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.

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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.

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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.

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With 2 Figures.

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

Local tomography troposphere model over mountains area

The term GNSS meteorology refers to the utilization of the Global Navigation Satellite System's (GNSS) radio signals to derive information about the state of the troposphere. GNSS tomography allows to resolve the spatial structure and temporal behavior of the tropospheric water vapor. This paper presents the verification of GNSS tomography over dense local GNSS network. The paper addresses the problem of obtaining a stable tomographic solution from an ill-conditioned system of linear equations. The main interests are in suitable horizontal and vertical resolution in given conditions. Here the Moore–Penrose pseudo inverse of variance–covariance matrix is used. The minimum constraints solution is obtained with no additional assumptions. The results are validated with the help of simulated weather conditions. Three various scenarios are tested. As general output of this paper the optimal model construction scheme is presented with possible further improvements. The verification of the tomography model based on the local GPS KARKONOSZE, situated in the Karkonosze mountains area in Poland.     

Observation of polar stratospheric clouds over Obninsk in December 2012

In late December 2012 a blocking anticyclone followed by the event of minor stratospheric warming, set in the troposphere over West Siberia and, after that, over the European part of Russia. As a result of the deformation of a polar stratospheric vortex, the temperature in the lower stratosphere over Obninsk dropped below the threshold of the formation of polar stratospheric clouds. The lidar measurements of temperature, ozone values, and aerosol characteristics in the middle atmosphere were carried out at the lidar station during this atmospheric event. In three cases, polar stratospheric clouds (PSCs) referred to NAT I_a type according to the sounding results, were registered at the height of about 20 km. No considerable decrease in the ozone concentration in the area of PSC formation was revealed in these measurements.     

青藏高原云的研究进展

青藏高原东侧九龙地区是西南涡多发区,利用该地区新型探测设备开展云探测,有助于增强对西南涡多发区云特征的认识。利用2018—2019年6—8月九龙站地基微波辐射计资料,分析了该地区夏季非降水云的出现率、液态水路径及过冷水路径的观测特征。结果表明：九龙夏季非降水云出现率月均值在67%~82%之间,以低云和中云为主,高云较少;低云出现率表现为白天低、夜间高,而中云和高云则相反;云出现率的垂直分布表现为单峰形态,在约2 km高度存在云出现率峰值8.1%;受大气热力层结日变化影响,云出现率的单峰垂直分布呈现日夜差异。另外,九龙夏季非降水云的液态水路径均值为0.433 kg·m^-2,其中低云、中云、高云的液态水路径均值分别为0.665、0.240、0.102 kg·m^-2;低云的液态水路径日变化特征与其出现率相似,而中云和高云的液态水路径日变化特征不明显。此外,九龙夏季非降水云中冷云的过冷水路径均值为0.154 kg·m^-2,其中低云、中云、高云的过冷水路径均值分别为0.065、0.166、0.102 kg·m^-2;总体上过冷水路径在液态水路径中的占比约为34.3%~38.8%,过冷水路径占比随云的高度而增大,这使得中云和高云的过冷水路径日变化与其液态水路径相似。与同纬度华中地区相比,九龙夏季非降水云具有明显不同的特征,这与两地之间的大气水汽特征差异密切相关。

     

Mid-level clouds over the Sahara in a convection-permitting regional model

Climate Dynamics - The simulation of Saharan mid tropospheric clouds is investigated with the weather research and forecasting (WRF) regional atmospheric model at convection permitting...     

Influence of aerosol on clouds over the Indo-Gangetic Plain, India

Using Total Ozone Mapping Spectrometer Aerosol Index (AI) and NCEP/NCAR reanalysis clouds data for the period 1979–1992, the influence of aerosol on the clouds (low and high cloud cover) over the Indo-Gangetic Plain (IGP) in India has been brought out for the first time in the present study. AI shows increasing tendency over the IGP suggesting that aerosol loading over this region increased significantly during the study period. In our analysis, High Cloud Cover (HCC) shows increasing trend and Low Cloud Cover (LCC) shows decreasing trend over the IGP during the same period. During pre-monsoon season when aerosol loading is more, HCC increases in positive correlation with AI. On the other hand, LCC show decreasing trend and is anti-correlated with AI. During summer monsoon, aerosol shows increasing trend but their effect on HCC and LCC is not seen to be significant. Similarly, the role of humidity on aerosol induced changes in HCC and LCC over the IGP region was also analyzed. In the low to moderate humid areas of IGP region (western and middle IGP), increasing AI leads to increase in HCC and decrease in LCC. On the other hand, in high humid areas (eastern IGP), increase in AI does not show any significant effect on HCC, but LCC shows positive trend. Therefore, we strongly argue that increasing aerosol loading enhances Cloud Condensation Nuclei over the region which in turn, alters the microphysical properties of clouds by reducing the size of cloud droplets, and atmospheric humidity controls the aerosol effect on clouds. During the recent period (2005–2010), similar features have also been observed over the IGP region.     

卷云对青藏高原的射出长波辐射影响的研究

本文叙述了一种求解卷云射出长波辐射的方法。通过建立卷云示迹模式,用累加法进行分析,并通过分析卷云的一些微物理特征,假设卷云含水量垂直分布模式,由三层近似化筒求得射出长波辐射通量密度。根据模式计算得到:1.与卫星观测青藏高原高云地区对应射出长波辐射通量密度基本一致的结果。2.卷云底高度及卷云厚度与射出长波辐射通量密度相关的一些有意义的结果,并讨论了青藏高原上卷云影响的特殊情况。     

Intercomparison of height assignment methods for opaque clouds over the tropics

Several methods of determining the height of opaque clouds over the tropics were compared using geostationary satellite measurements. The possible use of ozone channel measurements around the 9.7-μm ozone absorption band was examined in conjunction with the infrared window (IRW; 10.8 μm), H₂O (6.3 μm), and CO₂ (13.4 μm) channels, which are generally used for the assignment of cloud heights. Cloud top heights were retrieved from Meteosat-8 measurements with the aid of radiative transfer calculations using reanalysis data from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) as inputs. By using cloud top heights from collocated CloudSat observations as a reference, cloud top heights were determined from the one-channel radiance, two-channel brightness temperature difference (BTD), and two-channel radiance ratio methods, and the respective results were then compared for clouds with geometrical thicknesses of > 4 km. Overall, the retrievals from the CO₂-IRW ratio and O₃-CO₂ ratio methods are in substantial agreement with CloudSat observations, while the other methods either underestimate cloud top heights or demonstrate a lower ratio of successful height assignment. The O₃-CO₂ ratio method appears to be less practical than the CO₂-IRW ratio method because it requires two absorption channels. Our comparison also shows that the BTD between the ozone and IRW channels yields information that is similar to that of the IRW channel alone. It further shows that the O₃-IRW combination is not appropriate for the two-channel radiance ratio method. These results suggest that the inclusion of the ozone channel in BTD and ratio methods may not offer any significant improvement in convective cloud height retrieval over the tropics. In conclusion, the CO₂-IRW ratio method appears to provide the most accurate retrievals for opaque clouds.     

The Modified Envelope Orography and the Air Flow over and around Mountains

By use of the two-layer adiabatic globe spectral model and the zonally averaged climatic data of winter season as initial values, 10-day integrations are carried out based on three kinds of model topography (i.e., (1) the averaged topography; (2) the envelope topography; (3) the modified envelope topography). The results show that the orography of the Northern Hemisphere plays an important role in the simulation of large-scale weather patterns in winter season. The simulation based on the envelope topography developed by Wallace et al. has some improvements in the Rocky Mountains area. But this scheme causes very serious horizontal expansion around the Tibetan Plateau (hereafter referred to as the TV). A modified envelope topography scheme has been worked out that increases the slope of the TP by decreasing the horizontal expansion while keeping the maximum altitude. The results show some improvements of the scheme around the TP. By analysis of the mechanical effects of the large-scale orography on the     

Aerosol, clouds and rainfall: inter-annual and regional variations over India

Inter-annual and regional variations in aerosol and cloud characteristics, water vapor and rainfall over six homogeneous rainfall zones in India during the core monsoon month of July from 2000 to 2010, and their correlations are analyzed. Aerosol optical depth (AOD) and aerosol absorbing index (AAI) in July 2002, a drought year are higher over India when compared to normal monsoon years. The drier conditions that existed due to deficient rainfall in July 2002 could be responsible for raising more dust and smoke resulting in higher AODs over India. In addition, over India precipitation is not uniform and large-scale interruptions occur during the monsoon season. During these interruptions aerosols can build up over a region and contribute to an increase in AODs. This finding is supported by the occurrence of higher anomalies in AOD, AAI and rainfall over India in July 2002. Aerosol characteristics and rainfall exhibit large regional variations. Cloud effective radius (CER), cloud optical thickness and columnar water vapor over India are the lowest in July 2002. CER decreases as AOD and AAI increase, providing an observational evidence for the indirect effect of aerosols. Eighty percent of CER in northwest India, and 30% of CER over All India in July 2002 are <14 μm, the precipitation threshold critical cloud effective radius. Northeast India shows contrasting features of correlation among aerosols, clouds and rainfall when compared to other regions. These results will be important while examining the inter-annual variation in aerosols, cloud characteristics, rainfall and their trends.     

华东地区夏季云微物理结构的飞机观测分析

利用飞机搭载云粒子探头对2014年8月12-28日华东地区云的空间分布特征进行了探测,分析了云的垂直结构和水平分布特征,结合同时探测的气溶胶数据,探讨了云与气溶胶的相互作用关系。探测结果表明,安徽地区层状云云滴平均数浓度在24~297 cm^-3,液态含水量在0.04~0.13 g·m^-3,云滴数浓度随云底高度升高而减小,云滴粒径则随云底升高而增大。层积云(Sc)和雨层云(Ns)的云滴数浓度在云底最高,随高度上升浓度下降,液态含水量在云中部最高,云顶和云底处较低,高层云(As)云滴数浓度和液态含水量峰值均出现在云中上部。云的水平分布不均匀,云粒子双峰分布区域对应液态含水量高值区。Ns对气溶胶清除作用明显,清除方式以活化清除为主、碰并清除为辅。     

Identification of forcing mechanisms of convective initiation over mountains through high-resolution numerical simulations

正Convection and its ensuing severe weather,such as heavy rainfall,hail,tornado,and high wind,have significant impacts on our society and economy(e.g.,Cao et al.,2004;Fritsch and Carbone,2004;Verbout et al.,2006;Ashley and Black,2008;Cao,2008;Cao and Ma,2009;Zhang et al.,2014).Due to its localized and transient nature,the initiation of convection or convective initiation remains one of the least     

中国及周边海域对流云团的水平和垂直尺度

利用2007年1月-2010年12月CloudSat-CALIPSO二级云产品2B-CLDCLASS-LIDAR,统计中国及其周边海域对流云的发生频率,根据对流云发生频率的分布特征将中国及周边海域划分为青藏高原（TP）、东部陆地（EC）、南部海域（SO）和西北太平洋（WP）4个子区域,并研究了4个子区域积云团和深对流云团的水平尺度和垂直尺度。统计结果表明,海洋积云团的水平尺度约为2 km,陆地积云团的水平尺度约为1 km,海洋下垫面热力性质均匀,积云团尺度更大;陆地下垫面非均匀性强,积云团分布更为零散。深对流云团的水平尺度为10-50 km,东部陆地最大,约为45 km,西北太平洋最小,约为30 km。陆地深对流云团水平尺度较海洋上大,且多尺度特征显著,应该与深对流云发生的复杂天气背景有关。积云团的垂直尺度范围为0.24-2 km,4个区域无明显差异。垂直尺度海洋深对流云团大于陆地云团,其中在南部海域地区最大,约为15 km,青藏高原最小,约为10 km。与陆地云团相比,海洋深对流云团表现为水平尺度更小、垂直尺度更大的中尺度对流体特征。      

Deep convective clouds over the northern Pacific and their relationship with oceanic cyclones

Based on combined Cloud Sat/CALIPSO detections, the seasonal occurrence of deep convective clouds(DCCs) over the midlatitude North Pacific(NP) and cyclonic activity in winter were compared. In winter, DCCs are more frequent over the central NP, from approximately 30°N to 45°N, than over other regions. The high frequencies are roughly equal to those occurring in this region in summer. Most of these DCCs have cloud tops above a 12 km altitude, and the highest top is approximately 15 km. These wintertime marine DCCs commonly occur during surface circulation conditions of low pressure, high temperature, strong meridional wind, and high relative humidity. Further, the maximum probability of DCCs,according to the high correlation coefficient, was found in the region 10°–20° east and 5°–10° south of the center of the cyclones. The potential relationship between DCCs and cyclones regarding their relative locations and circulation conditions was also identified by a case study. Deep clouds were generated in the warm conveyor belt by strong updrafts from baroclinic flows. The updrafts intensified when latent heat was released during the adjustment of the cyclone circulation current. This indicates that the dynamics of cyclones are the primary energy source for DCCs over the NP in winter.     

Meteorological conditions and polar stratospheric clouds over Yakutsk in winter 2004/05

Regular observations of polar stratospheric clouds were started at a lidar stratospheric station in the city of Yakutsk. According to lidar measurements in the winter of 2004/05, thick aerosol layers in the lower stratosphere appeared over this region quite frequently. They were episodically observed as pearl clouds. In November 2004, polar stratospheric clouds were observed at the stratospheric temperatures that were much higher than those at which particles of the polar stratospheric clouds could condense. Analysis of air-mass trajectories that simultaneously passed over Yakutsk at different altitudes on the days of polar stratospheric cloud observations showed that clouds could be formed over the Norwegian Sea at altitudes of about 18–21 km, where the stratosphere was the coldest, and then were transported to Yakutsk by wind during 4–5 days.     

The breakup of marine boundary-layer clouds over an inhomogeneous sea surface temperature field

This numerical study examines the breakup of marine atmospheric boundary-layer (MABL) clouds through various physical processes over an inhomogeneous sea surface temperature (SST) field. Three regimes are identified under which the cloud layer will break up. (A) advection of drier air into the MABL for the California case. (B) daytime absorption of solar radiation, occurring most easily over the cold water. (C) mesoscale fluctuations in the flow, producing holes in the cloud layer.The budget study of these three situations concludes that large-scale subsidence, solar radiation, local mesoscale advection, and inhomogeneous surface fluxes cannot be neglected in modeling cloud breakup. This study also confirms the belief that the mixing process alone induced by evaporative entrainment is generally insufficient to predict the breakup of the cloud layer.Sections of this paper are based on an extended abstract by the author and Dr. Steven Stage for the Ninth Symposium on Turbulence and Diffusion held at Riso, Denmark, 1990.     

Intensified reduction in summertime light rainfall over mountains compared with plains in Eastern China

Based on daily rainfall data from 1960 to 2007, this study investigated the difference in rainfall trends between seven mountain stations and 21 nearby plain stations in eastern China for the months June–August. The amount and frequency of light rain (≤2.5 mm/day) over the mountain areas showed a greater decreasing trend than over the surrounding plain regions. The trend of light-rainfall frequency at mountain stations is ??4.8%/decade, approximately double that at plain stations (??2.3%/decade). The trend in light-rainfall amount at mountain stations is ??5.0%/decade, approximately three times that at plains station (??1.4%/decade). Reduced wind speed may explain the enhanced decrease in light rainfall over mountain areas through the weakened orographic lifting. Further study is needed to determine whether the precipitation difference between mountain and plain (urban) regions is exacerbated by air pollution in East China through its indirect effects and influence on regional air stability and wind speed.     

西北区一次联合探测层状云系云物理特征分析

对2003-09-17我国西北地区东部层状云降水天气过程的层状云系进行了联合探测。经观测对比分析发现：对同一天气系统开展多省联合探测．对认识云系发展变化、结构特征及降水潜力很有帮助；同一条锋面云系由于流场结构的差异，其不同位置的云层结构、温度、湿度、水汽含量存在一定差异；陇东和陕北虽然同处于副高西侧的偏南气流里，但是，低层天水处于反气旋区，抑制了水汽辐合，而延安处于横切变右侧的气旋性辐合区，有利于低层水汽辐合上升，促使该地区云系发展加强，其过冷层厚度大于天水地区；PMS粒子测量系统测得在整个过冷层中，小云粒子的数浓度平均值天水比延安少1个量级，2D—P测出延安的粒子谱较宽，说明上述条件下天水地区人工增雨潜力较小．延安地区相对条件较好，人工增雨潜力大。