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1.
Aircraft measurements were made from the NCAR Electra in stratus and stratocumulus clouds off the coast of California in June 1976. Several types of cloud conditions were observed, including (1) a broken layer less than 100 m thick, capped by an inversion at ~1000 m, (2) a broken stratocumulus layer ~300 m thick with an inversion at ~500 m, and (3) a solid stratocumulus layer ~250 m thick with an inversion at ~500 m. Although these observations indicate that a variety of cloud conditions may exist in mixed layers, simple one-dimensional mixed-layer models implicitly assume a solid cloud layer with no unsaturated region within the cloud. In order to generalize these simple models, a parametric representation of the heat and moisture fluxes is considered. In this scheme, the fluxes are parameterized in terms of the product of a cloud mass flux and the characteristic difference between the thermodynamic properties of an updraft-downdraft circulation. This representation allows for an explicit representation of the buoyancy flux when the downdraft has no liquid water.Data collected during these flights were used to calculate heat and moisture fluxes and to obtain the mean difference in the thermodynamic properties of the updrafts and downdrafts at a given level. The mass flux was calculated using updraft-downdraft differences and the fluxes. The mass fluxes obtained using various thermodynamic quantities are examined for consistency. The vertical distribution of the mass flux is determined. These results indicate that a mass flux formulation could prove to be useful in modeling applications where cloud conditions may vary between solid and broken.  相似文献   

2.
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.  相似文献   

3.
Tongwen Wu 《Climate Dynamics》2012,38(3-4):725-744
A simple mass-flux cumulus parameterization scheme suitable for large-scale atmospheric models is presented. The scheme is based on a bulk-cloud approach and has the following properties: (1) Deep convection is launched at the level of maximum moist static energy above the top of the boundary layer. It is triggered if there is positive convective available potential energy (CAPE) and relative humidity of the air at the lifting level of convection cloud is greater than 75%; (2) Convective updrafts for mass, dry static energy, moisture, cloud liquid water and momentum are parameterized by a one-dimensional entrainment/detrainment bulk-cloud model. The lateral entrainment of the environmental air into the unstable ascending parcel before it rises to the lifting condensation level is considered. The entrainment/detrainment amount for the updraft cloud parcel is separately determined according to the increase/decrease of updraft parcel mass with altitude, and the mass change for the adiabatic ascent cloud parcel with altitude is derived from a total energy conservation equation of the whole adiabatic system in which involves the updraft cloud parcel and the environment; (3) The convective downdraft is assumed saturated and originated from the level of minimum environmental saturated equivalent potential temperature within the updraft cloud; (4) The mass flux at the base of convective cloud is determined by a closure scheme suggested by Zhang (J Geophys Res 107(D14), doi:10.1029/2001JD001005, 2002) in which the increase/decrease of CAPE due to changes of the thermodynamic states in the free troposphere resulting from convection approximately balances the decrease/increase resulting from large-scale processes. Evaluation of the proposed convection scheme is performed by using a single column model (SCM) forced by the Atmospheric Radiation Measurement Program’s (ARM) summer 1995 and 1997 Intensive Observing Period (IOP) observations, and field observations from the Global Atmospheric Research Program’s Atlantic Tropical Experiment (GATE) and the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The SCM can generally capture the convective events and produce a realistic timing of most events of intense precipitation although there are some biases in the strength of simulated precipitation.  相似文献   

4.
GRAPES全球模式次网格对流过程对云预报的影响研究   总被引：4，自引：2，他引：2  
谭超  刘奇俊  马占山 《气象学报》2013,71(5):867-878
50 km分辨率下的GRAPES全球模式对赤道及低纬度地区云水、云冰、云量和格点降水的预报较实际观测偏少。为解决这一问题,在模式原有的格点尺度云方案基础上,将次网格对流过程的影响作为源汇项,加入到云水、云冰和总云量的预报方程中。结合云和地球辐射能量系统(CERES)与热带降雨测量(TRMM)等卫星云观测资料,进行了改进后的云方案与原云方案预报结果的对比分析。结果显示,考虑了对流对格点尺度云含水量和云量预报的影响后,GRAPES全球模式预报的云和格点降水在赤道及低纬度地区有明显改善,水凝物含水量和总云量的预报结果与实况较为接近,格点降水在总降水中的比例由原来的5%提高到25%。研究进一步表明,次网格对流过程对格点尺度云和降水的影响取决于上升气流质量通量的分布和强度,上升气流的质量通量在对流活动强烈的低纬度热带地区较强,其最大值出现在650—450 hPa高度,因此,次网格对流的卷出过程对中云的影响最为明显。对高云和低云也有一定程度的影响,使云顶变高,云底变低。  相似文献   

5.
The conditional sampling of coherent structures in large-eddy simulations of the convective boundary layer (Couvreux et al. Boundary-layer Meteorol 134:441–458, 2010) is used to propose and evaluate formulations of fractional entrainment and detrainment rates for mass-flux schemes. The proposed formulations are physically-based and continuous from the surface to the top of clouds. Entrainment is related to the updraft vertical velocity divergence, while detrainment depends on the thermal vertical velocity, on buoyancy and on the moisture contrast between the mean plume and its environment. The proposed formulations are first directly evaluated in simulations of shallow clouds. They are then tested in single-column simulations with the thermal plume model, a mass-flux representation of boundary-layer thermals.  相似文献   

6.
A simple closure scheme for nocturnal stratocumulus is proposed. The scheme is formulated in conserved variables. Cloud fraction and cloud water amount are diagnosed assuming a top-hat distribution for total water. Conversion of cloud water into rain water is parameterized in terms of cloud water and the incoming rain flux. Turbulence transport in the cloud layer is accounted for by a first-order vertical diffusion scheme with a profile-type diffusivity. The length scale corresponds to the thickness of the cloud layer. The turbulent velocity scale is directly related to the long wave radiative flux divergence in the cloud. Entrainment at cloud top is implicitly treated by extending the in-cloud mixing profile slightly beyond cloud top. The excess height is derived from the buoyancy frequency at cloud top and a radiative–convective velocity scale. The scheme is capable of simulating realistic profiles of the conserved variables and cloud parameters for a case of nocturnal stratocumulus prepared on the basis of ASTEX data.  相似文献   

7.
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.  相似文献   

8.
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.  相似文献   

9.
This paper aims at characterizing how different key cloud properties (cloud fraction, cloud vertical distribution, cloud reflectance, a surrogate of the cloud optical depth) vary as a function of the others over the tropical oceans. The correlations between the different cloud properties are built from 2?years of collocated A-train observations (CALIPSO-GOCCP and MODIS) at a scale close to cloud processes; it results in a characterization of the physical processes in tropical clouds, that can be used to better understand cloud behaviors, and constitute a powerful tool to develop and evaluate cloud parameterizations in climate models. First, we examine a case study of shallow cumulus cloud observed simultaneously by the two sensors (CALIPSO, MODIS), and develop a methodology that allows to build global scale statistics by keeping the separation between clear and cloudy areas at the pixel level (250, 330?m). Then we build statistical instantaneous relationships between the cloud cover, the cloud vertical distribution and the cloud reflectance. The vertical cloud distribution indicates that the optically thin clouds (optical thickness <1.5) dominate the boundary layer over the trade wind regions. Optically thick clouds (optical thickness >3.4) are composed of high and mid-level clouds associated with deep convection along the ITCZ and SPCZ and over the warm pool, and by stratocumulus low level clouds located along the East coast of tropical oceans. The cloud properties are analyzed as a function of the large scale circulation regime. Optically thick high clouds are dominant in convective regions (CF?>?80?%), while low level clouds with low optical thickness (<3.5) are present in regimes of subsidence but in convective regimes as well, associated principally to low cloud fractions (CF?<?50?%). A focus on low-level clouds allows us to quantify how the cloud optical depth increases with cloud top altitude and with cloud fraction.  相似文献   

10.
As part of the EUropean Cloud REsolving Modelling (EUCREM) model intercomparison project we compared the properties and development of stratocumulus as revealed by actual observations and as derived from two types of models, namely three-dimensional Large Eddy Simulations (LES) and one-dimensional Single Column Models (SCMs). The turbulence, microphysical and radiation properties were obtained from observations made in solid stratocumulus during the third flight of the first 'Lagrangian' experiment of the Atlantic Stratocumulus Transition Experiment (ASTEX). The goal of the intercomparison was to study the turbulence and microphysical properties of a stratocumulus layer with specified initial and boundary conditions.The LES models predict an entrainment velocity which is significantly larger than estimated from observations. Because the observed value contains a large experimental uncertainty no definitive conclusions can be drawn from this. The LES modelled buoyancy flux agrees rather well with the observed values, which indicates that the intensity of the convection is modelled correctly. From LES it was concluded that the inclusion of drizzle had a small influence (about 10%) on the buoyancy flux. All SCMs predict a solid stratocumulus layer with the correct liquid water profile. However, the buoyancy flux profile is poorly represented in these models. From the comparison with observations it is clear that there is considerable uncertainty in the parametrization of drizzle in both SCM and LES.  相似文献   

11.
Buoyancy fluxes in the marine atmospheric boundary layer (MABL) for the cloud street regime, observed during the Genesis of Atlantic Lows Experiment (GALE), have been analyzed using the technique of joint frequency distribution. For the lower half of the MABL, the results suggest that the buoyancy flux is mainly generated by the rising thermals and the sinking compensating ambient air, and is mainly consumed by the entrainment and detrainment of thermals, penetrative convection, and the entrainment from the MABL top.The results are compared to those from previous studies of mesoscale cellular convection (Air-Mass Transformation Experiment, AMTEX), the dry convective boundary layer, and the trade-wind MABL. For the lower MABL, the quadrant buoyancy fluxes, fractional coverages, and flux intensities are in good agreement with those of mesoscale cellular convection (AMTEX) and the dry convective boundary layer. The results suggest that, if the buoyancy flux is primarily driven by the temperature flux, the physical processes for generating buoyancy flux mentioned above are about the same for the lower boundary layers over land and ocean, even with different convective regimes. For the trade-wind MABL, the buoyancy flux is mainly driven by the moisture flux; the quadrant flux intensities are stronger than those of the other three studies except for the buoyant updrafts (thermals). These results suggest that the entrainment and detrainment of thermals are more effective in the trade-wind MABL than in the boundary layers driven by the temperature flux.Scale analysis of the buoyancy flux is in good agreement with that of AMTEX. For the lower half of the MABL, the buoyancy flux is mainly generated by the intermediate scale (200 m to 2 km), which includes the dominant convective thermals in the surface layer and the mixed layer. The scale smaller than 200 m is important only in the surface layer. The scale larger than 2 km, which includes the roll vortices, increases its significance upward. While most of the positive and negative fluxes are associated with the updrafts for the intermediate scale, the downdrafts are as important as updrafts for the larger scale.ST Systems Corporation, Lanham, MD, 20706, U.S.A.  相似文献   

12.
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.  相似文献   

13.
运用已建立的气溶胶核化清除的物理化学模式，研究了云的动力学因子（如：气块上升速度、夹卷作用）对云滴化学非均匀性的影响。计算结果表明：较强烈的云发展（较大的气块上升速度）可加强由于气溶胶核化和云滴凝结增长造成的云滴化学的非均匀程度。夹卷作用抑制了云的发展，因而减弱了这种非均匀程度。夹卷作用同时也造成总体液态水中Ｓ（ＶＩ）、Ｈ＋等浓度的增加，在Ｓｍａｘ附近可达１个量级。如果考虑气溶胶粒子的夹卷，则可使气块内云滴污染物浓度随云滴大小的变化更加复杂化，如：不仅云滴污染物浓度随云滴大小而变化，即使对于相同大小的云滴之间，其污染物浓度也可相差很大。  相似文献   

14.
The mechanism responsible for high rainfall over the Indian west coast region has been investigated by studying dynamical, thermodynamical and microphysical processes over the region for the monsoon season of 2009. The European Centre for Medium-Range Weather Forecasts wind and NCEP flux data have been used to study the large scale dynamical parameters. The moist adiabatic and multi-level inversion stratifications are found to exist during the high and low rainfall spells, respectively. In the moist adiabatic stratification regime, shallow and deep convective clouds are found coexisting. The Cloud Aerosol Interaction and Precipitation Enhancement EXperiment aircraft data showed cloud updraft spectrum ranging from 1 to 10 m s?1 having modal speed 1–2.5 m s?1. The low updrafts rates provide sufficient time required for warm rain processes to produce rainfall from shallow clouds. The low cloud liquid water is observed above the freezing level indicating efficient warm rain process. The updrafts at the high spectrum end go above freezing level to generate ice particles produced due to mixed-phase rainfall process from deep convective clouds. With aging, deep convection gets transformed into stratiform type, which has been inferred through the vertical distribution of the large scale omega and heating fields. The stratiform heating, high latent heat flux, strong wind shear in the lower and middle tropospheric levels and low level convergence support the sustenance of convection for longer time to produce high rainfall spell. The advection of warm dry air in the middle tropospheric regions inhibits the convection and produce low rainfall spell. The mechanisms producing these spells have been summarized with the block diagram.  相似文献   

15.
The marine atmospheric boundary layer is characterized by cool temperatures and high humidity. Clouds are observed over most of the oceans. It is generally accepted that these overcast cloud decks break up into scattered fragments due to cloud-top entrainment instability. That is, if the air above the boundary layer is sufficiently cool and dry relative to cloud top, the buoyancy flux will be directed upwards and entrainment can occur freely.A boundary-layer model is used to test the sensitivity of the model atmosphere to the various processes which promote the onset of cloud-top entrainment instability. It is found that the transition from a solid cloud deck to scattered cumulus clouds depends on a rate process. The cloud cover is sensitive to mesoscale variations in sea surface temperature only if the cloud-top inversion is sufficiently weak.  相似文献   

16.
A one-dimensional grid-level model including longwave radiative transfer and a level-4 second-order turbulent transfer closure which contains prognostic equations for turbulent quantities, is used to study the physics and dynamics of inversion-capped marine stratocumulus clouds.A set of numerical experiments had been performed to examined the role of sea surface temperature, large-scale vertical velocity, wind speed, and vertical wind shear in the formation and the structure of low-level clouds. For a given sea surface and geostrophic wind speed, stratocumulus clouds can grow higher with smaller large-scale subsidence as less dry air entrains into the cloud. Clouds grow higher with higher sea surface temperature for a given geostrophic wind speed and large-scale subsidence as a result of enhanced moist convection. In high wind speeds, the entire cloud deck is lifted up because of larger surface energy flux. In the budget studies of the turbulent kinetic energy (TKE), the buoyancy term is a major source term when the wind speed and the vertical shear are small across the inversion top. When the wind speed and the vertical wind shear across the inversion top become large, the mixed layer is decoupled into a cloud and a subcloud layer. In the TKE budget studies, the shear generation term becomes an important term in the budgets of the TKE and the variance of vertical velocity.  相似文献   

17.
利用辽宁阜新国家站(121.7458°E,42.0672°N)的毫米波云雷达(8 mm)和微雨雷达(12.5 mm)对2020年8月12-13日东北冷涡影响下的一次降水过程进行了观测,分析了云降水的垂直结构特征并探讨了降水机制。结果表明：本次过程中,云水平方向发展不均匀,以层状云和层积混合云为主,云内有时还嵌有对流泡。云降水阶段性变化明显,先后出现了层状云降水、层积混合云降水和对流云降水。层状云降水和层积混合云降水均表现出明显的亮带特征,但层积混合云降水的雷达回波强度、回波顶高和降水强度明显大于层状云降水。对流云降水的雷达回波会因强降水而产生明显衰减,因此回波顶高不能表示出实际的云顶情况。层状云降水阶段,云雷达反射率随高度降低增长缓慢,雨滴在下落过程中受蒸发和碰并的共同作用,反射率降低。与层状云降水相比,层积混合云降水的碰并效应强,且由于前期降水对近地面的增湿作用,使云下蒸发弱。对流云降水阶段,反射率的增长主要发生在冰水混合层,有利于大滴的产生,拓宽了云滴谱,提高了碰并效率。  相似文献   

18.
Stratocumulus-capped mixed layers derived from a three-dimensional model   总被引：22，自引：7，他引：22  
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.  相似文献   

19.
牛玺  马晓燕  贾海灵 《气象科学》2022,42(4):467-480
本文利用A-Train卫星队列中的Cloudsat卫星所提供的二级云分类产品资料(结合了CALIPSO卫星气溶胶激光雷达)2B-CLDCLASS-LIDAR,选取2007年3月至2017年2月的样本数据进行统计分析,研究北半球主要的气溶胶排放源区(中国东部,美国东部和欧洲西部)不同云型出现频率的分布特征。结果表明,在以单层云出现的8种云类中卷云,层积云和积云的发生频率总和高达50%~70%,其次为高层云、高积云和雨层云,而深对流云和层云这两种云仅占10%以下。各类云的发生频率的空间分布可看出卷云和层积云的发生频率可高达90%以上,高层云的发生频率在70%~80%左右,高积云和积云的发生频率则接近70%以上,深对流云和层云的发生频率则在20%以下。其中,卷云、深对流云和积云主要出现在低纬度的海洋上;高层云和高积云主要出现在中低纬工业发达的陆地上;层积云、层云和雨层云主要出现在中高纬地区,其中层积云和层云出现在海洋上居多,雨层云出现频次的海陆差别不大。不同云型在不同的季节出现频次差异明显,在夏季出现较多的云型以卷云、深对流云,积云和层云为主;在冬季则是高层云、层积云和雨层云这样稳定型的云型占据主导,同时还发现卷云和层积云发生频率的月变化相反,而高层云和雨层云发生频率的月变化相似。  相似文献   

20.
In numerical weather prediction and climate models, planetary boundary-layer (PBL) clouds are linked to subgrid-scale processes such as shallow convection. A comprehensive statistical analysis of large-eddy simulations (LES), obtained for warm PBL cloud cases, is carried out in order to characterize the distributions of the horizontal subgrid cloud variability. The production of subgrid clouds is mainly associated with the variability of the total water content. Nevertheless, in the case of PBL clouds, the temperature variability cannot be completely discarded and the saturation deficit, which summarizes both temperature and total water fluctuations, provides a better representation of the cloud variability than the total water content. The probability density functions (PDFs) of LES saturation deficit generally have the shape of a main asymmetric bell-shaped curve with a more or less distinct secondary maximum specific to each type of PBL clouds. Unimodal theoretical PDFs, even those with a flexible skewness, are not sufficient to correctly fit the LES distributions, especially the long tail that appears for cumulus clouds. They do not provide a unified approach for all cloud types. The cloud fraction and the mean cloud water content, diagnosed from these unimodal PDFs, are largely underestimated. The use of a double Gaussian distribution allows correction of these errors on cloud fields and provides a better estimation of the cloud-base and cloud-top heights. Eventually, insights for the design of a subgrid statistical cloud scheme are provided, in particular a new formulation for the weight of the two Gaussian distributions and for the standard deviation of the convective distribution.  相似文献   

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