Observation of a Self-Limiting,Shear-Induced Turbulent Inversion Layer Above Marine Stratocumulus

High-resolution measurements of thermodynamic, microphysical, and turbulence properties inside a turbulent inversion layer above a marine stratocumulus cloud layer are presented. The measurements are performed with the helicopter-towed measurement payload Airborne Cloud Turbulence Observation System (ACTOS), which allows for sampling with low true air speeds and steep profiles through cloud top. Vertical profiles show that the turbulent inversion layer consists of clear air above the cloud top, with nearly linear profiles of potential temperature, horizontal wind speed, absolute humidity, and concentration of interstitial aerosol. The layer is turbulent, with an energy dissipation rate nearly the same as that in the lower cloud, suggesting that the two are actively coupled, but with significant anisotropic turbulence at the large scales within the turbulent inversion layer. The turbulent inversion layer is traversed six times and the layer thickness is observed to vary between 37 and 85 m, whereas the potential temperature and horizontal wind speed differences at the top and bottom of the layer remain essentially constant. The Richardson number therefore increases with increasing layer thickness, from approximately 0.2 to 0.7, suggesting that the layer develops to the point where shear production of turbulence is sufficiently weak to be balanced by buoyancy suppression. This picture is consistent with prior numerical simulations of the evolution of turbulence in localized stratified shear layers. It is observed that the large eddy scale is suppressed by buoyancy and is on the order of the Ozmidov scale, much less than the thickness of the turbulent inversion layer, such that direct mixing between the cloud top and the free troposphere is inhibited, and the entrainment velocity tends to decrease with increasing turbulent inversion-layer thickness. Qualitatively, the turbulent inversion layer likely grows through nibbling rather than engulfment.     

‘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.     

The Sensitivity Of A Stratocumulus Transition: Model Simulations Of The Astex First Lagrangian

In this study, a one-dimensional ensemble-average model is used to simulatethe Atlantic Stratocumulus Transition Experiment firstLagrangian, where the same airmass was followed from the subtropical high pressure region en route towards the trade wind region. The airmass experiences increasing sea-surface temperature and achange from subsidence to weak ascent on its way south. Thiscauses the marine boundary layer (MBL)to grow and the cloud deck to change from a solid stratocumulus deck tomore broken stratocumulus clouds with cumulus cloudsdeveloping beneath, and reaching up into the stratocumulus clouds.A control run is analyzed and compared in detail with theobservations. Both a statistical evaluation and a more subjective evaluation are performed, where both establish confidencein the model performance. The model captures the MBL growth and the cloudliquid water, as well as the drizzle flux, is well predicted by the model.A sensitivity study was performed with the objective of examining theMBL and the cloud response to external and internal 'forces'.The results show that, if drizzle formation is not allowed,unrealistically high cloud liquid water mixing ratios are predicted. Even though the drizzle flux is very small, it is still important for the water budget of the MBL and for the boundary-layer dynamics.We also found that the sea-surface temperature increase is more important for the increasing cloud top height than the synoptic-scale divergence fields. However, the synoptic-scale subsidence is crucial during the first day, when the sea-surface temperature was constant, in keepingthe cloud top at a constant height. Drizzle evaporation below the cloud base seems to be important for below-cloud condensation. The drizzle predictions are significantly altered when the prescribed cloud droplet and/or drizzle drop numbers are altered.     

Air mass modification over Lake Michigan

The modification of a relatively cold air mass over the warm water of Lake Michigan is studied by using a two-dimensional nonlinear mesoscale model. Considerable amounts of heat and water vapor are supplied from the water surface to the lower atmosphere by turbulent eddies. A convective mixed layer develops and grows toward the downwind region with stratocumulus clouds over the lake.The model simulates the warming and moistening of the mixed layer, the development of a boundary layer, the divergence and convergence of wind near the coastlines, and the turbulent fluxes.The model warming of the mixed layer across the lake was about 2.2 °K and the moistening of the mixed layer was about 0.8 g kg^–1, which are comparable to 2.7 °K and 0.8 g kg^–1 observed by Lenschow (1973). The convective boundary layer, which includes the cloud layer, subcloud layer, and superadiabatic layer near the water surface, is well simulated. The tilt of the inversion which coincides with the cloud top is also well reproduced. When a prescribed cooling rate is applied at the cloud top, stronger turbulence and a deeper cloud layer are generated. Without the cooling, the cloud is shallow and the shape of the cloud base is determined by surface conditions. The rise of the inversion is due to upward vertical motion, and deepening of the convective layer in the downwind region.     

一次层积云发展过程对黑潮延伸体海洋锋强迫的响应研究——观测与机制分析

由于缺乏海上现场观测,对天气尺度扰动下,海表面温度锋 (海洋锋) 对海洋大气边界层 (MABL) 垂直结构和MABL内海洋性低云 (marine stratus) 的影响研究较少。2014年4月12日,中国海洋大学东方红2号科学考察船在黑潮延伸体海区的海洋锋附近捕捉到一次层积云的迅速发展。在比较稳定的天气形势下,由暖水侧向北穿越海洋锋时,云底和云顶高度升高,云区范围迅速扩大。本文利用多种大气-海洋联合观测数据,结合卫星观测和再分析资料,对此次层积云迅速发展的机理进行了综合分析。结果表明,在海上低压后部西北风控制下,在海洋锋的暖水侧 (下风方) 形成热通量大值中心和低压槽,有助于高空西风动量下传,进而又使得海气界面热通量增加,这种正反馈效应为MABL内混合层厚度加大和云底/顶高度在海洋锋的下风方升高提供有利背景条件。4月12日09:00~12:00(协调世界时),来自日本本州岛陆地的低空暖平流到达该热通量中心上空,暖平流与热通量中心的共同作用,导致该时段近海面暖中心强度异常增加,MABL中静力不稳定层加深和低压槽发展,综合作用的结果使得混合层厚度明显加深,云底高度升高,云区迅速发展。本研究有助于理解在复杂大气背景扰动下MABL对海洋强迫的响应机理。     

风切变对边界层对流影响的大涡模拟研究

利用"西北干旱区陆-气相互作用野外观测实验"加密观测期间在敦煌站的观测资料以及大涡模式,模拟了对流边界层的发展,以及示踪物从混合层向残留层传输的时空变化。模拟的对流边界层的结构及演变特征与实测结果基本一致。进一步通过有风切变和无风切变的敏感性数值试验,研究了风切变对垂直速度、位温和示踪物浓度的水平分布以及示踪物传输高度的影响。研究结果表明,在有风切变的试验中(甚至风切变仅存在于近地层中),对流边界层的增长加强,而且示踪物被传输的高度也较高。与浮力驱动的对流边界层相比,由浮力和风切变共同驱动的边界层中上升气流较弱而下沉气流较强,但前者的上升气流与下沉气流的分布在垂直方向上更为倾斜。由于夹卷作用的增强,浮力和风切变共同驱动的对流边界层较浮力驱动的对流边界层暖。在夹卷层,浮力和风切变共同驱动的边界层对流的上升气流和下沉气流都比浮力驱动的边界层对流中的强,而且垂直速度的概率密度函数分布也较对称,其位温和示踪物浓度的概率密度函数分布也比浮力驱动的边界层中的平直。对湍流动能收支的分析也表明风切变对湍流动能有重要影响,尤其对夹卷层中的湍流动能切变产生项影响较大。示踪物浓度的概率密度函数垂直分布显示,浮力驱动的边界层中示踪物浓度随高度变化较小,而浮力和风切变共同驱动的边界层中示踪物浓度随高度递减,但是示踪物传输的高度比较高。     

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.     

Effects of vertical wind shear, radiation and ice microphysics on precipitation efficiency during a torrential rainfall event in China

The effects of vertical wind shear, radiation and ice microphysics on precipitation efficiency （PE） were investigated through analysis of modeling data of a torrential rainfall event over Jinan, China during July 2007. Vertical wind shear affected PE by changing the kinetic energy conversion between the mean and perturbation circulations. Clou~radiation interaction impacted upon PE, but the relationship related to cloud radiative effects on PE was not statistically significant. The reduction in deposition processes as- sociated with the removal of ice microphysics suppressed efficiency. The relationships related to effects of vertical wind shear, radiation and ice clouds on PEs defined in cloud and surface rainfall budgets were more statistically significant than that defined in the rain microphysical budget.     

A process oriented characterization of tropical oceanic clouds for climate model evaluation, based on a statistical analysis of daytime A-train observations

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.     

The Coupling State of an Idealized Stable Boundary Layer

The coupling state between the surface and the top of the stable boundary layer (SBL) is investigated using four different schemes to represent the turbulent exchange. An idealized SBL is assumed, with fixed wind speed and temperature at its top. At the surface, two cases are considered, first a constant temperature, 20 K lower than the SBL top, and later a constant 2 K h⁻¹ cooling rate is assumed for 10 h after a neutral initial condition. The idealized conditions have been chosen to isolate the influence of the turbulence formulations on the coupling state, and the intense stratification has the purpose of enhancing such a response. The formulations compared are those that solve a prognostic equation for turbulent kinetic energy (TKE) and those that directly prescribe turbulence intensity as a function of atmospheric stability. Two TKE formulations are considered, with and without a dependence of the exchange coefficients on stability, while short and long tail stability functions (SFs) are also compared. In each case, the dependence on the wind speed at the SBL top is considered and it is shown that, for all formulations, the SBL experiences a transition from a decoupled state to a coupled state at an intermediate value of mechanical forcing. The vertical profiles of potential temperature, wind speed and turbulence intensity are shown as a function of the wind speed at the SBL top, both for the decoupled and coupled states. The formulation influence on the coupling state is analyzed and it is concluded that, in general, the simple TKE formulation has a better response, although it also tends to overestimate turbulent mixing. The consequences are discussed.     

Cloud Microphysical Budget Associated with Torrential Rainfall During the Landfall of Severe Tropical Storm Bilis (2006)

Effects of vertical wind shear, radiation, and ice clouds on cloud microphysical budget associated with torrential rainfall during landfall of severe tropical storm Bilis (2006) are investigated by using a series of analysis of two-day grid-scale sensitivity experiment data. When upper-tropospheric upward motions and lower-tropospheric downward motions occur on 15 July 2006, the removal of vertical wind shear and ice clouds increases rainfall contributions from the rainfall type (CM) associated with positive net condensation and hydrometeor loss/convergence, whereas the exclusion of cloud radiative effects and cloud-radiation interaction reduces rainfall contribution from CM. The elimination of vertical wind shear and cloud-radiation interaction increases rainfall contribution from the rainfall type (Cm) associated with positive net condensation and hydrometeor gain/divergence, but the removal of cloud radiative effects and ice clouds decreases rainfall contribution from Cm. The enhancements in rainfall contribution from the rainfall type (cM) associated with negative net condensation and hydrometeor loss/convergence are caused by the exclusion of cloud radiative effects, cloud-radiation interaction and ice clouds, whereas the reduction in rainfall contribution from cM results from the removal of vertical wind shear. When upward motions appear throughout the troposphere on 16 July, the exclusion of all these effects increases rainfall contribution from CM, but generally decreases rainfall contributions from Cm and cM.     

Wind shear at tilted inversions

Vertical wind shear at a temperature inversion can be caused by baroclinicity associated with a tilt of the inversion. Four observational cases of tilted inversions are presented. The tilts on horizontal scales of 20–100 km range from 2–10 × 10^-3 and the vertical wind shear is between 1 and 25 m/s per 100 m. In general, there is remarkable agreement between observed and geostrophic wind shear.The observations show that the inversion tilt is particularly strong at the edges of mesoscale cloud fields. The Richardson number can reach subcritical values. Cloud fields may be surrounded by a cyclonically rotating wind field and cloud gaps by an anticyclonically rotating wind field.     

Turbulence intensity and turbulent kinetic energy parameters over a heterogeneous terrain of Loess Plateau

A deep understanding of turbulence structure is important for investigating the characteristics of the atmospheric boundary layer, especially over heterogeneous terrain. In the present study, turbulence intensity and turbulent kinetic energy (TKE) parameters are analyzed for different conditions with respect to stability, wind direction and wind speed over a valley region of the Loess Plateau of China during December 2003 and January 2004. The purpose of the study is to examine whether the observed turbulence intensity and TKE parameters satisfy Monin-Obukhov similarity theory (MOST), and analyze the wind shear effect on, and thermal buoyancy function of, the TKE, despite the terrain heterogeneity. The results demonstrate that the normalized intensity of turbulence follows MOST for all stability in the horizontal and vertical directions, as well as the normalized TKE in the horizontal direction. The shear effect of the wind speed in the Loess Plateau region is strong in winter and could enhance turbulence for all stability conditions. During daytime, the buoyancy and shear effect together constitute the generation of TKE under unstable conditions. At night, the contribution of buoyancy to TKE is relatively small, and mechanical shearing is the main production form of turbulence.     

一次东北冷涡云降水垂直结构特征分析

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。     

CloudSat/CALIPSO卫星资料分析云的全球分布及其季节变化特征

全球气候模式（GCM）中云的参数化方案具有不确定性,了解云的时、空变化能为参数化方案提供有效参考。利用搭载在属于A-Train卫星序列的CloudSat和CALIPSO上的94 GHz云廓线雷达（CPR）以及正交极化云-气溶胶激光雷达（CALIOP）联合的2级云分类产品,分析了2007年3月-2010年2月8种云类及三相态的云量地理分布、纬向垂直分布的季节变化特征以及云层分布概率。结果发现,卷云的分布体系与深对流云相似,主要集中在西太平洋暖池、全球各季风区及赤道辐合带,分布格局与气压带、风带季节性移动一致。层云与层积云主要分布在中低纬度非季风区以及中高纬度的洋面上。高积云与高层云的分布形成明显的海陆差异,雨层云与积云的分布形成明显的纬度差异。冰云分布与卷云相似,云高随纬度递增而递减;水云分布与层积云相似,平均分布于2 km高度;混合云集中于高纬度地区及赤道辐合带,中纬度地区随纬度变化集中于海拔0-10 km的弧形带。层状云多以多层云形式出现,积状云多以单、双层云的形式出现,层状云的云重叠现象比积状云更显著。积状和层状云的分布特征与积云和层云降水的分布特征基本一致,验证了不同类型降水的卫星观测结果,同时为气候模式的云量诊断方案提供对比验证的数据。      

Enhanced aerosol-cloud relationships in more stable and adiabatic clouds

Modification of cloud microphysics and cloud albedo by cloud-active aerosol is generally identified and accepted, but the nature and magnitude of aerosol-cloud interactions are vaguely understood and thought to include a myriad of processes that vary regionally and confound the application of simple physical models of cloud-aerosol sensitivity. This paper presents observations demonstrating that cloud top stability through its regulation of mixing and vertical development is one of the critical mechanisms that regulate cloud response to cloud-active aerosol in some cloud systems. Strong above-cloud inversions are shown to buffer marine stratocumulus from the effects of mixing with drier, warmer inversion air. This buffering reduces the variability of the cloud liquid water path (LWP) and enables the clouds to remain nearly adiabatic. While weaker above-cloud inversions in continental stratocumulus promote variability in the LWP and sub-adiabatic LWPs, stronger inversions in marine stratocumulus enables a relatively adiabatic existence that increases the relationship of cloud microphysical alteration to cloud-active aerosol. This study has important implications for Geoengineering in that it demonstrates that cloud systems overlain by strong thermal inversions are more likely to respond predictably to intentional manipulation of the in-cloud concentration of cloud-active aerosol.     

Local Imbalance of Turbulent Kinetic Energy in the Surface Layer

We utilize experimental data collected in 2002 over an open field in Hanford, Washington, USA, to investigate the turbulent kinetic energy (TKE) budget in the atmospheric surface layer. The von Kármán constant was determined from the near-neutral wind profiles to be 0.36 ± 0.02 rather than the classical value of 0.4. The TKE budget was normalized and all terms were parameterized as functions of a stability parameter z/L, where z is the distance from the ground and L is the Obukhov length. The shear production followed the Businger–Dyer relation for −2 < z/L < 1. Contrary to the traditional Monin–Obukhov similarity theory (MOST), the shear, buoyancy and dissipation terms were found to be imbalanced due to a non-zero vertical transport over all stabilities. Motivated by this local imbalance, modified parameterizations of the dissipation and the turbulent transport were attempted and generated good agreement with the experimental data. Assuming stationarity and horizontal homogeneity, the pressure transport was estimated from the residual of the TKE budget.     

Intensive Radiosonde Measurements of Summertime Convection over the Inner Mongolia Grassland in 2014:Difference between Shallow Cumulus and Other Conditions

Using radiosonde measurements from 26 July to 30 July 2014 at Baiqi over the Inner Mongolia grassland of China, the vertical structure of shallow cumulus(SCu) clouds and associated environmental conditions were investigated. The cloud base height and the cloud top height of SCu was 3.4 km and 5 km, respectively. The temperature of the SCu layer was less than 0℃. The horizontal advection of specific humidity was smaller than the vertical transport in the atmosphere below 5 km.Above 5 km, the thermodynamic structure of the atmosphere remained stable. At the interface of the cloud layer and free air atmosphere, there was obvious wind shear and a temperature inversion(~2.9℃). Comparisons of environmental parameters associated with cumulus congestus, rain and clear days, showed that the formation of SCu was characterized by a higher Bowen ratio(high sensible heat flux and low latent heat flux), which indicated intensive turbulence in the boundary layer. The formation of SCu was associated with the boundary layer height exceeding the lifting condensation level. The maintenance of SCu was likely associated with the lower convective available potential energy, weak wind shear, and weak subsidence of the synoptic system, which did not favor the dramatic vertical development of SCu and thereby the transformation of SCu to cumulus congestus.     

Convection model for stratus cloud over a warm water surface

A numerical model is developed to simulate convective stratus cloud formation over the sea. The model is based on quasi-steady state moist plumes advecting over an area of increasing sea-surface temperature with the sea warmer than the air, but it is also a good approximation for non-steady states whenever the model is matched to air-sea temperature differences. Combining the effects of upward transfer of heat and moisture fluxes as well as adiabatic cooling, stratus cloud forms and spreads downward in this field. The depth of the convective field, the sea surface temperature gradient, the liquid water content at cloud top, and the horizontal pressure gradient are the four controlling parameters for the convective field. Alternatively, the wind speed, air-sea temperature difference, and the mixing ratio of the air, derived variables in this treatment, can be taken, with the depth, as the basic parameters.The entity type of convective model used here has the advantage that it models the transport and modification of air parcels and hence provides a method for studying drop size development in stratus clouds. It also uses relationships derived from water tank experiments with plumes and tested in dry convection and so needs no parameters specific to each situation. Its most important feature, however, is that the mean motion of plumes, rather than turbulent diffusion, transports the moisture.The upward growth and erosion of a temperature step increase to produce an inversion can be attributed to liquid water present in haze drops or cloud drops, rather than to turbulent diffusion. Radiative transfer is not a necessary requirement, but may either enhance or slow down the process.     

Implementation of turbulent mixing over a stratocumulus-topped boundary layer and its impact in a GCM

The effect of a vertical diffusion scheme over a stratocumulus topped boundary layer (STBL) was investigated using the YONU AGCM (Yonsei University Atmospheric General Circulation Model). To consider the impact of clouds on the turbulence production, the turbulence mixing term, driven by radiative cooling at the cloud top, is implemented as an extended non-local diffiusion scheme. In the model with this new scheme, the STBL parameterization significantly influences the lower atmosphere over the tropical and...