A numerical model of the partitioning of trace chemical solutes during drop freezing

Partitioning of volatile chemicals among the gas, liquid, and solid phases during freezing of liquid water in clouds can impact trace chemical distributions in the troposphere and in precipitation. We describe here a numerical model of this partitioning during the freezing of a supercooled liquid drop. Our model includes the time-dependent calculation of the coupled processes of crystallization kinetics, heat transport, and solute mass transport, for a freezing hydrometeor particle. We demonstrate the model for tracer partitioning during the freezing of a 1000 μm radius drop on a 100 μm ice substrate, under a few ambient condition scenarios. The model effectively simulates particle freezing and solute transport, yielding results that are qualitatively and quantitatively consistent with previous experimental and theoretical work. Results suggest that the ice shell formation time is governed by heat loss to air and not by dendrite propagation, and that the location of ice nucleation is not important to freezing times or the effective partitioning of chemical solutes. Even for the case of nucleation at the center of the drop, we found that dendrites propagated rapidly to form surface ice. Freezing then proceeded from the outside in. Results also indicate that the solid-liquid interfacial surface area is not important to freezing times or the effective partitioning of chemical solutes, and that the rate aspects of trapping are more important than equilibrium solid-liquid partitioning to the effective partitioning resulting from freezing.     

Numerical Simulations of the Physical Process for Hailstone Growth

1. IntroductionTheoretical and experimental studies on the phys-ical processes of hail growth (Schumann, 1938; Lud-lan, 1958; List, 1963) showed that its growth rate andstructural characteristics depend on the heat and masstransfers; its dynamic characteristics determine hail-stone's movement and stay in clouds and damage doneto ground bodies, actually controlling the growth in-side clouds. As we know, the heat transfers affectsdirectly hailstone's wet growth, melting and evapo-ration. In the…     

Mass transfer from discrete water drops on leaves in a cereal canopy

The boundary-layer resistance, r _d , for water vapour transfer from single drops on a wheat leaf was derived from field measurements of the evaporation rate, drop temperature and air humidity. Parameters are estimated in an equation to calculate r _d from drop diameter and wind speed. The relationship between resistance and wind speed is compared with that from other sources, and possible systematic errors in temperature measurements are examined using a model of the drop energy balance.On secondment from Department of Agricultural Sciences, University of Bristol, IACR, Long Ashton Research Station, Long Ashton, Bristol BS18 9AF, U.K.     

The energy balance of water drops on a leaf surface

The amount of heat conducted to an isolated drop of water on a leaf is described by an equation which includes an effective thermal conductivity coefficient. Measurements of the dimensions of water drops on a horizontal wheat leaf give relationships which allow the volume and exposed surface areas to be obtained from drop diameter. These relationships are used in the experimental determination of the drop boundary-layer resistance and the effective thermal conductivity coefficient for drops on a leaf in a chamber. The boundary-layer resistance of the drop appeared to be independent of drop size and the mean value was about 60% of that for one side of the leaf. For drops with diameters less than 1 mm, conduction of heat to the drop reduced the leaf-to-drop temperature difference by over 50% of the value expected without conduction. Conduction of heat to drops will significantly affect the evaporation rate of surface water from cereal canopies after rain.     

近40年京津冀蒸发皿蒸发量变化特征及影响因子

为了研究京津冀地区蒸发皿蒸发量的变化特征及成因,在京津冀地区200多个气象站中选择资料序列完整且具有较长时间序列、测站环境评分都在70分以上(按照中国气象局对测站探测环境评分标准评分)、均匀分布的87个气象站,利用1970—2013年京津冀地区87个气象站蒸发皿蒸发量以及其他气象要素的观测资料,采用线性倾向估计法和完全相关系数法,分析近44年来京津冀蒸发量变化特征及影响因子。结果表明,近44年来,京津冀地区年、季蒸发量呈明显下降趋势。全年蒸发量减少速率由大到小分别为:山前平原区太行山区冀东平原区燕山丘陵区冀北高原区(蒸发速率由北向南逐渐增大);四季中下降速率为:春季秋季冬季夏季。分析蒸发量与影响因子的完全相关系数发现,气温日较差、日照时数和平均风速是影响京津冀地区蒸发皿蒸发量变化的主要因子,在平原地区,平均风速是主导因子;在山区和高原地区,日照时数是主导因子。     

A method for determining thermophysical properties of organic material in aqueous solutions: Succinic acid

A method for determining evaporation rates and thermodynamic properties of aqueous solution droplets is introduced. The method combines evaporation rate measurements using modified TDMA technique with data evaluation using an accurate evaporation model. The first set of data has been collected and evaluated for succinic acid aqueous solution droplets.Evaporation rates of succinic acid solution droplets have been measured using a TDMA system at controlled relative humidity (65%) and temperature (298 K). A temperature-dependent expression for the saturation vapour pressure of pure liquid phase succinic acid at atmospheric temperatures has been derived by analysing the evaporation rate data with a numerical model. The obtained saturation vapour pressure of liquid phase succinic acid is ln(p) = 118.41 − 16204.8/T − 12.452ln(T). The vapour pressure is in unit of Pascal and the temperature in Kelvin. A linear expression for the enthalpy of vaporization for liquid state succinic acid is also presented.According to the results presented in the following, a literature expression for the vapour pressure of liquid phase succinic acid defined for temperatures higher than 461 K [Yaws, C.L., 2003. Yaws' Handbook of Thermodynamic and Physical Properties of Chemical Compounds, Knovel] can be extrapolated to atmospheric temperatures with very good accuracy. The results also suggest that at 298 K the mass accommodation coefficient of succinic acid is unity or very close to unity.     

A method for determining thermophysical properties of organic material in aqueous solutions: Succinic acid

A method for determining evaporation rates and thermodynamic properties of aqueous solution droplets is introduced. The method combines evaporation rate measurements using modified TDMA technique with data evaluation using an accurate evaporation model. The first set of data has been collected and evaluated for succinic acid aqueous solution droplets.Evaporation rates of succinic acid solution droplets have been measured using a TDMA system at controlled relative humidity (65%) and temperature (298 K). A temperature-dependent expression for the saturation vapour pressure of pure liquid phase succinic acid at atmospheric temperatures has been derived by analysing the evaporation rate data with a numerical model. The obtained saturation vapour pressure of liquid phase succinic acid is ln(p) = 118.41 − 16204.8/T − 12.452ln(T). The vapour pressure is in unit of Pascal and the temperature in Kelvin. A linear expression for the enthalpy of vaporization for liquid state succinic acid is also presented.According to the results presented in the following, a literature expression for the vapour pressure of liquid phase succinic acid defined for temperatures higher than 461 K [Yaws, C.L., 2003. Yaws' Handbook of Thermodynamic and Physical Properties of Chemical Compounds, Knovel] can be extrapolated to atmospheric temperatures with very good accuracy. The results also suggest that at 298 K the mass accommodation coefficient of succinic acid is unity or very close to unity.     

Simultaneous determinations of the sensible and latent heat transfer coefficients for tree leaves

The heat and mass transfer coefficients for exchange across the fluid dynamic boundary layer over tree leaves were simultaneously determined in a controlled environment chamber. The mass transfer coefficients were calculated from measured values of evaporation, air specific humidity and a value of leaf specific humidity at leaf temperature. The heat transfer coefficients were calculated from measured values of air temperature, leaf temperature and an estimate of the sensible heat flux density calculated as the measured net radiation at the leaf surfaces minus the latent heat flux density. The experiments described in this paper indicate that the equations based on laminar boundary-layer theory can give reasonable estimates of the transfer coefficients of real tree leaves for the velocities most commonly experienced in plant canopies, if they are adjusted by a constant multiplier greater than one. Calculations of local mass transfer coefficients based on temperature measurements at three locations at different distances from the leading edge of the leaves, indicate that the deviation from theory is probably the result of transition to turbulent boundary-layer flow at some distance from the leading edge.     

Experimental study of the transfer velocity for urban surfaces with a water evaporation method

A major problem in urban climate modelling is determining how the heat fluxes from various canyon surfaces are affected by canyon flow. To address this problem, we developed a water evaporation method involving filter paper to study the distribution of the convective transfer velocity in urban street canyons. In this method, filter paper is pasted onto a building model and the evaporation rate from the paper is measured with an electric balance. The method was tested on 2D (two-dimensional) street canyon models and 3D model arrangements. Moreover, in this technique, it is easy to restrict the flux within an arbitrary surface in question. That is, the evaporation distribution on a surface can be studied by using several small pieces of filter paper. In the 2D case, the wall transfer velocity was strongly dependent on the canyon aspect ratio for perpendicular wind directions and it varied widely with height within both windward and leeward wall surfaces. For 3D cubic arrays, the relation to canyon aspect ratio was largely different from that of the 2D canyon. And, as a case study, the variation of wind direction was investigated for a city-like setting. The area-averaged transfer velocity was insensitive to wind direction but its local deviation was significant. Finally, we measured the transfer velocity for a clustered block array surrounded by relatively wide streets. The effect of spatial heterogeneity on the transfer velocity was significant. Moreover, for a fixed total building volume, the transfer velocity was considerably larger when the building height varied than when it was uniform. Therefore, the water evaporation method with filter paper is expected to be useful for studying the transfer velocity and ventilation rates in urban areas with various canyon shapes.     

疏水性中空纤维膜组件操作过程模拟

采用疏水性中空纤维膜组件和去离子水分离混合气中CO2,研究了气液流速、混合气CO2浓度和操作温度以及膜形态等因素对总传质系数的影响。通过传质阻力层方程和质量微分方程的关联,建立了新型数学模型,模拟了各种条件下的传质过程。结果表明,流体力学状态的改变能够加强传质,但加强程度有限;提高气相CO2浓度能够提高总传质系数;具有高孔隙率的膜组件拥有高传质系数;提高操作温度能够促进扩散,提高传质系数,在较高温度下,存在膜孔湿润的现象。模型能够较好地模拟膜接触器—物理吸收过程,模型值能够较准确地反映疏水性中空纤维模组件传质过程。     

Some characteristics of cumulus convection over the Tibetan Plateau

The diagnostic model of the cumulus convection proposed by Yanai et al. (1973) was applied to the atmosphere over the Tibetan Plateau, and used to estimate the vertical mass flux, entrainment and detrainment, excess temperature and moisture, liquid water content, and condensation and precipitation rates of highland cloud clusters. The re-sults illustrated that in clouds over the Tibetan Plateau, the water vapor condensation rate, liquid water content, and efficiency of the rain generation process are less than those in the tropics (represented by the Marshall Islands region). Therefore, the condensational latent heat released over the Tibetan Plateau, overall, is much smaller than that in the tropics. The water vapor and liquid water detrainment from shallow nonprecipitating cumulus clouds, and their entrainment into deep cumulus clouds, serve as a growing mechanism for the deep precipitating cumulus towers over the Tibetan Plateau. It should be noted that there is a stronger detrainment of liquid water from cumulus clouds and a stronger re-evaporation rate in environment. The process of the condensation-detrainment-re-evaporation-entrainment is repeatedly in progress. It would play an important role in maintaining of cumulus convection on the condition that the supply of moisture is not plentiful over the Tibetan Plateau.The analyses also showed that the cloud mass flux Mc over the Tibetan Plateau is less, and the large-scale av-erage upward motion is much less than those over the Marshall Islands. Stronger compensating downward motion in the cloud environment over the Tibetan Plateau, responsible for the area’s strong environmental heating rate was re-vealed, and would link to the stability of the South Asian High in summer.     

Air-sea interaction including a shallow and coastal zone

We performed special experimental and theoretical research concerning the energy--mass exchange process in shallow waters, which took into account the influence of the basin depth on the evaporation and sensible heat exchange. Data was obtained from deep water basins, as well as from shallow waters, at an open sea, as well as in its coastal zone. A new parameterization model of evaporation and friction velocity from shallow water surfaces under different wind velocities was investigated. Results of models and measurements of the energy/mass exchange intensity of a small shallow lake (LITFASS-experiments) were compared. The validation of these models with the eddy-covariance measurements of the LITFASS-98 and LITFASS-2003 experiments showed good results for the wind sector, with good fetch conditions. Therefore, the models examined may be used for calculating the evaporation of lakes where a standard data set of wind velocity, air and water temperature, air moisture and the depth of the lake is available. It should be remembered that in a coastal zone the roughness of the water surface is transformed and therefore there are additional calculation difficulties for the intensity of the energy-mass exchange. Based on our experimental data of the basin depth influence on the water-atmosphere exchange, a new model for the calculation of the energy-mass exchange in a coastal zone was developed. Our new model in combination with the empirical dependence for the calculation of the energy-mass exchange in a coastal zone allows calculation of the momentum, heat and humidity fluxes values at different distances from a shore.     

Evaporation estimates from the Dead Sea and their implications on its water balance

The Dead Sea (DS) is a terminal hypersaline water body situated in the deepest part of the Jordan Valley. There is a growing interest in linking the DS to the open seas due to severe water shortages in the area and the serious geological and environmental hazards to its vicinity caused by the rapid level drop of the DS. A key issue in linking the DS with the open seas would be an accurate determination of evaporation rates. There exist large uncertainties of evaporation estimates from the DS due to the complex feedback mechanisms between meteorological forcings and thermophysical properties of hypersaline solutions. Numerous methods have been used to estimate current and historical (pre-1960) evaporation rates, with estimates differing by ??100%. Evaporation from the DS is usually deduced indirectly using energy, water balance, or pan methods with uncertainty in many parameters. Accumulated errors resulting from these uncertainties are usually pooled into the estimates of evaporation rates. In this paper, a physically based method with minimum empirical parameters is used to evaluate historical and current evaporation estimates from the DS. The more likely figures for historical and current evaporation rates from the DS were 1,500?C1,600 and 1,200?C1,250?mm per annum, respectively. Results obtained are congruent with field observations and with more elaborate procedures.     

华南秋季蒸发量的时空演变特征

利用华南区域66个气象站点1960～2004年的观测数据分析了华南秋季蒸发皿蒸发量和实际蒸发量的时空变化.分析结果表明:华南中部和西北部是华南秋季蒸发皿蒸发量的两个主要气候变异中心区,华南中部秋季蒸发皿蒸发量具有以年代际变化为主的特征,并且在45年内总体上呈下降趋势.在影响蒸发皿蒸发量的因子中,太阳辐射与蒸发皿蒸发量的相关性最好,呈显著的正相关.对实际蒸发量而言,华南中部和西部偏西地区则是两个主要的变异中心,两区域的秋季实际蒸发量具有以年际变化为主的特征,降水对华南秋季实际蒸发量的影响最为显著,华南秋季实际蒸发量一般都在蒸发皿蒸发量的40%左右,并且比值总体上呈现微弱的由南向北递增趋势.     

On the scavenging of SO2 by large and small rain drops: V. A wind tunnel and theoretical study of the desorption of SO2 from water drops containing S(IV)

An experimental and theoretical study has been carried out to investigate the rate of desorption of SO₂ from water drops falling at terminal velocity in air. The experiments were carried out in the Mainz vertical wind tunnel in which water drops of various sizes containing S(IV) in various concentrations were freely suspended in the vertical airstream of the tunnel. The results of these experiments were compared with the predictions of three theoretical models, and with the experiments of Walceket al. This comparison shows that the predictions of the diffusion model of Kronig and Brink in the formulation given by Walcek and Pruppacher agree well with the experimental results for all relevant large and small rain-drop sizes, and for all considered concentrations of S(IV) inside the drops. In contrast, the predictions of the diffusion model which assumes complete internal mixing inside a drop agrees with the experimental results only if the concentration of S(IV) inside the drop is less than that equivalent of an equilibrium SO₂ concentration of 15 ppbv. At larger concentrations, the theoretical predictions of the model for complete internal mixing progressively deviate from the experimental results. It is further shown that Barrie's double film model can be used to interpret the resistance to diffusion inside a drop in terms of a diffusion boundary layer inside the drop which increases in thickness with decreasing concentration of S(IV). Applying our results to the desorption of SO₂ from small and large rain drops falling below an assumed cloud base, shows that for typical contents of S(IV) inside the drops substantial amounts of SO₂ will desorb from these drops unless H₂O₂ is present in the surrounding air.     

On the scavenging of SO2 by cloud and rain drops: IV. A wind tunnel and theoretical study of the absorption of SO2 in the ppb(v) range by water drops in the presence of H2O2

An experimental study involving the Mainz vertical wind tunnel is described where the rate of SO₂ removed from the air by freely suspended water drops was measured for SO₂ concentrations in the gas phase ranging between 50 and 500 ppb, and for various H₂O₂ concentrations in the liquid phase. In a first set of experiments, the pH inside the SO₂ absorbing drops was monitored by means of colour pH indicators added to the drops. In a second set of experiments, the amount of SO₂ scavenged by the drops was determined as sulfate by an ionchromatograph after the drops had been removed from the vertical air stream of the wind tunnel after various times of exposure to SO₂. The results of our experimental study were compared with the theoretical gas diffusion model of Walcek and Pruppacher which was reformulated for the case of SO₂ concentrations in the ppbv(v) range for which the main resistance to diffusion lies in the gas phase surrounding the drop. Excellent agreement between experiment and theory was obtained. Encouraged by this agreement, the theory was used to investigate the rate of sulfate production inside a drop as a function of pH. The sulfate production rate, which includes transport and oxidation, was compared with the production rate based on bulk equilibrium, as cited 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.     

干旱及半干旱区土壤水热传输模式研究

本文发展了一个干旱地区土壤中水分及热量传输模式。首先从土壤孔隙内水汽运动研究入手,阐明了在土壤深层孔隙中水汽压与液面饱和值之间处在平衡态,而土壤表层则处在非平衡态,因而必要在计算表层蒸发的公式中引入必要的土壤表层阻力,才可使模式简单实用。在此基础上建立了考虑液态水及汽态水运动耦合的多层模式,并用HEIFE沙漠站的资料对模式进行了验证,结果表明此模式较好地再现土壤内及地气界面上的水热交换过程,并且也表明干旱地区土壤中水蒸汽输送对水分平衡及蒸发的计算是重要的。这种模式很易推广到气候研究的干旱土壤下垫面的模式中去。     

SPATIAL AND TEMPORAL VARIATIONS OF EVAPORATION OVER SOUTH CHINA IN AUTUMN

The spatial and temporal variations of the instrument-based evaporation and actual evaporation in autumn during a 45-year period from 1960 to 2004 are studied using the observation data from 66 stations over South China. The results reveal that there are two main anomalous centers of the instrument-based evaporation in autumn in the central and northwestern parts of South China respectively. The instrument-based evaporation over the central part of South China in autumn experiences not only a decreasing trend but also a main interdecadal variation. The solar radiation is best correlated with the instrument-based evaporation among all affecting factors. For the actual evaporation, two main anomalous centers are located at the central and western parts of the South China respectively. The actual evaporation over the two regions illustrates an interannual variation. Among the affecting factors, precipitation is the most remarkable. The actual evaporation is usually 40 percent of the instrument-based one, and the overall rate has a slightly increasing trend from the southern part to the northern part of the South China in autumn.     

吉林省一次层状云降水宏微观特征的观测研究

利用长春2004年7月5日一次降水过程的飞机观测资料,结合天气图、卫星云图及雷达回波等资料,综合分析了本次热带气旋影响下降水过程中云系的宏微观特征.研究表明,降水性层状云微观垂直结构配置可以分为4个发展层:云顶附近是核化和凝华增长区;-4～-7 ℃为云滴和冰粒子活跃增长层;0～-4 ℃为固态粒子聚合及云滴蒸发层;0 ℃层以下是雨滴碰并增长和云滴凝结增长区.2D-P观测的粒子的平均直径、最大直径、峰值直径的峰值集中在融化层附近,与融化层回波亮带对应.用同一种形式的密度分布函数N(r)=mrfexp(-ar br2-cr3)来拟合暖层小云滴、大云滴和雨滴的谱分布,拟合结果与观测的谱分布吻合较好,拟合出的平均直径、均方根直径、数浓度以及含水量与观测值也较接近,相对误差小.