperature. The results show that a near discontinuity occurs in variables across cloud top over an EVD of 0.3 m, but that above this, the air has already been modified by boundary-layer air. Thus, cloud-top is not the limit of mixing of boundary-layer air. This mixing may extend to tens of metres or more. The bulk Richardson number in the vicinity of cloud-top increases from near zero within the cloud to about 1.2 at an EVD of 3–6 m above cloud. Fluctuations of the three velocity components within cloud are nearly equal; above cloud the vertical component structure function is about half the horizontal components. The scalar structure functions are about an order of magnitude higher above cloud than in cloud. The structure parameters of temperature and humidity measured just below cloud-top agree reasonably well with predicted values based on a previously-developed model for the clear convective boundary layer. Above cloud, the scalar structure parameters are much larger, but their interpretation is questionable, since this region does notcontain isotropic turbulence.     

Stratocumulus-capped mixed layers derived from a three-dimensional model

Results of a three-dimensional numerical model are analysed in a study of turbulence and entrainment within mixed layers containing stratocumulus with or without parameterized cloud-top radiative cooling. The model eliminates most of the assumptions invoked in theories of cloud-capped mixed layers, but suffers disadvantages which include poor resolution and large truncation errors in and above the capping inversion.For relatively thick mixed layers with relatively thick capping inversions, the cloud-top radiative cooling is found to be lodged mostly within the capping inversion when the cooling is confined locally to the upper 50 m or less of the cloud. It does not then contribute substantially towards increased buoyancy flux and turbulence within the well mixed layer just below.The optimal means of correlating the entrainment rate, or mixed-layer growth rate, for mixed layers of variable amounts of stratocumulus is found to be through functional dependence upon an overall jump Richardson number, utilizing as scaling velocity the standard deviation of vertical velocity existing at the top of the mixed layer (near the center of the capping inversion). This velocity is found to be a fraction of the generalized convective velocity for the mixed layer as a whole which is greater for cloud-capped mixed layers than for clear mixed layers.     

A one-dimensional simulation of the stratocumulus-capped mixed layer

A marine stratocumulus model has been developed which has four major sub-models: (1) a one-dimensional version of the CSU cumulus model, (2) a partially-diagnostic higher-order turbulence model, (3) an atmospheric radiation model for both short-wave and long-wave radiation, and (4) a partial condensation scheme and cloud fractional parameterization. A set of numerical experiments have been performed to study the interactions among the turbulence, the long-wave radiation, the short-wave radiation, and the sub-grid condensation processes. The results indicate that surface sensible eddy heat flux and not radiative cooling is the major control on the rate of cloud-top entrainment. Cloud-top radiation cooling occurs principally within the upper part of the mixed layer. However, for the stratocumulus with numerous towers penetrated into the capping inversion, most of the long-wave radiation occurs within the capping inversion. It is found that cloud-top radiation cooling is balanced by turbulence transport of sensible heat from cloud-base levels.     

Current problems in the stratocumulus-topped atmospheric boundary layer

Extended sheets of stratocumulus (Sc) in the upper part of the atmospheric boundary layer (ABL) often occur under appropriate meteorological conditions. These cloud decks are important both in climate studies and in weather forecasting. We review the current knowledge of the turbulent structure of the ABL capped by a cloud deck, in the light of recent observations and model studies. The most important physical processes determining this structure are longwave radiative cooling at cloud top, shortwave radiative wanning by absorption in the cloud, surface buoyancy flux, and wind shear in the ABL. As a result, turbulence can cause entrainment against the buoyancy jump at cloud top. In cases where only longwave radiative fluxes and surface buoyancy fluxes are important, the turbulent structure is relatively well understood. When shortwave radiative fluxes and/or wind shear are also important, the resulting turbulent structure may change considerably. A decoupling of the cloud from the sub-cloud layer or of the top of the cloud from the rest of the ABL is then regularly observed. In no cases are the details of the entrainment at cloud top understood well enough to derive a relatively simple formulation that is consistent with observations. Cloud-top entrainment instability may lead to the break-up of a cloud deck (but also to cloud deepening). The role of mesoscale circulations in determining fractional cloudiness is not yet well understood.     

Do stratocumulus clouds detrain? FIRE I data revisited

Analyses of aircraft observations of the stratocumulus-topped boundary layer during the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE I) show the frequent presence of clear, but relatively moist, air patches near the stratocumulus cloud-top interface. A conditional sampling of measurements in these clear air patches shows that their thermodynamic properties do more resemble boundary-layer air characteristics than those of free troposphere air. From an aircraft leg through cloud tops it is demonstrated that turbulent mixing across the cloud-top interface can lead to the local dissipation of the cloud top. Analogous to the terminology used for shallow cumulus parameterizations this process can be considered as detrainment, with which we mean that after a mixing event across the cloud-top boundaries, mixed unsaturated parcels become part of the clear environment of the cloud.     

A treatment for the stratocumulus-to-cumulus transition in GCMs

Numerical models of climate have great difficulties with the simulation of marine low clouds in the subtropical Pacific and Atlantic Oceans. It has been especially difficult to reproduce the observed geographical distributions of the different cloud regimes in those regions. The present study discusses mechanisms proposed in previous works for changing one regime into another. One criterion is based on the theory of stratocumulus destruction through cloud top entrainment instability due to buoyancy reversal—situations in which the mixture of two air parcels becomes denser than either of the original parcels due to evaporation of cloud water. Another criterion is based on the existence of decoupling in the boundary layer. When decoupled, the stratocumulus regime changes to another in which these clouds can still exist together with cumulus. In a LES study, the authors have suggested that a combination of those two criteria can be used to diagnose whether, at a location, the cloud regime corresponds to a well-mixed stratocumulus regime, a shallow cumulus regime, or to a transitional regime where the boundary layer is decoupled. The concept is tested in the framework of an atmospheric general circulation model (GCM). It is found that several outstanding features of disagreement between simulation and observation can be interpreted as misrepresentations of the cloud regimes by the GCM. A novel criterion for switching among regimes is proposed to alleviate the effects of these misrepresentations.     

Impact of Turbulent Mixing in the Stratocumulus-Topped Boundary Layer on Numerical Weather Prediction

The impact of enhanced turbulent mixing induced by radiative cooling at the top of the stratocumulus-topped boundary layer (STBL) on numerical weather prediction is examined. An additional term involving top-down turbulent mixing via in-cloud radiative cooling is applied to the Yonsei University (YSU) planetary boundary layer (PBL) parameterization scheme using a top-down diffusivity profile and cloud-top entrainment. The modified scheme is evaluated in an advection fog case over the Yellow Sea of Korea using the Weather Research and Forecasting (WRF) model and in global medium-range forecasts using the Global/Regional Integrated Model system (GRIMs). In the fog case simulation, consideration of the additional top-down mixing parameterization in the YSU PBL simulates less formation and more rapid dispersion of the fog. As a result, the modified scheme simulates a drier and warmer boundary layer and a moister and cooler layer above the PBL. The modified algorithm also improves surface temperature prediction over the Yellow Sea accompanying early dissipation of the fog. In the global medium-range forecast experiment, the modified scheme simulates overall enhanced PBL mixing over the STBL in the tropics and subtropical ocean, showing drier and warmer regions near the surface and moister and cooler regions above the PBL, resulting in prediction of reduced low level cloud amount and increased downward shortwave radiation at the surface. The modified scheme appears to improve systematic bias in temperature and humidity in the lower troposphere compared to the control simulation.     

THERMODYNAMIC PROPERTIES AND ROLE OF THERMALS IN THE STRATUS-TOPPED BOUNDARY LAYER

From results of two large-eddy simulation of stratus-topped boundary layer,the structure,thermodynamic prop-erties and role of thermals are investigated by using conditional sampling methods,which divided the thermals into thewarm/moist,cool/dry,warm/dry and cool/moist events.The results show that the main turbulent circulation in thestratus-topped boundary layer is composed of the warm/moist updraft and cool/dry downdraft.Below entrainmentregion,the warm/moist updrafts and cool/dry downdrafts are,respectively,positively and negatively buoyant andcontribute most to total fluxes and variances.Evaporative cooling has important effect on the structure and thermodynamic properties of thermals instratus-topped boundary layer.     

THERMODYNAMIC PROPERTIES AND ROLE OF THERMALS IN THE STRATUS-TOPPED BOUNDARY LAYER*

From results of two large-eddy simulation of stratus-topped boundary layer,the structure,thermodynamic properties and role of thermals are investigated by using conditional sampling methods,which divided the thermals into the warm/moist,cool/dry,warm/dry and cool/moist events.The results show that the main turbulent circulation in the stratus-topped boundary layer is composed of the warm/moist updraft and cool/dry downdraft.Below entrainment region,the warm/moist updrafts and cool/dry downdrafts are,respectively,positively and negatively buoyant and contribute most to total fluxes and variances.Evaporative cooling has important effect on the structure and thermodynamic properties of thermals in stratus-topped boundary layer.     

Application of the Coupled Dynamical－Radiational Model of Stratocumulus

This paper presents some results concerning applications of the coupled dynamical-radiational model of stratocumulus Ye developed (Ye, 1990, hereafter referred to as the DRS model). Basically, calculations involved here relate to the time-integration of the DRS model to estimate some time constants associated with the boundary layer and cloud developments. To obtain cloud lifetime (CLT), the time interval from its formation to its dissipation, different cloud instability criteria are compared, the mechanism for dissipating cloud are discussed, and thereby two new criteria for judging whether cloud can maintain are established. Following these criteria, the sensitivities of CLT to parameters such as the air-sea temperature difference, subsidence, and surface wind are estimated. The main interesting results about the sensitivities of the forming time (CFT) are a stable environment, such as cool sea, low wind, and strong subsidence, would prolong the existence of Sc, which has been predicted by the model c     

大涡模式分辨率对海洋信风区大气边界层结构和演变模拟的影响

利用BOMEX（巴巴多斯海洋与气象学试验）的探空资料和LEM（大涡模式）,通过改变LEM水平分辨率的敏感性数值试验,对比分析不同尺度的湍涡对信风积云边界层中混合层和云层的结构、演变以及对流形式和强度的影响。结果表明,水平分辨率较高时模拟的湍涡尺度较小、混合层顶的夹卷作用较强,模拟的混合层较暖、较干,而且模拟的对流泡尺度较小、强度较大,能够模拟出较精细的边界层结构;而水平分辨率较低时则相反。模拟的湍涡尺度对海洋信风区边界层积云中液态水混合比的模拟结果影响较大:LEM模拟的湍涡尺度较小时模拟的信风积云形成的时间较早、云顶高度较高,单个云块的体积较小但数目较多,液态水含量较高;而模拟的湍涡尺度较大时则相反。虽然水平分辨率为50 m和125 m的试验都能模拟出较精细的信风边界层中混合层、云层的结构和演变特征,但是,考虑到提高分辨率在模拟过程中产生的噪音信号对结果的影响以及计算时间等问题,LEM采用125 m的水平网格距是对海洋信风边界层积云对流模拟较为理想的选择。      

A Mixed-Layer Model Of The Well-Mixed Stratocumulus-Topped Boundary Layer

Recently a range of sophisticated large-eddy simulations of thecloud-topped boundary layer have been intercompared and furthercompared with observations and single column models. Here we comparethese results with perhaps the simplest model of the cloud-toppedboundary layer, namely a mixed-layer model. Results from the model aredescribed with two aims in mind. Firstly, the good results act as areminder of the success of simple models, and, secondly, we suggestthat a simple mixed-layer model could be used as a baseline for futuremodel intercomparisons.The mixed-layer model is based on two assumptions that follow previousstudies. Firstly, the liquid-water potential temperature and the total waterspecific humidity are assumed to be constant with height in the boundarylayer. Secondly, turbulence entrains air across the inversion into the boundarylayer at a rate that is assumed to be proportional to the jump in radiative flux at the cloud top and inversely proportional to the jump in buoyancy at the inversion. The constant of proportionality is called the entrainment efficiency.Results from the model for the entrainment rate and height evolutionof the boundary layer are compared with the observations and modelsconsidered in a EUCREM intercomparison study. Thepresent mixed-layer model accurately predicts the observed heightevolution of the boundary layer, but over-estimates the entrainmentrate to a similar degree as the large-eddy simulations. We show that,if the subsidence rate is reduced to the value given by observationsrather than the value used in the EUCREM intercomparison study,then the model agrees well with observed value of the entrainment rateif the entrainment efficiency is taken to be 0.6. With this value, themodel also agrees well with a further case study byBechtold et al. An entrainment efficiency of 0.6 is a little higherthan suggested by large eddy simulations, but such simulations do notcurrently resolve the entrainment events explicitly. Hence this pointdeserves further study.     

A bulk mass flux convection scheme for climate model: description and moisture sensitivity

A convection scheme for climate model is developed based on Tiedtke’s (Mon Weather Rev 117:1779–1800, 1989) bulk mass flux framework and is evaluated with observational data and cloud resolving model simulation data. The main differences between the present parameterization and Tiedtke’s parameterization are the convection trigger, fractional entrainment and detrainment rate formulations, and closure method. Convection is triggered if the vertical velocity of a rising parcel is positive at the level at which the parcel is saturated. The fractional entrainment rate depends on the vertical velocity and buoyancy of the parcel as well as the environmental relative humidity. For the fractional detrainment rate, a linear decrease in the updraft mass flux above maximum buoyancy level is assumed. In the closure method, the cloud base mass flux is determined by considering both cloud layer instability and subcloud layer turbulent kinetic energy as controlling factors in the strength of the convection. The convection scheme is examined in a single column framework as well as using a general circulation model. The present bulk mass flux (BMF) scheme is compared with a simplified Relaxed Arakawa-Schubert (RAS) scheme. In contrast to the RAS, which specifies the cloud top, cloud top height in BMF depends on environmental properties, by considering the conditions of both the parcel and its environment in a fractional entrainment and detrainment rate formulations. As a result, BMF shows improved sensitivity in depth and strength of convection on environmental humidity compared to RAS, by strengthening coupling between cloud and environment. When the mid to lower troposphere is dry, the cloud resolving model and BMF produce cloud top around the dry layer and moisten the layer. In the framework of general circulation model, enhanced coupling between convection and environmental humidity in BMF results in improved representation of eastward propagating intraseasonal variability in the tropics—the Madden-Julian oscillation.     

深对流云输送对于对流层O3、NOx在分析的作用

利用一个冰雹云模式与云化学输送模块耦合而成的三维对流云化学／输送模式, 研究对流云对重要的大气污染物臭氧 （Ｏ３）、氮氧化物 （ＮＯｘ, 包括ＮＯ 和ＮＯ２） 的输送作用。模式较好地体现了一个单体积云的发展过程及其特征。云化学／输送模式的结果表明, 云内强烈的垂直输送能在３０ ｍ ｉｎ 左右, 把低层低体积分数的Ｏ３和高体积分数的ＮＯ２快速、有效地输送到对流层的上部, 造成化学物种的再分布。而在云顶附近, 由于对流穿透了对流层的顶部,造成了上层高体积分数Ｏ３的向下侵入,说明云的对流活动除了能把边界层内的污染物向上输送, 其夹卷作用还可以造成平流层和对流层化学物质的交换。     

Turbulence in Continental Stratocumulus,Part II: Eddy Dissipation Rates and Large-Eddy Coherent Structures

This study first illustrates the utility of using the Doppler spectrum width from millimetre wavelength radar to calculate the energy dissipation rate and then to use the energy dissipation rate to study turbulence structure in a continental stratocumulus cloud. It is shown that the turbulence kinetic energy dissipation rate calculated from the radar-measured Doppler spectrum width agrees well with that calculated from the Doppler velocity power spectrum. During the 16-h stratocumulus cloud event, the small-scale turbulence contributes 40 % of the total velocity variance at cloud base, 50 % at normalized cloud depth = 0.8 and 70 % at cloud top, which suggests that small-scale turbulence plays a critical role near the cloud top where the entrainment and cloud-top radiative cooling act. The 16-h mean vertical integral length scale decreases from about 160 m at cloud base to 60 m at cloud top, and this signifies that the larger scale turbulence dominates around cloud base whereas the small-scale turbulence dominates around cloud top. The energy dissipation rate, total variance and squared spectrum width exhibit diurnal variations, but unlike marine stratocumulus they are high during the day and lowest around sunset at all levels; energy dissipation rates increase at night with the intensification of the cloud-top cooling. In the normalized coordinate system, the averaged coherent structure of updrafts is characterized by low energy dissipation rates in the updraft core and higher energy dissipation rates surround the updraft core at the top and along the edges. In contrast, the energy dissipation rate is higher inside the downdraft core indicating that the downdraft core is more turbulent. The turbulence around the updraft is weaker at night and stronger during the day; the opposite is true around the downdraft. This behaviour indicates that the turbulence in the downdraft has a diurnal cycle similar to that observed in marine stratocumulus whereas the turbulence diurnal cycle in the updraft is reversed. For both updraft and downdraft, the maximum energy dissipation rate occurs at a cloud depth = 0.8 where the maximum reflectivity and air acceleration or deceleration are observed. Resolved turbulence dominates near cloud base whereas unresolved turbulence dominates near cloud top. Similar to the unresolved turbulence, the resolved turbulence described by the radial velocity variance is higher in the downdraft than in the updraft. The impact of the surface heating on the resolved turbulence in the updraft decreases with height and diminishes around the cloud top. In both updrafts and downdrafts, the resolved turbulence increases with height and reaches a maximum at cloud depth = 0.4 and then decreases to the cloud top; the resolved turbulence near cloud top, just as the unresolved turbulence, is mostly due to the cloud-top radiative cooling.     

解耦的海洋性层积云顶边界层湍流与云微物理特征

基于POST观测计划中获得的海洋性层积云顶边界层内高频气象资料和云微物理资料,在选取解耦个例基础上研究解耦边界层湍流和云微物理特征及成因。结果表明,过渡层的大气静力稳定度较强,抑制向上浮力做功,使得湍流动能迅速消耗殆尽,实现边界层解耦。湍流动能最大值出现在云内,主要与云顶降温、大云滴下落沉降拖曳带来的下沉气流增强及云底之上附近凝结增长潜热释放产生向上浮力作用有关。近地面层的浮力项和切变项对湍流动能都起到增强作用,并以切变项的贡献更为显著,云内的湍流动能是以浮力项贡献为主。过渡层附近存在向下的热通量,抑制了热量向上输送和向上浮力项的增强,促进解耦发生。云内存在向上感热通量,其最大值及其出现高度主要与云顶冷却和云中下部的凝结潜热加热有关。云顶之上湿层促进了潜热通量的向下输送,增强了云内水汽含量,为解耦边界层云的发展起到正反馈作用。云顶浮力倒转引起的云中湍流混合呈现非均匀性,并进一步导致绝热或超绝热液滴出现,促进凝结和碰并增长的增强,同时云顶之上湿层进一步对云中的微物理增长起到了重要的推动作用。云底因夹卷混合表现为均匀混合特征。     

Resolved Versus Parametrized Boundary-Layer Plumes. Part I: A Parametrization-Oriented Conditional Sampling in Large-Eddy Simulations

A conditional sampling based on the combination of a passive tracer emitted at the surface and thermodynamic variables is proposed to characterise organized structures in large-eddy simulations of cloud-free and cloudy boundary layers. The sampling is evaluated against more traditional sampling of dry thermals or clouds. It enables the characterization of convective updrafts from the surface to the top of the boundary layer (or the top of cumulus clouds), describing in particular the transition from the sub-cloud to the cloud layer, and retrieves plume characteristics, entrainment and detrainment rates, variances and fluxes. This sampling is used to analyze the contribution of boundary-layer thermals to vertical fluxes and variances.     

Observational Diagnosis of Cloud Phase in the Winter Antarctic Atmosphere for Parameterizations in Climate Models

The cloud phase composition of cold clouds in the Antarctic atmosphere is explored using data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instruments for the period 2000--2006. We used the averaged fraction of liquid-phase clouds out of the total cloud amount at the cloud tops since the value is comparable in the two measurements. MODIS data for the winter months (June, July, and August) reveal liquid cloud fraction out of the total cloud amount significantly decreases with decreasing cloud-top temperature below 0oC. In addition, the CALIOP vertical profiles show that below the ice clouds, low-lying liquid clouds are distributed over ～20% of the area. With increasing latitude, the liquid cloud fraction decreases as a function of the local temperature. The MODIS-observed relation between the cloud-top liquid fraction and cloud-top temperature is then applied to evaluate the cloud phase parameterization in climate models, in which condensed cloud water is repartitioned between liquid water and ice on the basis of the grid point temperature. It is found that models assuming overly high cut-offs (》-40oC) for the separation of ice clouds from mixed-phase clouds may significantly underestimate the liquid cloud fraction in the winter Antarctic atmosphere. Correction of the bias in the liquid cloud fraction would serve to reduce the large uncertainty in cloud radiative effects.