On convective entrainment in a mass flux cumulus parameterization

Summary A new entrainment/detrainment formulation in the Tiedtke’s mass flux cumulus parameterization is discussed here. Apart from validating it with observations both in one and three dimensional cases, it is also evaluated in the light of the results from the original Tiedtke scheme and another mass flux scheme due to Emanuel. The proposed analytical profiles of entrainment and detrainment, easier to implement in any mass flux scheme, give reasonable results in GCM experiments.     

On the modelling of mixed layers and entrainment in cumulus cloud

A boundary-layer model, which incorporates an eddy diffusivity for turbulent transports, is presented. It is shown that this model can predict the appropriate behaviour for mixed layers produced by surface shear, free convection and mechanical mixing. The model is used to consider the development of cumulus clouds, which are shown generally to be convectively unstable at cloud top. Thus the entrainment of dry air leads to a very low cloud-water content in cumulus cloud.     

The growth of cumulus over mountains

Summary In order to make the complicated process of thermal convection intelligible it is helpful if various component processes can be considered separately. It is well known that cumulus grow preferentially over mountains by day. The motion is analysed into thethermals, which are small scale, and thecirculations, which may be very large according to the topography. The circulations possess inertia which causes motion to persist into the night. The existence of clouds makes possible circulations in statically stable air and prevents the growth of clouds in other places. Nocturnal convection clouds are a complicated result of these and other minor effects. The forecasting of convection clouds is best done by comparison with previous occasions, preferably the previous day rather than by analysis of the present weather situation alone.

---

Zusammenfassung Um den komplizierten Vorgang der Wärmekonvektion verständlich zu machen, empfiehlt es sich, die verschiedenen Teilprozesse getrennt zu betrachten. Bekanntlich bilden sich Cumuli bevorzugt über Bergen bei Tage. Der Bewegungsvorgang läßt sich zerlegen in Wärmeströmungen von kleinen Dimensionen und in Zirkulationsströmungen, die entsprechend der Topographie sehr groß sein können. Die Zirkulationsvorgänge besitzen eine Trägheit, die die Bewegungsvorgänge bis in die Nacht andauern läßt. Das Vorhandensein von Wolken ermöglicht das Auftreten von Zirkulationen in statisch stabiler Luft und verhindert das Wachsen von Wolken an anderen Stellen; so bilden die nächtlichen Konvektionswolken ein kompliziertes Ergebnis dieser und anderer kleiner Effekte. Die Voraussage von Konvektionsbewölkung erfolgt am besten durch Vergleichung mit früheren Fällen, womöglich mit dem Vortage, eher als durch Analyse der momentanen Wettersituation allein.

---

Résumé Afin de rendre compréhensible le problème compliqué de la convection thermique, il est utile de considérer séparément les différents processus y prenant part. Généralement, les cumuli se forment de préférence en plein jour au-dessus des montagnes. Ce mouvement peut être divisé en courants thermiques de dimensions modestes et en courants de circulation qui, suivant la topographie du lieu, peuvent prendre une grande envergure. Ces courants de circulation possèdent une inertie qui permet à ces mouvements de persister pendant la nuit. La présence de nuages permet en outre des courants de circulation dans de l'air statiquement stable et peut empêcher des nuages de se former ailleurs. Les nuages de convection nocturnes sont, entre autre, le résultat de pareils effets. On sera mieux avisé de fonder la prévision des nuages de convection en comparant des cas précédents, notamment de la veille, plutôt qu'en analysant seulement la situation météorologique du moment.

---

---

With 2 Figures.

---

Presented at the 3rd International Meeting on Alpine Meteorology, Davos 12th–14th April, 1954.     

Les cumulus dans les Alpes valaisannes

Résumé Les cumulus de beau temps et les cumulonibus locaux ont leur base comprise entre 1500 et 4500 m. dans les Alpes valaisannes (Suisse); l'altitude moyenne s'établit à 3160 m. Variation diurne et saisonnière du niveau de condensation et spectre de fréquence.

---

Summary In the Alps of the Valais (Switzerland) the base of the height of the fine weather cumuli and of the cumulo-nimbi of local character is situated between 1500 and 4500 m. above sea level; the average height is 3160 m. The diurnal and the annual variation of the condensation height and the frequency distribution are discussed.

---

Zusammenfassung Die Basishöhe der Schönwettercumuli und der Cumulonimben lokalen Charakters liegt in den Walliser Alpen (Schweiz) zwischen 1500 und 4500 m über Meer; die mittlere Höhe beträgt 3160 m. Es werden die Tages- und die Jahresschwankung der Kondensationshöhe und Frequenzverteilung untersucht.

     

A model of fair-weather cumulus convection

By assuming that cumulus clouds grow from patches of air that extend from the well-mixed layer bear the surface, a model of fair-weather cumulus convection is developed. The model predicts the structure of the well-mixed layer and the cloud layer; in particular, cloud cover is estimated as a function of time. The model results are compared with laboratory and field observations.     

Cumulus mergers in the maritime continent region

Summary We examine a family of tall (up to 20 km) cumulonimbus complexes that develop almost daily over an adjacent pair of flat islands in the Maritime Continent region north of Darwin, Australia, and that are known locally as Hectors. Nine cases observed by a rawinsonde network, surface observations (including radiation and soil measurements), the TRMM/TOGA radar, and one day of aircraft photography are used to analyse the development, rainfall, surface energy budgets, and vertical structure of these convective systems.The systems undergo convective merging which is similar to that observed in previous Florida studies and is multiplicative in terms of rainfall. About 90% of the total rainfall comes from the merged systems, which comprise less than 10% of convective systems, and this has implications for the manner in which tropical rainfall is parameterised in largerscale numerical models. By comparison to the West Indies, GATE, and Florida, the Hector environment contains a weaker basic flow, with less vertical shear. The main thermodynamic difference is that the Darwin area has an unstable upper troposphere and very high tropopause. Numerical modelling results support earlier observations of updraughts in excess of 30 ms^–1 in this region, but show that only modest convective drafts are experienced below the freezing level (5 km).The surface fluxes over the islands are estimated from a Monash University study to be mainly in latent form from evapotranspiration, with a Bowen ratio only slightly larger than that commonly observed over oceans. These surface fluxes are crucial to the development of a suitable mixed layer to support deep convection. The flux estimates agree with the observed changes below the cloud base and provide sufficient information for calculations of the bounds on precipitation efficiency. Of particular interest are the observations of Hector development on a day when the islands were under a dense cirrus overcast. We find that the islands still provide sufficient net sensible and latent heat fluxes to initiate convection.With 10 Figures     

Impacts of cumulus momentum transport on MJO simulation

Vertical cumulus momentum transport is an important physical process in the tropical atmosphere and plays a key role in the evolution of the tropical atmospheric system. This paper focuses on the impact of the vertical cumulus momentum transport on Madden-Julian Oscillation (MJO) simulation in two global climate models (GCMs). The Tiedtke cumulus parameterization scheme is applied to both GCMs [CAM2 and Spectral Atmospheric general circulation Model of LASG/IAP (SAMIL)]. It is found that the MJO simulation ability might be influenced by the vertical cumulus momentum transport through the cumulus parameterization scheme. However, the use of vertical momentum transport in different models provides different results. In order to improve model's MJO simulation ability, we must introduce vertical cumulus momentum transport in a more reasonable way into models. Furthermore, the coherence of the parameterization and the underlying model also need to be considered.     

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.     

Analysis of cumulus solar irradiance reflectance (CSIR) events

Clouds are extremely important with regard to the transfer of solar radiation at Earth's surface. This study investigates Cumulus Solar Irradiance Reflection (CSIR) using ground-based pyranometers. CSIR events are short-term increases in solar radiation observed at the surface as a result of reflection off the sides of convective clouds. When Sun-cloud observer geometry is favorable, these occurrences produce characteristic spikes in the pyranometer traces and solar irradiance values may exceed expected clear-sky values. Ultraviolet CSIR events were investigated during the summer of 1995 using UVA and UVB pyranometers. Observed data were compared to clear-sky curves which were generated using a third degree polynomial best-fit line technique. Periods during which the observed data exceeded this clear-sky curve were identified as CSIR events. The magnitude of a CSIR event was determined by two different quantitative calculations. The MAC (magnitude above clear-sky) is an absolute measure of the difference between the observed and clear-sky irradiances. Maximum MAC values of 3.4 Win⁻² and 0.0169 Wm⁻² were observed at the UV-A and UV-B wavelengths, respectively. The second calculation determined the percentage above clear-sky (PAC) which indicated the relative magnitude of a CSIR event. Maximum UV-A and UV-B PAC magnitudes of 10.1% and 7.8%, respectively, were observed during the study. Also of interest was the duration of the CSIR events which is a function of Sun-cloud-sensor geometry and the speed of cloud propagation over the measuring site. In both the UV-A and UV-B wavelengths, significant CSIR durations of up to 30 minutes were observed. C 1997 Elsevier Science B.V.     

积云对流对涡度场和散度场的反馈作用

在涡度和散度方程简要讨论的基础上,计算得出长江中下游地区暴雨过程中涡度和散度收支各项的水平和垂直分布。结果表明,雨带附近对流层高层是源、低层是汇,尤其以散度场的不平衡更为显著,可见积云对流垂直输送涡度和散度是重要过程。     

Parameterization of a momentum source in a tropical cumulus ensemble

Summary An eddy effect of tropical deep convection on the large-scale momentum, resp vorticity budget is investigated. The process is specified by a simple parameterization approach which is based on a concept of rotating clouds exerting a momentum on the large-scale flow. The cloud rotation is associated with the thermal properties of a cloud ensemble by the principle of conservation of potential vorticity. A decomposition of cloud classes is applied in consistency with the thermodynamical parameterization scheme of Arakawa and Schubert (1974).The parameterization is tested with observations of GATE74, Phase III. The data are processed on a B/C-scale grid (55km) in a region within 9N and 16N, and between 21W and 27W, and with a vertical resolution of 41 levels. The parameterization results correspond to the observed patterns, especially in situations with strong large-scale wind shear. The findings suggest that certain large-scalle flow regimes provoke convective scale momentum generation rather than redistributing large-scale momentum by convective circulations.With 10 Figures     

西太平洋热带海域秋冬积云对流输送的初步研究

本文建立以考虑云侧边夹卷效应和微物理过程的一维半暖积云模式为基础的积云输送模式,并利用此模式和实测雷达资料,导出了计算西太平洋热带海域积云群体输送公式,对此进行了实际计算,还讨论了积云群体输送对大气状态的影响。     

一次山地积云并合扩展层化过程的数值模拟

因复杂地形下热力和动力抬升对近地面空气的扰动作用,贵州地区容易形成内部嵌有许多小对流单体的积层混合云.选取2005年5月29日发生在贵州省的一次积层混合云降水个例进行分析,并利用WRF模式模拟该云系的生成、发展过程.结果表明:积层混合云由积云并合扩展层化形成,其发展过程经历三个典型的并合阶段.云系的降水特点是降水范围很大,分布不均匀,雨区中存在多个强降水中心,降水量累计最大值可达60 mm,且强降水中心与云中小对流单体的位置对应;积层混合云形成过程中,地面产生强降水的最终原因是,云并合过程中释放的不稳定能量改变了云中的气流场和含水量场.     

An improvement in cumulus parameterization by invoking outgoing longwave radiation

Summary The Kuo-type cumulus parameterization and rainfall rate scheme is extended by including the outgoing longwave radiation (OLR), the local time rate of change of OLR, and the horizontal Laplacian of OLR to include the additional source of mesoscale moisture supply. The extended scheme remains simple and efficient. A series of 5-day forecasts of the 1979 monsoon for the tropical belt between the Arabian Sea and the western Pacific Ocean show improvement in the 3-day forecast in the daily accumulated precipitation in both the amount and location. Further RMS error analyses indicate that using OLR and the Laplacian of OLR also produces a significant improvement in a 3-day forecast. Additional tests of the extended scheme for the 1987 and 1988 monsoons produced consistent and characteristic areal rainfall rates in the tropics. The improvement in the OLR model forecasts is attributed to the improved skill of OLR forecast in the forecast model as the RMS errors for 5 consecutive days in the 6 experimental forecasts are smaller than those of the 3 control forecasts.With 12 Figures     

积云对流中扰动压力效应的诊断分析

本文用简单的结构模式分析诊断了扰动压力对积云对流的效应.结果表明:由浮力项、平流项和拖曳力项触发引起的扰动气压垂直梯度力与各对应源项具有同等量级,但与其源项的作用力方向相反.扰动气压梯度力在云的中上部为负力,它抑制了云的生长发展;在云的下部为正力,它使云向上的加速度增大.扰动气压梯度力对深对流的影响要大于对浅对流的影响.在云下部的扰动低压中心位于云边缘附近.浅对流与深对流的量级分别达-0.1— -0.2hPa和-0.2— -0.4hPa.     

Coupling mechanisms between equatorial waves and cumulus convection in an AGCM

In this study, we focused on the difference in appearances of the convectively coupled equatorial waves (CCEWs) in a simulation with the CCSR/NIES/FRCGC AGCM, between two experiments, one with and the other without implementation of the convective suppression scheme (CSS) in the prognostic Arakawa–Schubert cumulus parameterization. Realistic CCEW modes, i.e., Kelvin, Rossby, mixed Rossby-gravity (MRG), and n = 0 eastward inertio-gravity (EIG) wave modes, were reproduced in the with-CSS experiment, while only Rossby-wave-like signals appeared in the without-CSS experiment.By comparing the structures of the Kelvin wave mode and the Rossby wave mode in two runs, it was suggested that the structural difference between these two modes in conjunction with the difference in the controlling factor of cumulus convection determines the CCEW features. The CSS implemented here is such that cumulus convection is suppressed until the cloud-layer-averaged relative humidity exceeds the threshold of 80%. In the without-CSS model, only Rossby wave modes are coupled with the convection. This is because CAPE controls cumulus convection in this model, and the larger frictional convergence of Rossby wave mode prepares CAPE to generate favorable condition for cumulus convection. In the case of the with-CSS model, on the other hand, cumulus convection is largely controlled by the humidity in the free atmosphere. The convergence associated with the equatorial waves can produce the moisture anomaly to overcome the relative humidity threshold, and maintains the favorable condition for cumulus convection once it starts. In this case, not only Rossby waves but also Kelvin, MRG, and n = 0 EIG waves are reproduced more realistically. It is suggested that inclusion of some kind of mechanism connecting the free tropospheric moisture with the convection under the condition of abundant convective available potential energy could be a key factor for realistic coupling between large-scale atmospheric waves and convection.     

盐粉催化不同生命史的浓积云的数值模拟

用改进的一维时变参数化模式计算了浓积云自然发展和盐粉催化过程.在不同的地面温度条件下积云的动力过程和自然雨量有显著差异,其催化效果也不同.一般的自然降水浓积云,其动力过程演变较快,雨量较小,催化增雨量则较大,可达50％以上.动力过程长期维持的自然降水云,雨量较大,而催化增雨量较小,个别情况下催化后雨量稍减.自然不降水的浓积云催化后可能产生降水.它们的动力过程越稳定,催化后的雨量越大.文中还讨论了增雨机制,对盐粉的最佳粒度、剂量、播撒时机和部位也作了一些数值试验.     

The MJO in an AGCM with three different cumulus parameterization schemes

The Madden–Julian oscillation (MJO) is simulated using an AGCM with three different cumulus parameterization schemes: a moist convective adjustment (MCA) scheme, the Zhang–McFarlane (ZM) mass-flux scheme, and the Tiedtke scheme. Results show that the simulated MJO is highly dependent on the cumulus parameterization used. Among the three cumulus parameterizations, only the MCA scheme produces MJO features similar to observations, including the reasonable spatial distribution, intraseasonal time scales and eastward propagation. Meanwhile, the amplitude is too large and the eastward propagation speed too fast than observations and the relationship between precipitation and low-level wind anomaly is unrealistic with enhanced convection occurring within easterly anomalies instead of westerly anomalies as in observations. The over-dependence of precipitation on boundary convergence produced by the MCA scheme is presumably responsible for this unrealistic phase relation in the simulation. The other two schemes produce very poor simulations of the MJO: spectral power of westward propagation is larger than that of eastward propagation in zonal wind and precipitation, indicating a westward propagation of the intraseasonal variability.The mean state and vertical profile of diabatic heating are perhaps responsible for the differences in these simulations. The MCA scheme produces relatively realistic climate background. When either ZM or Tiedtke scheme is used, the observed extension of westerly winds from the western Pacific to the dateline is missing and precipitation over the equatorial region and SPCZ is dramatically underestimated. In addition, diabatic heating produced by both ZM and Tiedtke schemes are very weak and nearly uniform with height. The heating profile produced by the MCA scheme has a middle-heavy structure with much larger magnitude than those produced by the other two schemes. In addition, a very unrealistic boundary layer heating maximum produced by the MCA scheme induces too strong surface convergence, which perhaps contributes to the too strong intraseasonal variability in the simulation.     

强对流系统中对流云合并的观测分析

用雷达等观测资料以及LAPS(Local Analysis and Prediction System)中尺度再分析场资料,对2009年6月14日在河南、安徽等地引发冰雹大风天气的两个强对流系统的形成演变过程中的对流云合并现象进行观测分析.研究表明,不论是单体间的合并还是对流系统间的合并,合并后单体或对流系统都有显著的...     

盐粉催化积云降水的数值模拟

本文利用二维直角坐标非定常积云降水模式模拟播撒盐粉催化降水过程.播撒是在云中一个网格点中一次引入盐粉,盐粉进云后碰并云滴长大并随云中气流移动和沉降.在计算盐粉成雨的同时还计算自然成雨. 计算结果表明,在合适条件下,盐粉催化后约半小时可出现增雨,增加雨量为10％—50％,它是播撒量的几千倍.盐粉直径小的降雨效率高,但在云中生长时间长,一般采用几十微米至一百微米直径的盐粉为宜.